Patent ID: 12236989

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the following description and the drawings, the same symbols indicate the same or similar constituent elements, and redundant explanations for the same or similar constituent elements will be omitted.

Memory Element Configuration

FIG.1is a schematic cross-sectional view of the configuration of a magnetic memory element according to one embodiment of the present invention.

A magnetic memory element10according to one embodiment of the present invention comprises multiple first ferromagnetic layers1(1ato1d), multiple boundary layers2(2ato2d), a second ferromagnetic layer3, a first electrode4, an insulating film5, a third ferromagnetic layer6, and a second electrode7. The magnetic memory element10shown as an example has a three-dimensional structure in which the second ferromagnetic layer3, a layer structure9of the multiple boundary layers2and the multiple first ferromagnetic layers1, the insulating film5, and the third ferromagnetic layer6are stacked in this order from the bottom of the drawing between the first electrode4and the second electrode7.

The multiple first ferromagnetic layers1(1ato1d) are ferromagnetic layers with a switchable spin state. In the shown embodiment, the spin state can have two states; i.e., the arrows of spin can point upward or downward, for example. A single first ferromagnetic layer1functions as a memory cell for storing one bit of binary information. For example, the first ferromagnetic layer1may be formed using an elemental metal, such as iron or cobalt, or an alloy of these metals, such as Fe1-xNix, Fe1-xCox, Co1-xPtx, or CoFeB, wherein x is a composition ratio of the alloy and is a value in the range of 0<x<1.

The boundary layers2are each disposed between each pair of the multiple first ferromagnetic layers1to form a domain wall. In the shown embodiment, the spin state of the boundary layer2can have three states; i.e., the arrows of spin can point upward, downward, or horizontal, for example. When a domain wall is formed in the boundary layer2, the spin state of the boundary layer2is represented by a horizontal arrow. For convenience of explanation, the horizontal spin arrows only point to the right. In the present embodiment, the boundary layer2is formed using a non-magnetic material. For example, the non-magnetic material of the boundary layer2may be an elemental metal that is not ferromagnetic, such as copper or platinum, or an alloy of cobalt and platinum whose composition has been controlled as described below. In the present embodiment, although the boundary layer2is a non-magnetic material, since the thickness thereof is made thin, the boundary layer2is formed into a ferromagnetic material with a small exchange stiffness constant due to proximity effects caused by the ferromagnetic layers (the first ferromagnetic layers1or the second ferromagnetic layer3) adjacent to the boundary layer2.

The layer structure9is explained here, focusing on a single boundary layer2and a pair of the first ferromagnetic layers1(1a,1b) sandwiching the boundary layer2. In the layer structure9for the magnetic memory element10according to one embodiment, each of the boundary layers2generates ferromagnetic interaction (magnetic stiffness) Aex between the multiple first ferromagnetic layers1. More specifically, the boundary layer2has a thickness or composition that is sufficient to generate the ferromagnetic interaction Aex between the multiple first ferromagnetic layers1. The ferromagnetic interaction Aex is an interaction by which the spin direction is aligned. The ferromagnetic interaction Aex generated between the multiple first ferromagnetic layers1achieves a reduction in the drive current required for domain wall motion and improves the controllability of domain wall motion in the magnetic memory element10. The ferromagnetic interaction Aex is also generated between the first ferromagnetic layer1aand the second ferromagnetic layer3sandwiching the boundary layer2.

When an elemental metal is used for the non-magnetic material of the boundary layer2, an elemental metal with a thickness that is sufficient to generate the ferromagnetic interaction Aex between the multiple first ferromagnetic layers1is used to form the boundary layer2. For example, when copper is used to form the boundary layer2, the thickness of the boundary layer2is preferably a thickness within the range equivalent to one to three copper atoms. More preferably, the thickness of the boundary layer2is a thickness within the range equivalent to one to two copper atoms. For example, when platinum is used to form the boundary layer2, the thickness of the boundary layer2is preferably a thickness within the range equivalent to one to four platinum atoms. More preferably, the thickness of the boundary layer2is a thickness within the range equivalent to one to three platinum atoms.

