JP2008252037A - Magnetoresistive element, and magnetic memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further reduce a writing current required for magnetization reversal. <P>SOLUTION: A magnetoresistive element 10 is provided with a fixed layer 11 having a fixed magnetization direction; a free layer 13 having a magnetization direction variable due to energization in a direction perpendicular to a film surface; an assisting layer 15 having a magnetization direction variable due to energization in a direction perpendicular to the film surface, and assisting a change in magnetization direction of the free layer 13; an intermediate layer 12 provided between the fixed layer 11 and the free layer 13 and made of a non-magnetic body; and an intermediate layer 14 provided between the free layer 13 and the assisting layer 15 and made of a non-magnetic body. An energy barrier for magnetization reversal of the assisting layer 15 is smaller than an energy barrier for magnetization reversal of the free layer 13. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a magnetoresistive element and a magnetic memory, and more particularly, to a magnetoresistive element capable of recording information by supplying current bidirectionally and a magnetic memory using the same.

  In recent years, many solid-state memories that record information based on a new principle have been proposed. Among them, as a solid-state magnetic memory, a magnetic random access memory (MRAM: Magnetic Random) that uses a tunneling magnetoresistive (TMR) effect is proposed. Access Memory) is in the spotlight. The MRAM is characterized in that data is stored according to the magnetization state of an MTJ (Magnetic Tunnel Junction) element.

  In an MRAM in which writing is performed with a magnetic field using a conventional wiring current, the holding force Hc increases when the MTJ element size is reduced, and therefore, the current required for writing tends to increase. Actually, in order to manufacture an MRAM having a large capacity of 256 Mbits or more, it is necessary to reduce the chip size. In order to realize this, the cell array occupancy in the chip is increased and the reduction in the MTJ element size is suppressed. However, it is necessary to reduce the write current to the μA level. The reduction in the MTJ element size and the reduction in the write current are in a reciprocal relationship, and it is difficult for the conventional MRAM to achieve both a reduction in the cell size and a reduction in the current to increase the capacity exceeding 256 Mbits. .

  As a writing method for overcoming such a problem, an MRAM using a spin angular momentum transfer (SMT) writing method has been proposed (for example, Patent Document 1 and Non-Patent Document 1). In the spin angular momentum transfer (hereinafter also referred to as spin injection) magnetization reversal, the current Ic necessary for the magnetization reversal is defined by the current density Jc. Therefore, as the element area decreases, the current Ic required for magnetization reversal also decreases.

Compared to the above-described conventional magnetic field writing method, when writing with a constant current density, the write current is reduced as the MTJ element size is reduced, so that it is expected to have excellent scalability. However, in the current spin injection MRAM, the current density Jc necessary for the magnetization reversal is as large as 10 mA / cm ² or more, and even when a 100 nm ² size MTJ element is used, a write current of about 1 mA is required. Become.

  This is because the spin injection magnetization reversal method requires bidirectional energization, and the spin injection efficiency differs depending on the energization direction. That is, the spin injection magnetization reversal curve is asymmetric. This is because the current when the magnetization direction of the free layer is reversed so that the magnetization arrangement of the magnetization free layer and the magnetization fixed layer changes from parallel to antiparallel is about twice that when the magnetization direction of the free layer is inverted from antiparallel to parallel. It is necessary.

  As a problem associated with this asymmetry, when the MTJ element is written by energizing the MTJ element so that the magnetization arrangement of the free layer and the fixed layer is reversed from antiparallel to parallel, the current threshold is small and there is no problem. However, when the MTJ element is written by applying current so that the magnetization arrangement of the free layer and the fixed layer is reversed from parallel to antiparallel, a large write current is required. For this reason, if data is written to the MTJ element at a constant current density Ip-ap, the element resistance Rap at the time of antiparallel arrangement rises by an amount corresponding to the TMR effect, and as a result, the write voltage Vp -ap rises.

Accordingly, if the tunnel barrier layer does not have a sufficiently high breakdown voltage, the breakdown voltage Vbd of the tunnel barrier layer is reached before the magnetization arrangement becomes antiparallel, and the tunnel barrier layer breaks down. Further, there is a problem that operation reliability under a high voltage is not ensured even if dielectric breakdown does not occur.
US Pat. No. 6,256,223 C. Slonczewski, “Current-driven ecitation of magnetic multilayers”, JORNAL OF MAGNETISM AND MAGNETIC MATERIALS, VOLUME 159, 1996, pp.L1-L7

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a magnetoresistive element capable of further reducing a write current required for magnetization reversal and a magnetic memory using the magnetoresistive element. .

  A magnetoresistive element according to a first aspect of the present invention includes a fixed layer whose magnetization direction is fixed, a free layer whose magnetization direction can be changed by energization in a direction perpendicular to the film surface, and a direction perpendicular to the film surface The direction of magnetization can be changed by energizing the electrode, and is provided between an assist layer that assists in changing the direction of magnetization of the free layer, the fixed layer, and the free layer, and is made of a nonmagnetic material. A first intermediate layer; and a second intermediate layer provided between the free layer and the assist layer and made of a nonmagnetic material. The energy barrier for reversing the magnetization of the assist layer is smaller than the energy barrier for reversing the magnetization of the free layer.

  A magnetic memory according to a second aspect of the present invention is provided with the magnetoresistive element according to the first aspect and the first and the first elements provided so as to sandwich the magnetoresistive element and energizing the magnetoresistive element. And a memory cell including a second electrode.

  According to the present invention, it is possible to provide a magnetoresistive element capable of further reducing a write current required for magnetization reversal and a magnetic memory using the same.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, elements having the same function and configuration are denoted by the same reference numerals, and redundant description will be given only when necessary.

(First embodiment)
[1] Configuration of Magnetoresistive Effect Element (hereinafter referred to as MR Element) and Magnetization Reversal Operation FIG. 1 is a cross-sectional view showing the configuration of the MR element 10 according to the first embodiment of the present invention. FIG. 1 shows a basic configuration of the MR element 10 of the present embodiment. The arrow in the figure indicates the magnetization direction.

  The MR element 10 includes a magnetization fixed layer (pinned layer) 11, a first intermediate layer (nonmagnetic layer) 12, a magnetization free layer (also referred to as a free layer or a recording layer) 13, and a second intermediate layer (nonmagnetic layer). 14 and a magnetization reversal assist layer (assist layer) 15 are sequentially laminated. In this basic configuration, the stacking order may be reversed.

  Here, the MR element 10 detects a low resistance state and a high resistance state due to a difference in magnetization state between the pinned layer 11 and the free layer 13 sandwiching the intermediate layer 12, and determines the “1” state and the “0” state. Determine. When a material that exhibits ohmic conduction, for example, a metal material, is used as the intermediate layer 12, a giant magnetoresistive (GMR: Giant Magnetoresistive) effect appears. On the other hand, when an insulator or a semiconductor that exhibits tunnel conduction is used as the intermediate layer 12, a tunnel magnetoresistance (TMR) effect appears, and this MR element is used as a tunnel type magnetoresistance effect element (TMR element), or It is called a magnetoresistive effect type tunnel junction element (MTJ element).

  The pinned layer 11 has a fixed magnetization (or spin) direction. The free layer 13 and the assist layer 15 change (reverse) the magnetization direction. In FIG. 1, the easy magnetization directions of the pinned layer 11 and the free layer 13 are perpendicular to the film surface (or laminated surface) (hereinafter referred to as perpendicular magnetization). That is, the pinned layer 11 and the free layer 13 have perpendicular magnetic anisotropy. In the present embodiment, there is no particular problem even if the magnetization direction is parallel to the film surface (hereinafter referred to as in-plane magnetization). In this case, the pinned layer 11 and the free layer 13 have in-plane magnetic anisotropy.

  However, the easy magnetization directions of the pinned layer 11 and the free layer 13, that is, the direction perpendicular to or parallel to the film surface must be the same. FIG. 1 shows a case where a ferromagnetic material or a ferrimagnetic material is used for the assist layer 15. In this case, the easy magnetization directions of the pinned layer 11, the free layer 13, and the assist layer 15 are the same. There must be.

