JP2010219177A - Magnetic tunnel junction device, and magnetic random access memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the MR ratio of an MTJ element. <P>SOLUTION: The MTJ element (1) includes: a first magnetic layer (10); a second magnetic layer (20); a tunnel barrier layer (30) positioned between the first magnetic layer (10) and the second magnetic layer (20); and a first half metal layer (40) formed by a half metal material. The first half metal layer (40) is interposed between the first magnetic layer (10) and the tunnel barrier layer (30), abuts on the tunnel barrier layer (30), and magnetically couples with the first magnetic layer (10). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a magnetic tunnel junction device and a magnetic random access memory.

  A magnetic random access memory (MRAM) is a nonvolatile memory that stores data in a nonvolatile manner based on the magnetization direction of a magnetic material. The MRAM has features such as high-speed writing and unlimited number of writing. Therefore, MRAM is expected as a next-generation non-volatile memory, and its development is energetically advanced.

One of the powerful MRAM writing methods proposed in recent years is spin injection (spin momentum).
transfer) method. In spin injection magnetization reversal, a spin-polarized current is injected as a write current into the magnetization recording layer, thereby reversing the magnetization direction of the magnetization recording layer. In magnetization reversal by applying a current-induced magnetic field, which is well known in the past, the memory cell size decreases and the current required for magnetization reversal increases. In contrast, in spin transfer magnetization reversal, the memory cell size is reduced and the current required for magnetization reversal is reduced. Therefore, the spin injection method is considered to be an effective method for realizing a large-capacity MRAM.

  Furthermore, a “domain wall motion method” is also known as a kind of spin injection method. According to the domain wall motion method, by passing a spin-polarized current in the in-plane direction of the magnetization recording layer, the domain wall in the magnetization recording layer can be moved, thereby reversing the magnetization direction in the magnetization recording layer. Such a technique is disclosed in Patent Document 1 and Patent Document 2, for example.

In general, as a memory cell of MRAM, a magnetic tunnel junction (MTJ: Magnetic Tunnel)
Junction) elements are used. FIG. 1 is a cross-sectional view showing a configuration of a typical MTJ element 100. The MTJ element 100 includes a first ferromagnetic layer 110, a second ferromagnetic layer 120, and a tunnel barrier layer 130 sandwiched between the first ferromagnetic layer 110 and the second ferromagnetic layer 120. One of the first ferromagnetic layer 110 and the second ferromagnetic layer 120 is a pinned layer whose magnetization direction is fixed, and the other is a magnetization recording layer (free layer) whose magnetization direction changes. In the case of the domain wall motion type MRAM, the magnetization recording layer is a domain wall motion layer in which the domain wall moves.

  The resistance value of the MTJ element varies according to the magnetization state of the magnetization recording layer. Data is recorded as the magnitude of the resistance value, and data reading is performed by detecting the magnitude of the resistance value. Therefore, in order to improve the data read speed, it is desirable to increase the magnetoresistance ratio (MR ratio) of the MTJ element. It is assumed that the spin polarizabilities of the first ferromagnetic layer 110 and the second ferromagnetic layer 120 are P1 and P2, respectively (0 ≦ P1, P2 ≦ 1). At this time, the theoretical MR ratio of the MTJ element is given by the following formula (1) (generally called “Jullier's formula”).

  Formula (1): MR ratio = 2 × P1 × P2 / (1−P1 × P2)

  From this equation (1), it can be seen that the MR ratio increases as P1 and P2 increase. In particular, when P1 = P2 = 1, the MR ratio (theoretical value) is infinite. Therefore, in terms of MR ratio, it is preferable to use a material having a spin polarizability as close to 1 as possible for the magnetic layer.

  In the ferromagnetic tunnel junction device disclosed in Patent Document 3, a “half-metal ferromagnetic oxide layer” having a Curie point at room temperature or higher is sandwiched between both sides of an insulator barrier layer. The half metal oxide is, for example, magnetite. This ferromagnetic tunnel junction device exhibits a large MR ratio at room temperature or higher. Moreover, since this ferromagnetic tunnel junction element is entirely composed of an oxide, it is chemically stable.

