Magnetic memory element having an adjustment layer for reducing a leakage magnetic field from a reference layer and magnetic memory thereof

According to one embodiment, a magnetic memory element includes a memory layer, a first nonmagnetic layer, a reference layer, a second nonmagnetic layer, and an adjustment layer which are stacked. The adjustment layer is configured to reduce a leakage magnetic field from the reference layer. The adjustment layer is formed by stacking an interface layer provided on the second nonmagnetic layer, and a magnetic layer having magnetic anisotropy perpendicular to a film surface. Saturation magnetization of the interface layer is larger than that of the magnetic layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-231363, filed Oct. 21, 2011, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetic memory element and magnetic memory.

BACKGROUND

A magnetic random access memory (MRAM) uses, as a memory element, an MTJ (Magnetic Tunnel Junction) element using the magnetoresistive effect by which a resistance value changes in accordance with the direction of magnetization. The MTJ element has a three-layered structure including a reference layer, a memory layer, and an insulating layer that is sandwiched between the reference layer and memory layer and forms a tunnel barrier. The magnetization in the reference layer is fixed in one direction and does not reverse even when a write operation is performed. On the other hand, the magnetization in the memory layer reverses due to torque externally given by a write operation.

An MRAM using a spin-transfer torque writing method of writing data by directly supplying a current to the MTJ element is known. When a write current is supplied to the MTJ element, the resistance value of the MTJ element changes depending on the relative directions of magnetization in the two magnetic layers. That is, the resistance value of the MTJ element becomes low when the magnetization directions in the memory layer and reference layer are parallel, and becomes high when the magnetization directions are antiparallel. The MTJ element can be used as a memory element by making these low- and high-resistance states of the MTJ element correspond to binary data.

Generally, a magnetic layer having magnetic anisotropic energy higher than that of the memory layer is used as the reference layer, so a leakage magnetic field from the reference layer is large. Therefore, the leakage magnetic field from the reference layer acts on the memory layer, and the magnetic coercive force of the memory layer shifts. Consequently, a current for switching the magnetization in the memory layer increases, or the thermal stability of the MTJ element decreases. Also, as the micropatterning of the MTJ element advances, the leakage magnetic field from the reference layer increases. As a consequence, the magnetic coercive force of the memory layer largely shifts.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided a magnetic memory element comprising:

a memory layer having magnetic anisotropy perpendicular to a film surface and having a variable magnetization direction;

a first nonmagnetic layer provided on the memory layer;

a reference layer provided on the first nonmagnetic layer, having magnetic anisotropy perpendicular to a film surface, and having an invariable magnetization direction;

a second nonmagnetic layer provided on the reference layer; and

an adjustment layer provided on the second nonmagnetic layer and configured to reduce a leakage magnetic field from the reference layer,

wherein the adjustment layer is formed by stacking an interface layer provided on the second nonmagnetic layer, and a magnetic layer having magnetic anisotropy perpendicular to a film surface, and

saturation magnetization of the interface layer is larger than that of the magnetic layer.

The embodiments will be described hereinafter with reference to the accompanying drawings. In the description which follows, the same or functionally equivalent elements are denoted by the same reference numerals, to thereby simplify the description.

FIG. 1is a sectional view showing the arrangement of an MTJ element10as a magnetic memory element according to the first embodiment. The MTJ element10is formed by stacking a memory layer11, nonmagnetic layer (tunnel barrier layer)12, reference layer13, nonmagnetic layer (spacer layer)14, and adjustment layer15in this order from below. Note that the stacking order shown inFIG. 1may also be reversed. The arrows inFIG. 1represent magnetization.

Each of the memory layer11and reference layer13is made of a ferromagnetic material, has magnetic anisotropy in a direction perpendicular to the film surfaces, and has a direction of easy magnetization perpendicular to the film surfaces. That is, the MTJ element10is a so-called perpendicular magnetization type MTJ element in which the magnetization directions in the memory layer11and reference layer13are perpendicular to the film surfaces.