  In FIG. 1, a perpendicular magnetization film is used as the free layer 13. By using a perpendicular magnetization film for the free layer 13, the aspect ratio Ar of the MR element (ratio of the short side length to the long side length of the element, that is, Ar = long side length / short side length) is designed to be 1. Is possible. This is because, in the case of an in-plane magnetization film, the anisotropic magnetic field necessary for thermal stabilization is covered by the shape magnetic anisotropy energy, so that the aspect ratio of the MTJ element becomes larger than 1. On the other hand, in the case of a perpendicular magnetization film, the anisotropic magnetic field necessary for thermal stabilization is covered by the magnetocrystalline anisotropy energy. Because it does not depend. As a result, when a perpendicular magnetization film is used for the MR element, the MR element size can be reduced. In this case, if a TMR film having the same current density Jc required for magnetization reversal is used, the magnetization reversal current Ic between the in-plane magnetization film and the perpendicular magnetization film having the same MR element width is the aspect ratio of the perpendicular magnetization film. Since the ratio Ar is small, it is reduced.

  In the MR element 10 configured as described above, data is written as follows. In the present embodiment, the current refers to the flow of electrons. First, the MR element 10 is energized in both directions in a direction perpendicular to the film surface.

  As a result, electron spin polarized in majority and minority is supplied to the free layer 13. Then, when the spin angular momentum of the majority electron spin moves to the free layer 13, a spin torque is applied to the free layer 13 to induce magnetization rotation of the free layer 13. Since the spin torque is represented by the outer product of the unit vectors in the magnetization direction of the pinned layer (or assist layer) and the free layer, the spin torque can be applied to the free layer from both the pinned layer and the assist layer. Thereby, the magnetization reversal current by spin injection can be reduced.

  In the present embodiment, it is necessary to fix the magnetization of the assist layer 15 as compared with an MR element having a conventional dual pin structure (a structure in which two pin layers are arranged on both sides of the free layer via intermediate layers, respectively). And there is an effect that high spin injection efficiency can be obtained as in the conventional dual pin structure. This facilitates the design of the magnetization arrangement of the assist layer 15.

  Hereinafter, the magnetization reversal mechanism when the effect of the present embodiment is exhibited will be described in more detail and specifically. In this embodiment, the energy barrier (E_ast) that must be exceeded when the magnetization of the assist layer 15 is reversed is set to be smaller than the energy barrier (E_free) that must be exceeded when the magnetization of the free layer 13 is reversed.

  Here, if the effective magnetic anisotropy energy is Keff and the magnetic body volume is V, the energy barrier E that must be exceeded at the time of magnetization reversal is expressed by the following equation.

E = Keff · V
Therefore, the relationship between the energy barrier (E_ast) of the assist layer 15 and the energy barrier (E_free) of the free layer 13 is expressed by the following equation.

(Keff · V) ast <(Keff · V) free
Further, the energy barrier (E_free) that must be exceeded when the magnetization of the free layer 13 is reversed is set to be smaller than the energy barrier (E_pin) that must be exceeded when the magnetization of the pinned layer 11 is reversed. Therefore, the relationship between the energy barrier (E_pin) of the pinned layer 11 and the energy barrier (E_free) of the free layer 13 is expressed by the following equation.

(Keff · V) free <(Keff · V) pin
Thereby, the relationship between the energy barriers of the pinned layer 11, the free layer 13, and the assist layer 15 is expressed by the following equation.

(Keff · V) ast <(Keff · V) free <(Keff · V) pin
As described above, the MR element 10 of the present embodiment is a design policy of an MR element having a conventional dual pin structure.
(Keff · V) ast> (Keff · V) free and (Keff · V) free <(Keff · V) pin
Is different.

  In order to realize the relationship of E_ast <E_free, the material design of the free layer 13 and the assist layer 15 may be as follows. When the free layer 13 and the assist layer 15 have the same magnetic anisotropy energy (Keff_free = Keff_ast), the relationship between the volume V_free of the free layer 13 and the volume V_ast of the assist layer 15 is expressed by the following equation. .

V_ast <V_free
When the free layer 13 and the assist layer 15 have the same area (S_free = S_ast), the relationship between the film thickness t_free of the free layer 13 and the film thickness t_ast of the assist layer 15 is expressed by the following equation.

t_ast <t_free
When the free layer 13 and the assist layer 15 have the same volume (V_free = V_ast), the effective anisotropic energy Keff_free of the free layer 13 and the effective anisotropic energy Keff_ast of the assist layer 15 The relationship is expressed by the following equation.

Keff_ast <Keff_free
Here, in the case of the in-plane magnetization film, the anisotropic energy Keff is expressed by the following equation.

Keff = Kc + Kshape = Kc + Ms · 4πCshapeMs
Kshape: shape anisotropy energy, Kc: magnetocrystalline anisotropy energy, Cshape: demagnetizing field coefficient of in-plane magnetization reversal according to shape, and Ms: saturation magnetization.

  In the case of a perpendicular magnetization film, the anisotropic energy Keff is expressed by the following equation.

Keff = Kc-2πMs · Ms
Therefore, when the magnetocrystalline anisotropy energy Kc of the free layer 13 and the assist layer 15 is the same, the saturation magnetization Ms_free of the in-plane magnetization assist layer 15 is larger than the saturation magnetization Ms_ast of the in-plane magnetization assist layer 15. Is set as follows.

Ms_ast <Ms_free
On the other hand, when the magnetocrystalline anisotropy energy Kc of the free layer 13 and the assist layer 15 is the same, the saturation magnetization of the perpendicular magnetization free layer 13 is set to be smaller than the saturation magnetization of the perpendicular magnetization assist layer 15. The

Ms_ast> Ms_free
The assist layer 15 can move freely in magnetization. Further, the relationship between the magnetization direction of the assist layer 15 and the magnetization direction of the free layer 13 is not uniquely defined. That is, the unit of the free layer 13, the intermediate layer 14, and the assist layer 15 of the present embodiment is distinguished from a free layer having a SAF (Synthetic Anti-Ferromagnet) structure that is always antiferromagnetically coupled. More specifically, when no current is passed, the magnetic coupling energy (for example, interlayer coupling energy or magnetostatic coupling energy) does not exceed the magnetization reversal energy of the assist layer 15. That is, the assist layer 15 does not have a magnetization direction defined by magnetic coupling with the free layer 13 when no current is passed.

  Next, magnetization reversal from the antiparallel magnetization state (high resistance state) to the parallel magnetization state (low resistance state) of the free layer 13 will be described with reference to FIG. In this case, electrons are supplied to the MR element 10 from the pinned layer 11 side. A critical current at which the magnetization of the free layer 13 is reversed is defined as Ic. Further, the critical current at which the magnetization of the assist layer is reversed is defined as Iast. Here, the effective current I passed through the MR element 10 is described as I1, I2, I3 (I1 <I2 <I3) in the order of change for convenience.

  First, a spin current I1 is injected from the pinned layer 11 into the free layer 13. At this time, I1 <Iast <Ic. Since the angular momentum of the spin current I1 is moved to the free layer 13, the electrons passing through the free layer 13 are spin-polarized according to the magnetization direction of the free layer 13, and the spin-polarized electrons are converted into the assist layer. 15 (see FIG. 2A).

  Next, when a certain spin current I2 is injected from the free layer 13 into the assist layer 15, the magnetization is reversed so that the assist layer 15 is aligned with the same magnetization direction as the free layer 13 (see FIG. 2B). At this time, Iast <I2 <Ic. Next, spins of electrons reflected by the assist layer 15 are injected into the free layer 13. When a spin current I3 exceeding the critical current Ic is injected from the pinned layer 11 and the assist layer 15 into the free layer 13, the magnetization is reversed so that the free layer 13 is aligned with the same magnetization direction as the pinned layer 11 (FIG. 2). (See (c)).

  In the magnetization arrangement of the pinned layer 11 and the assist layer 15 immediately before the magnetization reversal of the free layer 13, both the spin injection effect from the pinned layer 11 and the effect of spin reflection from the assist layer 15 (spin accumulation effect) exist. . Thereby, the magnetization directions of the pinned layer 11 and the free layer 13 are arranged in parallel. In this parallel arrangement, the resistance value of the MR element 10 is the smallest, and this case is defined as data “0”.

  Next, magnetization reversal from the parallel magnetization state (low resistance state) to the antiparallel magnetization state (high resistance state) of the free layer 13 will be described with reference to FIG. In this case, electrons are supplied to the MR element 10 from the assist layer 15 side.

