JP4970113B2 - Magnetoresistive element and magnetic memory - Google Patents

Description

  The present invention relates to a magnetoresistive element and a magnetic memory, and more particularly to a magnetoresistive element and a magnetic memory capable of recording information by supplying a current in both directions.

  Magnetic random access memory (MRAM) using a ferromagnetic material is expected as a non-volatile memory having non-volatility, high-speed operation, large capacity, and low power consumption. The MRAM includes an MTJ (Magnetic Tunnel Junction) element that uses a tunneling magnetoresistive (TMR) effect as a storage element, and stores information according to the magnetization state of the MTJ element.

  In a conventional MRAM in which writing is performed with a magnetic field due to a wiring current, the current value that can be passed through the wiring is reduced with miniaturization, so that it is difficult to supply a sufficient current magnetic field to the MTJ element. Furthermore, the magnitude of the current magnetic field required for recording information on the MTJ element increases with miniaturization. For this reason, it is considered that the conventional MRAM has a theoretical limit in the 126 Mbits to 256 Mbits generation.

Therefore, an MRAM using a writing method using spin angular momentum transfer (SMT) has been proposed (for example, Patent Document 1). Magnetization reversal by spin angular momentum transfer (hereinafter referred to as spin injection) has the advantage that high-efficiency writing is possible because the current density required for magnetization reversal hardly increases even if the element is miniaturized. Have.
US Pat. No. 6,256,223

  The present invention provides a magnetoresistive element and a magnetic memory that can further reduce the current density required for magnetization reversal by applying spin torque to a free layer with high efficiency.

A magnetoresistive element according to one aspect of the present invention has a first free layer having a magnetization perpendicular to the film surface and the direction of magnetization changed, and a magnetization perpendicular to the film surface, the magnetization direction being fixed. A spin diffusion length that is provided between the first free layer and the first free layer, the magnetization direction of which is independent of the magnetization direction of the first free layer, A second free layer having the following thickness; a first nonmagnetic layer provided between the first free layer and the second free layer; the second free layer; And a second nonmagnetic layer provided between the layers. Then, the product Ku1 × V1 of the magnetic anisotropy constant Ku1 of the first free layer and the activation volume V1, and the product Ku2 of the magnetic anisotropy constant Ku2 of the second free layer and the activation volume V2. The relationship with × V2 satisfies Ku1 × V1> Ku2 × V2, and the first and second nonmagnetic layers are each made of oxide, nitride, fluoride, selenide, or compound semiconductor . A first write operation in which electrons flow from the first free layer to the fixed layer to change the magnetization of the fixed layer and the first free layer from a parallel state to an antiparallel state is performed by the second free layer. Including a step in which the magnetization of the layer is parallel to the magnetization of the fixed layer, and the magnetization of the first free layer is antiparallel to the magnetization of the second free layer. The second write operation in which electrons flow from the fixed layer to the first free layer to change the magnetization of the fixed layer and the first free layer from an antiparallel state to a parallel state is performed by the second free operation. Including a step in which the magnetization of the layer is parallel to the magnetization of the pinned layer and the magnetization of the first free layer is parallel to the magnetization of the second free layer.

The magnetic memory according to an aspect of the present invention, a memory comprising said magnetoresistive element, wherein provided so as to sandwich the magnetoresistive element, and the first and second electrodes performing the energization to said magnetoresistive element A cell is provided.

  According to the present invention, it is possible to provide a magnetoresistive element and a magnetic memory that can further reduce the current density required for magnetization reversal by applying spin torque to the free layer with high efficiency.

  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)
In order to realize a 1 Gbit class MRAM using spin injection magnetization reversal, it is necessary to reverse the magnetization with a current value of several tens of μA or less. For example, assuming that the magnetoresistive element is a rectangle having a width of 45 nm and an aspect ratio of 2, the area is about 4,000 nm ² . If the current value that can be passed through the transistor is about 100 μA, the maximum value of the current density that can be passed through the magnetoresistive element is estimated to be about 2.5 × 10 ^{6 A} / cm ² .

On the other hand, when the magnetization reversal current density Jc of CoFeB / MgO / CoFeB, which has recently obtained a high MR (Magnetoresistance) ratio, is calculated based on the spin torque model proposed by Slancewski in 1996, Jc is 1 × 10 ⁷ A / Cm ² is expected. Here, an attenuation constant of 0.01, a saturation magnetization of 1300 emu / cc, a spin polarizability of 0.5, a magnetic anisotropic magnetic field of 30 Oe, and a thermal fluctuation resistance (KuV / KbT) = 100 were used. Ku is the magnetic anisotropy constant, V is the activation volume, T is the temperature, and KbT is the thermal vibration energy. In order to put magnetization reversal using spin torque into practical use as an MRAM, the current density Jc needs to be halved or less.

