JP5050853B2 - MRAM - Google Patents

Description

  The present invention relates to an MRAM (Magnetic Random Access Memory), and more particularly to an MRAM that uses a magnetoresistive element that uses a SAF (Synthetic Anti-Ferromagnet) as a magnetization free layer as a memory cell.

  The MRAM is a nonvolatile memory capable of high-speed writing / reading. Most typically, an MRAM includes a magnetoresistive element composed of a magnetization free layer in which magnetization can be reversed, a magnetization fixed layer in which magnetization is fixed, and a nonmagnetic layer interposed therebetween. Use as Data is stored as the magnetization direction of the magnetization free layer. When the nonmagnetic layer is composed of an extremely thin insulator, the magnetoresistive element exhibits a TMR (Tunnel MagnetoRegistance) effect, and the magnetoresistive element thus configured is often called an MTJ (Magnetic Tunnel Junction) element. . On the other hand, when the nonmagnetic layer is formed of a nonmagnetic conductor, the magnetoresistive element exhibits a GMR (Giant MagnetoRegistance) effect, and the magnetoresistive element configured as described above is a CPP-GMR (Current Perpendicular). to Plane Giant MagnetoResistive) element.

  Data is written by applying an external magnetic field to the magnetization free layer, thereby reversing the magnetization of the magnetization free layer in a desired direction. On the other hand, data reading uses the magnetoresistive effect exhibited by the magnetoresistive element. Regardless of whether the TMR effect or the GMR effect is used, the resistance of the magnetoresistive element changes according to the magnetization direction of the magnetization free layer. Using the change in resistance of the magnetoresistive element, the magnetization direction of the magnetization free layer, that is, the written data is determined.

  One problem with MRAM is the selectivity of memory cells in the write operation. In the most conventional MRAM, writing is performed using the fact that the writing threshold curve for the external magnetic field of the magnetization free layer becomes an asteroid curve. Specifically, word lines and bit lines orthogonal to each other are provided in the vicinity of each magnetoresistive element, and these write lines extend in the easy axis and hard axis directions of the magnetization free layer of the magnetoresistive element. Be placed. Then, a current is passed through the word line and bit line intersecting the selected memory cell, and the generated current-induced magnetic field is simultaneously applied to the memory cell. When the magnitude of the synthetic magnetic field at this time reaches the outside of the threshold curve (asteroid) determined by the magnitude of the magnetic anisotropy of the element, the magnetization of the magnetization free layer is two in parallel with the easy axis direction. Flip to face one of the two orientations. At this time, a write current value is set to such an extent that the magnetization of the non-selected memory cell in which current is supplied only to one of the corresponding word line and bit line is not reversed. Since there is a variation in the write magnetic field in an actual memory cell, the selective write by such a method does not have good memory cell selectivity. In the worst case, there may be a case where there is no appropriate selection current value margin, and there is a problem that erroneous writing is likely to occur.

  As one method for improving the memory cell selectivity during the write operation, a “toggle write method” is known (see, for example, US Pat. No. 6,545,906). The toggle write method is a technique for performing a write operation with high selectivity by using SAF for the magnetization free layer and changing the direction of the write magnetic field in time series; where SAF is a plurality of ferromagnetic It is a structure having a layer and in which adjacent ferromagnetic layers are magnetically antiferromagnetically coupled.

  FIG. 1 is a plan view showing a typical configuration of an MRAM employing a toggle writing method. In the memory array of the MRAM, a word line 103 and a bit line 102 orthogonal to the word line 103 are extended. An MTJ element 101 used as a memory cell is provided at each position where the word line 103 and the bit line 102 intersect. As shown in FIG. 2, the MTJ element 101 typically includes a lower electrode layer 111, an antiferromagnetic layer 112, a magnetization fixed layer 113, a barrier layer 114, and a magnetization free layer 115 provided above the substrate 100. , And an upper electrode layer 116. As shown in FIG. 1, the MTJ element 101 is configured so that the easy axes of the magnetization fixed layer 113 and the magnetization free layer 115 form an angle of 45 ° with the word line 103 and the bit line 102, that is, the MTJ element 101. Are arranged such that the longitudinal direction forms an angle of 45 ° with the word line 103 and the bit line 102.

  Referring to FIG. 2 again, the magnetization free layer 115 is composed of SAF. More specifically, the magnetization free layer 115 includes ferromagnetic layers 121 and 122 and a nonmagnetic layer 123 interposed therebetween. The nonmagnetic layer 123 is configured to cause an antiferromagnetic RKKY (Rudermann, Kittel, Kasuya, Yoshida) interaction between the ferromagnetic layers 121 and 122. It is well known to those skilled in the art that by appropriately selecting the material and film thickness of the nonmagnetic layer, the ferromagnetic layers bonded to each other can be antiferromagnetically coupled by the RKKY interaction.

FIG. 9 is a graph showing the dependence of the strength of the binding energy due to the RKKY interaction on the film thickness of the nonmagnetic layer. Here, data is shown when the ferromagnetic layers 121 and 122 are formed of NiFe films and the nonmagnetic layer 123 is formed of a Ru film. J _{saf in} FIG. 9 is an “antiparallel coupling energy constant”, and represents the energy per unit area (antiferromagnetic coupling energy) in which the magnetization of the ferromagnetic layer in the SAF is parallel or antiparallel coupled. Show. Note that the binding energy is defined as positive when the ferromagnetic layers are antiferromagnetically coupled. The binding energy constant _Jsaf due to the RKKY interaction oscillates with an increase in the film thickness of the nonmagnetic layer, exhibits an antiferromagnetic RKKY interaction in a certain range, and exhibits a ferromagnetic property in the other range. RKKY interaction is expressed. At a certain film thickness, no magnetic interaction occurs. The film thickness of the nonmagnetic layer 123 is selected so that they exhibit an antiferromagnetic RKKY interaction. The residual magnetization of the entire magnetization free layer 115 (that is, the magnetization of the entire magnetization free layer 115 when the external magnetic field is 0) is as close to 0 as possible. This condition can be satisfied, for example, by forming the two ferromagnetic layers 121 and 122 with the same material and having the same magnetic volume.

In the toggle writing method, when the two ferromagnetic materials of SAF are composed of substantially equivalent ferromagnetic layers, selective data writing is performed using the property that SAF exhibits spin flop. FIG. 3 shows the magnetization curve of SAF composed of two ferromagnetic layers that develop spin flops. Even if an external magnetic field is applied in the easy axis direction of the SAF, the magnetization of the SAF remains zero if the external magnetic field is small. When the external magnetic field increases and reaches the magnetic field H _flop , magnetization suddenly appears in the SAF. At this time, the magnetizations of the two ferromagnetic layers are arranged so that they are magnetically coupled so that they form an angle smaller than 180 °, and the resultant magnetization is in the direction of the external magnetic field. This phenomenon is called “spin flop”, and the magnetic field H _flop that the spin flop develops is called “flop magnetic field”. Note that the spin flop appears only if the overall remanent magnetization of the SAF is sufficiently small. When the external magnetic field is further increased, the magnetizations of the two ferromagnetic layers are eventually arranged completely in parallel. This magnetic field is called “saturation magnetic field Hs”.

The saturation magnetic field Hs is represented by the following formula:
Hs = 2J _saf / (M · t) −2K / M = Hj−Hk (1)
Hj = 2J _saf / (M · t) (2)
Hk = 2K / M (3)

Here, Hk is “anisotropic magnetic field”, and K is “anisotropic energy”. J _saf is an “antiparallel bond energy constant” as described above and takes a positive value. When the relative magnetization angle of the two ferromagnetic layers is θ, the energy resulting from the antiparallel coupling is represented by J _saf cos θ. The energy resulting from the antiparallel coupling is the smallest at −J _{saf at} θ = 180 degrees which is the antiparallel arrangement, and in that case, the SAF is most stable. Hj is an “anti-parallel coupling magnetic field”, and corresponds to an external magnetic field value that can be resisted until the magnetizations of the ferromagnetic layers coupled in an anti-parallel arrangement by anti-parallel coupling energy become a parallel arrangement. In other words, the antiparallel coupling magnetic field Hj is an amount obtained by converting the antiparallel coupling force of the SAF into a magnetic field.

Further, the flop magnetic field H _flop is expressed as follows by using the saturation magnetic field Hs and the anisotropic magnetic field Hk described above:
H _flop = 2 / M [K (J _saf / t−K)] ^0.5 (4a)
= (Hs · Hk) ^0.5 (4b)
= [(Hj−Hk) · Hk] ^0.5 (4c)
= [(2J _saf / (M · t) −Hk) · Hk] ^0.5 (4d)

FIG. 4 is a conceptual diagram for explaining the procedure of the toggle writing method, and FIG. 5 is a graph showing waveforms of currents flowing through the word line 103 and the bit line 102 when data writing is performed by toggle writing. It should be noted in FIG. 4 that the magnetizations of the ferromagnetic layers 121 and 122 of the magnetization free layer 115 are referred to by symbols M ₁ and M ₂ , respectively.

Data writing by the toggle writing method is performed by rotating the direction of the magnetic field applied to the magnetization free layer 115 in the plane and directing the magnetizations of the ferromagnetic layers 121 and 122 in a desired direction. Specifically, first, a write current is supplied to the word line 103, thereby generating a magnetic field _{HWL in} a direction perpendicular to the word line 103 (time t ₁ ). Subsequently, the write current is supplied to the bit line 102 while the write current is supplied to the word line 103 (time t ₂ ). As a result, a magnetic field H _WL + H _BL is generated in a direction that forms an angle of 45 ° with both the word line 103 and the bit line 102. Subsequently, the supply of the write current to the word line is stopped while the write current is supplied to the bit line (time t ₃ ). As a result, a magnetic field _HBL is generated in a direction perpendicular to the bit line 102 (that is, a direction parallel to the word line 103). When a write current is passed through the word line 103 and the bit line 102 in such a procedure, the magnetic field applied to the magnetization free layer 115 is rotated, thereby rotating the magnetizations of the ferromagnetic layers 121 and 122 by 180 °. be able to.

In the toggle write method, the magnetic field applied to the magnetization free layer 115 when a write current is passed through the word line 103 and the bit line 102 must be larger than the above-described flop magnetic field H _{flop and} smaller than the saturation magnetic field H _s. I must. Otherwise, SAF does not indicate a spin flop and toggle writing cannot be performed.

The advantage of the toggle writing method is that the selectivity of the memory cell is high in principle. FIG. 6 shows a magnetization reversal mode for write current magnetic fields H _WL and H _BL by a bit line and a word line. As shown in FIG. 6, according to the toggle writing method, when a write current is applied only to one of the word line 103 or the bit line 102, the magnetization of the SAF is not reversed in principle (No Switching). In other words, the magnetization of the half-selected memory cell does not reverse undesirably in principle. This effectively improves the write selectivity and reliability of the MRAM. When an external magnetic field corresponding to a toggle inversion region (Toggle Switching) is applied to the magnetization free layer 115, SAF indicates a spin flop and toggle writing is performed.

  Further, as shown in FIG. 6, there is a “direct switching region” adjacent to the origin side of the toggle inversion region. In the direct inversion mode, toggle writing inversion by spin flop is not performed. Instead, the magnetization of the magnetization free layer 115 is directly reversed to either the antiparallel or parallel arrangement with respect to the magnetization of the magnetization fixed layer 113. In toggle writing, it is necessary to avoid a writing magnetic field from entering such a direct inversion region. If the write magnetic field enters the direct inversion region, forcible magnetization reversal in an undesired magnetization direction occurs and erroneous writing occurs.

  This direct inversion region is generated because the ferromagnetic layers 121 and 122 included in the magnetization free layer 115 are not magnetically equivalent. For example, the direct inversion region is said to be generated depending on the difference in the magnetization amounts of the ferromagnetic layers 121 and 122. As the amount of magnetization of one of the ferromagnetic layers increases, the direct inversion region in FIG. 6 expands and the toggle inversion region decreases. When the direct inversion region widens, the threshold value at which toggle inversion starts is shifted to the high magnetic field side. This is a problem due to an increase in the write magnetic field and a decrease in the write magnetic field margin. Therefore, it is said that the difference between the magnetization amounts of the ferromagnetic layers 121 and 122 should be made as small as possible to reduce the direct inversion region.

  Reducing the direct inversion region is one of the most important issues in developing a toggle writing MRAM.

  Now, SAF is described in various documents. The number of ferromagnetic layers included in the SAF of the magnetization free layer may be three or more. The SAF having such a structure is hereinafter referred to as “multilayer SAF”. The following are known as conventional techniques related to the multilayer SAF.

  According to the technique disclosed in US Pat. No. 6,545,906, the number of ferromagnetic layers included in SAF is 3 or more, and the residual magnetization of SAF is within 10% of saturation magnetization. It is configured.