In the memory layer11, the magnetization direction is variable (reverses). In the reference layer13, the magnetization direction is invariable (fixed). The reference layer13is so set as to have perpendicular magnetic anisotropic energy much higher than that of the memory layer11. The magnetic anisotropy can be set by adjusting the material configuration and/or thickness. Thus, the magnetization switching current of the memory layer11is decreased, thereby making the magnetization switching current of the reference layer13larger than that of the memory layer11. This makes it possible to implement the MTJ element10including the memory layer11having a variable magnetization direction and the reference layer13having an invariable magnetization direction, with respect to a predetermined write current.

As the nonmagnetic layer12, it is possible to use, e.g., a nonmagnetic metal, nonmagnetic semiconductor, or insulator. The nonmagnetic layer12is called a tunnel barrier layer when an insulator is used. As the tunnel barrier layer12, magnesium oxide (MgO) or the like is used.

This embodiment uses a spin-transfer torque writing method by which a write current is directly supplied to the MTJ element10and the magnetized state of the MTJ element10is controlled by using this write current. The MTJ element10can take one of a low-resistance state and high-resistance state in accordance with whether the relative magnetization directions in the memory layer11and reference layer13are parallel or antiparallel.

When a write current flowing from the memory layer11to the reference layer13is supplied to the MTJ element10, the relative magnetization directions in the memory layer11and reference layer13become parallel. In this parallel state, the MTJ element10has the lowest resistance value and is set in the low-resistance state. The low-resistance state of the MTJ element10is defined as, e.g., data “0”.

On the other hand, when a write current flowing from the reference layer13to the memory layer11is supplied to the MTJ element10, the relative magnetization directions in the memory layer11and reference layer13become antiparallel. In this antiparallel state, the MTJ element10has the highest resistance value and is set in the high-resistance state. The high-resistance state of the MTJ element10is defined as, e.g., data “1”.

Thus, the MTJ element10can be used as a memory element capable of storing one-bit data (binary data). It is possible to freely set the allocation between the resistance states of the MTJ element10and data.

When reading out data from the MTJ element10, a read voltage is applied to the MTJ element10, and the resistance value of the MTJ element10is detected based on the read current flowing through the MTJ element10in this state. The read voltage is set at a value much smaller than the threshold value of magnetization reversal caused by spin-transfer torque.

FIG. 2is an exemplary view for explaining leakage magnetic fields in the MTJ element10. The memory layer11and reference layer13forming the MTJ element10are made of magnetic materials and hence generate magnetic fields outside. In the perpendicular magnetization type MTJ element, a leakage magnetic field generated from the reference layer13is generally larger than that in an in-plane magnetization type element. Also, the memory layer11having a magnetic coercive force smaller than that of the reference layer13is strongly affected by the leakage magnetic field from the reference layer13. More specifically, the coercive magnetic force (or magnetization curve) of the memory layer11shifts under the influence of the leakage magnetic field from the reference layer13, thereby increasing the magnetization switching current and/or decreasing the thermal stability. The adjustment layer15of this embodiment is formed to reduce the leakage magnetic field applied from the reference layer13to the memory layer11.

The adjustment layer15has a multilayered structure in which an interface layer16formed on the nonmagnetic layer14and a magnetic layer17are stacked. The magnetic layer17is made of a ferromagnetic material, has magnetic anisotropy perpendicular to the film surfaces, and has a direction of easy magnetization perpendicular to the film surfaces. Like the reference layer13, the magnetic layer17has an invariable magnetization direction. The magnetic layer17antiferromagnetically couples with the reference layer13, and the magnetization directions in the magnetic layer17and reference layer13are set antiparallel. The interface layer16is made of a ferromagnetic material. The saturation magnetization of the interface layer16is set larger than that of the magnetic layer17. The interface layer16itself need not have magnetic anisotropy perpendicular to the film surfaces; the magnetization in the interface layer16need only be set perpendicular to the film surfaces by exchange coupling with the magnetic layer17. As a consequence, the adjustment layer15can have magnetization perpendicular to the film surfaces as a whole.