  According to the spin torque model, the current density Jc depends on the saturation magnetization, activation volume, anisotropic magnetic field, spin polarizability, and damping constant of the magnetic film cell. Reduction of the current density Jc by various control factors other than the spin polarizability leads to an increase in inversion time and a decrease in resistance to thermal fluctuations. Therefore, there is a limit to the reduction of the current density Jc by these factors. Regarding the spin polarizability P, it has been proposed that P = 1 by using a half metal in the magnetic layer, but there is no report that it has been realized at room temperature.

  Therefore, in a structure of a free layer, a tunnel barrier layer, and a fixed layer in which a general tunnel magnetoresistance effect can be obtained, a magnetic layer having a smaller magnetic anisotropy energy U than the free layer between the tunnel barrier layer and the free layer We have invented a structure that improves the spin transfer efficiency by using a structure that sandwiches.

  FIG. 1 is a cross-sectional view showing a configuration of an MTJ element (magnetoresistance element) 10 according to the first embodiment of the present invention. The arrows in the figure indicate the direction of magnetization. The MTJ element 10 includes a fixed layer (or pinned layer) 11, a tunnel barrier layer (nonmagnetic layer) 12, a second free layer 13, a tunnel barrier layer (nonmagnetic layer) 14, a first free layer ( Alternatively, it is also referred to as a free layer or a recording layer. In this laminated structure, the lamination order may be reversed.

  In the present embodiment, an MTJ element 10 having a single pinned layer structure (a structure in which one fixed layer is arranged with respect to a free layer) will be described as an example. However, the present invention is not limited to this, and can be applied to a magnetoresistive element having a dual pinned layer structure (a structure in which two fixed layers are arranged on both sides of the free layer).

  The fixed layer 11 has a fixed magnetization (or spin) direction. The first free layer 15 and the second free layer 13 change (reverse) the magnetization direction. The easy magnetization directions of the fixed layer 11, the second free layer 13, and the first free layer 15 are each perpendicular to the film surface (hereinafter referred to as perpendicular magnetization). That is, the MTJ element 10 is a so-called perpendicular magnetization type MTJ element in which the magnetization directions of the fixed layer 11, the second free layer 13, and the first free layer 15 are perpendicular to the film surface. The present embodiment is not limited to perpendicular magnetization, and the easy magnetization directions of the fixed layer 11, the second free layer 13, and the first free layer 15 are in-plane with respect to the film surface. (Hereinafter referred to as in-plane magnetization).

  Note that the easy magnetization direction is a direction in which the internal energy is lowest when the spontaneous magnetization is directed in a state without an external magnetic field, assuming a certain size of ferromagnetic material. The difficult magnetization direction is a direction in which the internal energy is maximized when the spontaneous magnetization is directed in a state without an external magnetic field, assuming a ferromagnetic material of a certain size.

  Incidentally, the product (magnetic anisotropy energy U1) of the magnetic anisotropy constant Ku1 of the magnetic material of the first free layer 15 and the activation volume V1 and the magnetic anisotropy of the magnetic material of the second free layer 13 are shown. The product of the constant Ku2 and the activation volume V2 (magnetic anisotropy energy U2) has a relationship of U1> U2. By satisfying such a relationship, when a write current is supplied to the MTJ element 10, the second free layer 13 undergoes magnetization reversal earlier than the first free layer 15. In the present embodiment, when data is written in the MTJ element 10, the magnetization state corresponding to the data is recorded in the first free layer 15. The second free layer 13 functions as a layer for filtering electron spin at the time of data writing and reading.

  For example, the same material is used for the first free layer 15 and the second free layer 13, and the film thickness t1 of the first free layer 15 and the film thickness t2 of the second free layer 13 are such that t1> t2. The first free layer 15 and the second free layer 13 satisfying U1> U2 can be manufactured. Further, the first free layer 15 and the second free layer 13 may not be the same material. In that case, the saturation magnetization or the magnetocrystalline anisotropy of the first free layer 15 and the second free layer 13 may be adjusted so as to satisfy the relationship U1> U2.

  As the fixed layer 11 and the first free layer 15, a ferromagnetic material is used. For example, iron (Fe), cobalt (Co), nickel (Ni), platinum (Pt), manganese (Mn), palladium (Pd ) And a ferromagnetic material containing one or more elements of chromium (Cr).