  First, an electron spin current I1 reflected by the free layer 13 is injected into the assist layer 15 (see FIG. 3A). At this time, I1 <Iast <Ic. When the spin current injected into the assist layer 15 becomes I2, the assist layer 15 has a magnetization arrangement antiparallel to the free layer 13. At this time, Iast <I2 <Ic. Next, an electron spin current I 3 spin-polarized by the assist layer 15 is injected into the free layer 13. Further, the electron spin current I3 reflected by the pinned layer 11 is injected into the free layer 13 (see FIG. 3B). Thereby, the magnetization reversal is performed so that the free layer 13 is aligned in the opposite magnetization direction to the pinned layer 11 (see FIG. 3C). At this time, Iast <Ic <I3.

  In the magnetization arrangement of the pinned layer 11 and the assist layer 15 immediately before the magnetization reversal of the free layer 13, both the spin injection effect from the assist layer 15 and the effect of spin reflection from the pinned layer 11 exist. Thereby, the magnetization directions of the pinned layer 11 and the free layer 13 are antiparallel. In this antiparallel arrangement, the resistance value of the MR element 10 is the largest, and this case is defined as data “1”.

  Next, data reading is performed as follows. A read current Ir is passed through the MR element 10 to detect a change in the resistance value of the MR element 10. This read current Ir is set to a value smaller than the write current. More specifically, the read current Ir is preferably set to a current sufficiently smaller than the magnetization reversal current Ic of the free layer 13. The “sufficiently small current” referred to here is a current smaller than Iast, and is preferably about 1/2 or less of Iast.

[2] Other configuration examples of assist layer 15 In the case of a general dual pin structure, a unit of free layer 13 / intermediate layer 12 / pinned layer 11 and a unit of assist layer 15 / intermediate layer 14 / free layer 13 There is a problem that the maximum MR of the MR element is lowered because the MRs that are expressed cancel each other.

  FIG. 4 is a cross-sectional view showing another configuration of the assist layer 15. In the present embodiment, a superparamagnetic material (superparamagnetic material) having a very small magnetization reversal energy is used for the assist layer 15. The superparamagnetic material has magnetization locally, but the residual magnetization is zero and isotropic (random). Therefore, a magnetoresistive effect is exhibited between the free layer 13 and the pinned layer 11 via the intermediate layer 12, but a magnetoresistive effect via the intermediate layer 14 is provided between the free layer 13 and the assist layer 15 having a random magnetization arrangement. Not expressed. This is a great advantage, and it is possible to avoid the deterioration of the read output due to the second pinned layer, which is a problem in the MR element having the dual pin structure.

Therefore, when the same material, for example, an insulator such as MgO (magnesium oxide) or AlO _x (aluminum oxide) is used for both the intermediate layer 12 and the intermediate layer 14, the pin layer and the assist layer provide high spin injection efficiency. At the same time, the MR ratio does not cancel because the magnetoresistive effect appears only in one of the intermediate layers 12.

  Next, magnetization reversal from the antiparallel magnetization state (high resistance state) to the parallel magnetization state (low resistance state) of the free layer 13 will be described with reference to FIG. In this case, electrons are supplied to the MR element 10 from the pinned layer 11 side. Here, the critical current at which the free layer 13 undergoes magnetization reversal is defined as Ic. In addition, a current in which the magnetization of the assist layer 15 made of a superparamagnetic material is arranged in one direction and the magnetization direction can be defined is defined as Iast. The effective current I passed through the MR element 10 is described as I1, I2, I3 (I1 <I2 <I3) in the order of change for convenience.

  First, a spin current I1 is injected from the pinned layer 11 into the free layer 13. At this time, I1 <Iast <Ic. Since the angular momentum of the spin current I1 is moved to the free layer 13, the electrons passing through the free layer 13 are spin-polarized according to the magnetization direction of the free layer 13, and the spin-polarized electrons are converted into the assist layer. 15 (see FIG. 5A).

  Subsequently, the magnetization direction of the assist layer 15 is aligned with the magnetization direction of the free layer 13 by the spin current I2 from the free layer 13 (see FIG. 5B). At this time, Iast <I2 <Ic. Next, spins of electrons reflected by the assist layer 15 are injected into the free layer 13. When a spin current exceeding the critical current Ic is injected from the pinned layer 11 and the assist layer 15 to the free layer 13, the magnetization is reversed so that the free layer 13 is aligned with the same magnetization direction as the pinned layer 11 (FIG. 5C). reference). At this time, Iast <Ic <I3. Thereafter, when the supply of electrons to the MR element 10 is stopped, the magnetization of the assist layer 15 becomes random.

  Next, magnetization reversal from the parallel magnetization state (low resistance state) to the antiparallel magnetization state (high resistance state) of the free layer 13 will be described with reference to FIG. In this case, electrons are supplied to the MR element 10 from the assist layer 15 side.

  First, an electron spin current I1 reflected by the free layer 13 is injected into the assist layer 15 (see FIG. 6A). At this time, I1 <Iast <Ic. When the spin current injected into the assist layer 15 becomes I2, the assist layer 15 has a magnetization arrangement antiparallel to the free layer 13. At this time, Iast <I2 <Ic. Next, an electron spin current I 3 spin-polarized by the assist layer 15 is injected into the free layer 13. Further, the electron spin current I3 reflected by the pinned layer 11 is injected into the free layer 13 (see FIG. 6B). Thereby, the magnetization reversal is performed so that the free layer 13 is aligned in the magnetization direction opposite to that of the pinned layer 11 (see FIG. 6C). At this time, Iast <Ic <I3. Thereafter, when the supply of electrons to the MR element 10 is stopped, the magnetization of the assist layer 15 becomes random.

  In the conventional dual pin structure, although a high spin injection efficiency can be obtained, a reciprocal magnetoresistive effect appears in both the intermediate layer 12 and the intermediate layer 14, so that the TMR ratio required at the time of reading is lowered. Although a problem has occurred, the use of a super paramagnetic material for the assist layer 15 can avoid such a problem.

[3] MTJ Laminate Configuration A main configuration example of the MTJ film will be described with reference to the drawings.

  FIG. 7 shows a basic laminated structure of the MR element 10. The basic configuration including the pinned layer 11, the intermediate layer 12, the free layer 13, the intermediate layer 14, and the assist layer 15 is the same as that shown in FIG. A base layer 16 for controlling the crystal orientation or crystallinity of the basic structure is provided in the lowermost layer on the substrate (not shown) side. The underlayer 16 is made of a nonmagnetic metal. The uppermost layer is provided with a cap layer 17 for protecting the basic structure from deterioration such as oxidation and corrosion. A nonmagnetic metal is used for the cap layer 17. As these nonmagnetic metals, W (tungsten), Al (aluminum), Cu (copper), AlCu, or the like is used.

  FIG. 8 shows an MR element 10 including an interface free layer, an interface pinned layer, and an interface assist layer. An interface pinned layer 21 is provided between the pinned layer 11 and the intermediate layer 12. An interface free layer 22 is provided between the intermediate layer 12 and the free layer 13. An interface free layer 23 is provided between the free layer 13 and the intermediate layer 14. An interface assist layer 24 is provided between the intermediate layer 14 and the assist layer 15. The interface free layer, the interface pinned layer, and the interface assist layer have the effect of increasing the magnetoresistance effect, and further have the effect of reducing the write current during spin injection writing.

  The configuration shown in FIG. 8 is mainly used for a coercive force difference type MR element. In this case, the magnetization direction may be perpendicular magnetization or in-plane magnetization. When a superparamagnetic material is used for the assist layer 15, the interface assist layer 24 is not inserted. FIG. 9 shows the MR element 10 when the assist layer 15 is a superparamagnetic material.

  When interfacial free layers are inserted on both sides of the free layer, the substantial free layer thickness is the sum of the free layer and interface free layer thicknesses. When comparing the film thickness of the free layer 13 or the product of the saturation magnetization and the film thickness, the sum of the film thicknesses of the free layer and the interface free layer is used. Regarding the magnetic characteristics, the overall characteristics of the interface free layer and the free layer are the substantial magnetic characteristics of the free layer.

  Similarly, when the interface pinned layer is inserted, the substantial film thickness of the pinned layer is the sum of the film thicknesses of the pinned layer and the interface pinned layer. When comparing the film thickness of the pinned layer or the product of the saturation magnetization and the film thickness, the sum of the film thicknesses of the pinned layer and the interface pinned layer is used. Regarding the magnetic characteristics, the overall characteristics of the interface pinned layer and the pinned layer are the substantial magnetic characteristics of the pinned layer. The same applies to the interface assist layer.