When an alloy is used for the non-magnetic material of the boundary layer2, an alloy with a composition that is sufficient to generate the ferromagnetic interaction Aex between the multiple first ferromagnetic layers1is used to form the boundary layer2. By controlling the composition ratio of the alloy used to form the boundary layer2, the magnitude of the ferromagnetic interaction Aex that is generated between the multiple first ferromagnetic layers1is controlled. The Curie temperature Tc refers to a transition temperature at which a material changes from ferromagnetic to paramagnetic. Thus, whether the alloy exhibits properties of ferromagnetic materials or paramagnetic materials (i.e. non-magnetic materials) can be controlled by controlling the Curie temperature Tc of the alloy. The Curie temperature Tc and the ferromagnetic interaction Aex are proportionate. On the other hand, the Curie temperature Tc of the alloy can be controlled by controlling the composition ratio of the alloy. For example, the compositional dependence of the Curie temperature Tc in Ni1-xCuxalloy is described in FIG. 3 in S. A. Ahern, M. J. C. Martin and Willie Sucksmith, “The spontaneous magnetization of nickel+copper alloys,” Proc. Math. Phys. Eng. Sci., United Kingdom, The Royal Society, 11 Nov. 1958, Volume 248, Issue 1253, pp. 145-152, https://doi.org/10.1098/rspa.1958.0235. The same control can be applied not only to the Ni1-xCuxalloys disclosed as an example in this article but also to Co1-xPtxalloys. That is, by controlling the composition ratio of the alloy used to form the boundary layer2, the Curie temperature Tc of the alloy can be controlled, and whether the alloy exhibits properties of ferromagnetic materials or paramagnetic materials can be controlled, whereby the magnitude of the ferromagnetic interaction Aex can be controlled.

The second ferromagnetic layer3is a ferromagnetic layer with a switchable spin state. The second ferromagnetic layer3is disposed on the side of the first ferromagnetic layer1a, which is located on the lower side of drawing in the layer structure9, via one of the boundary layers2. The second ferromagnetic layer3functions as a layer for writing one bit of binary information to the first ferromagnetic layer1a. For example, the second ferromagnetic layer3may be formed using an alloy of cobalt and platinum or an alloy of iron and nickel. The material of the second ferromagnetic layer3for use may be various materials used for fixed magnetic layers in magnetoresistive random-access memories (MRAMs).

The second ferromagnetic layer3has coercivity higher than that of the first ferromagnetic layers1. For example, the second ferromagnetic layer3has coercivity higher than that of the first ferromagnetic layers1when at least one of the following three conditions is satisfied. The first condition is that the second ferromagnetic layer3and the first ferromagnetic layer1are formed using the same material, and that the second ferromagnetic layer3is thicker than the first ferromagnetic layer1. The second condition is that the thickness of the second ferromagnetic layer3and the thickness of the first ferromagnetic layer1are the same, and that the second ferromagnetic layer3is formed using a material with magnetic anisotropy higher than that of the first ferromagnetic layer1. The third condition is that the second ferromagnetic layer3is thinner than the first ferromagnetic layer1, that the second ferromagnetic layer3is formed using a material with magnetic anisotropy sufficiently higher than that of the first ferromagnetic layer1, and that the coercivity of the second ferromagnetic layer3is thus higher than that of the first ferromagnetic layer1.

The first electrode4is disposed adjacent to the second ferromagnetic layer3to switch the spin state of the second ferromagnetic layer3by spin-orbit torque. The first electrode4comprises a spin-orbit torque (SOT) layer41and two bottom electrodes42(42a,42b) electrically connected to the spin-orbit torque layer41.

When a drive current for switching the spin state of the second ferromagnetic layer3is passed between terminals11and12of the first electrode4, a write current Iw shown in the drawing as a single dotted line passes through the spin-orbit torque layer41, and the spin state of the second ferromagnetic layer3is switched by spin-orbit torque generated by spin orbital interaction. The spin state of the second ferromagnetic layer3is determined according to the direction of the write current Iw. In the present embodiment, the write current Iw is pulsed. For example, the spin-orbit torque layer41may be formed using a heavy metal, such as platinum. The bottom electrodes42may be formed using various conductive metals, such as gold and copper.