In the case of a perpendicular magnetization type MTJ element, examples of the ferromagnetic material include Fe ₅₀ Pt ₅₀ , Fe ₅₀ Pd ₅₀ , Co ₅₀ Pt ₅₀ , Co ₅₀ Pd ₅₀ , Fe ₃₀ Ni ₂₀ Pt ₅₀ , Co ₃₀ Fe ₂₀ Pt ₅₀ , Co Examples thereof include ₃₀ Ni ₂₀ Pt ₅₀ , Mn ₅₀ Al ₅₀ , and Fe ₅₀ Ni ₅₀ ordered alloys. The composition ratio of these ordered alloys is an example, and is not limited to the above composition ratio. These ordered alloys include Cu (copper), Zn (zinc), Ag (silver), Ni (nickel), Co (cobalt), Fe (iron), Mn (manganese), Cr, (chromium), V By adding an impurity element such as (vanadium), Ti (titanium), Os (osmium) or an alloy thereof, or an insulator, magnetic anisotropy energy and saturation magnetization can be adjusted to be low. It is also possible to use a L1 ₂ type ordered alloy by adjusting the composition ratio.

Using _Fe 50 _{Pt 50} about as a free layer 15, for example a thickness of 3 nm, when using a _Fe 50 _{Pt 50} as well fixed layer 11, the fixed layer 11 must be greater magnetization reversal current as compared to the free layer 15 Therefore, it is necessary to make it thicker than the thickness of the free layer 15. For example, if a film thickness of 10 to 20 nm is used, it can be used as the fixed layer 11 in which the magnetization direction is not reversed when information is written.

The conditions required for the fixed layer 11 are that the anisotropic magnetic field is larger than that of the free layer 15, the saturation magnetization is large, the film thickness is large, and the damping constant is large. If one or more of these conditions are satisfied, so _good, in addition to the Fe 50 _{Pt 50,} the free layer having a thickness of thicker L1 ₀ type than 15 crystalline material may be used an L1 ₂ type crystal material. Further, as the magnetic material of the free layer 15 and the fixed layer 11, the following materials (1) to (3) may be used.

(1) Irregular alloy Co (cobalt) as the main component, Cr (chromium), Ta (tantalum), Nb (niobium), V (vanadium), W (tungsten), Hf (hafnium), Ti (titanium), An alloy containing one or more elements of Zr (zirconium), Pt (platinum), Pd (palladium), Fe (iron), and Ni (nickel). Examples thereof include a CoCr alloy, a CoPt alloy, a CoCrTa alloy, a CoCrPt alloy, a CoCrPtTa alloy, and a CoCrNb alloy. These alloys can adjust the magnetic anisotropy energy and saturation magnetization by increasing the proportion of nonmagnetic elements.

(2) Artificial lattice Fe (iron), Co (cobalt), Ni (nickel) any one element or an alloy containing one or more elements, Cr (chromium), Pt (platinum), Pd (palladium) ), Ir (iridium), Rh (rhodium), Ru (ruthenium), Os (osmium), Re (rhenium), Au (gold), Cu (copper), or one or more elements. A laminated film in which alloys containing it are alternately laminated. Examples thereof include a Co / Pt artificial lattice, a Co / Pd artificial lattice, a CoCr / Pt artificial lattice, a Co / Ru artificial lattice, a Co / Os artificial lattice, a Co / Au artificial lattice, and a Ni / Cu artificial lattice. These artificial lattices can adjust magnetic anisotropy energy and saturation magnetization by adjusting the addition of elements to the magnetic layer and adjusting the film thickness ratio of the magnetic layer and the nonmagnetic layer.

(3) Ferrimagnetic material A ferrimagnetic material made of an alloy of a rare earth metal and a transition metal. For example, an amorphous alloy composed of one or more elements of Tb (terbium), Dy (dysprosium), and Gd (gadolinium) and one or more elements of a transition metal. Specific examples include TbFe alloys, TbCo alloys, TbFeCo alloys, DyTbFeCo alloys, GdTbCo alloys, and the like. These alloys can adjust magnetic anisotropy energy and saturation magnetization by adjusting the composition ratio.

On the other hand, in the case of an in-plane magnetization type MTJ element, the free layer 15 may be made of a magnetic metal made of one or more elements of about 2 nm of Fe, Co, and Ni or an alloy containing this one element. . In addition to oxides such as magnetite, CrO ₂ , RXMnO _{3 -y} (R: rare earth element, X: Ca (calcium), Ba (barium), Sr (strontium)) having high spin polarizability, NiMnSb, PtMnSb, etc. A Heusler alloy or the like may be used. These metals include Ag (silver), Cu (copper), Au (gold), and Al (within a range not losing ferromagnetism in order to adjust saturation magnetization, magnetocrystalline anisotropy, attenuation constant, and the like. Aluminum), Mg (magnesium), Si (silicon), Bi (bismuth), Ta (tantalum), B (boron), C (carbon), O (oxygen), N (nitrogen), Pd (palladium), Pt ( It may contain a nonmagnetic element such as platinum, Zr (zirconium), Ir (iridium), W (tungsten), Mo (molybdenum), Nb (niobium) or the like.