  According to the configuration disclosed in US Pat. No. 6,714,446, the number of ferromagnetic layers included in SAF is four. Also, the saturation magnetic field related to the antiferromagnetic coupling between the first ferromagnetic layer and the second ferromagnetic layer, and the antimagnetic coupling between the third ferromagnetic layer and the fourth ferromagnetic layer. Compared to the saturation magnetic field related to the ferromagnetic coupling, the saturation magnetic field related to the antiferromagnetic coupling between the second ferromagnetic layer and the third ferromagnetic layer is small.

  According to the technique disclosed in Japanese Patent Laid-Open No. 2002-151758, the SAF has a laminated structure of at least five layers in order to improve the stability against thermal fluctuation.

  According to the technique disclosed in Japanese Patent Application Laid-Open No. 2005-85951, the multilayer SAF is composed of a plurality of magnetic layers divided by a nonmagnetic layer. The sum of the magnetization amounts of the even-numbered magnetic layers is substantially equal to the sum of the magnetization amounts of the odd-numbered magnetic layers.

  According to the configuration disclosed in Japanese Patent Laid-Open No. 2005-86015, the number of ferromagnetic layers included in the SAF of the magnetization free layer is four. Further, the strength of the antiferromagnetic interaction between the first ferromagnetic layer and the second ferromagnetic layer is such that the third ferromagnetic layer and the fourth ferromagnetic layer It is almost equal to the strength of the antiferromagnetic interaction between them. In addition, the magnetization amounts of the first and fourth ferromagnetic layers are approximately equal, and the magnetization amounts of the second and third ferromagnetic layers are approximately equal. Further, a configuration is disclosed in which the number of ferromagnetic layers included in the SAF of the magnetization free layer is an even number of four or more, and the thickness of each ferromagnetic layer is substantially symmetric in the vertical direction.

  However, according to experiments conducted by the inventor, even if a multilayer SAF is actually manufactured according to US Pat. No. 6,545,906, it has hardly worked well by itself. Most magnetization free layers did not toggle at all for the applied magnetic field. Even if a configuration that partially operates is obtained, there is a problem that the operation rate is low. In addition, when a writing magnetic field within the operation margin of the multilayer SAF is applied, an element that seems to work well at first glance may not show a toggle operation for a specific applied magnetic field. It was also found out. Such an element is a defective element. These are phenomena that have not been observed in a normal two-layer SAF.

  Further, in order to actually manufacture a multilayer SAF according to JP-A-2005-86015, a nonmagnetic layer between the first ferromagnetic layer and the second ferromagnetic layer, a third ferromagnetic layer, and a fourth ferromagnetic layer are provided. Even when the material and film thickness of the nonmagnetic layer in the meantime were set to be the same, it was still rare to operate by the toggle writing method.

  As described above, it has been said that the direct inversion region is generated depending on the difference in the magnetization amount of the ferromagnetic layer. The amount of magnetization of a ferromagnetic layer is defined as the product of the volume of the ferromagnetic layer that effectively contains magnetization (hereinafter referred to as “magnetic volume V”) and the magnetization M of the ferromagnetic layer. (Magnetization amount = magnetic volume V × magnetization M). A value obtained by dividing the magnetic volume V by the average area of the magnetization free layer is a film thickness that effectively includes magnetization, and is referred to as an “effective film thickness t”. The product of the magnetization M and the effective film thickness t of the ferromagnetic layer is referred to as “magnetization film thickness product” (magnetization film thickness product = magnetization M × effective film thickness t). It can be said that the magnetization film thickness product is the amount of magnetization per unit area.

  In order to reduce the direct region, when adjusting the magnetization amount of each ferromagnetic layer in the SAF of the actual device, it is difficult to realize the same magnetic volume. This is because a dead layer is formed depending on the material of the nonmagnetic layer with which each ferromagnetic layer is in contact. The dead layer is a region in which a certain amount of magnetization disappears due to diffusion or the like around the interface between the ferromagnetic layer and the nonmagnetic layer. This dead layer changes the magnetic volume of each ferromagnetic layer. Further, when processing the magnetization free layer in the processing process, it is difficult to process the side surface completely perpendicularly. For example, when the junction side wall is inclined, even if a ferromagnetic layer having the same film thickness is formed at the time of film formation, the effective magnetic volume of each ferromagnetic layer is different.

  Further, in the magnetization free layer of the actual device, it is desirable to make the film quality of the ferromagnetic layers whose magnetizations are antiparallel to each other as much as possible in the residual magnetization state. For example, when the crystal orientation of the ferromagnetic layer is different, the magnetic properties of the ferromagnetic layer itself such as the magnetocrystalline anisotropy magnetic field and magnetostriction constant are different. As a result, a difference occurs in the magnetic characteristics between the ferromagnetic layers that are paired in the spin flop. If a difference occurs in magnetic characteristics, for example, magnetic anisotropy, between the ferromagnetic layers paired by the spin flop, and the magnetic energy is not equivalent, a direct inversion region is likely to occur. Furthermore, such non-uniformity of the magnetic characteristics of the ferromagnetic layer can be a cause of variation in magnetization reversal characteristics such as a flop magnetic field and a saturation magnetic field.

  In a SAF consisting of two normal ferromagnetic layers, these problems in real devices are not yet serious. This is because the number of ferromagnetic layers is 2 and they are always adjacent to each other via a nonmagnetic layer, so that these ferromagnetic layers are very close to each other in terms of film quality. Also, a large difference is unlikely to occur with respect to the difference in magnetic volume due to the processed shape of the joint side wall described above. Even if there is a difference in magnetic volume, the direct inversion region can be greatly reduced by fixing the thickness of one ferromagnetic layer and fine-tuning the thickness of the other ferromagnetic layer. It is.

  On the other hand, in a multilayer SAF, these problems in a real device are serious. This is because there are three or more layers whose magnetic volume should be adjusted in the magnetization free layer, and the total film thickness is also increased. According to the experiments by the present inventors, there was a large difference in crystallinity between the lowermost part and the uppermost part in the multilayer SAF. In particular, there was a large difference between the film quality of the nonmagnetic layer and its antiparallel coupling force. When a multilayer SAF is actually manufactured, in addition to the difference in characteristics of each ferromagnetic layer and nonmagnetic layer already formed at the time of film formation, there is also a difference in magnetic volume due to the processed shape of the junction side wall. . Furthermore, there is no guarantee that the direct region can be reduced even if only the volume of one ferromagnetic layer is finely adjusted, as in the case of a two-layer SAF. It is difficult to grasp which ferromagnetic layer is actually in what state.

  The multi-layer SAF composed of four layers (or more layers) disclosed in Japanese Patent Application Laid-Open No. 2005-86015 contains many difficulties of such a multi-layer SAF. As pointed out in the literature, the pair of ferromagnetic layers in the remanent magnetization state where the magnetic amounts are equal and the magnetizations are antiparallel are the first and fourth layers. And a second ferromagnetic layer and a third ferromagnetic layer. However, in such a structure, in an actual device, the farthest ferromagnetic layers such as the first ferromagnetic layer and the fourth ferromagnetic layer must always be equivalent to each other. Therefore, the volume difference inevitably caused by processing and the difference in magnetic properties due to different film quality are serious. This becomes more serious when the number of ferromagnetic layers is further increased in order to increase the thermal disturbance resistance in order to cope with a smaller MTJ. This is because the ferromagnetic layers that are required to have equivalent magnetic properties are arranged further apart from each other, and a plurality of ferromagnetic layers are provided.

  As described above, when the conventional multilayer SAF is made into an actual device and adapted to MRAM, it becomes difficult to increase the direct region and control the antiparallel coupling force due to the difference in the magnetic characteristics of the ferromagnetic layers in the multilayer SAF. Problems arise. These cause a decrease in the operation rate of the MRAM (occurrence of a minority bit error) due to generation of defective elements. Further, an increase in the direct inversion region and an increase in the variation in the flop magnetic field cause an increase in the write current value. These are serious problems in MRAM development.

In addition to the toggle writing method, improving the spin-flop characteristics is important for improving the performance of the MRAM using the multilayer SAF as the magnetization free layer. In other words, in the MRAM using the multilayer SAF as the magnetization free layer, it is important from the viewpoint of reducing the write magnetic field (reversal magnetic field) to reduce the flop magnetic field H _flop while maintaining a large saturation magnetic field Hs. The write current is reduced due to the reduction of the reversal magnetic field, which leads to higher performance of the MRAM. The decrease in SAF reversal magnetic field is not limited to the writing method using a current-induced magnetic field, but a “spin transfer method” that causes magnetization reversal by supplying a spin-polarized current to SAF. Is also important because it leads to a reduction in write current.

  A technique capable of operating an MRAM using a multilayer SAF as a magnetization free layer with high yield is desired.

  Accordingly, an object of the present invention is to provide a multilayer SAF capable of improving the operation rate of the MRAM. Another object of the present invention is to provide an MRAM in which defective bits are reduced using the multilayer SAF as a magnetization free layer.

  Another object of the present invention is to provide a multilayer SAF capable of facilitating controllability of magnetic characteristics on an actual device. Another object of the present invention is to provide an MRAM using the multilayer SAF as a magnetization free layer and having improved write characteristics.

  Still another object of the present invention is to provide a technique capable of increasing the write margin of the MRAM.

  The inventor of the present application has found out the essential points in operating the multilayer SAF with high yield through experiments. That is, the antiparallel coupling through the uppermost and lowermost nonmagnetic layers in the multilayer SAF is set so that it can be solved simultaneously with respect to the external magnetic field. For example, the distribution of the magnetization amount of each ferromagnetic layer in the multilayer SAF as pointed out conventionally is not an essential point. Generally, with respect to the uppermost layer and the lowermost ferromagnetic layer, there is only one coupling with a nonmagnetic layer that exhibits antiparallel coupling. For this reason, the antiparallel coupling to the uppermost and lowermost ferromagnetic layers is unstable with respect to the external magnetic field and is most likely to come off. At this time, if the antiparallel coupling to the uppermost layer and the lowermost ferromagnetic layer is not released at the same time and only one of the antiparallel couplings is released, the SAF ferromagnetic layer pair in the removed part immediately becomes parallel arrangement, and the entire multilayer SAF Large remanent magnetization occurs. When the next weak antiparallel coupling is released in a state where the remanent magnetization is large, it is no longer easy to cause a spin flop. The inventor of the present application has determined that this is one of the causes of a decrease in operating rate. If it is understood that the antiparallel coupling via the uppermost and lowermost nonmagnetic layers can be solved at the same time, a new structure that can avoid or reduce the problems contained in the conventional structure becomes possible.

  The MRAM according to the present invention includes a substrate and a magnetoresistive element. A magnetoresistive element includes a magnetization fixed layer having fixed magnetization, a magnetization free layer having reversible magnetization, and a nonmagnetic layer interposed between the magnetization fixed layer and the magnetization free layer and exhibiting a magnetoresistance effect With. The magnetization free layer includes first to Nth ferromagnetic layers (N is 4 or an integer of 6 or more) and first to N-1th non-ferromagnetic layers formed to exhibit antiferromagnetic RKKY interaction. And a magnetic layer. The kth nonmagnetic layer (k is an arbitrary integer between 1 and N−1) of the first to N−1 nonmagnetic layers is the kth ferromagnetic layer of the first to Nth ferromagnetic layers. Between the layer and the (k + 1) th ferromagnetic layer. The first nonmagnetic layer is located closest to the substrate among the first to N-1 nonmagnetic layers, and the N-1 nonmagnetic layer is the first to N-1 nonmagnetic layers. It is located farthest from the substrate.

The volume (magnetization volume) of the k-th ferromagnetic layer is expressed as V _k , and its magnetization is expressed as M _k . Divided by the average area of a plane direction of the magnetization free layer the volume V _k is the effective thickness t _k of the k-th ferromagnetic layer. The relative angle between the magnetization directions of the first k ferromagnetic layer via the first k nonmagnetic layer (k + 1) th ferromagnetic layer is represented by theta _k. The total antiparallel coupling energy of the kth ferromagnetic layer and the (k + 1) th ferromagnetic layer through the kth nonmagnetic layer is expressed as J _k cosθ _k using an antiparallel coupling energy constant J _k having a positive value. expressed.

According to the present invention, M ₁ × t ₁ and M ₂ × t ₂ are approximately equal, and M _N−1 × t _N−1 and M _N × t _N are approximately equal. In addition, one of the following relationships is satisfied:
(A) M ₁ × t ₁ > M _N × t _N and J ₁ > J _N-1
(B) M ₁ × t ₁ <M _N × t _N and J ₁ <J _N-1
Here, the parameter M _k × t _k is the magnetization film thickness product of the k-th ferromagnetic layer.

  Further, it is preferable that N = 4. In that case, there are the effects that the number of stacked magnetization free layers is the smallest and the write margin is increased. The effect has been demonstrated in actual devices by the inventors.

The present invention also provides a magnetization free layer including N ferromagnetic layers, wherein N is an odd number of 3 or more. In that case, the sum of M ₁ × t ₁ and M _N × t _N is approximately equal to M _{(N + 1) / 2} . In addition, one of the following relationships is satisfied:
(A) M ₁ × t ₁ > M _N × t _N and J ₁ > J _N-1
(B) M ₁ × t ₁ <M _N × t _N and J ₁ <J _N-1
Here, the parameter M _k × t _k is the magnetization film thickness product of the k-th ferromagnetic layer.