JP 2006-73930 A JP 2007-317895 A JP-A-11-97766

  In a typical MTJ element 100, a metal alloy made of a magnetic metal such as Fe, Co, or Ni is used as the material for the first ferromagnetic layer 110 and the second ferromagnetic layer 120. In such a metal alloy, the up-spin and down-spin state densities have an existence probability in the vicinity of the Fermi surface. Since spins in the respective directions exist at a constant ratio, the spin polarizability does not become 1, but is about 0.4 to 0.7. In that case, the upper limit of the actual MR ratio is about several tens of percent at most.

  On the other hand, for example, in order to realize a reading operation of 500 MHz or more required for the embedded memory, a reading speed on the order of nanoseconds is necessary. For this purpose, a high MR ratio exceeding 100% is required. However, it is difficult to achieve a high MR ratio exceeding 100% with the structure shown in FIG. In particular, in the MTJ element used in the domain wall motion type MRAM, the crystal structure and the crystal orientation plane of the domain wall motion layer are inconsistent with the tunnel magnetoresistance effect through the insulating layer. Therefore, it is difficult to realize a high MR ratio exceeding 100%.

  One object of the present invention is to increase the MR ratio of the MTJ element.

  The MTJ element according to the present invention includes a first magnetic layer, a second magnetic layer, a tunnel barrier layer positioned between the first magnetic layer and the second magnetic layer, and a first half formed of a half metal material. A metal layer. The first half metal layer is interposed between the first magnetic layer and the tunnel barrier layer, contacts the tunnel barrier layer, and is magnetically coupled to the first magnetic layer.

  According to the present invention, the MR ratio of the MTJ element can be increased.

FIG. 1 is a cross-sectional view showing the structure of a typical MTJ element. FIG. 2 is a cross-sectional view showing the structure of the MTJ element according to the first embodiment of the present invention. FIG. 3 is a cross-sectional view showing the structure of the MTJ element according to the second embodiment of the present invention. FIG. 4 is a cross-sectional view showing the structure of an MTJ element according to the third embodiment of the present invention. FIG. 5 is a cross-sectional view showing the structure of an MTJ element according to the fourth embodiment of the present invention. FIG. 6 is a cross-sectional view showing the structure of an MTJ element according to the fifth embodiment of the present invention. FIG. 7 is a schematic diagram showing the configuration of the MRAM according to the embodiment of the present invention.

  In order to improve the MR ratio of the MTJ element, it is preferable to use a material having a spin polarizability as close to 1 as possible for the magnetic layer. In the present embodiment, “half-metal material” is used as such a material having a high spin polarizability. The half-metal is a magnetic material in which one electron spin has a metallic band structure and the other electron spin has an insulating band structure. In half metal, one of upspin and downspin occupies a lower energy than the Fermi surface, and the other has a state on the Fermi surface. That is, in a half-metal material, Fermi energy electrons can take only one spin state. Therefore, the spin polarizability of the half metal material is theoretically 1 and is ideal.

  On the other hand, the magnetic anisotropy of the half metal material is very small. For example, in the L21 structure full Heusler alloy having a very high spin polarizability, the magnetic anisotropy becomes isotropic due to the high symmetry. Therefore, when the ferromagnetic layers 110 and 120 shown in FIG. 1 are simply replaced with a half metal layer, the MTJ element does not function. That is, it is difficult to apply a half metal material as it is to a magnetic device such as an MRAM that uses strong magnetic anisotropy.