The spacer layer14is formed to prevent ferromagnetic coupling between the adjustment layer15and reference layer13. The spacer layer14has a heat resistance that prevents mixing of the adjustment layer15and reference layer13in a heating step, and also has a function of controlling the crystal orientation when forming the adjustment layer15. As the spacer layer14, it is possible to use a nonmagnetic metal such as ruthenium (Ru), platinum (Pt), silver (Ag), or copper (Cu).

Magnetic materials meeting the saturation magnetization conditions of the interface layer16and magnetic layer17will be explained below.

The interface layer16is made of one element selected from the group consisting of cobalt (Co), iron (Fe), and nickel (Ni), or an alloy containing at least one element selected from the group consisting of cobalt (Co), iron (Fe), and nickel (Ni). The interface layer16made of this material has magnetic anisotropy in the in-plane direction when no external magnetic field is applied, and has a direction of easy magnetization in the in-plane direction.

The magnetic layer17is made of an alloy containing at least one element selected from the group consisting of cobalt (Co) and iron (Fe), and at least one element selected from the group consisting of platinum (Pt), palladium (Pd), and chromium (Cr). Also, the magnetic layer17is formed by alternately stacking an alloy containing at least one element selected from the group consisting of cobalt (Co) and iron (Fe), and an alloy containing at least one element selected from the group consisting of platinum (Pt), palladium (Pd), and chromium (Cr).

Generally, the leakage magnetic field from the reference layer13increases when the MTJ element10is micropatterned. When the adjustment layer15is formed by a single magnetic layer, it is necessary to increase the thickness and/or saturation magnetization of the adjustment layer15in order to reduce the leakage magnetic field applied to the memory layer as micropatterning advances. If the thickness of the adjustment layer15is simply increased, however, the height of the MTJ element in the direction perpendicular to the film surfaces increases, and this makes micropatterning difficult. Also, if the saturation magnetization of the adjustment layer15is increased, the magnetic anisotropy generally decreases, and this makes the adjustment layer magnetically unstable.

On the other hand, when the adjustment layer15has the multilayered structure including the interface layer16and magnetic layer17as in this embodiment, it is possible to efficiently reduce the leakage magnetic field to be applied to the memory layer by forming the interface layer16having large saturation magnetization in a position close to the memory layer. Also, since it is unnecessary to forcedly raise the saturation magnetization of the magnetic layer17, a material that maximizes the magnetic anisotropy can be used. That is, the functions are separated such that the interface layer16close to the memory layer adjusts the saturation magnetization of the adjustment layer15, which greatly helps reduce the leakage magnetic field, thereby efficiently reducing the leakage magnetic field acting on the memory layer, and the magnetic layer17adjusts the magnetic stability of the adjustment layer15. Even when the MTJ element10is micropatterned, therefore, it is possible to reduce the thickness of the whole adjustment layer15and at the same time reduce the leakage magnetic field to be applied to the memory layer. This is the feature of this embodiment.

EXAMPLES

FIG. 3is a view for explaining the effect of reducing the thickness of the adjustment layer15. The ordinate inFIG. 3indicates the total thickness of the adjustment layer15including the interface layer16.

In the MTJ element10of Example 1, cobalt (Co) having a thickness of about 1 nm and a saturation magnetization of about 1,400 emu/cc was used as the interface layer16of the adjustment layer15, and a perpendicular magnetization film having a saturation magnetization of about 900 emu/cc was used as the magnetic layer17of the adjustment layer15.

In the MTJ element10of Example 2, cobalt (Co) having a thickness of about 2 nm and a saturation magnetization of about 1,400 emu/cc was used as the interface layer16of the adjustment layer15, and a perpendicular magnetization film having a saturation magnetization of about 900 emu/cc was used as the magnetic layer17of the adjustment layer15.

FIG. 3also shows a comparative example in which the adjustment layer15was a single layer made of the same material as that of the magnetic layer17. The arrangements of the memory layer11and reference layer13were the same in Examples 1 and 2 and the comparative example. The memory layer11was, e.g., a perpendicular magnetization film having a thickness of about 2 nm and a saturation magnetization of about 800 emu/cc. The reference layer13was, e.g., a perpendicular magnetization film having a thickness of about 8.5 nm and a saturation magnetization of about 691 emu/cc. The diameter of the MTJ element10was about 41 nm.