  It is necessary for realizing the spin-flop operation that the remanent magnetization of these magnetization free layers is within 10% of the saturation magnetization. The toggle write MRAM has the above-described magnetoresistive element arranged as a memory element at the intersection of the word line, the bit line orthogonal to the word line, and the word line and the bit line. The direction of the easy axis of the magnetization free layer of the magnetoresistive element is set to 45 degrees with respect to the extending direction of the word line or bit line.

  The most important point in configuring the magnetization free layer of the present invention is that the magnetic fields at which antiparallel coupling starts to be solved with respect to the external magnetic field are set to be approximately equal for the first ferromagnetic layer and the Nth ferromagnetic layer. is there. That is, the external magnetic field at which the antiparallel coupling to the first ferromagnetic layer starts to dissolve is substantially equal to the external magnetic field at which the antiparallel coupling to the Nth ferromagnetic layer starts to dissolve.

For this purpose, it is preferable that J ₁ / (M ₁ × t ₁ ) and J _N−1 / (M _N × t _N ) are substantially equal with respect to the uppermost lowermost ferromagnetic layer and nonmagnetic layer. . More quantitatively, the ratio of {J ₁ / (M ₁ × t ₁ )} / {J _N−1 / (M _N × t _N )} is preferably 0.8 or more and 1.2 or less. .

The cases where the total anisotropic magnetic fields of the first, second, N-1, and N ferromagnetic layers are greatly different are as follows. The average total anisotropic magnetic field of the kth ferromagnetic layer and the (k + 1) th ferromagnetic layer through the kth nonmagnetic layer is _denoted by Hkk. At this time, referring to Equation 4 (d), the SAF flop magnetic field via the first nonmagnetic layer is given by [{2J ₁ / (M ₁ × t ₁ ) −Hk ₁ } × Hk ₁ ]. . Further, flop field of the SAF through the first N-1 nonmagnetic layer _is given by _{[{2J N-1 / (} M N × t N) -Hk N-1} × Hk N-1]. _{[{2J 1 / (M 1} × t 1) -Hk 1} × H k1] and _{[{2J N-1 / (} M N × t N) -Hk N-1} × Hk N-1] is approximately Set to be equal. More _{quantitatively, [{2J 1 / (M} 1 × t 1) -Hk 1} × Hk 1] / [{2J N-1 / (M N × t N) -Hk N-1} × Hk N ₋₁ ] is preferably 0.8 or more and 1.2 or less.

According to the configuration of the magnetization free layer of the present invention, the magnetization film thickness product of the uppermost ferromagnetic layer and the lowermost ferromagnetic layer may not be equivalent (M ₁ × t ₁ ≠ M _N × t _N , J ₁ ≠ J _N−1 , [J ₁ / (M ₁ × t ₁ )] ≈ [J _N−1 / (M _N × t _N )]). Therefore, the multilayer SAF as a whole can be made equivalent to the magnetization film thickness product of the magnetic layers taking antiparallel arrangement in the residual magnetization state, and even to the magnetic energy. This is a great advantage in real devices. Similarly, [J ₁ / (M ₁ × t ₁ )] ≈ “J _N−1 / (M _N × t _N )” is satisfied in the four-layer SAF disclosed in Japanese Patent Application Laid-Open No. 2005-86015. . However, always _{M 1 × t 1 ≒ M N} × t N, because it is _{J 1} ≒ _{J N-1,} in the actual device, the magnetization film thickness product of the magnetic layer between taking the anti-parallel arrangement, until more magnetic energy Are also difficult to equalize.

  One of the advantages of the multilayer SAF according to the present invention is that the magnetization film thickness product and the magnetic energy of the ferromagnetic layers taking the antiparallel arrangement in the remanent magnetization state can be made equivalent. The following can be considered as the magnetization free layer configuration capable of further drawing out the advantages. That is, when N is an even number of 4 or more, the k-th and (k + 1) -th ferromagnetic layers (k is an odd number) are set to have substantially the same magnetization film thickness product.

In addition, when N is an even number of 4 or more, it is preferable that the antiparallel coupling energy constant J _k is set to be the largest when K = N / 2. In that case, the saturation magnetic field can be further increased. More quantitatively, the N / 2 antiparallel coupling energy constant J _{N / 2} of the SAF through the nonmagnetic layer, the antiparallel coupling energy constant first N-1 nonmagnetic layer of the top has J _N-1 Is the following parameter [J _{N / 2} {1 / (M _{N / 2} × t _{N / 2} ) + 1 / (M _{N / 2 + 1} × t _{N / 2 + 1} )}] / [2J _N−1 / (M _N × t _N )] is set to be large. The parameter is preferably set to be larger than 1. As the ratio is set larger, the saturation magnetic field of the multilayer SAF magnetization free layer extends, and the write margin increases. According to the inventor's experiment, the effect has been demonstrated in the range where the parameter is less than 4. Larger values are possible, although not proven. From at least the SAF portion composed of the first and second ferromagnetic layers sandwiching the first nonmagnetic layer, and the N-1 and Nth ferromagnetic layers sandwiching the N-1 nonmagnetic layer. The saturation magnetic field of the SAF part is set larger than the magnetic field at which the SAF part of the N / 2 ferromagnetic layer and the (N / 2 + 1) th ferromagnetic layer sandwiching the N / 2 non-magnetic layer starts to deviate from the external magnetic field. This is preferable because the spin flop in the multi-layer SAF is not interrupted. The same applies to the lowermost first nonmagnetic layer. In this case, as the above parameter, [J _{N / 2} {1 / ( _{MN / 2} × tN _{/ 2} ) + 1 / ( _{MN / 2 + 1} × tN _{/ 2 + 1} )}] / [2J ₁ / (M ₁ × t ₁ )] is used.

More preferably, for all N−1 nonmagnetic layers, antiparallel coupling energy constants J _k other than the lowermost and uppermost nonmagnetic layers (k is a value other than 1 and N−1). It is set to more than the size equivalent compared to that of the bottom and top layers (J ₁ or J _N-1). In such a configuration, since the lowermost layer and the uppermost ferromagnetic layer are always easily reversed first with respect to the external magnetic field, a stable operation of the magnetization free layer is realized.

The following is conceivable as a film configuration suitable for realizing the magnetic configuration of the magnetization free layer of the present invention. Let α _k be the order of the antiferromagnetic peak of the RKKY interaction of the k-th nonmagnetic layer. The first nonmagnetic layer has a thickness in a range corresponding to the α _first- order antiferromagnetic peak of the RKKY interaction. The N-1th nonmagnetic layer has a film thickness in a range corresponding to the α _N-1 order antiferromagnetic peak of the RKKY interaction. When J ₁ > J _N−1 , the relationship of α ₁ <α _N−1 is satisfied, and when J ₁ <J _N−1 , the relationship of α ₁ > α _N−1 is satisfied. Preferably, when J ₁ > J _N−1 , the relationship α _N−1 = α ₁ +1 is satisfied, and when J ₁ <J _N−1 , the relationship α ₁ = α _N−1 +1 is satisfied. It is. More specifically, when J ₁ > J _N−1 , the first nonmagnetic layer is formed of a ruthenium layer having a thickness of 1.8 nm to 2.5 nm, and the N−1 nonmagnetic layer is It is preferable that the layer is formed of a ruthenium layer having a thickness of 3.1 nm to 3.9 nm. In the case of J ₁ <J _N-1 , the first nonmagnetic layer is formed of a ruthenium layer having a thickness of 3.1 nm to 3.9 nm, and the N-1 nonmagnetic layer is 1.8 nm to 2. It is preferable that the layer is formed of a ruthenium layer having a thickness of 5 nm.

Furthermore, in the magnetization free layer of the present invention, compared with the coupling force of the uppermost and lowermost nonmagnetic layers, the amount of antiparallel coupling of the nonmagnetic layer on the central side is further increased, thereby further saturation. Extending the magnetic field can be expected. For this purpose, when J ₁ > J _N−1 , at least one nonmagnetic layer of the second to (N−2) th nonmagnetic layers has a lower order than the N−1th nonmagnetic layer. It is preferable to have a film thickness in a range corresponding to the antiferromagnetic peak. In that case, the antiparallel coupling force at the center of the magnetization free layer is more than twice as strong as the top. In the case of J ₁ <J _N−1 , at least one of the second to (N−2) nonmagnetic layers corresponds to a lower-order antiferromagnetic peak than the first nonmagnetic layer. It is preferable to have a film thickness in the range. In that case, the antiparallel coupling force at the center of the magnetization free layer is more than twice as strong as the lowest part.

It is desirable to set the nonmagnetic layer having such strong antiparallel coupling force as the centermost layer of the magnetization free layer. That is, when N is an even number, it is desirable to set the (N / 2) nonmagnetic layer to have a strong antiparallel coupling force. Preferably, in the case of J ₁ > J _N−1 , the relationship of α ₁ <α _N-1 and α _{N / 2} <α _N-1 is satisfied. In the case of J ₁ <J _N−1 , the relationship of α ₁ > α _N−1 and α _{N / 2} <α ₁ is satisfied. More preferably, when J ₁ > J _N−1 , the relationship α _{N / 2} + 1 = α _N−1 is satisfied, and when J ₁ <J _N−1 , α _{N / 2} + 1 = α ₁ The relationship is satisfied. Specifically, the (N / 2) nonmagnetic layer is formed of a ruthenium layer having a thickness of 1.8 nm to 2.5 nm, and the first nonmagnetic layer or the N−1 nonmagnetic layer is 3 Formed with a ruthenium layer having a thickness of .1 nm to 3.9 nm. Alternatively, the (N / 2) non-magnetic layer is formed of a ruthenium layer having a thickness of 0.7 nm to 1.2 nm, and the first non-magnetic layer or the N-1 non-magnetic layer is 1.8 nm to It is formed of a ruthenium layer having a thickness of 2.5 nm.

  In the magnetization free layer according to the present invention, the first nonmagnetic layer and the N-1th nonmagnetic layer may have different structures. For example, the film thickness and crystal orientation of the first nonmagnetic layer and the N-1th nonmagnetic layer are different. In that case, it is preferable that the N-1th nonmagnetic layer is thicker than the first nonmagnetic layer. Furthermore, the crystal orientations of the first ferromagnetic layer and the N-1th ferromagnetic layer that exist directly under these nonmagnetic layers may be different. In addition, at least one of the first ferromagnetic layer, the second ferromagnetic layer, the N-1th ferromagnetic layer, and the Nth ferromagnetic layer includes two layers having different constituent elements or constituent element compositions. It is a laminated film laminated as described above. By changing the thickness ratio of the laminated film, the antiparallel coupling force of the first nonmagnetic layer and the N-1th nonmagnetic layer is controlled to a desired value. Thereby, it is possible to realize the magnetization free layer according to the present invention. As such a laminated film, a laminated film of a NiFe film and a CoFe film is suitable.

  Further, the first nonmagnetic layer and the (N-1) th nonmagnetic layer may have substantially the same structure. In that case, the elemental composition ratios of all parts directly in contact with the upper and lower interfaces of the first nonmagnetic layer are different from the element composition ratios of all parts directly in contact with the upper and lower interfaces of the N-1 nonmagnetic layer. By setting as described above, the antiparallel coupling force can be adjusted. Also in this case, at least one of the first ferromagnetic layer, the second ferromagnetic layer, the N-1th ferromagnetic layer, and the Nth ferromagnetic layer has a constituent element or constituent element composition. This is a laminated film in which two or more different films are laminated. By changing the thickness ratio of the laminated film, the antiparallel coupling force of the first nonmagnetic layer and the N-1th nonmagnetic layer is controlled to a desired value. Thereby, it is possible to realize the magnetization free layer according to the present invention. As such a laminated film, a laminated film of a NiFe film and a CoFe film is suitable.