  In the MTJ film using the in-plane magnetization film, a ferromagnetic film having soft magnetism is used for the pinned layer. In this case, an antiferromagnetic layer 25 is inserted between the underlayer 16 and the pinned layer 11. Thereby, the magnetization of the pinned layer 11 is fixed in one direction by exchange coupling between the pinned layer 11 and the antiferromagnetic layer 25. At the same time, due to exchange coupling between the pinned layer 11 and the antiferromagnetic layer 25, high magnetic anisotropy energy is imparted to the pinned layer 11 and the function as the pinned layer 11 is imparted. As shown in FIG. 10, the pinned layer 11 may be pinned by the antiferromagnetic layer 25 even in the MTJ film using the perpendicular magnetization film.

  Further, the assist layer 15 may use exchange coupling with an antiferromagnetic material. However, this is limited to the case where the magnetization reversal energy barrier (E_ast) of the assist layer 15 does not exceed the magnetization reversal energy barrier (E_free) of the free layer 13.

  Further, as shown in FIG. 11, the pinned layer 11 is composed of a magnetic layer 11C / nonmagnetic layer 11B / magnetic layer 11A, and has a pinned layer of SAF structure having a magnetically antiferromagnetic magnetization direction. 11 may be used. In the description of the laminated film, the left side of “/” represents the upper layer and the right side represents the lower layer.

  For the SAF nonmagnetic layer 11B, a metal material such as Ru (ruthenium) or Os (osmium) is used, and the film thickness is set to 3 nm or less. This is to obtain sufficiently strong antiferromagnetic coupling between the magnetic layers 11C and 11A via the nonmagnetic layer 11B. The assist layer 15 may also use a SAF structure. By using this SAF structure, the resistance of the assist layer 15 to the external magnetic field and the thermal stability can be adjusted.

  Needless to say, the MR element 10 may be configured by arbitrarily combining the configurations shown in FIGS. 7 to 11. Further, the interface free layer, the interface pinned layer, and the interface assist layer do not have to include all of the interface layers shown in FIG. 8, and even when some interface layers are included, the characteristics of the MR element 10 can be improved. Can be improved.

[4] Cross-sectional structure of MR element As described above, the cross-sectional shape of the MR element 10 is optimized as a method of creating the relationship between the magnetization reversal energy barriers of the pinned layer 11, the free layer 13, and the assist layer 15. It is also good. That is, the energy barrier E is
E = Keff · V
Therefore, the energy barrier E can also be adjusted by adjusting the volume of each layer of the pinned layer 11, the free layer 13, and the assist layer 15. Therefore, the energy barrier E can be adjusted by adjusting the area of each layer of the element.

  FIG. 12 schematically shows a cross-sectional view of the MR element 10 when an MTJ film is ideally processed. The planar shape of the MR element 10 is, for example, a circle or an ellipse. The angle of the side surface of the MR element 10 is defined as θ. As shown in FIG. 12, assuming that θ≈90 ° ideally, the magnetization reversal energy barrier E is adjusted by the magnetocrystalline anisotropy energy Kc, saturation magnetization Ms, and film thickness t of each layer. .

  FIG. 13 schematically shows a cross-sectional view of the MR element 10 when θ <90 °. As shown in FIG. 13, the side surface of the MR element 10 is inclined so that θ <90 °. That is, the cross-sectional shape of the MR element 10 is a tapered shape that becomes narrower upward.

If processing is performed so that the angle of the side surface of the MR element 10 is less than 90 °, even if the pinned layer 11, the free layer 13, and the assist layer 15 have the same film thickness,
(Keff · V) ast <(Keff · V) free <(Keff · V) pin
MR elements satisfying the above relationship can be formed. This facilitates formation of an MR element that satisfies the energy barrier relationship.

  FIG. 14 schematically shows a cross-sectional view of the MR element 10 when the cross-sectional shape forms a step shape. The area of the free layer 13 is smaller than the area of the pinned layer 11. The area of the assist layer 15 is smaller than the area of the free layer 13. In other words, the length of the free layer 13 in the film surface direction is shorter than the length of the pinned layer 11 in the film surface direction. The length of the assist layer 15 in the film surface direction is shorter than the length of the free layer 13 in the film surface direction. Further, the side surfaces of the pinned layer 11, the free layer 13, and the assist layer 15 are substantially vertical. By optimizing the patterning process and the processing process, it is possible to form the MR element 10 whose cross-sectional shape forms a step shape.

In the MR element 10 shown in FIG. 14, even when the pinned layer 11, the free layer 13, and the assist layer 15 have the same film thickness,
(Keff · V) ast <(Keff · V) free <(Keff · V) pin
It is possible to satisfy the relationship.

[5] Materials Used for MR Element 10 Hereinafter, materials of each layer constituting the MR element 10 will be described in order.

[5-1] Materials Used for Intermediate Layer 12 and Intermediate Layer 14 In the MR element 10 of the present embodiment, an insulator or a semiconductor is used for the intermediate layer 12. In this case, the tunnel magnetoresistive effect appears in the unit of the free layer 13 / intermediate layer 12 / pinned layer 11. Therefore, at the time of reading, the magnetization direction of the pinned layer 11 and the free layer 13 becomes parallel or antiparallel, so that the resistance value of the MR element 10 becomes low resistance or high resistance. It is determined that the data is “1”.

  When a super paramagnetic material is used for the assist layer 15, the tunnel magnetoresistive effect does not appear in the unit of the assist layer 15 / intermediate layer 14 / free layer 13. This is because there is almost no magnetization (residual magnetization) in the residual magnetization state of the assist layer 15 in the perpendicular magnetization direction and the in-plane magnetization direction, that is, the random array magnetization direction. In this case, the net Ms of the assist layer 15 is almost zero.

  Therefore, any of a metal conductor, an insulator, and a semiconductor may be used for the intermediate layer 14. However, when an insulator and a semiconductor are used, the resistance value of the MR element 10 is increased, and therefore a metal conductor is preferably used.

Here, as a metal conductor used for the intermediate | middle layer 14, Cu (copper), Al (aluminum), Ag (silver), Au (gold), etc. are preferable. Furthermore, a granular structure material of a conductive metal phase such as MgO—Cu or AlO _x —Cu and an insulating phase may be used. When the granular structure material is used, the current flowing through the intermediate layer 14 flows through a path having the smallest tunnel barrier. The current density can be locally improved by this current confined path (CCP) effect. As a result, the magnetization reversal current of the free layer 13 can be reduced.

The film thicknesses of the intermediate layers 12 and 14 are set to be 3 nm or less when the tunnel magnetoresistance effect is used. This is because it is necessary to flow a tunnel current of about 1 × 10 ⁵ to 1 × 10 ⁷ A / cm ² at the time of writing information, so that the resistance × area (RA) of MR element 10 is 100 Ωμm ² or less. This is because it is necessary to make it small.

Examples of the insulator used for the intermediate layers 12 and 14 include Al ₂ O ₃ (aluminum oxide), MgO (magnesium oxide), CaO (calcium oxide), SrO (strontium oxide), TiO (titanium oxide), and EuO (eurobium oxide). ), ZrO (zirconium oxide) or HfO (hafnium oxide). Examples of the semiconductor include a semiconductor such as Ge (germanium) or Si (silicon), a compound semiconductor such as GaAs (gallium arsenide) or InAs (indium arsenide), and an oxide semiconductor such as TiO2 (titanium oxide). .

Among these, MgO having a NaCl structure is a preferable material for the intermediate layer. This is because the MR ratio is maximized when MgO is used. When MgO is used, it is possible to obtain an MR ratio of 100% or more when the resistance and area product (RA) of the MR element 10 is in the range of 5 Ωμm ^{2 to} 1000 Ωμm ² . This MgO has a NaCl structure, and the (100) plane orientation is most preferable as the crystal orientation from the viewpoint of MR ratio. In addition, when the MgO layer is formed, the MR ratio can be further improved by inserting a Mg layer of 1 nm or less above or below the MgO.