The fixed layer 11 may be made of one or more elements of Fe, Co, and Ni or a magnetic metal made of an alloy containing the one element. Alternatively, a magnetite having a high spin polarizability, an oxide such as CrO ₂ , RXMnO _{3 -y} (R: rare earth element, X: Ca, Ba, Sr), or a Heusler alloy such as NiMnSb or PtMnSb may be used. . In addition, these metals include Ag, Cu, Au, Al, Mg, Si, Bi, Ta, B, and the like within the range in which ferromagnetism is not lost in order to adjust saturation magnetization, magnetocrystalline anisotropy, attenuation constant, and the like. , C, O, N, Pd, Pt, Zr, Ir, W, Mo, Nb, and other nonmagnetic elements may be included. However, as the fixed layer 11, a magnetic layer having a larger reversal current than that of the first free layer 15 is used.

  When the magnetization reversal is caused by spin-polarized electrons, the reversal current is proportional to the saturation magnetization, the anisotropic magnetic field, and the volume. Therefore, these are appropriately adjusted so that the first free layer 15 and the fixed layer 11 Thus, the magnetization direction of the fixed layer 11 can be fixed. Further, an antiferromagnetic layer (PtMn, IrMn, FeMn, or the like) may be added to the fixed layer 15 to fix the magnetization direction of the fixed layer 11.

  As the second free layer 13, it is possible to freely select the ferromagnetic material mentioned in this embodiment as the material of the first free layer 15. However, it is necessary to select materials for the first free layer 15 and the second free layer 13 that satisfy the relationship of U1> U2. For example, when the same material as the first free layer 15 is used for the second free layer 13, the thickness of the second free layer 13 may be designed to be smaller than the thickness of the first free layer 15. Further, the second free layer 13 may be a so-called paramagnetic material in which magnetization becomes zero at a room temperature and an external magnetic field is zero.

  Further, the second free layer 13 may have a granular structure. FIG. 2 is a cross-sectional view showing an example of the second free layer 13. The second free layer 13 includes an insulating phase 13-1 and a plurality of granular magnetic phases 13-2 provided in the insulating phase 13-1. In the second free layer 13 configured as described above, the current flowing through the second free layer 13 flows through a path having the smallest tunnel barrier. Due to the current confined path (CCP) effect, the current density flowing through the second free layer 13 can be improved. As a result, the MTJ element 10 can be designed so as to obtain a current density sufficient to reverse the magnetization of the portion having high magnetic anisotropy energy. For the magnetic layer 13-2, the materials listed in the first free layer 15 can be freely selected. For example, MgO may be used for the insulating layer 13-1, and CoFe or CoFeB may be used for the magnetic layer 13-2 to form a granular structure.

  Moreover, the activation volume of the 2nd free layer 13 can be reduced by using the 2nd free layer 13 of a granular structure. In this case, even when the film thicknesses of the second free layer 13 and the first free layer 15 are the same, the magnetic anisotropy energy U2 of the second free layer 13 is changed to the magnetic force of the first free layer 15. It can be made smaller than the anisotropic energy U1. Thereby, the design of the magnetic anisotropy energy U2 of the second free layer 13 is facilitated.

  Note that as the film thickness of the second free layer 13 increases, the current value required to reverse the magnetization of the second free layer 13 increases. Further, when the film thickness of the second free layer 13 becomes longer than the spin diffusion length, the spin torque is relaxed in the second free layer 13 and is necessary for reversing the magnetization of the second free layer 13. The current value will increase. For this reason, it is preferable that the film thickness of the second free layer 13 is equal to or less than the spin diffusion length of the second free layer 13. For the same reason, the film thickness of the first free layer 15 is preferably equal to or less than the spin diffusion length of the first free layer 15.

The tunnel barrier layer 12 includes an oxide such as MgO, AlO _x , SiO _x , BiO _x , SrTiO _x , and AlLaO _x , a nitride such as TiN and VN, a fluoride such as MgF _x and CaF _x , or ZrSe. And selenides such as ZnSe and compound semiconductors such as AlAs. Therefore, the MTJ element 10 of this embodiment has a TMR (Tunneling Magnetoresistive) effect. Among these, MgO which is an oxide insulator is a preferable material for the tunnel barrier layer 12. This is because the MR ratio becomes larger when MgO is used.