FIG. 1 is a plan view showing a typical configuration of an MRAM corresponding to the toggle writing method. FIG. 2 is a cross-sectional view showing a typical configuration of an MTJ element incorporated in an MRAM that supports the toggle writing method. FIG. 3 is a graph showing a typical magnetization curve of SAF expressing a spin flop. FIG. 4 is a conceptual diagram showing a data writing procedure by the toggle writing method. FIG. 5 is a graph showing a waveform of a write current that flows in the bit line and the word line when data writing is performed by the toggle writing method. FIG. 6 is a graph showing the write magnetic field value dependency of the write characteristics of the MRAM employing the toggle write method. FIG. 7A is a cross-sectional view showing the configuration of the MTJ element of the MRAM according to the first embodiment of the present invention. FIG. 7B is a cross-sectional view showing the configuration of the MTJ element of the MRAM according to the second exemplary embodiment of the present invention. FIG. 7C is a cross-sectional view showing the configuration of the MTJ element of the MRAM according to the third exemplary embodiment of the present invention. FIG. 8A is a cross-sectional view of an actual device of the MTJ element of the MRAM according to the fourth embodiment of the present invention. FIG. 8B is a cross-sectional view of an actual MTJ element as a comparative example for an MRAM according to the fourth embodiment of the present invention. FIG. 9 is a graph showing the dependence of the binding energy constant of the RKKY interaction on the thickness of the nonmagnetic layer when NiFe is a ferromagnetic layer and Ru is a nonmagnetic layer. FIG. 10A is a table showing configurations of the magnetization free layers of the samples of Comparative Examples 1 to 7 and Examples 1 to 4. FIG. 10B is a table showing configurations of the magnetization free layers of the samples of Comparative Examples 1 to 7 and Examples 1 to 4. FIG. 11 is a table showing toggle writing characteristics of 0.6 × 1.2 μm ² oval MTJ devices of the samples of Comparative Examples 1-7 and Examples 1-4. FIG. 12 is a table showing detailed toggle writing characteristics of the oblong MTJ devices of the samples of Comparative Examples 1 and 2 and Examples 2 and 4. FIG. 13 compares the saturation magnetic field exhibited only by the SAF via the first nonmagnetic layer of the samples of Comparative Examples 3-7 and Examples 1-4, and the saturation magnetic field exhibited by only the SAF via the third nonmagnetic layer. FIG. FIG. 14 shows the toggle writing operation rate of the MTJ device of 0.6 × 1.2 μm ² obtained from the samples of Comparative Examples 3 to 7 and Examples 1 to 4 and FIG. 13, and [J ₁ / (M _{1 × t 1)] / [J} 3 / (M 4 × t 4)] is a diagram comparing the ratio.

  The best mode for carrying out the invention will be described below. In the present embodiment, in order to clarify the essence of the present invention, a simplified case is considered that the shape magnetic anisotropy is weak and need not be considered. It should be noted that for finer elements, correction must be made by the shape magnetic anisotropy of the ferromagnetic layer and antiferromagnetic magnetostatic coupling between the ferromagnetic layers.

(First embodiment)
FIG. 7A is a cross-sectional view showing the structure of the MTJ element 1A employed in the memory cell of the MRAM according to the first embodiment of the present invention. The MTJ element 1 </ b> A includes a lower electrode layer 11, an antiferromagnetic layer 12, a magnetization fixed layer 13, a barrier layer 14, a magnetization free layer 15 </ b> A, a cap layer 16, and an upper electrode layer 17.

  The MTJ element 1A is arranged so as to correspond to the toggle writing method, for example. Specifically, similar to the MTJ element 101 of the conventional MRAM shown in FIG. 1, the MTJ element 1A has an angle of 45 ° with respect to the word line (and the bit line perpendicular thereto). It is arranged to make. Thereby, the easy axis of the ferromagnetic layer constituting the magnetization fixed layer 13 and the magnetization free layer 15A is oriented in a direction that forms an angle of 45 ° with respect to the word line (and the bit line perpendicular thereto). Hereinafter, the configuration of the MTJ element 1A will be described in detail.

  The lower electrode layer 11 is formed on a substrate 10 on which MOS transistors (not shown) are integrated, and functions as a path for providing an electrical connection to the magnetization fixed layer 13. The lower electrode layer 11 is made of, for example, Ta, TaN, Ti, TiN, and Nb.

  The antiferromagnetic layer 12 is formed of an antiferromagnetic material such as PtMn, IrMn, or NiMn, for example, and has a role of fixing the magnetization of the magnetization fixed layer 13.

  The magnetization fixed layer 13 is formed of a magnetically hard ferromagnetic material such as CoFe. The magnetization of the magnetization fixed layer 13 is fixed by the exchange interaction in which the antiferromagnetic layer 12 acts. The magnetization fixed layer 13 may be configured by the above-described SAF. For example, the magnetization fixed layer 13 can be composed of two CoFe films and a Ru film inserted between them. In this case, the Ru film is formed so as to have a film thickness that exhibits an antiferromagnetic RKKY interaction.

The barrier layer 14 is often an amorphous insulator film that is thin enough to allow a tunnel current to flow. When the barrier layer 14 is amorphous, as will be described later, it greatly affects the crystallinity of the film constituting the magnetization free layer 15A. More specifically, the barrier layer 14 includes, for example, alumina (AlO _x ), magnesium oxide (MgO), zirconium oxide (ZrO ₂ ), hafnium oxide (HfO ₂ ), silicon oxide (SiO ₂ ), aluminum nitride (AlN). ) And the like. The barrier layer 14 does not need to be amorphous, and may be formed of, for example, single crystal MgO having a NaCl structure.

  The magnetization free layer 15A is composed of SAF having four ferromagnetic layers. More specifically, the magnetization free layer 15 </ b> A includes ferromagnetic layers 21 to 24 and nonmagnetic layers 31 to 33 interposed therebetween. The ferromagnetic layer 21 is formed on the barrier layer 14, and the nonmagnetic layer 31 is formed on the ferromagnetic layer 21. On the nonmagnetic layer 31, the ferromagnetic layer 22, the nonmagnetic layer 32, the ferromagnetic layer 23, the nonmagnetic layer 33, and the ferromagnetic layer 24 are sequentially formed in this order. The magnetic volumes of the ferromagnetic layer 21 and the ferromagnetic layer 22 are substantially equal to the magnetic volumes of the ferromagnetic layer 23 and the ferromagnetic layer 24, and the magnetic volumes of the ferromagnetic layer 21 and the ferromagnetic layer 24 are different from each other. This is one of the important points of the present invention. In the present embodiment, it is assumed that the magnetic volume of the ferromagnetic layer 24 is smaller than that of the ferromagnetic layer 21. At this time, the antiparallel coupling energy of the nonmagnetic layer 33 must be set smaller than that of the nonmagnetic layer 31. Otherwise, the antiparallel coupling with respect to the ferromagnetic layer 21 is first removed by a low external magnetic field.

The magnetization free layer 15A according to the present embodiment has the following film configuration from the ferromagnetic layer 21 on the tunnel barrier layer 14 to the ferromagnetic layer 24 below the cap layer 16:
Tunnel barrier layer / NiFe (4.8 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (4.8 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (3. 3 nm) / CoFe (0.25 nm) / Ru (3.5 nm) / NiFe (3.7 nm) / cap layer.

  In the above configuration, the ferromagnetic layer 21 and the ferromagnetic layer 22 correspond to NiFe (4.8 nm) / CoFe (0.35 nm), and the ferromagnetic layer 23 and the ferromagnetic layer 24 each have NiFe (3.3 nm). ) / CoFe (0.25 nm) and NiFe (3.7 nm). The magnetization film thickness product of the ferromagnetic layer 21 and the ferromagnetic layer 22 is about 4.72 Tnm, and the magnetization film thickness product of the ferromagnetic layer 23 and the ferromagnetic layer 24 is about 3.15 Tnm. The ferromagnetic layer 21 has a magnetization film thickness product 1.5 times that of the ferromagnetic layer 24. The nonmagnetic layers 31 to 33 correspond to Ru (2.1 nm), Ru (2.1 nm), and Ru (3.5 nm).

  In the present embodiment, as means for setting antiparallel coupling energy via the nonmagnetic layers 31 to 33, (I) the ferromagnetic layers 21 to 24 are composed of a plurality of materials, and the interface material of the nonmagnetic layer is Control by changing and (II) control by directly changing the film thickness of the nonmagnetic layer are combined.

Regarding the former, specifically, the ferromagnetic layer is formed to have a two-layer or three-layer structure such as NiFe / CoFe, CoFeB / CoFeNi, CoFe / NiFe / CoFe, etc. Among them, the nonmagnetic layers 31 to 33 are formed. The film thickness of the layer (CoFe or CoFeNi) in contact with is controlled. This utilizes the fact that the strength of the RKKY interaction differs depending on the material in direct contact with the nonmagnetic layers 31 to 33. Specifically, a larger J _SAF is obtained with a Co-rich material than with a Ni-rich material. Therefore, NiFe is the main component of the ferromagnetic layer, and CoFe of 1 nm or less is thinly formed on the nonmagnetic layer side. It is possible to increase J _SAF as the film thickness of CoFe inserted into the NiFe / Ru interface increases. Similarly, in the case of an alloy such as CoFeNi, the J _SAF decreases as the Ni-rich ferromagnetic film. Moreover, it is not limited to CoFe and NiFe, and may be composed of a laminated film or an alloy film containing a plurality of other elements (including nonmagnetic elements). In the present specification, the ferromagnetic layer means a layer having the same direction of ferromagnetic magnetization as a whole, and should be interpreted as being composed of a single ferromagnetic film. Don't be. For example, a laminated film composed of a plurality of ferromagnetic films such as NiFe / CoFe is also one ferromagnetic layer. Also, a laminate composed of two ferromagnetic films and a nonmagnetic film interposed between the two ferromagnetic films is also one ferromagnetic layer. The magnetization film thickness product in the case of such a laminated film of a plurality of ferromagnetic films is defined as the sum of the magnetization film thickness products of each ferromagnetic film.

  Regarding the latter, the film thickness of the nonmagnetic layer must be set in consideration of the crystallinity of the nonmagnetic layer. As shown in FIG. 9, the strength of the RKKY interaction depends on the material and film thickness of the nonmagnetic layer. Therefore, in order to make the strength of the antiferromagnetic RKKY interaction expressed by the nonmagnetic layer 31 and the nonmagnetic layer 33 the same, the nonmagnetic layer 31 and the nonmagnetic layer 33 are made of the same material. It may be considered that the thickness should be the same. However, according to experiments by the inventors, in an actually integrated MRAM, even if the nonmagnetic layer 31 and the nonmagnetic layer 33 are formed with the same material and the same film thickness, the nonmagnetic layer 31 and the nonmagnetic layer The strength of the RKKY interaction expressed by 33 is not the same. This is because the nonmagnetic layer 31 and the nonmagnetic layer 33 have different crystallinity. The magnetization free layer 15 </ b> A has a ferromagnetic layer 21, a nonmagnetic layer 31, a ferromagnetic layer 22, a nonmagnetic layer 32, a ferromagnetic layer 23, a nonmagnetic layer 33, and a ferromagnetic layer 24 sequentially on the barrier layer 14. Since it is formed by stacking, Ru of the nonmagnetic layer 33 formed later has a crystal orientation of HCP (hexagonal close packed) <0001> rather than Ru of the nonmagnetic layer 31 formed earlier. high. The strength of the RKKY interaction is stronger as the crystal orientation is better. When the nonmagnetic layer 31 and the nonmagnetic layer 33 are formed with the same material and the same film thickness, the nonmagnetic layer 31 is stronger than the nonmagnetic layer 31. No. 33 will express a stronger RKKY interaction. In order to make rough adjustment so that the strength of the RKKY interaction is not too far away, rather, the nonmagnetic layer 31 is formed so as to have a film thickness in a range corresponding to a relatively low order peak. It is preferable to form the layer 33 so as to have a film thickness in a range corresponding to a relatively high-order peak. Thereby, the effect that the strength of the RKKY interaction expressed by the nonmagnetic layer 33 is strengthened by good crystallinity, and the effect that it is weakened by having a film thickness corresponding to a relatively high-order peak Is canceled and the antiferromagnetic RKKY interaction strength expressed by the nonmagnetic layer 31 and the nonmagnetic layer 33 can be roughly adjusted so as not to be too far away.

Specifically, in the present embodiment, the barrier layer 14 is made of amorphous AlO _x , and the nonmagnetic layer 31 of the magnetization free layer 15A grown on the barrier layer 14 is a second antiferromagnetic peak (antireflection layer) of the RKKY interaction. The nonmagnetic layers 32 and 33 have a film thickness in the range corresponding to the third antiferromagnetic peak (antiferromagnetic 3rd peak). ing. More specifically, when the nonmagnetic layers 31 to 33 are formed of ruthenium, the nonmagnetic layer 31 is formed so that the film thickness exceeds 1.8 nm and is less than 2.5 nm. The nonmagnetic layer 33 is formed so that the film thickness is more than 3.1 nm and less than 3.9 nm. Most preferably, the nonmagnetic layer 31 is formed to have a film thickness of 2.1 nm corresponding to the antiferromagnetic 2nd peak, and the nonmagnetic layer 33 is formed of 3.5 nm corresponding to the antiferromagnetic 3rd peak. It is formed to have a film thickness. Such a combination of film thicknesses is effective for coarsely adjusting the strength of the antiferromagnetic RKKY interaction expressed by the nonmagnetic layer 31 and the nonmagnetic layer 33. Therefore, in this embodiment, the nonmagnetic layer 31 is formed of Ru (2.1 nm), and the nonmagnetic layer 33 is formed of Ru (3.5 nm). In addition, not only when the nonmagnetic layer 31 is set to the antiferromagnetic 2nd peak, but generally, the antiferromagnetic peak applied to the nonmagnetic layer 31 is antiferromagnetic applied to the nonmagnetic layer 33. If the magnetic peaks are set to a higher order by the first order, their antiparallel coupling forces can be adjusted to the same order.