The MgO layer is formed by sputtering with a rare gas (Ar (argon), Ne (neon), Kr (krypton), or Xe (xenon)) using an MgO target, or using an Mg target. It is formed by an oxidation reactive sputtering method in an O ₂ atmosphere. Alternatively, the Mg layer can be formed by oxidizing with oxygen radicals, oxygen ions, ozone, or the like. It can also be formed by epitaxial growth using molecular beam epitaxy (MBE) method using MgO or electron beam evaporation method.

  Here, in order to obtain a large MR ratio, it is necessary to improve the degree of orientation of MgO. Based on the plane orientation of MgO, the orientation of the magnetic layer serving as the underlayer to be selected is determined. MgO preferably has a (100) plane orientation. In order to orient (100) plane preferential orientation of MgO, its underlayer (free layer, pinned layer, assist layer, interface layer, etc.) has a BCC (Body-Centered Cubic) structure (100) plane orientation, FCC (Face). -Centered Cubic) structure (100) orientation or amorphous structure is preferable.

Examples of the material of the BCC structure include BCC-Fe _100-x Co _x (0 ≦ x ≦ 70 atomic%), BCC-Co of 1 nm or less epitaxially grown on the BCC structure, and the like. Alternatively, BCC-Fe _100-x (CoNi) _x (0 ≦ x ≦ 70 atomic%) or the like may be used. In this case, the effect of increasing the MR ratio of 10 to 20% can be obtained by adding dilute Ni of 10 atomic% or less. Examples of the amorphous material include a Co—Fe—B alloy and an Fe—Co—Zr alloy.

[5-2] Magnetic material used for the perpendicular magnetization free layer, the perpendicular magnetization pinned layer, and the perpendicular magnetization assist layer In the present embodiment, a perpendicular magnetization film is used for the pinned layer 11, the free layer 13, and the assist layer 15. . When the in-plane magnetization free layer is used, the switching magnetic field strongly depends on the size of the MR element, but by using the perpendicular magnetization free layer, the size dependence of the MR element is reduced.

  In the case of in-plane magnetization, the switching magnetic field changes depending on the element shape and element size in order to maintain the stability of magnetization by the shape magnetic anisotropy energy using saturation magnetization. On the other hand, in the case of perpendicular magnetization, the change in switching magnetic field depends on the element shape and element size in order to reduce the saturation magnetization and maintain the stability of magnetization by the magnetocrystalline anisotropy energy independent of the element shape and element size. do not do. Accordingly, the use of the perpendicular magnetization free layer solves the problem of the MR element using the in-plane magnetization film that the switching magnetic field is increased when the MR element is reduced, and is preferable for miniaturization of the MR element.

  The perpendicular magnetization film used in the MR element 10 of the present embodiment includes at least one of Fe (iron), Co (cobalt), Ni (nickel), and Mn (manganese), Pt (platinum), and Pd. (Palladium), Ir (iridium), Rh (rhodium), Os (osmium), Au (gold), Ag (silver), Cu (copper), and a magnetic material containing at least one kind of Cr (chromium) Is used.

  Furthermore, B (boron), C (carbon), Si (silicon), Al (aluminum) are used for adjustment of saturation magnetization, control of magnetocrystalline anisotropy energy, adjustment of crystal grain size and inter-grain bond. At least one element selected from Mg, magnesium (magnesium), Ta (tantalum), Zr (zirconium), Ti (titanium), Hf (hafnium), Y (yttrium), and rare earth elements. Also good. By adding these elements, the saturation magnetization and the magnetocrystalline anisotropy energy can be lowered without impairing the perpendicular magnetization, and the fragmentation and refinement of crystal grains can be promoted.

Examples of the magnetic material containing Co as a main component include a Co—Cr—Pt alloy, a Co—Cr—Ta alloy, and a Co—Cr—Pt—Ta alloy having an HCP (Hexagonal Closest Packing) structure. “-” Means an alloy. By adjusting the composition of each element, it is possible to adjust the magnetocrystalline anisotropy energy within the range of 1 × 10 ⁵ erg / cc or more and less than 1 × 10 ⁷ erg / cc. When these materials are used for the pinned layer, it is preferable to use Ru (ruthenium) having an HCP structure as the underlayer.

The Co—Pt alloy forms an L1 ₀ —CoPt ordered alloy with a composition in the vicinity of Co ₅₀ Pt ₅₀ (atomic%). This ordered alloy has an FCT (Face-Centered Tetragonal) structure. When MgO (100) is used as the intermediate layer 12, a (001) -oriented FCT-CoPt ordered alloy is preferable because the interface misfit with the intermediate layer 12 can be reduced. Further, even when an interface layer is inserted between the intermediate layer and the free layer, the interface layer is easily (100) -plane oriented.

Examples of the magnetic material containing Fe as a main component include an Fe—Pt alloy and an Fe—Pd alloy. Among this, Fe-Pt alloy composition is ordered in _Fe 50 _{Pt 50} (atomic%), having an L1 ₀ structure as a basic structure FCT structure. Further, Fe-Pt alloy composition is ordered in _Fe 75 _{Pt 25} (atomic%), having an L1 ₂ structure as a basic structure FCT structures _(Fe 3 Pt structure). Thereby, a large magnetocrystalline anisotropy energy of 1 × 10 ⁷ erg / cc or more can be expressed.

Fe ₅₀ Pt ₅₀ alloy, prior to ordering the L1 ₀ structure and has a FCC structure. In this case, the magnetocrystalline anisotropy energy is about 1 × 10 ⁶ erg / cc. Therefore, the crystal magnetic anisotropy energy is adjusted within the range of 5 × 10 ⁵ erg / cc to 5 × 10 ⁸ erg / cc by adjusting the annealing temperature, composition, control of the order by the layer structure, and additive elements. can do. The saturation magnetization is approximately 800 to 1100 emu / cc before the addition of the element, but can be reduced to 800 emu / cc or less by adding the element. When this magnetic material is used for the free layer, it is preferable in terms of reducing the spin injection current density Jc.

Specifically, the Fe-Pt alloy having an L1 ₀ ordered structure, Cu (copper), Ti (titanium), V (vanadium), Mn (manganese), or Cr (chromium) range of 30 atomic% or less, and more It is possible to control the saturation magnetization and the magnetocrystalline anisotropy energy of the Fe—Pt alloy. V (vanadium) has an effect of lowering a damping constant (magnetization braking constant) important in spin injection magnetization reversal and an effect of reducing a magnetization reversal current.

L1 ₀ or Fe-Pt alloy ordered to L1 ₂ structure has a FCT structure, prior to ordering has a FCC structure. Therefore, it is very consistent with MgO (100). Specifically, (100) plane-oriented growth is grown on MgO (100) by growing BCC-Fe with (100) plane orientation on MgO (100) and stacking Pt (100) thereon. it is possible to form the L1 ₀ structure or L1 ₂ structure Fe-Pt ordered alloy. Further, when BCC-Cr is formed between the Fe—Pt ordered alloy and MgO (100), the (100) plane orientation of the Fe—Pt ordered alloy is more preferred.

Further, when forming an Fe—Pt ordered alloy having an L1 ₀ or L1 ₂ structure, if a multilayer structure of [Fe / Pt] n (n is an integer of 1 or more) is formed, L1 ₀ or L1 ₂ close to an ideal rule. An Fe—Pt ordered alloy having a structure can be formed. The order of lamination of Fe and Pt does not matter. In this case, it is desirable to set the film thickness of Fe / Pt to be 0.1 nm or more and 3 nm or less. This is to create a uniform composition state is essential, thereby, when the ordering of the L1 ₀ or L1 ₂ structure Fe-Pt alloy, martensitic transformation from FCC structure to FCT structure (martensitic transformation) This is important because this transformation is promoted.

Further, ordering temperature of L1 ₀ or L1 ₂ structure Fe-Pt alloys is as high as more than 500 degrees, is excellent in heat resistance. This point has resistance to annealing treatment in a later process. Further, the ordering temperature can be lowered by the additive element in the range of 30 atomic% or less such as Cu and Pd described above.

  Examples of other perpendicular magnetization films used in the MR element 10 of this embodiment include ferrimagnetic materials containing at least one of Fe, Co, Ni, Mn, Cr, and rare earth elements. As rare earth elements, La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (bromethium), Sm (samarium), Eu, Gd (gadolinium), Tb (terbium), Dy (dysprosium) ), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), or Lu (lutetium).