  As the tunnel barrier layer 12, Al (aluminum), Au (gold), As (arsenic), Ag (silver), Be (beryllium), Ga (calcium), P (phosphorus), Pt (platinum), Pd ( Palladium), Ir (iridium), Ru (ruthenium), Rh (rhodium), Cu (copper), V (vanadium), Cr (chromium), Nb (niobium), Mo (molybdenum), Ta (tantalum), W ( A non-magnetic metal containing one or more elements of tungsten) may be used. When this nonmagnetic metal is used, a GMR (Giant Magnetoresistive) effect can be used.

As the tunnel barrier layer 14, an oxide such as MgO, AlO _x , SiO _x , BiO _x , SrTiO _x , and AlLaO _x , a nitride such as TiN and VN, a fluoride such as MgFx and CaFx, or ZrSe and ZnSe And selenides such as AlAs and compound semiconductors such as AlAs. Alternatively, one or more of Al, Au, As, Ag, Be, Ga, P, Pt, Pd, Ir, Ru, Rh, Cu, V, Cr, Nb, Mo, Ta, and W are used as the tunnel barrier layer 14. A nonmagnetic metal containing any of these elements may be used. Among these, oxide insulators, nitride insulators, or compound semiconductors are more preferable materials. By using such a material, the spin polarizability due to the TMR effect can be improved. Further, MgO that is an oxide insulator is a preferable material for the tunnel barrier layer 14. This is because when MgO is used, the spin polarizability becomes larger.

  Note that when the same material is used for the tunnel barrier layer 12 and the tunnel barrier layer 14, the tunnel barrier layer 14 is set to be thinner than the tunnel barrier layer 12. This is to provide a difference in MR ratio when reading data.

  Next, an information writing operation of the MTJ element 10 configured as described above will be described. The MTJ element 10 is energized in both directions in a direction perpendicular to the film surface. The current here refers to the flow of electrons (e).

  First, the case where the magnetization directions of the fixed layer 11 and the first free layer 15 are changed from the parallel state to the antiparallel state will be described. FIG. 3 is a diagram for explaining the magnetization state of the MTJ element 10 when the magnetization directions of the fixed layer 11 and the first free layer 15 are changed from the parallel state to the antiparallel state. In this case, electrons traveling from the first free layer 15 to the fixed layer 11 are supplied to the MTJ element 10.

  When electrons are supplied from the first free layer 15 side, the spin-polarized electrons by the first free layer 15 are injected into the second free layer 13 (see FIG. 3A). Thereby, the magnetization of the second free layer 13 is aligned in the same direction as the magnetization of the first free layer 15 (see FIG. 3B).

  Furthermore, electrons that are spin-polarized in the direction opposite to the magnetization of the second free layer 13 due to the reflection of electrons by the second free layer 13 are injected into the first free layer 15. Thereby, the magnetization of the first free layer 15 is aligned in the direction opposite to the magnetization direction of the fixed layer 11 (see FIG. 3C). As a result, the magnetization directions of the fixed layer 11 and the first free layer 15 are antiparallel.

  Next, a case where the magnetization directions of the fixed layer 11 and the first free layer 15 are changed from the antiparallel state to the parallel state will be described. FIG. 4 is a diagram illustrating the magnetization state of the MTJ element 10 when the magnetization direction of the fixed layer 11 and the first free layer 15 is changed from the antiparallel state to the parallel state. In this case, electrons traveling from the fixed layer 11 to the first free layer 15 are supplied to the MTJ element 10.

  When electrons are supplied from the fixed layer 11 side, electrons that are spin-polarized by the fixed layer 11 are injected into the second free layer 13 (see FIG. 4A). In this case, the magnetization direction of the second free layer 13 is aligned with the same direction as the magnetization direction of the fixed layer 11 (see FIG. 4B).

  Further, electrons spin-polarized by the second free layer 13 are injected into the first free layer 15, and the magnetization of the first free layer 15 is aligned in the same direction as the magnetization direction of the fixed layer 11 ( (Refer FIG.4 (c)). Thereby, the magnetization directions of the fixed layer 11 and the first free layer 15 are arranged in parallel.

  Next, the data read operation of the MTJ element 10 will be described. Data is read by supplying a read current to the MTJ element 10. This read current is set to a value smaller than the write current, and is set to a current that does not reverse the magnetization of the first free layer 15.

  First, a data read operation in the case where the magnetization directions of the fixed layer 11 and the first free layer 15 are antiparallel will be described. FIG. 5 is a diagram for explaining the magnetization state of the MTJ element 10 at the time of data reading when the magnetization directions of the fixed layer 11 and the first free layer 15 are antiparallel.

  For example, when a read current from the first free layer 15 toward the fixed layer 11 is supplied to the MTJ element 10, electrons spin-polarized by the first free layer 15 are injected into the second free layer 13 (FIG. 5). (See (a)). Thereby, the magnetization of the second free layer 13 is aligned in the same direction as the magnetization direction of the first free layer 15 (see FIG. 5B).