  In the stacked magnetization free layer shown in FIG. 7A, not only the nonmagnetic layers 31 to 33 but also the ferromagnetic layers 21 to 24, the crystallinity is improved as the upper layer is increased. For example, when permalloy is used as the ferromagnetic layers 21 to 24 and ruthenium is used as the nonmagnetic layers 31 to 33, the permalloy of the ferromagnetic layer 23 is less than the FCC (face center cubic) < 111> degree of orientation is high. Therefore, regarding the ruthenium of the nonmagnetic layer 31 and the nonmagnetic layer 33 grown immediately above, the degree of orientation of HCP <0001> is higher in the ruthenium in the nonmagnetic layer 33 than in the nonmagnetic layer 31.

Such a technique is particularly effective when the laminated magnetization free layer is formed on the AlO _x barrier layer as described above, but is not limited thereto. For example, even when the magnetization free layer is disposed below the tunnel barrier layer, it is possible to use a microcrystalline or amorphous material for the underlayer of the magnetization free layer in order to obtain flatness of the underlayer. In addition, there may be a case where preparation of a sufficient base layer is restricted due to the restriction of the film thickness. Even in such a case, the technique according to the present invention is effective. Further, even when the crystallinity of the underlayer of the magnetization free layer is good, the magnetization free layer to be grown thereon does not always grow while having a suitable crystal orientation. Rather, in general, undesired orientation on crystal planes or uneven growth occurs due to lattice matching or mismatching. Even in that case, the present technology is particularly effective. Since the lowermost nonmagnetic layer of the magnetization free layer cannot avoid the influence of crystal growth or uneven growth on an undesired crystal plane, it has undesired crystallinity and weakens the RKKY interaction. However, until the uppermost nonmagnetic layer grows, it is expected that the uppermost nonmagnetic layer recovers the desired crystallinity by the ferromagnetic / nonmagnetic laminated base, thereby enhancing the RKKY interaction. Is done.

As a result of adjusting the antiparallel coupling force as described above, in the present embodiment, the J _SAFs of the nonmagnetic layers 31 and 33 are about 0.015 erg / cm ² and 0.011 erg / cm ² , respectively. . Hj (= 2J _SAF / (M × t): refer to formula (2)) obtained by converting these antiparallel coupling forces into the magnitude of the magnetic field is 81 Oe and 86 Oe, respectively, and is set almost equal. According to the inventor's experiment, more quantitatively, it is desirable that the difference in Hj falls within about 20%. That is, the parameter [J ₁ / (M ₁ × t ₁ )] / [J ₃ / (M ₄ × t ₄ )] corresponding to the ratio of Hj between the nonmagnetic layer 31 and the nonmagnetic layer 33 is 0.8 or more. It is desirable to set it to 1.2 or less. Further, considering the difference in magnetic anisotropy more strictly, the average anisotropic magnetic field Hk ₁ of the ferromagnetic layers 21 and 22 is 9.5 Oe, and the average anisotropic magnetic field Hk of the ferromagnetic layers 23 and 24 is ₃ is 7Oe. In this case, the flop magnetic fields [{(Hj−Hk) × Hk}] ^ 0.5 (see Equation 4 (c)) of the nonmagnetic layers 31 and 33 are 26 Oe and 23.5 Oe, respectively, and there is a slight difference. spread. In this case, if J _{SAF of the} nonmagnetic layer 33 is slightly increased, it is possible to operate more suitably.

In the present embodiment, as a further aim to bring out additional effects, the central nonmagnetic layer 32 is set to exhibit the largest J _SAF . Regarding the nonmagnetic layer 32, its own material configuration and the configuration of the ferromagnetic layer interface in contact with the upper and lower sides are almost the same as the nonmagnetic layer 31. However, J _{SAF of the} nonmagnetic layer 32 is about 0.038 erg / cm ^2, which is the largest among all the nonmagnetic layers in the SAF. That is, Ru (2.1 nm) of the nonmagnetic layer 32 is NiFe (4.8 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (4.8 nm) / CoFe (0.35 nm). This is because it is formed on the ground and the crystal orientation of HCP <0001> is increased. Since the magnetic field from which the antiparallel coupling through the nonmagnetic layer 32 is released is larger than that in the nonmagnetic layer 31 and the nonmagnetic layer 33, the saturation magnetic field of the magnetization free layer is greatly increased (stretched). As a result, the write margin is increased.

  Here, the stronger the antiparallel coupling through the nonmagnetic layer 32, the better. This is because if the antiparallel coupling is too strong, the magnetization of the ferromagnetic layers via the nonmagnetic layer 31 or 33 is saturated before the ferromagnetic layers via the nonmagnetic layer 32 cause spin flops. Because. In this case, the spin flop of the entire magnetization free layer is interrupted, causing a write failure. Therefore, in order to obtain a suitable antiparallel coupling force, it is desirable that the nonmagnetic layer 32 uses only one lower-order antiferromagnetic peak than the nonmagnetic layer 33 that flops at the lowest magnetic field. More specifically, when the nonmagnetic layer 32 and the nonmagnetic layer 33 are formed of ruthenium, the nonmagnetic layer 32 is formed so that the film thickness is more than 1.8 nm and less than 2.5 nm. The nonmagnetic layer 33 is formed so that its film thickness is more than 3.1 nm and less than 3.9 nm. Alternatively, when the nonmagnetic layer 32 is formed to have a thickness exceeding 0.7 nm and less than 1.2 nm, the nonmagnetic layer 33 has a thickness exceeding 1.8 nm. It is formed to be less than 5 nm.

  Most preferably, the nonmagnetic layer 31 is formed to have a film thickness of 2.1 nm corresponding to the antiferromagnetic 2nd peak, and the nonmagnetic layer 33 is formed of 3.5 nm corresponding to the antiferromagnetic 3rd peak. It is formed to have a film thickness. Alternatively, the nonmagnetic layer 31 is formed to have a thickness of 0.9 nm corresponding to the antiferromagnetic 1st peak, and the nonmagnetic layer 33 has a thickness of 2.1 nm corresponding to the antiferromagnetic 2nd peak. Formed as follows.

The cap layer 16 is a layer for protecting the magnetization fixed layer 13, the barrier layer 14, and the magnetization free layer 15A. The cap layer 16 is made of Ta or Ru, for example. The cap layer 16 can also be formed of AlO _x that is extremely thin to the extent that a tunnel current flows.

  The upper electrode layer 17 functions as a path that provides an electrical connection to the magnetization free layer 15A. The upper electrode layer 17 is made of, for example, Ta, TaN, TiN, Cu, or Al.

(Second Embodiment)
FIG. 7B is a cross-sectional view showing the configuration of the MTJ element 1B according to the second embodiment of the present invention. As in the MTJ element 1A shown in FIG. 7A, the MTJ element 1B shown in FIG. 7B includes four ferromagnetic layers 21 to 24 and three nonmagnetic layers 31 to 31. 33.

The difference is the setting of the magnetic volume of the ferromagnetic layers 21 to 24 and the setting of the antiparallel coupling force via the nonmagnetic layers 31 to 33 in the magnetization free layer. In the second embodiment, the magnetization free layer 15B has the following film configuration from the ferromagnetic layer 21 on the tunnel barrier layer 14 to the ferromagnetic layer 24 below the cap layer 16:
Tunnel barrier layer / NiFe (3 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (3 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (4.6 nm) / CoFe (0.45 nm) / Ru (3.5 nm) / NiFe (4.6 nm) / CoFe (0.45 nm) / cap layer.

The ferromagnetic films 21 and 22 have the same magnetization film thickness product of 3.15 Tnm, and the ferromagnetic films 23 and 24 have the same magnetization film thickness product of 4.72 Tnm. Compared with the ferromagnetic layer 21, the magnetic film thickness of the ferromagnetic layer 24 is about 1.5 times as large as the magnetic film thickness product. Further, anti-parallel coupling energy _{J 1} of 31 of the non-magnetic layer is about 0.013erg / cm ^2, the antiparallel coupling energy _{J 3} of the non-magnetic layer 32 is about 0.021erg / cm ^2. At this time, Hj related to the nonmagnetic layer 31 is set to 107 Oe, and Hj related to the nonmagnetic layer 33 is set to 114 Oe, which are substantially equal. As a result, the antiparallel coupling between the ferromagnetic layer 21 and the ferromagnetic layer 24 can be removed almost simultaneously, and the operation is preferably performed. Further, in the present embodiment, the ferromagnetic layers 21 and 22 and the ferromagnetic layers 23 and 24 are preferable because they have the same configuration and are close in crystallinity.

  In the second embodiment, since the antimagnetic coupling force stronger than the nonmagnetic layer 31 is required for the nonmagnetic layer 33, the nonmagnetic layer 33 has a Ru (2.1 nm) corresponding to the 2nd peak. It is good. In that case, if the nonmagnetic layer 33 is replaced with Ru (2.1 nm) as it is, the antiparallel coupling force becomes too strong. For example, the CoFe (0.45 nm) of the ferromagnetic layer 23 is reduced, and the ferromagnetic layer 23 is reduced. By comprising almost only NiFe, the antiparallel coupling force of the nonmagnetic layer 33 can be finely adjusted to a suitable value. At this time, the nonmagnetic layer 31 and the nonmagnetic layer 33 are composed of substantially the same film thickness and material, but the element composition ratios of all the portions that are in direct contact with the upper and lower interfaces of the nonmagnetic layer 31 and the nonmagnetic layer 33. Is different. Specifically, elements such as Ni, Fe, and Co are in direct contact with the upper and lower interfaces of the nonmagnetic layer 31, but the nonmagnetic layer 33 is only Ni and Fe. Compared with the nonmagnetic layer 33, the elemental composition ratio of the portion in contact with the nonmagnetic layer 31 is Co rich.

(Third embodiment)
The present invention is also applicable when the number of ferromagnetic layers included in the SAF constituting the magnetization free layer is different from the first and second embodiments, or when the number is not an even number but an odd number. is there. For example, as shown in FIG. 7C, the magnetization free layer 15C of the MTJ element 1C is formed of three ferromagnetic layers 21 to 23 and nonmagnetic layers 31 and 32 inserted therebetween. An example will be described.

In this case, as an example of the film configuration from the ferromagnetic layer 21 on the tunnel barrier layer 14 to the ferromagnetic layer 23 below the cap layer 16, the following configuration can be given:
Tunnel barrier layer / NiFe (4.6 nm) / CoFe (0.45 nm) / Ru (2.1 nm) / NiFe (8.4 nm) / CoFe (0.4 nm) / Ru (3.5 nm) / NiFe (3. 7 nm) / cap layer.

  With the above configuration, the ferromagnetic layer 21 corresponds to NiFe (4.6 nm) / CoFe (0.45 nm), the ferromagnetic layer 23 corresponds to NiFe (3.7 nm), and the ferromagnetic layer 22 corresponds to NiFe (8 .4 nm) / CoFe (0.4 nm).

  For the same reason as in the first embodiment, Ru (2.1 nm) antiferromagnetic 2nd-peak is used for the nonmagnetic layer 31 with poor crystal orientation of HCP <0001>, and the crystal orientation The antiparallel coupling force is roughly adjusted by using an antiferromagnetic 3rd-peak of Ru (3.5 nm) with respect to the nonmagnetic layer 32 whose properties are slightly improved. The magnetization film thickness product is about 4.72 Tnm for the ferromagnetic layer 21, about 7.9 Tnm for the ferromagnetic layer 22, and about 3.15 Tnm for the ferromagnetic layer 23. Thus, the magnetization film thickness product of the ferromagnetic layer 22 is set to be the sum of the magnetization film thickness products of the ferromagnetic layers 21 and 23.

  More generally, in the case of a multilayer SAF magnetization free layer of N ferromagnetic layers (N is an odd number of 3 or more), the Nth ferromagnetic layer that is the uppermost layer of the magnetization free layer and the first ferromagnetic layer that is the lowermost layer. The magnetization film thickness product of the ((1 + N) / 2) ferromagnetic layer at the center may be set so as to be substantially the same as the sum of the magnetization film thickness products of the layers. The reason is as follows. In the multilayer SAF in which N is an odd number, the uppermost and lowermost ferromagnetic layers are always arranged in parallel in the residual magnetization state. Therefore, the role of canceling the magnetization film thickness product of the Nth ferromagnetic layer and the first ferromagnetic layer is assigned to the most central ferromagnetic layer in the multilayer SAF, so that the spin flop of the entire magnetization free layer is reduced. Stabilized. When N is an odd number, the magnetization of the ((1 + N) / 2) ferromagnetic layer that is the centermost layer in the residual magnetization state is always antiparallel to the magnetizations of the Nth ferromagnetic layer and the first ferromagnetic layer. It becomes. In particular, in a multilayer SAF with N = 3 and N = 5, unless the magnetization of the uppermost and lowermost ferromagnetic layers is canceled by the magnetic layer at the center, the magnetization of the entire magnetization free layer is changed to the residual magnetization state. Cannot cancel at.