  Therefore, in the present embodiment, a part that imparts a device function to the MTJ element and a part that improves the MR ratio are divided. More specifically, a half metal layer is inserted between the magnetic layer having strong magnetic anisotropy and the tunnel barrier layer. “Exchange coupling” or “magnetostatic coupling” works between magnetic materials. When magnetic bodies are magnetically coupled to each other, the direction of magnetization is governed by a magnetic body having stronger magnetic anisotropy. This tendency is particularly significant in a nanometer-order magnetic material such as MTJ. Therefore, when the magnetic layer having strong magnetic anisotropy and the isotropic half metal layer are magnetically coupled, the magnetization direction as a whole is defined by the former. As a result, the MTJ element functions as a device. Further, since a half metal layer having a high spin polarizability is provided at the tunnel barrier interface, the MR ratio is remarkably increased. As a result, the read signal becomes large and high-speed data reading is realized.

1. First Embodiment FIG. 2 shows a cross-sectional structure of an MTJ element 1 according to the first embodiment. The MTJ element 1 includes a first ferromagnetic layer 10, a second ferromagnetic layer 20, a tunnel barrier layer 30, a first half metal layer 40, and a second half metal layer 50. The first ferromagnetic layer 10, the first half metal layer 40, the tunnel barrier layer 30, the second half metal layer 50, and the second ferromagnetic layer 20 are stacked in this order. That is, the tunnel barrier layer 30 is located between the first ferromagnetic layer 10 and the second ferromagnetic layer 20, and is sandwiched between the first half metal layer 40 and the second half metal layer 50. Yes. The first half metal layer 40 is interposed between the first ferromagnetic layer 10 and the tunnel barrier layer 30 and is in contact with both the first ferromagnetic layer 10 and the tunnel barrier layer 30. The second half metal layer 50 is interposed between the second ferromagnetic layer 20 and the tunnel barrier layer 30 and is in contact with both the second ferromagnetic layer 20 and the tunnel barrier layer 30.

  The first ferromagnetic layer 10 and the second ferromagnetic layer 20 have strong magnetic anisotropy. The magnetic anisotropy may be in-plane magnetic anisotropy or perpendicular magnetic anisotropy. One of the first ferromagnetic layer 10 and the second ferromagnetic layer 20 is a pinned layer whose magnetization direction is fixed in one direction. The other of the first ferromagnetic layer 10 and the second ferromagnetic layer 20 is a magnetization recording layer (free layer) whose magnetization direction changes. In the case of the domain wall motion type MRAM, the magnetization recording layer is a domain wall motion layer in which the domain wall moves.

  The first half metal layer 40 and the second half metal layer 50 are formed of a half metal material. The first half metal layer 40 and the second half metal layer 50 have a high spin polarizability close to 1. That is, the spin polarizabilities of the first half metal layer 40 and the second half metal layer 50 are higher than the spin polarizabilities of the first ferromagnetic layer 10 and the second ferromagnetic layer 20. On the other hand, the magnetic anisotropy of the first half metal layer 40 and the second half metal layer 50 is smaller than the magnetic anisotropy of the first ferromagnetic layer 10 and the second ferromagnetic layer 20. Typically, the first half metal layer 40 and the second half metal layer 50 have isotropic magnetic anisotropy.

  The first half metal layer 40 interposed between the first ferromagnetic layer 10 and the tunnel barrier layer 30 is magnetically coupled to the first ferromagnetic layer 10. As a result, the magnetization direction of the first half metal layer 40 is determined depending on the magnetization direction of the first ferromagnetic layer 10. In addition, the second half metal layer 50 interposed between the second ferromagnetic layer 20 and the tunnel barrier layer 30 is magnetically coupled to the second ferromagnetic layer 20. As a result, the magnetization direction of the second half metal layer 50 is determined depending on the magnetization direction of the second ferromagnetic layer 20.

  Examples of the insulating material that is a material of the tunnel barrier layer 30 include crystalline MgO or amorphous Al—O.