  As a result, the magnetization directions of the fixed layer 11 and the second free layer 13 are antiparallel. In this antiparallel arrangement, the MTJ element 10 has the largest resistance value, and this case is defined as data “1”.

  Next, a data read operation when the magnetization directions of the fixed layer 11 and the first free layer 15 are in a parallel state will be described. FIG. 5 is a diagram for explaining the magnetization state of the MTJ element 10 at the time of data reading when the magnetization directions of the fixed layer 11 and the first free layer 15 are parallel.

  For example, when a read current from the first free layer 15 toward the fixed layer 11 is supplied to the MTJ element 10, electrons that are spin-polarized by the first free layer 15 are injected into the second free layer 13 (FIG. 6). (See (a)). Thereby, the magnetization of the second free layer 13 is aligned in the same direction as the magnetization direction of the first free layer 15 (see FIG. 6B).

  As a result, the magnetization directions of the fixed layer 11 and the second free layer 13 are in a parallel arrangement. In this parallel arrangement, the resistance value of the MTJ element 10 is the smallest, and this case is defined as data “0”. By detecting a change in the resistance value of the MTJ element 10, data recorded in the MTJ element 10 is read.

  Hereinafter, another configuration example of the MTJ element 10 will be described. First, another configuration of the second free layer 13 will be described as a first example. The second free layer 13 has a laminated structure in which a plurality of magnetic layers and tunnel barrier layers (nonmagnetic layers) are laminated. FIG. 7 is a cross-sectional view showing another configuration example of the MTJ element 10.

  As shown in FIG. 7, the second free layer 13 is formed by laminating a magnetic layer 13A, a tunnel barrier layer 13B, and a magnetic layer 13C. As the material of the tunnel barrier layer 13B, the same material as that of the tunnel barrier layer 14 is used. Further, as described above, a ferromagnetic material or a paramagnetic material is used as the magnetic layers 13A and 13C constituting the second free layer 13. Further, the magnetic anisotropy energy of the magnetic layers 13A and 13C is set smaller than the magnetic anisotropy energy U1 of the first free layer 15, respectively.

  As a specific example of the layer constituting the MTJ element 10, for example, FePt (2 nm) / MgO (0.7 nm) / CoFeB (1 nm) / MgO (0.7 nm) / CoFeB (1 nm) / MgO (1 nm) / CoFeB ( 2 nm) / FePt (10 nm). CoFeB (2 nm) is not shown, but corresponds to the interface layer, and is provided between the fixed layer 11 and the tunnel barrier layer 12. In the description of the laminated film, the left side of “/” represents the upper layer and the right side represents the lower layer.

  By forming the second free layer 13 by stacking a plurality of magnetic layers and tunnel barrier layers, it is possible to further improve the spin polarizability at the interface between the first free layer 15 and the tunnel barrier layer 14. Become. Thereby, since the spin transfer torque can be efficiently applied to the first free layer 15, the write current necessary for the magnetization reversal can be reduced. In addition, the magnetic layer which comprises the 2nd free layer 13 may be three or more layers.

  A nonmagnetic metal layer may be used as the material of the tunnel barrier layer 13B. In this case, the GMR effect is used. It is possible to further improve the spin polarizability at the interface between the first free layer 15 and the tunnel barrier layer 14 by forming the second free layer 13 by laminating a plurality of magnetic layers and nonmagnetic metal layers. It becomes. Thereby, since the spin transfer torque can be efficiently applied to the first free layer 15, the write current necessary for the magnetization reversal can be reduced.

  Next, another configuration of the first free layer 15 will be described as a second example. FIG. 8 is a cross-sectional view showing another configuration example of the MTJ element 10. The first free layer 15 has a laminated structure of magnetic layer 15C / nonmagnetic layer 15B / magnetic layer 15A. The magnetization directions of the magnetic layer 15A and the magnetic layer 15C are set antiparallel to each other. The magnetic layer 15A and the magnetic layer 15C are antiferromagnetically coupled with the nonmagnetic layer 15B interposed therebetween. As the nonmagnetic layer 15B, for example, a nonmagnetic metal is used.

  The laminated structure of the first magnetic layer / nonmagnetic layer / second magnetic layer in which the directions of magnetization are antiparallel through the nonmagnetic layer is referred to as a synthetic anti-ferromagnet (SAF) structure. By using this SAF structure, the resistance to the external magnetic field and the thermal stability of the first free layer 15 can be improved.