The insulating film5and the third ferromagnetic layer6are used in combination with the first ferromagnetic layer1dlocated on the upper side of the drawing in the layer structure9to function as a magnetic tunnel junction (MTJ) for reading out the spin state of the first ferromagnetic layer1d. The first ferromagnetic layer1dfunctions as the free layer of the magnetic tunnel junction. The spin state of the first ferromagnetic layer1dis read out by measuring the magnitude of the current passing through the first ferromagnetic layer1d, the insulating film5, and the third ferromagnetic layer6. The method for reading out the spin state with a magnetic tunnel junction is well known, and a further detailed description therefor is thus omitted here.

The insulating film5functions as the tunnel layer of the magnetic tunnel junction. The insulating film5is disposed between the first ferromagnetic layer1dlocated on the upper side of the drawing in the layer structure9and the second electrode7. The third ferromagnetic layer6is a layer with a fixed spin state (in the shown embodiment, the arrow points upward) and functions as the fixed layer of the magnetic tunnel junction. In the present embodiment, the arrow of spin representing the spin state of the third ferromagnetic layer6is fixed to point upward. The third ferromagnetic layer6is disposed between the insulating film5and the second electrode7. The insulating film5may be formed using an oxide film, such as magnesium oxide (MgO). The third ferromagnetic layer6may be formed using, for example, CoFeB, which is an alloy of cobalt, iron, and boron. The material of the third ferromagnetic layer6for use may be various materials used for fixed magnetic layers in MRAMs.

The second electrode7reads out the spin state of the first ferromagnetic layer1dlocated on the upper side of the drawing in the layer structure9. The second electrode7is disposed adjacent to the third ferromagnetic layer6and on the side of the first ferromagnetic layer1d, which is located on the upper side of the drawing.

A drive current for moving domain walls is passed between either the terminal11or12of the first electrode4and the terminal13of the second electrode7to allow a domain wall drive current Id shown in the drawing as a single dotted line to pass between the second electrode7and the first electrode4. This can allow domain walls to move between the multiple boundary layers2(2ato2d) located between the second electrode7and the first electrode4, and allow the spin state of each of the multiple first ferromagnetic layers1(1ato1d) to be shifted in a racetrack fashion to be transferred sequentially. The insulating film5and the third ferromagnetic layer6function as a magnetic tunnel junction for readout, whereby the spin state of the first ferromagnetic layer1d, which is located on the upper side of the drawing in the layer structure9, is read out through the second electrode7. In the present embodiment, the domain wall drive current Id is pulsed. For example, the second electrode7may be formed using various conductive metals, such as gold and copper.

Operation of Memory Element

FIGS.2and3are schematic diagrams for explaining the operation of a magnetic memory element according to one embodiment of the present invention.FIG.4is a flowchart for explaining the procedure for storing data in a magnetic memory element according to one embodiment of the present invention.

A method for storing data in the magnetic memory element10according to one embodiment comprisespassing a current between the bottom electrode42aand42bof the first electrode4to set the spin state of the second ferromagnetic layer3by spin-orbit torque (step S1), andpassing a domain wall drive current Id between the second electrode7and the first electrode4to transfer the spin state of the second ferromagnetic layer3to the first ferromagnetic layer1(id) by spin-transfer torque (step S2).

Referring now toFIGS.2and3, the procedure shown inFIG.4for storing data in the magnetic memory element10according to one embodiment and the procedure for reading data from the magnetic memory element10are explained. In the embodiment shown as an example, the magnetic memory element10comprises four memory cells (cell No. 1 to cell No. 4) and stores four bits of binary information in total. For convenience of explanation, an upward-pointing spin arrow represents the value “0,” while a downward-pointing spin arrow represents the value “1.”

Initialization of Data

FIG.2(A)shows the state in which the magnetic memory element10is initialized. In each of the four first ferromagnetic layers1and the second ferromagnetic layer3, the spin arrow is pointing upward, indicating that the value “0” is stored. In each of the four boundary layers2, the spin arrow is also pointing upward, indicating that no domain walls are formed in the boundary layers2in the initialized state. The initialization of the magnetic memory element10can be done, for example, by continuously passing, a predetermined number of times, a pulsed domain wall drive current Id from the first electrode4to the second electrode7. The spin state of the third ferromagnetic layer6is fixed upward in the present embodiment and is not changed by initialization.