Ferrimagnetic materials containing rare earth elements have an amorphous structure. In this ferrimagnetic material, the saturation magnetization can be reduced to 400 emu / cc or less and the magnetocrystalline anisotropy energy can be improved to 1 × 10 ⁶ erg / cc or more by adjusting the composition.

  Further, as the perpendicular magnetization film used in the MR element 10 of the present embodiment, a ferromagnetic material made of a mixed crystal of a metal magnetic phase and an insulating phase may be used. In this case, the metal magnetic phase includes at least one of Fe, Co, Ni, and Mn and at least one of Pt, Pd, Ir, Rh, Os, Au, Ag, Cu, Cr, Ta, and rare earth elements. It is comprised from the ferromagnetic material containing 1 or more types. The insulating phase is an oxide, nitride, or oxynitride containing at least one element selected from B, C, Si, Al, Mg, Ta, Cr, Zr, Ti, Hf, Y, and rare earth elements Consists of

  Ferromagnetic material consisting of mixed crystal of metal magnetic phase and insulating phase is separated into conductive metal magnetic part and non-conductive insulator part, so current concentrates on metal magnetic part As a result, the current-carrying area is reduced and the local current density is increased. This has the effect of reducing the required reversal current substantially.

  In order to obtain such a CCP effect, it is necessary to control crystallinity. As the two-phase separation structure, there are a granular (crystal grain dispersion) structure, an island (island shape) structure, and a columner (columnar shape) structure. In the case of the columnar structure, since the metal magnetic part penetrates up and down the layer, the current confinement effect is easily obtained. In the case of the granular structure and the island structure, since the current flows through the path having the smallest tunnel barrier, the CCP effect can be obtained similarly to the columnar structure.

In addition, examples of the perpendicular magnetization film used in the MR element 10 of the present embodiment include a Mn ferromagnetic alloy or a Cr ferromagnetic alloy. Examples of the Mn-based ferromagnetic alloy include a Mn—Al alloy, a Mn—Au alloy, a Mn—Zn alloy, a Mn—Ga alloy, a Mn—Ir alloy, and a Mn—Pt ₃ alloy, which have a regular lattice. There are features. Examples of the Cr-based ferromagnetic alloy include a Cr—Pt ₃ alloy. This has an L1 ₀ ordered lattice and has characteristics as a ferrimagnetic material.

[5-3] Superparamagnetic material used for assist layer 15 The energy required for a magnetic material to maintain the magnetization direction depends on the ambient temperature at which the magnetic material is placed. As described above, the energy Est of the magnetic body in the static state is expressed by the following equation.

Est = Keff · V
On the other hand, the thermal energy Eenv at a certain environmental temperature T is expressed by the following equation, assuming that the Boltzmann coefficient is kB.

Eenv = kB · T
Therefore, if the energy of the magnetic material is larger than the thermal energy, the magnetic material can define the magnetization direction. However, when the thermal energy is lower, the static magnetization direction of the magnetic material is not determined. This state is called “superparamagnetic state”, and the magnetic body in this state is called “superparamagnetic body”. Since the superparamagnetic material is not a nonmagnetic material and has a magnetization locally, the magnetization is defined in the magnetic field application direction in the magnetic field. That is, in principle, the magnetization is directed in the direction in which torque acts.

  The size of the magnetic material to become a superparamagnetic material depends on the magnetic anisotropy energy, but in an actual system, the average diameter of the magnetic material is about 3 nm. Therefore, a film in which the average crystal grain size is 3 nm or less and the crystal grains are magnetically separated may be formed.

Examples of the superparamagnetic film used in the present embodiment include a granular film in which an oxide in which magnetic particles such as FeCoNi—SiO _x and FeCoNi—MgO are dispersed serves as a parent phase. Here, the composition ratio of Fe, Co, and Ni may be arbitrary. The parent phase may be a nitride as well as an oxide. Examples of the oxide parent phase include oxides of Ca, Ti, V, Cr, Y, Zr, Nb, Hf, Ta, or W in addition to SiO _x , MgO, or AlO _x . The valence of the oxide does not matter. Further, Si, Al, Ca, Ti, V, Cr, Y, Zr, Nb, Hf, Ta, or W nitride may be used.

  Further, there are FeCoNi-Cu or FeCoNi-Au granular films in which a nonmagnetic metal serves as a parent phase. The nonmagnetic metal is preferably in a non-solid relationship with Fe, Co, or Ni. When Fe, Co, or an FeCo alloy is used, Cu, Au, or Ag is preferable as the nonmagnetic metal. From the viewpoint of resistance control, it can be said that the parent phase is preferably a nonmagnetic metal.

  In the sense that the condition that the average diameter of the magnetic particles is about 3 nm is satisfied, when the assist layer 15 made of a superparamagnetic material is used, even a continuous film having a film thickness of 3 nm or less is a magnet that becomes a magnetization reversal unit at the time of spin injection magnetization reversal. Since the body is less than 3 nm, it can be a superparamagnetic behavior. Actually, even in a continuous Co—Fe—Ni film originally having in-plane magnetization, a superparamagnetic behavior is observed at 1.5 nm or less even when a non-current is applied. Here, the composition ratio of Co, Fe, and Ni is arbitrary. At this time, the CoFeNi alloy layer having a thickness of 1.5 nm or less preferably has an average crystal grain size of 3 nm or less.

  In the present embodiment, since the assist layer 15 is formed adjacent to the intermediate layer 14, the second intermediate layer can be formed by making the material of the granular layer mentioned above and the material of the intermediate layer 14 the same. 14 may be used. Thereby, it can be expected that the spin injection efficiency from the assist layer 15 is improved.

[5-4] Magnetic material used for in-plane magnetization free layer, in-plane magnetization pinned layer, and in-plane magnetization assist layer In this embodiment, the magnetization directions of free layer 13, pinned layer 11, and assist layer 15 are It may be in-plane. From the viewpoint of ease of formation, an in-plane magnetization film is often used.

As the in-plane magnetization film used in the MR element 10 of the present embodiment, a ferromagnetic material including at least one of Fe, Co, and Ni is used. _{Examples of the} ferromagnetic material include an Fe _x Co _y Ni _z alloy (x, y, z ≧ 0, x + y + z = 1) having an FCC or BCC structure.

For the free layer 13 and the assist layer 15, an Fe—Co—Ni alloy is mainly used. Further, in order to reduce the saturation magnetization of the Fe-Co-Ni _{alloy, (Fe x Co y Ni z} ) 100-a X a alloy (x, y, z ≧ 0 , x + y + z = 1, a> 0, a is Atomic%, X is an additive element) is also preferable. By reducing the saturation magnetization, the reversal current can be greatly reduced.

  Examples of additive elements that do not destroy the BCC structure and can reduce saturation magnetization, that is, a complete solid solution that can be dissolved in a substitutional form or an additive element that has a solid solution source to some extent include V (vanadium), Nb (niobium), Ta (Tantalum), W (tungsten), Cr (chromium), Mo (molybdenum), Si (silicon), Ga (gallium), or Ge (germanium). Among these, V is effective because it also has an effect of reducing a damping constant (magnetization braking constant).

Also, an interstitial element such as B (boron), C (carbon), or N (nitrogen) is added, or Zr, Ta, Ti, Hf, Y, or a rare earth element having almost no solid solution source is added. When added, saturation magnetization can be reduced by changing the crystal structure to an amorphous structure. As such a material, for example, it has an amorphous structure _{(Fe x Co y Ni z)} 100-b X b alloy (x, y, z ≧ 0 , x + y + z = 1, b> 0, b atomic%, X B, C, N, Zr, Ta, Ti, Hf, Y, or additional elements such as rare earth elements).

An example of the material containing Mn is a Mn-based ferromagnetic Heusler alloy. Heusler alloys are materials that exhibit half-metal conduction characteristics. Here, the Mn-based ferromagnetic Heusler alloy is a BCC-based alloy having an ordered lattice represented by A ₂ MnX. The element A is a material selected from Cu, Au, Pd, Ni, and Co. The X element is a material selected from Al (aluminum), In (indium), Sn (tin), Ga (gallium), Ge (germanium), Sb (antimony), and Si (silicon). Among Heusler alloys, a Co ₂ MnAl alloy having a BCC structure or the like has good consistency with MgO (100) by being oriented in the BCC (100) plane.