As described above, this embodiment is the same as the first, second, and fourth embodiments except for the magnetization adjustment method. The uppermost and lowermost ferromagnetic layers may be set so as to be simultaneously removed from the external magnetic field. In the example shown in this embodiment, the antiparallel coupling energy constants of the nonmagnetic layer 31 and the nonmagnetic layer 32 are about J ₁ = 0.02 erg / cm ² and J ₂ = 0.014 erg / cm ² , respectively. The parameters J ₁ / (M ₁ × t ₁ ) and J ₂ / (M ₃ × t ₃ ) obtained by converting them into magnetic fields are set to be approximately equal to 53 Oe and 56 Oe, respectively.

(Fourth embodiment)
More generally, the configuration necessary for the operation of the present invention will be described when N is an integer of 4 or 6 and the multilayer SAF is composed of N ferromagnetic layers and N-1 nonmagnetic layers. . The lowermost ferromagnetic layer is the first ferromagnetic layer, the uppermost ferromagnetic layer is the Nth ferromagnetic layer, the nonmagnetic layer immediately above the lowermost ferromagnetic layer is the first nonmagnetic layer, and the uppermost strong layer is strong. The nonmagnetic layer immediately below the magnetic layer is referred to as the (N-1) th nonmagnetic layer. As a minimum necessary condition, M ₁ × t ₁ and M ₂ × t ₂ are equal, and M _N−1 × t _N−1 and M _N × t _N are required to be equal. Further, M ₁ × t ₁ > M _N × t _N and J ₁ > J _N , or M ₁ × t ₁ <M _N × t _N and J ₁ <J _N It is necessary to be. Furthermore, if the parameters J ₁ / (M ₁ × t ₁ ) and J _N / (M _N × t _N ) are substantially equal, the operation is more preferably performed.

The average anisotropic magnetic field Hk _{1 of} the first ferromagnetic layer and the second ferromagnetic layer is equal to the average anisotropic magnetic field Hk _{N− of} the (N−1) th ferromagnetic layer and the Nth ferromagnetic layer. If the difference is significantly different from ₁ , the parameter [{2J ₁ / (M ₁ × t ₁ ) −Hk ₁ } × Hk ₁ corresponding to the flop magnetic field when the uppermost layer and the lowermost layer SAF coupling portion exist independently. ] And [{2J _N−1 / (M _N × t _N ) −Hk _N−1 } × Hk _N−1 ] need to be approximately equal. By changing the film thickness and material of the nonmagnetic layer, the interface material of the magnetic layer / nonmagnetic layer, the film thickness of the magnetic layer, etc., the parameters of the magnetization free layer are controlled so as to satisfy such requirements. Just do it.

  In the case of a magnetization free layer having a large N such as 7 layers, 9 layers, 11 layers, etc., where N is an odd number, the fourth embodiment is more preferable than the third embodiment. This is because according to the present embodiment, the most adjacent ferromagnets (the first and second ferromagnetic layers and the (N-1) th ferromagnet and the Nth ferromagnet layer) are set to be magnetically equivalent. This is because it is easy to make the ferromagnetic layers which are antiparallel in the remanent magnetization state more magnetically equivalent in the magnetization free layer.

The J _SAF of the second to (N−2) th nonmagnetic layers is preferably set to be equal to or greater than that of the first and (N−1) th nonmagnetic layers. Furthermore, in order to extend the saturation magnetic field of the magnetization free layer further, it is effective to intentionally provide a nonmagnetic layer having a strong antiparallel coupling force in the second to (N-2) nonmagnetic layers. It is. Such a nonmagnetic layer is desirably located at the centermost portion. When N is an even number, it is preferable that the antiparallel coupling force of the (N / 2) th nonmagnetic layer is the largest. Specifically, the film thickness of each nonmagnetic layer is such that the order of the antiferromagnetic peak of the (N / 2) th nonmagnetic layer is smaller by one than the order of the (N-1) th nonmagnetic layer. It is preferable to set the thickness.

As a specific example, a cross section of the MRAM storage bit portion 40A of the present invention is shown in FIG. 8A. The MRAM storage bit unit 40A has a magnetization free layer 15D composed of six ferromagnetic layers 21 to 26 and five nonmagnetic layers 31 to 35 inserted therebetween. As an example of a suitable form from the ferromagnetic layer 21 on the tunnel barrier layer 14 to the ferromagnetic layer 26 below the cap layer 16, the following film configuration can be given:
Tunnel barrier layer / NiFe (4 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (4 nm) / CoFe (0.35 nm) / Ru (3.5 nm) / NiFe (2 nm) / CoFe (0 .25 nm) / Ru (2.1 nm) / NiFe (2 nm) / CoFe (0.25 nm) / Ru (3.5 nm) / NiFe (3 nm) / CoFe (0.25 nm) / Ru (3.5 nm) / NiFe (3 nm) / CoFe (0.25 nm) / cap layer.

As shown in FIG. 8A, generally in an actual device, the side wall portion of the magnetization free layer in the MRAM storage bit portion 40A is not vertically processed but has a slope. At this time, the junction area that determines the magnetic resistance is defined by the area of the interface between the tunnel barrier layer and the first ferromagnetic layer. When the ferromagnetic layers are formed to have exactly the same thickness, the effective magnetic film thicknesses t _{1 to} t ₆ of the ferromagnetic layers are t ₁ > t ₂ > t ₃ > t ₄ > t ₅ > t. ₆ Therefore, it is necessary to adjust the film thickness at the time of forming the ferromagnetic layers 21 to 26 in consideration of the magnetic volume difference resulting from the processing of the actual device. According to this embodiment, the _J 1 / of SAF over the Ru (2.1 nm) which is a non-magnetic layer _{21 (M 1 × t 1)} , a Ru (3.5 nm) which is a non-magnetic layer 25 The SAF of J ₅ / (M ₆ × t ₆ ) is set to be substantially equal. The pairs that require equality of the ferromagnetic layers are the adjacent ferromagnetic layer 21 and ferromagnetic layer 22, ferromagnetic layer 23 and ferromagnetic layer 24, and ferromagnetic layer 25 and ferromagnetic layer 26. As is clear from FIG. 8A, regarding these sets, the magnetic volume difference due to processing is difficult to occur, and the difference in magnetic properties due to crystallinity is also small. For the nonmagnetic layer 23 at the center, the 2nd antiferromagnetic peak of Ru 2.1 nm is used and the crystallinity is good, so that the greatest antiferromagnetic coupling force can be obtained. Therefore, the saturation magnetic field of the magnetization free layer is extended, and the write margin is widened.

For comparison, a cross-section of an MRAM storage bit portion 40B made according to the prior art is shown in FIG. 8B. Similarly, this MRAM storage bit unit 40B has a magnetization free layer 15E composed of six ferromagnetic layers 21 to 26 and five nonmagnetic layers 31 to 35 inserted therebetween. An example of a suitable film configuration from the ferromagnetic layer 21 on the tunnel barrier layer 14 to the ferromagnetic layer 26 below the cap layer 16 includes the following configuration:
Tunnel barrier layer / NiFe (4 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (2 nm) / CoFe (0.35 nm) / Ru (3.5 nm) / NiFe (3 nm) / CoFe (0 .25 nm) / Ru (2.1 nm) / NiFe (3 nm) / CoFe (0.25 nm) / Ru (3.5 nm) / NiFe (2 nm) / CoFe (0.35 nm) / Ru (3.5 nm) / NiFe (4 nm) / CoFe (0.35 nm) / cap layer.

  The antiparallel coupling forces of the nonmagnetic layers 31 and 35 are substantially equal, and the magnetization film thickness products of the ferromagnetic layers 21 to 26 are set symmetrically in the vertical direction. However, even if it is formed so as to have such a magnetic film thickness product at the time of film formation, as is apparent from FIG. 8B, a pair (ferromagnetic layer 21 and ferromagnetic layer 26) requiring equivalence of the ferromagnetic layer is required. As for the ferromagnetic layer 22 and the ferromagnetic layer 25, and the ferromagnetic layer 23 and the ferromagnetic layer 24), the volume difference caused by the element processing process greatly impairs the magnetic volume equivalence. In addition, even if the position in the SAF where these ferromagnetic layers are formed is considered, a difference in magnetic characteristics due to crystallinity can also occur significantly. Regarding the difference in magnetic volume, it is difficult to grasp which layer has how much magnetic volume, and it is impossible to grasp the magnetic volume when the shape of the joining side wall is more complicated. Thus, it is not easy to cancel the magnetic volume difference and film quality difference of the multilayer SAF of the prior art.

  As described above, in the description of the embodiment, an example in which the film thickness of the (N-1) -th nonmagnetic layer is thicker than that of the first non-magnetic layer is shown. You may comprise so that the film thickness of a 1st nonmagnetic layer may become thick with respect to a (N-1) nonmagnetic layer. In addition, for example, when the film quality difference is reduced by devising film formation conditions, etc., or the film thickness controllability of the nonmagnetic layer is improved, and the strict antiferromagnetic coupling energy can be controlled only by the nonmagnetic film thickness. In such a case, such a configuration may not be adopted. The important point is not the film form but the magnitude of the effective antiferromagnetic coupling force.

  Hereinafter, examples of operation experiments for MRAMs having various magnetization free layer structures are shown as examples.

1. Evaluation of flop magnetic field, saturation magnetic field, and write margin Magnetize a normal SAF consisting of two ferromagnetic layers and one nonmagnetic layer, and a multilayer SAF consisting of four ferromagnetic layers and three nonmagnetic layers A plurality of MTJs as free layers were produced on an 8-inch substrate. With respect to 100 to 120 MTJ elements, toggle writing was examined, and a flop magnetic field, a saturation magnetic field, a write margin, and an operation rate were evaluated. Samples using the conventional technique and the technique of the present invention were prepared for the four-layer SAF of the magnetization free layer, and the characteristics were compared. The planar shape of the MTJ element is mainly an ellipse of 0.6 × 1.2 μm ² . In addition, evaluation was performed on an oval element having a part of 0.4 × 0.8 μm ² .

Here, Al (αnm) O _x means an AlO _x film formed by oxidizing an Al film of α (nm). In addition, a Ta film, a PtMn film, a CoFe / Ru / CoFe laminate, and an Al (0.9 nm) O _x film sequentially formed on the substrate are a lower electrode layer 11, an antiferromagnetic layer 12, and a magnetization, respectively. Al (0.7 nm) O _x film and Ta film formed on the magnetization free layer correspond to the fixed layer 13 and the barrier layer 14, and correspond to the cap layer 16 and the upper electrode layer 17. Please note that.

The structure of the MTJ produced in this example is as follows:
Substrate / Ta (20 nm) / PtMn (20 nm) / CoFe (2.5 nm ₎ / Ru (0.9 nm ₎ / CoFe (2.5 nm) / Al (0.9 nm) O _x / Free magnetization layer / Al (0. 7 nm) O _x / Ta (100 nm).

10A and 10B are tables showing the configuration of the magnetization free layer of each sample. The number of ferromagnetic layers included in the magnetization free layer is 2 or 4, and the material and film thickness of the ferromagnetic layer are finely adjusted so that the residual magnetization as the whole magnetization free layer is 0. All the nonmagnetic layers of the magnetization free layer are made of ruthenium. Here, permalloy (Ni ₈₁ Fe ₁₉ ) is used as NiFe. In the following, the characteristics of each sample are schematically described.

  The magnetization free layer of the MTJ element of Comparative Example 1 is formed of SAF including two ferromagnetic layers and one nonmagnetic layer. The film thickness of the nonmagnetic layer is 2.1 nm corresponding to the antiferromagnetic 2nd peak. All other samples are formed of SAF including four ferromagnetic layers and three nonmagnetic layers. The first nonmagnetic layer uses 2.1 nm ruthenium corresponding to the antiferromagnetic 2nd peak. For the second nonmagnetic layer, ruthenium of 2.1 nm is used, and this nonmagnetic layer has a good crystal orientation and an antiferromagnetic 2nd peak, so that it is more than three times that of other nonmagnetic layers. Has antiferromagnetic binding energy. The film thickness of the third nonmagnetic layer located away from the substrate is 3.5 nm corresponding to the antiferromagnetic 3rd peak.

  In the magnetization free layer of Comparative Example 2, the magnetization film thickness products of all the ferromagnetic layers are substantially equal, and the antiparallel coupling forces of the first nonmagnetic layer and the third nonmagnetic layer are substantially equal. The magnetization film thickness product of each ferromagnetic layer is 3.15 Tnm.

  In Comparative Examples 3 to 7 and Examples 1 to 4, the magnetization film thickness product of the first ferromagnetic layer and the second ferromagnetic layer is approximately 4.72 Tnm, and the third ferromagnetic layer and the fourth ferromagnetic layer are the same. Are equal to approximately 3.15 Tnm, and the first ferromagnetic layer has a magnetization thickness product 1.5 times larger than that of the fourth ferromagnetic layer. In the samples of Comparative Examples 3-6 and Examples 1-2, only the antiparallel coupling energy of the first nonmagnetic layer is changed by changing the ratio of NiFe / CoFe in the first ferromagnetic layer. In the samples of Comparative Example 7 and Examples 3 to 4, the ratio of NiFe / CoFe in the third ferromagnetic layer is changed to change only the antiparallel coupling energy of the third nonmagnetic layer. In any case, as the CoFe film thickness increases, the antiparallel coupling force of the upper nonmagnetic layer increases. FIG. 10B shows the thickness of CoFe existing at the ferromagnetic layer interface immediately below the first and third nonmagnetic layers.