Writing of Data

Writing of data in the magnetic memory element10is done through the second ferromagnetic layer3. In the present embodiment, the second ferromagnetic layer3is disposed below the layer structure9. For this reason, the writing of data is done from the first ferromagnetic layer1acorresponding to cell No. 1. To write the value “1” in cell No. 1, first, the value “1” is set in the second ferromagnetic layer3. Next, the value “1” set in the second ferromagnetic layer3is transferred to cell No. 1.

As shown inFIG.2(B), when the write current Iw is passed to the right through the first electrode4, the spin state of the second ferromagnetic layer3is switched with the spin arrow being changed from pointing upward to pointing downward by spin-orbit torque generated by the write current Iw. The value “1” is set and stored in the second ferromagnetic layer3accordingly. Along with this, a domain wall is formed in the boundary layer2a, which is located between the second ferromagnetic layer3and the first ferromagnetic layer1a.

As shown inFIG.2(C), when a pulsed domain wall drive current Id is passed from the second electrode7to the first electrode4, the domain wall formed in the boundary layer2amoves to the boundary layer2b, which is located between the first ferromagnetic layer1aand the first ferromagnetic layer1b. Thus, the domain wall motion induced by spin-transfer torque generated by the domain wall drive current Id sequentially transfers the spin state of each of the second ferromagnetic layer3and the four first ferromagnetic layers1(1ato1d) to the upper layer shown in the drawing. Accordingly, the sequential spin state transfer accompanied by domain wall motion induced by spin-transfer torque causes the spin state of the second ferromagnetic layer3to be transferred to the first ferromagnetic layer1a; thus, the spin state of the first ferromagnetic layer1ais switched with the spin arrow being changed from pointing upward to pointing downward. The value “1” set in the second ferromagnetic layer3is written to the first ferromagnetic layer1a, and the value “1” is stored in cell No. 1. Likewise, the sequential spin state transfer causes the value stored in cell No. 1 to be transferred to and stored in cell No. 2, the value stored in cell No. 2 to be transferred to and stored in cell No. 3, and the value stored in cell No. 3 to be transferred to and stored in cell No. 4.

If the pulsed domain wall drive current Id is passed from the first electrode4to the second electrode7in the direction opposite to that shown in the drawing, the spin state of each of the second ferromagnetic layer3and the four first ferromagnetic layers1(1ato1d) will be sequentially transferred to the lower layer in the drawing by domain wall motion induced by spin-transfer torque generated by the domain wall drive current Id. In the present embodiment, the second ferromagnetic layer3for use in writing data is disposed below the layer structure9, and the insulating film5and the third ferromagnetic layer6for use in reading data as described below are disposed above the layer structure9. Therefore, the domain wall drive current Id is passed from the second electrode7to the first electrode4to sequentially transfer the spin state to the upper layer in the drawing.

As shown inFIG.3(D), when the pulsed domain wall drive current Id is continuously passed from the second electrode7to the first electrode4, the domain wall formed in the boundary layer2bmoves to the boundary layer2c, which is located between the first ferromagnetic layer1band the first ferromagnetic layer1c, and the sequential spin state transfer described with reference toFIG.2(C)also continues. The spin state of the first ferromagnetic layer1ais transferred to the first ferromagnetic layer1b, and the spin state of the first ferromagnetic layer1bis switched with the spin arrow being changed from pointing upward to pointing downward. The value “1” stored in the first ferromagnetic layer1ais transferred to the first ferromagnetic layer1b, and the value “1” is stored in cell No. 2.

Accordingly, while the value “1” is set in the second ferromagnetic layer3, the value “1” set in the second ferromagnetic layer3is written to the first ferromagnetic layer1a, and the value “1” is stored in cell No. 1. Likewise, the sequential spin state transfer causes the value stored in cell No. 2 to be transferred to and stored in cell No. 3, and the value stored in cell No. 3 to be transferred to and stored in cell No. 4. The value stored in cell No. 4 is sequentially read by passing the domain wall drive current Id from the second electrode7to the first electrode4.

To write the value “0” in cell No. 1, the value “0” is first set in the second ferromagnetic layer3. Next, the value “0” set in the second ferromagnetic layer3is transferred to cell No. 1.

As shown inFIG.3(E), when the write current Iw is passed to the left through the first electrode4, the spin state of the second ferromagnetic layer3is switched with the spin arrow being changed from pointing downward to pointing upward by spin-orbit torque generated by the write current Iw. The value “0” is thus set and stored in the second ferromagnetic layer3. Along with this, a domain wall is formed in the boundary layer2a, which is located between the second ferromagnetic layer3and the first ferromagnetic layer1a.