The material used for the pinned layer 11 is preferably a half-metal material having a high polarizability and capable of realizing a polarizability of 100% in principle. As a material containing Mn, a Mn-based ferromagnetic Heusler alloy can be used as a half metal. Here, the Mn-based ferromagnetic Heusler alloy is a BCC-based alloy having an ordered lattice represented by A ₂ MnX. The element A is a material selected from Cu, Au, Pd, Ni, and Co. The X element is a material selected from Al, In, Sn, Ga, Ge, Sb, and Si. Among Heusler alloys, a Co ₂ MnAl alloy having a BCC structure or the like has good consistency with MgO (100) by being oriented in the BCC (100) plane.

  The film thickness of the ferromagnetic layer in the pinned layer needs to be 1 nm or more. This is because if the thickness is less than the above-mentioned thickness, the ferromagnetic layer does not become a continuous film, and the characteristics as a magnetic layer are not sufficiently exhibited, and a sufficient magnetoresistance effect ratio (TMR ratio or GMR ratio) cannot be obtained. The maximum film thickness is desirably 3 nm or less. This is because when the film thickness exceeds 3 nm, the current threshold necessary for spin injection magnetization reversal is significantly increased because the coherent spin precession length is far exceeded.

Also, if the in-plane magnetization pinned layer described above is the base layer of the MgO intermediate _layer, Fe x _Co y Ni _z alloy (x, y, z ≧ 0 , x + y + z = 1) materials, have a (100) -oriented And preferably has a BCC structure. Furthermore, it is also preferable to add B, C, N, or the like to the Fe _x Co _y Ni _z alloy at a concentration of 30 atomic% or less to form an amorphous structure. This is because MgO tends to be preferentially oriented on the (100) plane on a film having an amorphous structure.

[5-5] Materials used for interface free layer, interface pinned layer, and interface assist layer In the present embodiment, between the intermediate layer 12 and the free layer 13, and between the free layer 13 and the intermediate layer 14, That is, an interface free layer may be inserted at the interface. Further, an interface pinned layer may be inserted between the pinned layer 11 and the intermediate layer 12. Further, an interface assist layer may be inserted between the intermediate layer 14 and the assist layer 15.

  The interface pinned layer, interface free layer, and interface assist layer (hereinafter referred to as interface layer) have the effect of increasing the magnetoresistive effect, and further have the effect of reducing the write current during spin injection writing. The interface layer that increases the magnetoresistive effect desirably has a large bulk polarizability of the material itself, and further, the material is preferably selected so that the interface polarizability with the intermediate layer is increased.

As the interface layer used in the MR element 10 of the present embodiment, a ferromagnetic material including at least one of Fe, Co, Ni, Mn, and Cr is used. _{Examples of} the material mainly composed of Fe, Co, or Ni include a Fe _x Co _y N _z alloy having an FCC or BCC structure. Further, in order to reduce the saturation magnetization of the Fe-Co-Ni _{alloy, (Fe x Co y Ni z} ) 100-a X a alloy (x, y, z ≧ 0 , x + y + z = 1, a> 0, a is Atomic%, X is an additive element) is also preferable. By reducing the saturation magnetization, the reversal current can be greatly reduced.

  Examples of additive elements that can reduce the saturation magnetization without breaking the BCC structure, that is, the total solid solution that can be solid-dissolved in the substitution type or the additive elements that have a solid solution source to some extent include V, Nb, Ta, W, Cr, Mo , Si, Ga, Ge or the like. Among these, V is effective because it also has an effect of reducing a damping constant (magnetization braking constant).

Further, by adding interstitial elements such as B, C, or N, or adding Zr, Ta, Ti, Hf, Y, or rare earth elements having almost no solid solution source, the crystal structure is made amorphous. Saturation magnetization can be reduced by changing the structure. As such a material, for example, it has an amorphous structure _{(Fe x Co y Ni z)} 100-b X b alloy (x, y, z ≧ 0 , x + y + z = 1, b> 0, b atomic%, X is B, C, N, Zr, Ta, Ti, Hf, Y, or additional elements such as rare earth elements). However, in order to obtain a certain TMR ratio, it is important to promote recrystallization partially, that is, at the interface with MgO.

An example of the material containing Mn is a Mn-based ferromagnetic Heusler alloy. Here, the Mn-based ferromagnetic Heusler alloy is a BCC-based alloy having an ordered lattice represented by A ₂ MnX. The element A is a material selected from Cu, Au, Pd, Ni, and Co. The X element is a material selected from Al, In, Sn, Ga, Ge, Sb, and Si. Among Heusler alloys, a Co ₂ MnAl alloy having a BCC structure or the like has good consistency with MgO (100) by being oriented in the BCC (100) plane. An Mn-based Heusler alloy may exhibit half-metal conduction characteristics.

An oxide material can also be used as the interface layer. As the oxide material, a half metal such as Fe ₂ O ₃ is applicable as the interface layer.

  The film thickness of the interface layer formed on the magnetic layer such as the free layer or the pinned layer needs to be 0.5 nm or more. Similarly, the film thickness of the interface layer formed on the insulating layer or the semiconductor layer is also required. 0.5 nm or more is necessary. This is because if the thickness is less than the above-described thickness, the interface layer does not become a continuous film, and the characteristics as the interface layer are not sufficiently exhibited, and a sufficient magnetoresistance effect ratio (TMR ratio or GMR ratio) cannot be obtained. The maximum film thickness of the interface layer is desirably 5 nm or less. This is because if the film thickness exceeds 5 nm, the current threshold required for spin injection magnetization reversal becomes significantly large because the precession length of the coherent spin is far exceeded.

  As described in detail above, according to the present embodiment, when the magnetization of the free layer 13 is reversed, the spin injection effect and the spin accumulation effect from the pinned layers 11 and 15 can be used. As a result, the spin injection efficiency can be improved, and the write current required for magnetization reversal can be reduced.

  When a superparamagnetic material is used as the assist layer 15, a magnetoresistive effect is exhibited between the free layer 13 and the pinned layer 11 via the intermediate layer 12. In between, the magnetoresistive effect through the intermediate layer 14 does not appear. Thereby, it is possible to suppress a decrease in MR ratio at the time of data reading.

  Further, since it is not necessary to fix the magnetization of the assist layer 15, the design of the magnetization arrangement of the assist layer 15 is facilitated. Furthermore, since the assist layer 15 can be made thinner than the conventional dual pin structure, the MR element 10 can be miniaturized in the thickness direction.

  Further, it is possible to use a conductor such as a metal for the intermediate layer 14 that does not exhibit the magnetoresistive effect. Thereby, the resistance value of the MR element can be reduced, and the parasitic resistance contributing to the series circuit of the intermediate layer 14 is reduced, so that the MR ratio can be increased.

(Second Embodiment)
The second embodiment shows an example in which an MRAM is configured using the MR element 10 described above.

  FIG. 15 is a circuit diagram showing a configuration of the MRAM according to the second embodiment of the present invention. The MRAM includes a memory cell array 30 having a plurality of memory cells MC arranged in a matrix. In the memory cell array 30, a plurality of bit line pairs BL, / BL are arranged so as to extend in the column direction. In the memory cell array 30, a plurality of word lines WL are arranged so as to extend in the row direction.

  Memory cells MC are arranged at the intersections between the bit lines BL and the word lines WL. Each memory cell MC includes an MR element 10 and a selection transistor 31. One end of the MR element 10 is connected to the bit line BL. The other end of the MR element 10 is connected to the drain terminal of the selection transistor 31. The gate terminal of the selection transistor 31 is connected to the word line WL. The source terminal of the selection transistor 31 is connected to the bit line / BL.

  A row decoder 32 is connected to the word line WL. A write circuit 34 and a read circuit 35 are connected to the bit line pair BL, / BL. A column decoder 33 is connected to the write circuit 34 and the read circuit 35. Each memory cell MC is selected by the row decoder 32 and the column decoder 33.

  Data is written to the memory cell MC as follows. First, in order to select a memory cell MC for data writing, the word line WL connected to the memory cell MC is activated. As a result, the selection transistor 31 is turned on.

  Here, the bidirectional write current Iw is supplied to the MR element 10. Specifically, when the write current Iw is supplied to the MR element 10 from left to right, the write circuit 34 applies a positive potential to the bit line BL and applies a ground potential to the bit line / BL. When supplying the write current Iw to the MR element 10 from right to left, the write circuit 34 applies a positive potential to the bit line / BL and applies a ground potential to the bit line BL. In this way, data “0” or data “1” can be written in the memory cell MC.