Examples of the high spin polarizability material (half metal material) that is a material of the half metal layers 40 and 50 include a Heusler alloy exhibiting half metal properties. In particular, a full Heusler alloy represented by “Co ₂ MX” having isotropic magnetic anisotropy is suitable. Here, M is preferably at least one element selected from a set of first transition elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc.). X is preferably at least one element selected from the group of 3B group elements and 4B group elements (Al, Si, Ga, Ge, In, Sn, Tl, Pb, etc.). For example, Co ₂ MnSi, Co ₂ FeAlSi, or Co ₂ CrFeAl is used as the half metal material.

  The materials of the magnetization recording layer (one of the first ferromagnetic layer 10 and the second ferromagnetic layer 20) and the pinned layer (the other of the first ferromagnetic layer 10 and the second ferromagnetic layer 20) are as follows. is there.

  In the case of in-plane magnetic anisotropy, alloy materials such as NiFe, CoFe, and CoFeB are used. The pinned layer may have a stacked structure in which an antiferromagnetic layer (PtMn), a ferromagnetic layer (CoFe), a nonmagnetic layer (Ru), and a ferromagnetic layer (CoFe) are sequentially stacked.

  In the case of perpendicular magnetic anisotropy, a laminated film such as Co / Ni, Co / Pd, and Co / Pt is used. Alternatively, alloy materials such as Fe—Pt, Co—Pt, Co—Pt—Cr, Co—Pd, Co—Re, Co—Re—Ni, Tb—Fe—Co, and Gd—Tb—Fe—Co are used. These perpendicular magnetic anisotropic materials are excellent in domain wall motion characteristics and are particularly suitable for domain wall motion type MRAM. The pinned layer may have a stacked structure in which an antiferromagnetic layer (PtMn), a perpendicular magnetization layer, a nonmagnetic layer (Ru), and a perpendicular magnetization layer are sequentially stacked.

  As described above, according to the present embodiment, the half-metal layer with high spin polarizability that is magnetically coupled to the magnetic material having strong magnetic anisotropy is provided at the tunnel barrier interface. Therefore, the MR ratio of the MTJ element 1 is improved without impairing the function as a device. When such a half metal layer is not provided, the MR ratio is several tens of percent, but according to the present embodiment, an MR ratio exceeding 100% can be realized. As a result, the read signal becomes large and high-speed data reading is realized. In particular, even in the case of a domain wall motion type MRAM having characteristics such as a low write current and a large capacity, it is possible to realize an MR ratio exceeding 100% without impairing the domain wall motion characteristics. That is, the domain wall motion type MRAM can be further increased in speed.

2. Second Embodiment From the structure shown in FIG. 2, either the first half metal layer 40 or the second half metal layer 50 may be omitted. For example, as shown in FIG. 3, the second half metal layer 50 is omitted from the structure shown in FIG. In this case, the first ferromagnetic layer 10 is connected to the tunnel barrier layer 30 via the first half metal layer 40, but the second ferromagnetic layer 20 is in direct contact with the tunnel barrier layer 30. Even with such a structure, the same effects as those of the first embodiment can be obtained to some extent.

3. Third Embodiment FIG. 4 shows a cross-sectional structure of an MTJ element 1 according to a third embodiment. The MTJ element 1 shown in FIG. 4 further includes a first magnetic coupling layer 60 and a second magnetic coupling layer 70 in addition to the structure shown in FIG. The first magnetic coupling layer 60 is sandwiched between the first ferromagnetic layer 10 and the first half metal layer 40, and the second magnetic coupling layer 70 is composed of the second ferromagnetic layer 20 and the second half metal layer. 50. Examples of the material of the magnetic coupling layers 60 and 70 include Ru, Ta, Cu, Al, Os, and Re. In this case, the first ferromagnetic layer 10 and the first half metal layer 40 are magnetically coupled via the first magnetic coupling layer 60. Further, the second ferromagnetic layer 20 and the second half metal layer 50 are magnetically coupled via the second magnetic coupling layer 70. Even with such a structure, the same effect as in the first embodiment can be obtained. Further, by providing the magnetic coupling layers (60, 70), it becomes possible to control the magnetic coupling and crystallinity between the ferromagnetic layers (10, 20) and the half metal layers (40, 50). .