As shown inFIG.3(F), when the pulsed domain wall drive current Id is passed from the second electrode7to the first electrode4, the domain wall formed in the boundary layer2amoves to the boundary layer2b, and the domain wall formed in the boundary layer2cmoves to the boundary layer2d, which is located between the first ferromagnetic layer1cand the first ferromagnetic layer1d. Thus, the domain wall motion induced by spin-transfer torque generated by the domain wall drive current Id sequentially transfers the spin state of each of the second ferromagnetic layer3and the four first ferromagnetic layers1(1ato1d) to the upper layer shown in the drawing. The spin state of the second ferromagnetic layer3is transferred to the first ferromagnetic layer1a, and the spin state of the first ferromagnetic layer1ais switched with the spin arrow being changed from pointing downward to pointing upward. The value “0” set in the second ferromagnetic layer3is written to the first ferromagnetic layer1a, and the value “0” is stored in cell No. 1. Likewise, the sequential spin state transfer causes the value stored in cell No. 1 to be transferred to and stored in cell No. 2, the value stored in cell No. 2 to be transferred to and stored in cell No. 3, and the value stored in cell No. 3 to be transferred to and stored in cell No. 4.

By sequentially applying the write operation to the magnetic memory element10as described above, a four-bit data of values “0”, “1”, “1”, and “0” in this order can be written in the magnetic memory element10comprising four memory cells (cell No. 1 to cell No. 4).

Read of Data

Read of data from the magnetic memory element10is done through the insulating film5and the third ferromagnetic layer6. In the present embodiment, the insulating film5and the third ferromagnetic layer6are disposed above the layer structure9. For this reason, read of data is done from the first ferromagnetic layer1dcorresponding to cell No. 4. The spin state of the first ferromagnetic layer1dis read out by measuring the magnitude of the readout current Ir passing through the first ferromagnetic layer1d, the insulating film5, and the third ferromagnetic layer6using a magnetic tunnel junction. Since the sequential spin state transfer occurs per pulse, a single magnetic tunnel junction is required per memory element10to read out the spin state.

In the state of the magnetic memory element10shown inFIG.3(F), the value “0” is first read by measuring the spin state of the first ferromagnetic layer1dcorresponding to cell No. 4 using a magnetic tunnel junction.

Next, the pulsed domain wall drive current Id is passed from the second electrode7to the first electrode4. This causes the spin state to move sequentially to the upper layer in the drawing by domain wall motion induced by spin-transfer torque. The value “0” stored in cell No. 1 is transferred to and stored in cell No. 2, the value “1” stored in cell No. 2 is transferred to and stored in cell No. 3, and the value “1” stored in cell No. 3 is transferred to and stored in cell No. 4. Although the value stored in cell No. 4 is destroyed by passing the pulsed domain wall drive current Id, the value stored in cell No. 4 has already been read using a magnetic tunnel junction before passing the pulsed domain wall drive current Id.

Subsequently, by repeating the read operation comprising the step of measuring the spin state of the first ferromagnetic layer1dcorresponding to cell No. 4 using a magnetic tunnel junction and the step of passing the pulsed domain wall drive current Id from the second electrode7to the first electrode4, the four-bit data of values “0,” “1,” “1,” and “0” in this order can be read from the memory element10.

In the example of the operation of the magnetic memory element10described above, the read of data from the first ferromagnetic layer1dis a destructive read process similar to that of dynamic random access memories (DRAMs); thus, data may be written again after the read operation.

Configuration of Magnetic Memory Device

FIG.5is a schematic diagram of the configuration of a magnetic memory device according to one embodiment of the present invention.

The magnetic memory device20according to one embodiment comprises a magnetic memory element10, a current source14, and a sensor15.