  Next, data reading from the memory cell MC is performed as follows. First, the selection transistor 31 of the selected memory cell MC is turned on. The read circuit 35 supplies the MR element 10 with a read current Ir that flows from right to left, for example. The read circuit 35 detects the resistance value of the MR element 10 based on the read current Ir. In this way, data stored in the MR element 10 can be read.

  Next, the structure of the MRAM will be described. FIG. 16 is a cross-sectional view of the MRAM centered on the MR element 10. The MR element 10 is formed above an select transistor 31 formed on a semiconductor substrate made of silicon or the like via an interlayer insulating layer.

  An MR element 10 is provided on the extraction electrode (lower electrode) 37. The extraction electrode 37 is electrically connected to the drain region of the selection transistor 31 via the via plug 36. A hard mask 38 is provided on the MR element 10. The hard mask 38 functions as an upper electrode. A bit line BL is provided on the hard mask 38.

  As the via plug 36, the extraction electrode 37, the hard mask 38, and the bit line BL, W, Al, Cu, AlCu, or the like is used. In the case of a metal wiring layer or via plug using Cu, a Cu damascene or Cu dual damascene process is used.

  FIG. 17 is a cross-sectional view of another configuration example of the MRAM centered on the MR element 10. The MR element 10 is provided directly on the via plug 36. That is, in the MRAM in FIG. 17, the extraction electrode 37 is omitted compared to the MRAM in FIG. A hard mask 38 is provided on the MR element 10. A bit line BL is provided on the hard mask 38.

  The MR element 10 may be electrically connected to the via plug 36 by the lead electrode 37 as shown in FIG. 16, or may be formed directly on the via plug 36 as shown in FIG. When the configuration of FIG. 17 is used, it is preferable that the MR element size is smaller than the via size.

Assuming that the minimum processing dimension determined by lithography, etching technology, etc. is F (Minimum Feature Size), the minimum cell size is 8F ² when the layout of FIG. 16 is used. On the other hand, when the layout of FIG. 17 is used, the minimum cell size can be reduced to 4F ² .

  In the MRAM configured as described above, the writing speed when data is written to the MR element 10 can be improved. Specifically, spin injection writing can be performed with a current having a pulse width of several nanoseconds to several microseconds as a writing speed.

  It is desirable that the read current Ir supplied to the MR element 10 at the time of reading has a smaller pulse width than the write current Iw supplied to the MR element 10 at the time of writing. This is because the absolute value of the write current value increases as the pulse width of the write current Iw is shorter. Thereby, erroneous writing with the read current Ir can be reduced.

  The present invention is not limited to the above-described embodiment, and can be embodied by modifying the constituent elements without departing from the scope of the invention. In addition, various inventions can be configured by appropriately combining a plurality of constituent elements disclosed in the embodiments. For example, some constituent elements may be deleted from all the constituent elements disclosed in the embodiments, or constituent elements of different embodiments may be appropriately combined.

1 is a cross-sectional view showing a configuration of an MR element 10 according to a first embodiment of the present invention. FIG. 3 is a diagram for explaining an operation in which magnetization of the free layer 13 shown in FIG. 1 is reversed from an antiparallel magnetization state to a parallel magnetization state. FIG. 3 is a diagram for explaining an operation in which the free layer 13 shown in FIG. 1 undergoes magnetization reversal from a parallel magnetization state to an antiparallel magnetization state. Sectional drawing which shows the other structure of the assist layer. FIG. 5 is a diagram for explaining an operation in which the free layer 13 shown in FIG. 4 undergoes magnetization reversal from an antiparallel magnetization state to a parallel magnetization state. FIG. 5 is a diagram for explaining an operation in which the free layer 13 shown in FIG. 4 undergoes magnetization reversal from a parallel magnetization state to an antiparallel magnetization state. FIG. 6 is a cross-sectional view showing another configuration example of the MR element 10. FIG. 3 is a cross-sectional view showing a configuration of an MR element 10 including an interface free layer, an interface pinned layer, and an interface assist layer. FIG. 3 is a cross-sectional view showing a configuration of the MR element 10 when an interface free layer and an interface pinned layer are provided and the assist layer 15 is a superparamagnetic material. FIG. 3 is a cross-sectional view showing a configuration of the MR element 10 including an antiferromagnetic layer. FIG. 3 is a cross-sectional view showing the configuration of the MR element 10 when the pinned layer 11 has a SAF structure. FIG. 3 is a cross-sectional view of an MR element 10 formed by processing an MTJ film. FIG. 3 is a cross-sectional view showing a configuration of the MR element 10 having a tapered cross-sectional shape. FIG. 3 is a cross-sectional view showing a configuration of the MR element 10 having a step shape in cross section. The circuit diagram which shows the structure of MRAM which concerns on the 2nd Embodiment of this invention. FIG. 3 is a cross-sectional view of the MRAM mainly showing the MR element 10. FIG. 6 is a cross-sectional view of another configuration example of the MRAM mainly showing the MR element 10.

Explanation of symbols

  MC ... memory cell, BL ... bit line, WL ... word line, 10 ... MR element, 11 ... magnetization fixed layer, 11A, 11C ... magnetic layer, 11B ... nonmagnetic layer, 12 ... first intermediate layer, 13 ... magnetization Free layer, 14 ... second intermediate layer, 15 ... magnetization reversal assist layer, 16 ... underlayer, 17 ... cap layer, 21 ... interface pinned layer, 22,23 ... interface free layer, 24 ... interface assist layer, 25 ... Antiferromagnetic layer, 30 ... memory cell array, 31 ... select transistor, 32 ... row decoder, 33 ... column decoder, 34 ... write circuit, 35 ... read circuit, 36 ... via plug, 37 ... lead electrode, 38 ... hard mask.

Claims (13)

A fixed layer with a fixed magnetization direction;
A free layer whose magnetization direction can be changed by energization in a direction perpendicular to the film surface;
An assist layer that is capable of changing the direction of magnetization by energization in a direction perpendicular to the film surface, and that assists in changing the direction of magnetization of the free layer;
A first intermediate layer provided between the fixed layer and the free layer and made of a nonmagnetic material;
A second intermediate layer provided between the free layer and the assist layer and made of a nonmagnetic material;
Comprising
2. The magnetoresistive element according to claim 1, wherein an energy barrier for reversing the magnetization of the assist layer is smaller than an energy barrier for reversing the magnetization of the free layer.   The magnetoresistive element according to claim 1, wherein the assist layer is made of a superparamagnetic material.   The magnetoresistive element according to claim 2, wherein the free layer and the fixed layer have magnetic anisotropy in a direction perpendicular to a film surface. The assist layer is made of a ferromagnetic material or a ferrimagnetic material,
The magnetoresistive element according to claim 1, wherein the magnetic anisotropy of the assist layer is smaller than the magnetic anisotropy of the free layer. The assist layer is made of a ferromagnetic material or a ferrimagnetic material,
The magnetoresistive element according to claim 1, wherein the product of the saturation magnetization and the film thickness of the assist layer is smaller than the product of the saturation magnetization and the film thickness of the free layer.   The magnetoresistive element according to claim 4, wherein the fixed layer, the free layer, and the assist layer have magnetic anisotropy in a direction perpendicular to a film surface. The fixed layer, the free layer, and the assist layer have magnetic anisotropy in a direction perpendicular to the film surface,
The magnetoresistive element according to claim 1, wherein the saturation magnetization of the assist layer is larger than the saturation magnetization of the free layer. The fixed layer, the free layer, and the assist layer have in-plane magnetic anisotropy,
The magnetoresistive element according to claim 1, wherein a saturation magnetization of the assist layer is smaller than a saturation magnetization of the free layer.   The magnetoresistive element according to claim 1, wherein the first intermediate layer is made of an insulator or a semiconductor.   The magnetoresistive element according to claim 1, wherein the second intermediate layer is made of a conductor.   11. A memory cell comprising: the magnetoresistive element according to claim 1; and first and second electrodes provided so as to sandwich the magnetoresistive element and energizing the magnetoresistive element. A magnetic memory comprising:   12. The magnetic memory according to claim 11, further comprising a write circuit that is electrically connected to the first and second electrodes and that supplies current to the magnetoresistive element bidirectionally.   The magnetic memory according to claim 12, wherein the memory cell includes a selection transistor electrically connected between the second electrode and the write circuit.

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