As can be understood fromFIG. 3, the thickness of the adjustment layer15of each of Examples 1 and 2 was smaller than that of the comparative example in which the adjustment layer15was a single perpendicular magnetic film. Also, the thickness of the adjustment layer15of Example 2 in which the thickness of the interface layer16was larger than that of Example 1 was smaller than that of Example 1, and smaller by about 4.6 nm than that of the comparative example.

In the first embodiment as described in detail above, the MTJ element10includes the adjustment layer15for reducing the leakage magnetic field from the reference layer13, in addition to the memory layer11and reference layer13having magnetic anisotropy perpendicular to the film surfaces. The adjustment layer15is stacked on the spacer layer14on the reference layer13, and the magnetization direction in the adjustment layer15is set antiparallel to that in the reference layer13. Also, the adjustment layer15is formed by stacking the interface layer16formed on the spacer layer14, and the magnetic layer17having magnetic anisotropy perpendicular to the film surfaces. The saturation magnetization of the interface layer16is set larger than that of the magnetic layer17.

Accordingly, the first embodiment can prevent the leakage magnetic field of the reference layer13from acting on the memory layer11, thereby preventing the shift of the magnetic coercive force of the memory layer11. This makes it possible to reduce the magnetization switching current, and improve the thermal stability of the MTJ element10.

Also, even when the MTJ element10is micropatterned, the increase in thickness of the adjustment layer15can be prevented. That is, the leakage magnetic field from the reference layer13can efficiently be reduced because the interface layer16having a large saturation magnetization is positioned close to the reference layer13. This makes it possible to reduce the thickness of the adjustment layer15and at the same time prevent the shift of the magnetic coercive force of the memory layer11.

Furthermore, since the thickness of the whole MTJ element10can be reduced, the processing of the MTJ element10is easy. Even when the MTJ element10is micropatterned, therefore, it is readily possible to process the MTJ element10into a pillar shape.

In the second embodiment, two adjustment layers are prepared to reduce a leakage magnetic field from a reference layer, and an MTJ element10is formed by sandwiching the reference layer and a memory layer between the two adjustment layers.FIG. 4is a sectional view showing the arrangement of the MTJ element10according to the second embodiment.

A nonmagnetic layer (spacer layer)18is formed under a memory layer11. The material of the spacer layer18is the same as that of the spacer layer14explained in the first embodiment.

An adjustment layer19for reducing a leakage magnetic field from a reference layer13is formed under the spacer layer18. The adjustment layer19has a multilayered structure in which an interface layer20formed on the spacer layer18and a magnetic layer21are stacked. The magnetic layer21is made of a ferromagnetic material, has magnetic anisotropy perpendicular to the film surfaces, and has a direction of easy magnetization perpendicular to the film surfaces. Like the reference layer13, the magnetic layer21has an invariable magnetization direction. The magnetization direction in the magnetic layer21is set antiparallel to that in the reference layer13. The interface layer20is made of a ferromagnetic material and has saturation magnetization set larger than that of the magnetic layer21. The adjustment layer15has magnetization perpendicular to the film surfaces as a whole due to the magnetization of the magnetic layer21. The materials of the interface layer20and magnetic layer21are respectively the same as those of the interface layer16and magnetic layer17explained in the first embodiment.

The MTJ element10arranged as described above can further reduce the influence the leakage magnetic field from the reference layer13on the memory layer11. This makes it possible to prevent the shift of the magnetic coercive force of the memory layer11. Also, even when the MTJ element10is micropatterned, the increase in thickness of an adjustment layer15and the adjustment layer19can be prevented.

The third embodiment is a configuration example when an MRAM (magnetic memory) is formed by using an MTJ element10described above. As the MTJ element10, either of the MTJ elements explained in the first and second embodiments can be used.