The current source14passes current between the bottom electrode42aand the bottom electrode42bof the first electrode4of the magnetic memory element10, and from the second electrode7to the first electrode4. The sensor15reads data represented by a spin state stored in the magnetic memory element10. The sensor15can measure the current value of the readout current Ir passing through the magnetic memory element10and, from that value, detect the resistance value to read out the spin state in the magnetic memory element. The current source14and the sensor15are connected to a memory controller16. The memory controller16controls the operation of the current source14and the sensor15to thus control the write operation to the magnetic memory element10and the read operation from the magnetic memory element10as explained above with reference toFIGS.2and3. The data read through the sensor15is transmitted and received via a data bus17. The connection from the current source14to the first electrode4or the second electrode7of the magnetic memory element10is switched, for example, using switches18aand18b. The operation of the switches18aand18bis controlled, for example, by the memory controller16. A plurality of the magnetic memory element10may be arranged in an array to form a memory array.

Numerical Simulation Regarding Layer Structure

FIG.6is a schematic diagram showing the layer structure used for numerical simulation in terms of a magnetic memory element according to one embodiment of the present invention, andFIG.7is a graph showing the results of numerical simulation in terms of the layer structure for a magnetic memory element according to one embodiment of the present invention.

The vertical axis of the graph inFIG.7represents the current density Jc (A/m2) required to operate the layer structure, while the horizontal axis of the graph represents the magnitude of the ferromagnetic interaction Aex (×1012J/m) introduced between the multiple first ferromagnetic layers1. In the numerical simulation, the magnitude of the current density Jc required for the operation represented by the vertical axis is calculated using the magnitude of the ferromagnetic interaction Aex represented by the horizontal axis as a free parameter.

The conditions of the numerical simulation are as follows. The parameters for the simulation are determined with the assumption of typical materials for MRAMs.

Layer Shape and Size

Each layer has a disk shape with a diameter of 20 nm and a thickness of 3 nm and is formed of a cell (1 nm×1 nm×1 nm).

Number of Layers and Layer Structure

The layer structure shown inFIG.6has 12 layers in total. The parameters of the material for each layer are as follows. Below, Ms represents the magnitude of saturation magnetization, Ku represents the magnitude of magnetic anisotropy, and Aex represents the magnitude of ferromagnetic interaction. Since the value of Ku in the domain wall layers is zero, a domain wall is trapped in the domain wall layers instead of the recording layers. This improves the controllability of the location of domain walls.

Layers3,5,7,9, and11(Domain Wall Layers)

Ms=8×105[A/m], Ku=0

The domain wall layers corresponds to the boundary layers2in the magnetic memory element10according to one embodiment of the present invention. The Aex introduced to the domain wall layers is a free parameter in the simulation and corresponds to the horizontal axis of the graph shown inFIG.7.

Layers2,4,6,8,10, and12(Recording Layers)

Ms=8×105[A/m], Ku=1×106[J/m3], Aex=1×10−11[J/m]

The recording layers correspond to the first ferromagnetic layers1in the magnetic memory element10according to one embodiment of the present invention.

Layer1(Pin Layer)

Ms=8×105[A/m], Ku=1×107[J/m3], Aex=1×10−11[J/m]

The pin layer corresponds to the second ferromagnetic layer3in the magnetic memory element10according to one embodiment of the present invention.

The graph shown inFIG.7is analyzed here. In the graph inFIG.7showing the results of numerical simulation, the magnitude of the operating current density Jc is approximately 1014when the value of the ferromagnetic interaction Aex is zero; thus, in conventional magnetic memory elements in which the ferromagnetic interaction Aex is not introduced, the magnitude of the operating current density Jc is in the order of 1014. In contrast, in the magnetic memory element10according to one embodiment of the present invention, in which the ferromagnetic interaction Aex is introduced between the multiple first ferromagnetic layers1, the magnitude of the operating current density Jc is in the order of 1010. This indicates that introduction of the ferromagnetic interaction Aex between the multiple first ferromagnetic layers1can reduce the operating current density Jc to approximately 1/10000, at maximum, of that of the conventional products, or at least about 1/100 of that of the conventional products. The reduction in the operating current density Jc achieves a reduction in the drive current required for domain wall motion and improves the controllability of domain wall motion.

As described above, the present invention provides a layer structure for a magnetic memory element in which the drive current required for domain wall motion is reduced, and the controllability of domain wall motion is improved, and provides a magnetic memory element comprising the layer structure. In the layer structure9for the magnetic memory element10according to one embodiment, the boundary layer2generates the ferromagnetic interaction Aex between the multiple first ferromagnetic layers1. The ferromagnetic interaction Aex generated between the multiple first ferromagnetic layers1achieves a reduction in the drive current required for domain wall motion and improves the controllability of domain wall motion in the magnetic memory element10.