  As a specific example of the layer constituting the MTJ element 10, for example, FePt (2 nm) / Ru (0.8) / FePt (2 nm) / MgO (0.7 nm) / CoFeB (1 nm) / MgO (1 nm) / CoFeB ( 2 nm) / FePt (10 nm). CoFeB (2 nm) is not shown, but corresponds to the interface layer, and is provided between the fixed layer 11 and the tunnel barrier layer 12.

  Next, another configuration of the fixed layer 11 will be described as a third example. FIG. 9 is a cross-sectional view showing another configuration example of the MTJ element 10. The fixed layer 11 has a laminated structure of magnetic layer 11C / nonmagnetic layer 11B / magnetic layer 11A. The magnetization directions of the magnetic layer 11A and the magnetic layer 11C are set antiparallel to each other. The magnetic layer 11A and the magnetic layer 11C are antiferromagnetically coupled with the nonmagnetic layer 11B interposed therebetween. For example, a nonmagnetic metal is used as the nonmagnetic layer 11B.

  By using the SAF structure for the fixed layer 11, the magnetization fixing force of the fixed layer 11 can be enhanced, and the resistance to an external magnetic field and the thermal stability can be improved. In the SAF structure, when the saturation magnetization of the magnetic layer 11C is Ms1, the film thickness is t1, the saturation magnetization of the magnetic layer 11A is Ms2, and the film thickness is t2, Ms1 · t1≈Ms2 · t2 is set. The product Ms · t of the apparent saturation magnetization and the film thickness of the layer 11 can be made almost zero. Thereby, the fixed layer 11 becomes difficult to react to an external magnetic field, and can further improve external magnetic field resistance.

  Specific examples of the layers constituting the MTJ element 10 include, for example, FePt (2 nm) / MgO (0.7 nm) / CoFeB (1 nm) / MgO (1 nm) / CoFeB (2 nm) / FePt (8 nm) / Ru (0. 8) / FePt (10) can be used. CoFeB (2 nm) is not shown, but corresponds to the interface layer, and is provided between the fixed layer 11 and the tunnel barrier layer 12.

  As described in detail above, in the present embodiment, the second free layer 13 is provided between the fixed layer 11 and the first free layer 15, and the magnetic anisotropy energy U2 of the second free layer 13 is set to the first. The magnetic anisotropy energy U1 of the free layer 15 is set to be smaller.

  Therefore, according to the present embodiment, the spin polarizability at the interface between the first free layer 15 and the tunnel barrier layer 14 can be improved. Thereby, since the spin transfer torque can be efficiently applied to the first free layer 15, the write current necessary for the magnetization reversal can be reduced. Further, since the MTJ element 10 can be miniaturized, a large capacity (for example, 256 Mbits or more) MRAM can be realized using the MTJ element 10.

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

  FIG. 10 is a cross-sectional view showing the configuration of the MTJ element 10 according to the second embodiment of the present invention. The basic configuration in which the fixed layer 11, the tunnel barrier layer 12, the second free layer 13, the tunnel barrier layer 14, and the first free layer 15 are sequentially stacked is the same as that of the first embodiment. An upper electrode 17 and a lower electrode 16 are provided so as to be sandwiched between the upper and lower sides. As the upper electrode 17 and the lower electrode 16, for example, tantalum (Ta) or titanium nitride (TiN) is used.

  FIG. 11 is a circuit diagram showing a configuration of the MRAM. The MRAM includes a memory cell array 20 having a plurality of memory cells MC arranged in a matrix. In the memory cell array 20, a plurality of bit line pairs BL, / BL are arranged so as to extend in the column direction. In the memory cell array 20, 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 MTJ element 10 and a selection transistor 21. The upper electrode 17 of the MTJ element 10 is connected to the bit line BL. The lower electrode 16 of the MTJ element 10 is connected to the drain terminal of the selection transistor 21. The gate terminal of the selection transistor 21 is connected to the word line WL. The source terminal of the selection transistor 21 is connected to the bit line / BL.

  A row decoder 22 is connected to the word line WL. A write circuit 24 and a read circuit 25 are connected to the bit line pair BL, / BL. A column decoder 23 is connected to the write circuit 24 and the read circuit 25. Each memory cell MC is selected by the row decoder 22 and the column decoder 23.

  Data writing to the memory cell MC is performed 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 21 is turned on.

  Here, the bidirectional write current Iw is supplied to the MTJ element 10. Specifically, when supplying the write current Iw to the MTJ element 10 from left to right, the write circuit 24 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 from the right to the left to the MTJ element 10, the write circuit 24 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 21 of the selected memory cell MC is turned on. The read circuit 25 supplies the MTJ element 10 with a read current Ir that flows from right to left, for example. Then, the read circuit 25 detects the resistance value of the MTJ element 10 based on the read current Ir. In this way, data stored in the MTJ element 10 can be read.