FIG. 11 shows the operation rate, flop magnetic field, and saturation magnetic field of the 0.6 × 1.2 μm ² toggle MRAM device using the magnetization free layers of Comparative Examples 1 to 7 and Examples 1 to 4. Comparing Comparative Example 1 and Comparative Example 2, the flop magnetic field is distributed around 38 (Oe) and 37 (Oe), respectively, and the saturation magnetic field is distributed around 157 (Oe) and 570 (Oe), respectively. It was. By changing the ferromagnetic layer from two layers to four layers, the write margin is greatly increased.

In the samples of Comparative Examples 3 to 6 and Examples 1 and 2, the antiparallel coupling force (antiparallel coupling energy constant J ₃ ) of the _third nonmagnetic layer is fixed, and the antiparallel coupling force (antireflection) of the first nonmagnetic layer is fixed. Only the parallel binding energy constant J ₁ ) corresponds to the increasing sample. In Comparative Example 3 the most J ₁ corresponds to a small sample, although elements showing a direct inversion a small fraction of the elements were present at all did not exist element exhibiting toggle inversion. As Comparative Examples 4 and 5 and J ₁ increase, the toggle operation rate is improved. In the samples of Examples 1 and 2, the toggle operation rate is 98% and 100%, respectively, and the toggle operation rate is very high. The result of writing was obtained. In Comparative Example 6 is a further sample J ₁ is increased, the toggle operation rate has been reduced 65%. In the sample of Example 1, the average flop magnetic field was 42 (Oe), and the saturation magnetic field was 602 (Oe). In the sample of Example 2, the average flop magnetic field was 44 (Oe), and the saturation magnetic field was 606 (Oe).

  In the samples of Comparative Example 5, Example 4, Example 3, and Comparative Example 7, the antiparallel coupling force of the first nonmagnetic layer is fixed, and only the antiparallel coupling force of the third nonmagnetic layer is reduced. It corresponds to the sample. As the antiparallel coupling force of the third nonmagnetic layer decreases, the toggle operation rate improves, and the results of the operation rates of 93% and 100% are obtained in the antiparallel coupling forces of Examples 3 and 4, respectively. It was. In the sample of Example 3, the average flop magnetic field is 36 (Oe), and the saturation magnetic field is 565 (Oe). In the sample of Example 4, the average flop magnetic field was 37 (Oe), and the saturation magnetic field was 576 (Oe). The ratio corresponding to the write margin (saturation magnetic field / flop magnetic field) is as small as 4 in Comparative Example 1, but in Examples 1 to 4, the ratio is greatly increased to 10 or more. According to the present invention, it has been demonstrated that the write margin can be remarkably improved while the toggle operation rate is high and the flop magnetic field is substantially constant.

Furthermore, the result of having compared the toggle operation characteristic of Comparative Examples 1 and 2, Example 2, and Example 4 in detail is shown by FIG. FIG. 12 shows a flop magnetic field of 0.6 × 1.2 μm ² elements, a standard deviation corresponding to the variation, a saturation magnetic field, a write margin, and a toggle write operation rate. Furthermore, the difference in direct inversion region width between 0.6 × 1.2 μm ² and 0.4 × 0.8 μm ² elements is shown. The direct region width is also measured with respect to the easy axis direction, like the flop magnetic field and the saturation magnetic field.

The variation of the flop magnetic field (standard deviation σ _flop ) is 3.3 (Oe) in Comparative Example 2, which is a four-layer SAF magnetization free layer according to the conventional technique. In contrast, in Examples 2 and 4, which are the four-layer SAF magnetization free layer according to the present invention, the magnitude of the variation in the flop magnetic field is as small as 2.5 (Oe) and 2.1 (Oe), respectively. Consider a case where these samples are applied to, for example, a 1 Mbit MRAM. When the writing magnetic field value is “average flop magnetic field value + 5 × σ _flop ”, the writing magnetic field value is 53.5 (Oe) for the sample of Comparative Example 2 and 56.5 (Oe) for the sample of Example 2. In the sample of Example 4, it becomes 47.5 (Oe). In particular, in the sample of Example 4, since the flop magnetic field is small and variation is small, the writing magnetic field is most reduced.

Further, regarding the difference in direct region width between 0.6 × 1.2 μm ² and 0.4 × 0.8 μm ² elements, the difference is 2. in the first comparative example consisting of a normal toggle magnetization free layer composed of two layers of SAFs. 2Oe is the smallest. In Examples 2 and 4, the difference is as small as 2.8 (Oe) and 3.1 (Oe), respectively. On the other hand, in Comparative Example 2, the difference was the largest at 5.8 (Oe). The fact that the direct inversion region width varies depending on the element size means that the difference in magnetic volume of each ferromagnetic layer in the SAF varies depending on the element size. This is considered to be due to, for example, the influence of the magnetic volume non-uniformity of the magnetization free layer side wall portion caused by the processing process as shown in FIGS. 8A and 8B. As described above, according to the multilayer SAF magnetization free layer according to the present invention, the flop magnetic field variation is small compared to the multilayer SAF magnetization free layer according to the prior art, and the size of the direct inversion region also affects the element size. It is hard to reduce the direct inversion area more easily.

2. Evaluation of antiferromagnetic coupling by RKKY interaction The relationship between the results of toggle writing characteristic evaluation, the antiparallel coupling force of the magnetization free layer and the magnetization film thickness product was investigated.

In order to investigate the antiparallel coupling force through the “first nonmagnetic layer” of the magnetization free layers of Comparative Examples 3-6 and Examples 1-4, the following samples were prepared and the magnetization curves were evaluated:
Substrate / Ta (20 nm ₎ / PtMn (20 nm) / CoFe (2.5 nm) / Ru (0.9 nm) / CoFe (2.5 nm ₎ / Al (0.9 nm) O _x / NiFe / CoFe / Ru (2. 1 nm) / NiFe (4.8 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / Ta (10 nm).

In order to investigate the antiparallel coupling force through the “third nonmagnetic layer” of the magnetization free layers of Comparative Examples 3 to 6 and Examples 1 to 4, the following samples were prepared and the magnetization curves were evaluated:
Substrate / Ta ₍ 20 nm) / PtMn ₍ 20 nm) / CoFe ₍ 2.5 nm) / Ru ₍ 0.9 nm) / CoFe ₍ 2.5 nm) / Al ₍ 0.9 nm) O _x / NiFe ₍ 4.8 nm _{) /} CoFe (0.35 nm) / Ru (2.5 nm) / NiFe (4.8 nm) / CoFe (0.35 nm) / Ru (2.5 nm) / NiFe / CoFe / Ru (3.5 nm) / NiFe (3.7 nm) ) / Al (0.7 nm) O _x / Ta (10 nm)

  In this sample, the Ru thickness of the first and second nonmagnetic layers was 2.5 nm, and the antiparallel coupling force was made substantially zero, so that the sample was composed of the third nonmagnetic layer and the third and fourth ferromagnetic layers. The magnetization curve of only the SAF part can be evaluated. At this time, the crystallinity of the third nonmagnetic layer is substantially the same as in Comparative Examples 3 to 6 and Examples 1 to 4.

  FIG. 13 shows the relationship between the saturation magnetic field obtained from the magnetization curve of the SAF part and the CoFe film thickness directly under each Ru nonmagnetic layer. It can be seen that the saturation magnetic fields of the SAF portions of the first nonmagnetic layer and the third nonmagnetic layer increase almost in proportion to the increase of the CoFe thickness at the immediately lower ferromagnetic layer interface. The saturation magnetic field almost reflects the magnitude of the antiparallel coupling energy, and it is shown that the antiparallel coupling energy of each nonmagnetic layer increases as the CoFe thickness increases.

Also, in order to evaluate the magnetization film thickness product and the anisotropic magnetic field of each ferromagnetic layer in the SAF, a laminated film having the following film configuration was produced:
Substrate / Ta (20 nm) / Al ₍ 0.9 nm) O _x / seed layer / NiFe / CoFe / cap layer / Ta (10 nm).
A sample in which this laminated film was cut into 1 × 1 cm ² squares was produced, and the magnetization curve was evaluated. In the above sample, Ru (2.1 nm) was used or not used for the seed layer depending on the ferromagnetic layer to be evaluated. For the cap layer, either Ru (2.1 nm) or Al (0.7 nm) O _x was selected depending on the ferromagnetic layer to be evaluated.

From the magnetization film thickness product, the anisotropic magnetic field, and the saturation magnetic field shown in FIG. 13, J ₁ , J ₃ , M ₁ × t ₁ , M in the magnetization free layers of Comparative Examples 3 to 6 and Examples 1 to 4 A value of ₄ × t ₄ was determined. The relationship between the ratio [J ₁ / (M ₁ × t ₁ )] / [J ₃ / (M ₄ × t ₄ )] for each sample and the toggle operation rate with 0.6 × 1.2 μm ² elements, It is shown in FIG. Than 14, toggling rate of Comparative Examples 3-6 and Examples 1-4 all elements and ratios _{[J 1 / (M 1 ×} t 1)] / [J 3 / (M 4 × t 4)] and It can be seen that there is a clear correlation between. Even though the values of J ₁ / (M ₁ × t ₁ ) and J ₃ / (M ₄ × t ₄ ) are different for all samples, [J ₁ / (M ₁ × t ₁ )] / [J ₃ / (M ₄ × t ₄ )] = 1, the toggle operation rate changes to have a peak. Further, the toggle operation rate with respect to the ratio [J ₁ / (M ₁ × t ₁ )] / [J ₃ / (M ₄ × t ₄ )] is on the same curve. When [J ₁ / (M ₁ × t ₁ )] / [J ₃ / (M ₄ × t ₄ )] = 1, the toggle operation rate is 100%. In particular, in the range of 0.8 <[J ₁ / (M ₁ × t ₁ )] / [J ₃ / (M ₄ × t ₄ )] <1.2, the toggle operation rate is 100% or close to it. The value is obtained. The range is a range corresponding to Examples 1 to 4, and the device characteristics are very good. These experimental results show that, in the multilayer toggle of the present invention, J _SAF / (M × t) corresponding to the amount converted to the magnetic field of the lowermost layer and the uppermost nonmagnetic layer is equal, in other words, the lowermost layer and the lowermost layer. This suggests that the magnetization free layer composed of the multilayer SAF can be toggled more favorably as the antiparallel coupling of the upper ferromagnetic layer is set to be simultaneously removed from the external magnetic field.

Finally, the magnitude of the antiparallel coupling force of the second nonmagnetic layer realizing a large saturation magnetic field in the magnetization free layers of Examples 1 to 4 was evaluated using the following samples:
Substrate / Ta ₍ 20 nm) / PtMn ₍ 20 nm) / CoFe ₍ 2.5 nm) / Ru ₍ 0.9 nm) / CoFe ₍ 2.5 nm) / Al ₍ 0.9 nm) O _x / NiFe ₍ 4.8 nm _{) /} CoFe (0.35 nm) / Ru (2.5 nm) / NiFe (4.8 nm) / CoFe (0.35 nm) / Ru (2.1 nm) / NiFe (3 nm) / CoFe (0.35 nm) / Ru (3. 5 nm) / Ta (10 nm).
In this sample, the Ru thickness of the first nonmagnetic layer is 2.5 nm, and the antiparallel coupling force is substantially zero, so that only the SAF portion composed of the second nonmagnetic layer and the second and third ferromagnetic layers is used. The magnetization curve of can be evaluated.

The saturation magnetic field of the above SAF was approximately 255 Oe. Since this saturation magnetic field value is equal to J ₂ [1 / (M ₂ × t ₂ ) + 1 / (M ₃ × t ₃ )], when converted to an antiparallel coupling energy constant, J ₂ = 0.038 erg / cm ² . Equivalent to. In Examples 1 to 4, Hj = 2J ₃ / (M ₄ × t ₄ ) indicated by the SAF part of the third nonmagnetic layer is in the range of 68 Oe to 132 Oe, and the ratio {J ₂ [1 / (M ₂ × t ₂ ) + 1 / (M ₃ × t ₃ )]} / {2J ₃ / (M ₄ × t ₄ )} has been demonstrated to operate favorably in the range of 1.9 to 3.75. . From this result, the ratio of the antiparallel coupling energy constant between the second nonmagnetic layer and the third nonmagnetic layer, {J ₂ [1 / (M ₂ × t ₂ ) + 1 / (M ₃ × t ₃ )]} / It can be said that {2J ₃ / (M ₄ × t ₄ )} can be set within a range of 4 or less. In this embodiment, by setting the antiparallel coupling force in this way, a large saturation magnetic field value, that is, a write margin is realized with a low flop magnetic field.