FIG. 5is a circuit diagram showing the arrangement of an MRAM30according to the third embodiment. The MRAM30includes a memory cell array32including a plurality of memory cells MC arranged in a matrix. Note thatFIG. 5shows (2×2) memory cells MC as an example. The memory cell array32includes a plurality of pairs of bit lines BL and /BL running in the column direction. The memory cell array32also includes a plurality of word lines WL running in the row direction.

The memory cells MC are arranged at the intersections of the bit lines and word lines. Each memory cell MC includes the MTJ element10and a selection transistor31. The selection transistor31is, e.g., an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). One terminal of the MTJ element10is connected to the bit line BL. The other terminal of the MTJ element10is connected to the drain of the selection transistor31. The gate of the selection transistor31is connected to the word line WL. The source of the selection transistor31is connected to the bit line /BL.

A row decoder33is connected to the word lines WL. The row decoder33selects one of the plurality of word lines WL based on a row address.

A write circuit36and read circuit37are connected to the pairs of bit lines BL and /BL via a column selector35. The column selector35includes, e.g., N-channel MOSFETs equal in number to all the bit lines, and selects a pair of bit lines BL and /BL necessary for an operation in accordance with an instruction from a column decoder34. The column decoder34decodes a column address, and supplies the decoded signal to the column selector35.

The write circuit36receives externally supplied write data. The write circuit36applies a write voltage to a pair of bit lines BL and /BL connected to a selected memory cell as a write target. The write circuit36then writes the data in the selected memory cell by supplying a write current to the selected memory cell.

The read circuit37applies a read voltage to a selected memory cell as a read target. Then, the read circuit37detects data stored in the selected memory cell based on a read current flowing through the selected memory cell. The data read by the read circuit37is output outside.

Data write to the memory cell MC is performed as follows. First, to select the memory cell MC as a data write target, the row decoder33activates the word line WL connected to the selected memory cell MC. This turns on the selection transistor31. In addition, the column decoder34selects the pair of bit lines BL and /BL connected to the selected memory cell MC.

In this state, one of bidirectional write currents is supplied to the MTJ element10in accordance with write data. More specifically, when supplying the write current to the MTJ element10from the left to the right inFIG. 5, the write circuit36applies a positive voltage to the bit line BL and the ground voltage to the bit line /BL. When supplying the write current to the MTJ element10from the right to the left inFIG. 5, the write circuit36applies a positive voltage to the bit line /BL and the ground voltage to the bit line BL. Consequently, data “0” or “1” can be written in the memory cell MC.

Data read from the memory cell MC is performed as follows. First, the selection transistor31of the selected memory cell MC is turned on in the same manner as in data write. The read circuit37supplies a read current flowing, e.g., from the right to the left inFIG. 5to the MTJ element10. This read current is set at a value much smaller than the threshold value of magnetization reversal caused by spin-transfer torque. Then, the read circuit37detects the resistance value of the MTJ element10based on the read current. Consequently, data stored in the MTJ element10can be read.

Next, a structure example of the MRAM30will be explained.FIG. 6is a sectional view showing the arrangement of the MRAM30. An element isolation insulating layer42having an STI (Shallow Trench Isolation) structure is formed in a P-type semiconductor substrate41. An N-channel MOSFET as the selection transistor31is formed in an element region (active area) surrounded by the element isolation insulating layer42. The selection transistor31includes a source region43and drain region44formed apart from each other in the element region, a gate insulating film45formed on a channel region between the source region43and drain region44, and a gate electrode46formed on the gate insulating film45. The gate electrode46corresponds to the word line WL shown inFIG. 5. Each of the source region43and drain region44is an N-type diffusion region.

A contact plug47is formed on the source region43. The bit line /BL is formed on the contact plug47. A contact plug48is formed on the drain region44. An extraction electrode49is formed on the contact plug48. The MTJ element10is formed on the extraction electrode49. The bit line BL is formed on the MTJ element10. An interlayer dielectric layer50fills the portion between the semiconductor substrate41and bit line BL.

In the third embodiment as described in detail above, the MRAM30can be formed by using either of the MTJ elements10explained in the first and second embodiments.