  As described above in detail, according to the present embodiment, the MRAM can be configured using the MTJ element 10 shown in the first embodiment. Further, by using the MTJ element 10 shown in the first embodiment, it is possible to configure an MRAM that can be miniaturized and can reduce the inversion current density.

  The present invention is not limited to the above-described embodiment, and can be embodied by modifying the components 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 MTJ element 10 according to a first embodiment of the present invention. FIG. 4 is a cross-sectional view showing an example of a second free layer 13. The figure explaining the magnetization state of the MTJ element 10 in the case of changing the magnetization direction of the fixed layer 11 and the first free layer 15 from the parallel state to the antiparallel state. The figure explaining the magnetization state of the MTJ element 10 in the case of changing the magnetization direction of the fixed layer 11 and the first free layer 15 from the antiparallel state to the parallel state. The figure explaining the magnetization state of the MTJ element 10 at the time of data reading in the direction of magnetization of the fixed layer 11 and the first free layer 15 in an antiparallel state. The figure explaining the magnetization state of the MTJ element 10 at the time of data reading in the state where the magnetization directions of the fixed layer 11 and the first free layer 15 are parallel. FIG. 6 is a cross-sectional view showing another configuration example of the MTJ element 10. FIG. 6 is a cross-sectional view showing another configuration example of the MTJ element 10. FIG. 6 is a cross-sectional view showing another configuration example of the MTJ element 10. Sectional drawing which shows the structure of the MTJ element 10 which concerns on the 2nd Embodiment of this invention. A circuit diagram showing composition of MRAM concerning a 2nd embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... MTJ element, 11 ... Fixed layer, 11C, 11A ... Magnetic layer, 11B ... Nonmagnetic layer, 12 ... Tunnel barrier layer, 13 ... Second free layer, 13-1 ... Insulating phase, 13-2 ... Magnetic phase , 13A, 13C ... magnetic layer, 13B ... tunnel barrier layer, 14 ... tunnel barrier layer, 15 ... first free layer, 15C, 15A ... magnetic layer, 15B ... nonmagnetic layer, 16 ... lower electrode, 17 ... upper electrode 20 ... memory cell array, 21 ... select transistor, 22 ... row decoder, 23 ... column decoder, 24 ... write circuit, 25 ... read circuit, MC ... memory cell, WL ... word line, BL ... bit line.

Claims (8)

A first free layer having magnetization perpendicular to the film surface and changing the direction of magnetization;
A fixed layer having magnetization perpendicular to the film surface and having a fixed magnetization direction;
Provided between the first free layer and the fixed layer, the magnetization direction changes independently of the magnetization direction of the first free layer, and has a film thickness equal to or less than its own spin diffusion length A second free layer;
A first nonmagnetic layer provided between the first free layer and the second free layer;
A second nonmagnetic layer provided between the second free layer and the pinned layer;
Comprising
The product Ku1 × V1 of the magnetic anisotropy constant Ku1 of the first free layer and the activation volume V1, and the product Ku2 × V2 of the magnetic anisotropy constant Ku2 of the second free layer and the activation volume V2. Satisfies the relationship Ku1 × V1> Ku2 × V2,
Each of the first and second nonmagnetic layers is made of an oxide, nitride, fluoride, selenide, or compound semiconductor,
A first write operation in which electrons flow from the first free layer to the fixed layer to change the magnetization of the fixed layer and the first free layer from a parallel state to an antiparallel state,
The magnetization of the second free layer is parallel to the magnetization of the pinned layer;
The magnetization of the first free layer is anti-parallel to the magnetization of the second free layer;
Including
A second write operation in which electrons flow from the fixed layer to the first free layer to change the magnetization of the fixed layer and the first free layer from an antiparallel state to a parallel state,
The magnetization of the second free layer is parallel to the magnetization of the pinned layer;
The magnetization of the first free layer is parallel to the magnetization of the second free layer;
Magnetoresistive element characterized by comprising a.   The magnetoresistive element according to claim 1, wherein each of the first free layer and the second free layer is made of a ferromagnetic material.   The magnetoresistive element according to claim 1, wherein the second free layer has a stacked structure in which magnetic layers and nonmagnetic layers are alternately stacked.   The magnetoresistive element according to claim 1, wherein the second free layer has a granular structure in which a magnetic phase is dispersed in an insulating phase.   5. The magnetoresistive element according to claim 1, wherein a film thickness of the second free layer is smaller than a film thickness of the first free layer.   6. 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:   The magnetic memory according to claim 6, 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 7, wherein the memory cell includes a selection transistor electrically connected between the second electrode and the write circuit.

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