  As described above, according to the present invention, an MRAM using a multilayer SAF including three or more ferromagnetic layers as a magnetization free layer is provided. The operation rate of the MRAM is improved and defective bits are reduced. Further, according to the present invention, controllability of magnetic characteristics on an actual device is facilitated. In addition, according to the present invention, it is possible to improve the write characteristics of the MRAM and reduce the write current value. Furthermore, according to the present invention, the write margin of the MRAM can be increased.

Claims (34)

A substrate,
Comprising a magnetoresistive element,
The magnetoresistive element is
A magnetization fixed layer having a fixed magnetization;
A magnetization free layer having reversible magnetization;
A nonmagnetic layer interposed between the magnetization fixed layer and the magnetization free layer,
The magnetization free layer is
First to Nth ferromagnetic layers (N is an integer of 4 or 6);
Comprising first to (N-1) -th nonmagnetic layers formed to exhibit antiferromagnetic RKKY interaction,
The kth nonmagnetic layer (k is an integer of 1 to N-1) of the first to N-1 nonmagnetic layers is the kth ferromagnetic layer of the first to Nth ferromagnetic layers. Between the layer and the (k + 1) th ferromagnetic layer,
The first nonmagnetic layer is located closest to the substrate among the first to N-1 nonmagnetic layers, and the N-1 nonmagnetic layer is the first to N-1 Of the non-magnetic layer, located farthest from the substrate,
Volume of the k-th ferromagnetic layer, the magnetization and the effective thickness is represented by V _k, M _k and t _k, respectively, wherein the effective film thickness t _k is the volume V _k of the magnetization free layer Divided by the average area in the plane direction,
The relative angle between the magnetization direction of the kth ferromagnetic layer and the magnetization direction of the (k + 1) th ferromagnetic layer is represented by θ _k .
Wherein the first k the k-th ferromagnetic layers via a nonmagnetic layer (k + 1) th antiparallel coupling energy of the ferromagnetic layer, using the anti-parallel coupling energy constant J _k, represented by J _k cos [theta] _k When
M ₁ × t ₁ and M ₂ × t ₂ are substantially equal,
M _N−1 × t _N−1 and M _N × t _N are substantially equal,
Also, any of the following relationships:
M ₁ × t ₁ > M _N × t _N and J ₁ > J _N-1 , or
M ₁ × t ₁ <M _N × t _N and J ₁ <J _N−1
Is satisfied MRAM. MRAM of claim 1 wherein N is four. A substrate,
Comprising a magnetoresistive element,
The magnetoresistive element is
A magnetization fixed layer having a fixed magnetization;
A magnetization free layer having reversible magnetization;
A nonmagnetic layer interposed between the magnetization fixed layer and the magnetization free layer,
The magnetization free layer is
First to Nth ferromagnetic layers (N is an odd number of 3 or more);
Comprising first to (N-1) -th nonmagnetic layers formed to exhibit antiferromagnetic RKKY interaction,
The kth nonmagnetic layer (k is an integer of 1 to N-1) of the first to N-1 nonmagnetic layers is the kth ferromagnetic layer of the first to Nth ferromagnetic layers. Between the layer and the (k + 1) th ferromagnetic layer,
The first nonmagnetic layer is located closest to the substrate among the first to N-1 nonmagnetic layers, and the N-1 nonmagnetic layer is the first to N-1 Of the non-magnetic layer, located farthest from the substrate,
Volume of the k-th ferromagnetic layer, the magnetization and the effective thickness is represented by V _k, M _k and t _k, respectively, wherein the effective film thickness t _k is the volume V _k of the magnetization free layer Divided by the average area in the plane direction,
The relative angle between the magnetization direction of the kth ferromagnetic layer and the magnetization direction of the (k + 1) th ferromagnetic layer is represented by θ _k .
Wherein the first k the k-th ferromagnetic layers via a nonmagnetic layer (k + 1) th antiparallel coupling energy of the ferromagnetic layer, using the anti-parallel coupling energy constant J _k, represented by J _k cos [theta] _k When
The sum of M ₁ × t ₁ and M _N × t _N is substantially equal to M _{(N + 1) / 2} × t _{(N + 1) / 2} ,
Also, any of the following relationships:
M ₁ × t ₁ > M _N × t _N and J ₁ > J _N-1 , or
M ₁ × t ₁ <M _N × t _N and J ₁ <J _N−1
Is satisfied MRAM. A MRAM according to any one of claims 1 to 3,
The residual magnetization of the magnetization free layer is within 10% of the saturation magnetic field MRAM. A MRAM according to any one of claims 1 to 4,
A word line,
A bit line orthogonal to the word line;
A storage element provided at an intersection of the word line and the bit line and including the magnetoresistive element;
The easy axis direction of the magnetization free layer of the magnetoresistive element forms an angle of 45 degrees with respect to the extending direction of the word line or the bit line. MRAM. A MRAM according to claim 1 or 2,
The external magnetic field at which the antiparallel coupling to the first ferromagnetic layer begins to dissolve is substantially equal to the external magnetic field at which the antiparallel coupling to the Nth ferromagnetic layer begins to dissolve. A MRAM according to claim 3,
The external magnetic field at which the antiparallel coupling to the first ferromagnetic layer begins to dissolve is substantially equal to the external magnetic field at which the antiparallel coupling to the Nth ferromagnetic layer begins to dissolve. A MRAM according to claim 6 or 7,
_{J 1 / (M 1 × t} 1) and _{J N-1 / (M N} × t N) substantially equal to an MRAM with. A MRAM according to claim 8,
_{{J 1 / (M 1 ×} t 1)} / {J N-1 / (M N × t N)} is 0.8 or more and 1.2 or less MRAM. A MRAM according to claim 6 or 7,
When the average total anisotropic magnetic field of the kth ferromagnetic layer and the (k + 1) th ferromagnetic layer through the kth nonmagnetic layer is represented by Hk _k ,
_{[{2J 1 / (M 1} × t 1) -Hk 1} × Hk 1] and _{[{2J N-1 / (} M N × t N) -Hk N-1} × Hk N-1] and parenchyma Equal MRAM. A MRAM according to claim 10,
_{[{2J 1 / (M 1} × t 1) -Hk 1} × Hk 1] / [{2J N-1 / (M N × t N) -Hk N-1} × Hk N-1] is 0. MRAM which is 8 or more and 1.2 or less. A MRAM according to claim 6,
N is an even number;
K is an odd number greater than or equal to 1,
The MRAM, wherein the kth ferromagnetic layer and the k + 1th ferromagnetic layer have substantially the same magnetic volume. A MRAM according to claim 6,
N is an even number of 4 or more,
The antiparallel bond energy constant _Jk is the largest when k = N / 2. MRAM. A MRAM according to claim 13,
The following relationships:
_{[J N / 2 {1 /} (M N / 2 × t N / 2) + 1 / (M N / 2 + 1 × t N / 2 + 1)}] / [2J N-1 / (M N × t N)] < 4
Is satisfied MRAM. A MRAM according to claim 13 or 14,
SAF composed of the first ferromagnetic layer and the second ferromagnetic layer sandwiching the first nonmagnetic layer, the N-1 ferromagnetic layer and the Nth ferromagnetic layer sandwiching the N-1 nonmagnetic layer The saturation magnetic field exhibited by the SAF consisting of layers is
An MRAM larger than an external magnetic field at which an antiparallel coupling between the N / 2 ferromagnetic layer and the N / 2 + 1 ferromagnetic layer starts to break. A MRAM according to claim 6 or 7,
The antiparallel coupling energy constant J _k (where k is other than 1, N−1) is equal to or greater than J ₁ or J _N−1 MRAM. A MRAM according to claim 6 or 7,
The order of the antiferromagnetic peak of the RKKY interaction for the kth nonmagnetic layer is represented by α _k ,
The first nonmagnetic layer has a thickness in a range corresponding to the α _1st- order antiferromagnetic peak,
The N-1th nonmagnetic layer has a thickness in a range corresponding to the α _N- 1th order antiferromagnetic peak,
When J ₁ > J _N−1 , the relationship α ₁ <α _N−1 is satisfied,
MRAM satisfying the relationship of α ₁ > α _N−1 when J ₁ <J _N−1 . A MRAM according to claim 17,
If J ₁ > J _N−1 , then the relationship α _N−1 = α ₁ +1 is satisfied,
If J ₁ <J _N−1, the MRAM satisfying the relationship of α ₁ = α _N−1 +1. A MRAM according to claim 18,
If J ₁ > J _N−1 ,
The first nonmagnetic layer is formed of a ruthenium layer having a thickness of 1.8 nm to 2.5 nm,
The N-1 nonmagnetic layer is formed of a ruthenium layer having a thickness of 3.1 nm to 3.9 nm,
If J ₁ <J _N−1 ,
The first nonmagnetic layer is formed of a ruthenium layer having a thickness of 3.1 nm to 3.9 nm,
The N-1th nonmagnetic layer is an MRAM formed of a ruthenium layer having a thickness of 1.8 nm to 2.5 nm. A MRAM according to any one of claims 17 to 19,
If J ₁ > J _N−1 ,
At least one nonmagnetic layer of the second to N-2 nonmagnetic layers has a thickness in a range corresponding to a lower order antiferromagnetic peak than the N-1 nonmagnetic layer,
If J ₁ <J _N−1 ,
At least one nonmagnetic layer of the second to N-2 nonmagnetic layers has a thickness in a range corresponding to a lower order antiferromagnetic peak than the first nonmagnetic layer. A MRAM according to claim 6,
N is an even number;
The order of the antiferromagnetic peak of the RKKY interaction for the kth nonmagnetic layer is represented by α _k ,
The first nonmagnetic layer has a thickness in a range corresponding to the α _1st- order antiferromagnetic peak,
The N-1th nonmagnetic layer has a thickness in a range corresponding to the α _N- 1th order antiferromagnetic peak,
In the case of J ₁ > J _N−1 , the relationship of α ₁ <α _N-1 and α _{N / 2} <α _N-1 is satisfied,
In the case of J ₁ <J _N−1 , an MRAM in which α ₁ > α _N−1 and α _{N / 2} <α ₁ are satisfied. A MRAM according to claim 21,
If J ₁ > J _N−1 , then the relationship α _{N / 2} + 1 = α _N−1 is satisfied,
If J ₁ <J _N−1 , then the relationship of α _{N / 2} + 1 = α ₁ is satisfied MRAM. A MRAM according to claim 22,
The (N / 2) nonmagnetic layer is formed of a ruthenium layer having a thickness of 1.8 nm to 2.5 nm,
The first nonmagnetic layer or the N-1th nonmagnetic layer is an MRAM formed of a ruthenium layer having a thickness of 3.1 nm to 3.9 nm. A MRAM according to claim 22,
The (N / 2) nonmagnetic layer is formed of a ruthenium layer having a thickness of 0.7 nm to 1.2 nm,
The first nonmagnetic layer or the N-1th nonmagnetic layer is an MRAM formed of a ruthenium layer having a thickness of 1.8 nm to 2.5 nm. A MRAM according to any one of claims 1 to 11,
The first nonmagnetic layer and the N-1th nonmagnetic layer have different structures. A MRAM according to claim 25,
The MRAM in which a film thickness of the first nonmagnetic layer is different from a film thickness of the N-1 nonmagnetic layer. A MRAM according to claim 25,
The MRAM in which the crystal orientation of the first nonmagnetic layer is different from the crystal orientation of the N-1 nonmagnetic layer. A MRAM according to claim 27,
The crystal orientation of the N-1 nonmagnetic layer is higher than the crystal orientation of the first nonmagnetic layer, and the N-1 nonmagnetic layer is higher than the first nonmagnetic layer. MRAM with thicker film. A MRAM according to claim 27,
The MRAM in which the crystal orientation of the first ferromagnetic layer is different from the crystal orientation of the N-1th ferromagnetic layer. A MRAM according to claim 25,
The first ferromagnetic layer, the second ferromagnetic layer, the N-1th ferromagnetic layer, and the Nth ferromagnetic layer
An MRAM in which at least one of the ferromagnetic layers is composed of a laminated film. A MRAM according to claim 30,
The stacked film includes an MRAM including a NiFe film and a CoFe film. A MRAM according to any one of claims 1 to 11,
The first nonmagnetic layer and the N-1 nonmagnetic layer have substantially the same structure, and the element composition ratios of all portions in direct contact with the upper and lower interfaces of the first nonmagnetic layer are The MRAM is different from the elemental composition ratio of all portions in direct contact with the upper and lower interfaces of the N-1 nonmagnetic layer. A MRAM according to claim 32,
The first ferromagnetic layer, the second ferromagnetic layer, the N-1th ferromagnetic layer, and the Nth ferromagnetic layer
An MRAM in which at least one of the ferromagnetic layers is composed of a laminated film. A MRAM according to claim 33,
The stacked film includes an MRAM including a NiFe film and a CoFe film.

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