4). Fourth Embodiment From the structure shown in FIG. 4, either the first magnetic coupling layer 60 or the second magnetic coupling layer 70 may be omitted. For example, as shown in FIG. 5, the second magnetic coupling layer 70 is omitted from the structure shown in FIG. In this case, the first ferromagnetic layer 10 is connected to the first half metal layer 40 via the first magnetic coupling layer 60, but the second ferromagnetic layer 20 is in direct contact with the second half metal layer 50. Even with such a structure, the same effect as in the third embodiment can be obtained.

5). Fifth Embodiment A first magnetic coupling layer 60 may be added to the structure shown in FIG. 3 (second embodiment). That is, as shown in FIG. 6, the first magnetic coupling layer 60 is sandwiched between the first ferromagnetic layer 10 and the first half metal layer 40. In this case, the first ferromagnetic layer 10 and the first half metal layer 40 are magnetically coupled via the first magnetic coupling layer 60. Even with such a structure, the same effect as in the third embodiment can be obtained.

6). MRAM
FIG. 7 schematically shows the MRAM according to the present embodiment. The MRAM includes a plurality of memory cells arranged in an array. Any one of the above-described MTJ elements 1 is used as a memory cell. Since the MR ratio of the MTJ element 1 is high, a high read output and a high speed read operation are realized. The data writing method is, for example, a domain wall motion method. In that case, a reduction in write current, a higher density of memory cells, and a larger capacity of MRAM are realized.

  The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the above-described embodiments, and can be appropriately changed by those skilled in the art without departing from the scope of the invention.

DESCRIPTION OF SYMBOLS 1 MTJ element 10 1st ferromagnetic layer 20 2nd ferromagnetic layer 30 Tunnel barrier layer 40 1st half metal layer 50 2nd half metal layer 60 1st magnetic coupling layer 70 2nd magnetic coupling layer

Claims (8)

A first magnetic layer;
A second magnetic layer;
A tunnel barrier layer located between the first magnetic layer and the second magnetic layer;
A first half metal layer formed of a half metal material,
The first half metal layer is interposed between the first magnetic layer and the tunnel barrier layer, contacts the tunnel barrier layer, and is magnetically coupled to the first magnetic layer. Magnetic tunnel junction element. The magnetic tunnel junction device according to claim 1,
Furthermore, a second half metal layer formed of a half metal material is provided,
The second half metal layer is interposed between the second magnetic layer and the tunnel barrier layer, contacts the tunnel barrier layer, and is magnetically coupled to the second magnetic layer. Magnetic tunnel junction element. The magnetic tunnel junction device according to claim 2,
And a first magnetic coupling layer sandwiched between the first magnetic layer and the first half metal layer,
The magnetic tunnel junction element, wherein the first magnetic layer and the first half metal layer are magnetically coupled via the first magnetic coupling layer. The magnetic tunnel junction device according to claim 3,
And a second magnetic coupling layer sandwiched between the second magnetic layer and the second half metal layer,
The magnetic tunnel junction element, wherein the second magnetic layer and the second half metal layer are magnetically coupled via the second magnetic coupling layer. The magnetic tunnel junction device according to any one of claims 1 to 4,
The half metal material is a Heusler alloy magnetic tunnel junction element. The magnetic tunnel junction device according to claim 5,
The Heusler alloy is represented by Co ₂ MX,
M is the first transition element,
X is a group 3B element or a group 4B element. Magnetic tunnel junction element. The magnetic tunnel junction element according to any one of claims 1 to 6,
One of the first magnetic layer and the second magnetic layer is a pinned layer having a fixed magnetization direction,
The other of the first magnetic layer and the second magnetic layer is a domain wall moving layer in which a domain wall moves. Magnetic tunnel junction element.   A magnetic random access memory comprising the magnetic tunnel junction element according to claim 1 as a memory cell.

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