OTHER EMBODIMENTS

Although the present invention is described above according to a specific embodiment, the present invention is not limited to the embodiment described above.

The number of memory cells in the magnetic memory element10is not limited to four, and the magnetic memory element10can include a greater number of memory cells.

In the embodiment described above, the boundary layer2is formed using a non-magnetic material; however, the material of the boundary layer2is not limited to non-magnetic materials. The boundary layer2, which forms a domain wall, may be formed using a ferromagnetic material of a type different from that of the first ferromagnetic layer1, or a ferromagnetic material whose composition is different from that of the first ferromagnetic layer1. For example, a CoFeB alloy may be used to form the first ferromagnetic layer1, and an elemental metal of cobalt may be used to form the boundary layer2. In such a case, the boundary layer2may be referred to as a “fourth ferromagnetic layer” in the magnetic memory element10according to the embodiment described above. Examples of ferromagnetic materials that can be used as materials of the boundary layer2include Ni1-xCuxalloys, Co1-xCuxalloys, and Co1-xPtxalloys. As described above, by controlling the composition ratio of these Ni1-xCuxalloys and Co1-xPtxalloys, the Curie temperature Tc of the alloys can be controlled, and whether the alloys exhibit properties of ferromagnetic materials or paramagnetic material can be controlled, whereby the magnitude of the ferromagnetic interaction Aex can be controlled. Thus, the alloys whose composition ratios are controlled to exhibit the properties of ferromagnetic materials and to generate an appropriate magnitude of the ferromagnetic interaction Aex can be used as the material of the boundary layer2.

In the embodiment described above, read of data from the magnetic memory element10is a destructive read process; however, the process can be modified to a non-destructive read process by changing the locations of the insulating film5and the third ferromagnetic layer6. For example, a third ferromagnetic layer6that has a ring shape and is concentric to the layer structure9may be disposed in the center of the height direction of the layer structure9, and the insulating film5may be disposed between the wall of the layer structure9and the third ferromagnetic layer6. According to this structure, the first ferromagnetic layers1located below the third ferromagnetic layer6in the layer structure9can serve as a data buffer.

In the embodiment described above, read of data from the magnetic memory element10is done by moving data in each single memory cell with the pulsed domain wall drive current Id and reading the value stored in the memory cell one by one by a magnetic tunnel junction; however, the procedure for reading data from the magnetic memory element10is not limited to this. By passing multiple pulsed domain wall drive currents Id, multiple data stored in multiple memory cells can be moved at once, and multiple data can be read sequentially by a magnetic tunnel junction using the domain wall drive current Id. When data are read using current, the time variation of the magnitude of the domain wall drive current Id can be used. If the magnitude of the domain wall drive current Id does not vary with time, then the time variation of the magnitude of the voltage of the magnetic tunnel junction can be used.

In the embodiment described above, the magnetic memory element10comprises the insulating film5, and the spin state of the first ferromagnetic layer1dis read out by a TMR (tunnel magnetoresistance) effect; however, the method for reading out the spin state of the first ferromagnetic layer1dis not limited to this. For example, a layer of a non-magnetic metal, such as copper, may be provided, instead of the insulating film5, in the magnetic memory element to read out the spin state of the first ferromagnetic layer1dby a GMR (giant magnetoresistance) effect. The use of the GMR effect in the readout of the spin state allows a relatively large current to pass through.

In the embodiment described above, the structure of the magnetic memory element10is a three-dimensional structure with various layers stacked vertically; however, the structure of the magnetic memory element can also have a structure with a region corresponding to the various layers arranged horizontally to be mounted on a plane. The layer structure recited in the claims not only means a three-dimensional structure with various layers stacked vertically, but also means a structure with a region corresponding to the various layers arranged horizontally to be mounted on a plane as stated above.

DESCRIPTION OF THE REFERENCE NUMERALS

1(1ato1d) First ferromagnetic layers2Boundary layer3Second ferromagnetic layer4First electrode5Insulating film6Third ferromagnetic layer7Second electrode9Layer structure10Magnetic memory element11to13Terminals14Current source15Sensor16Memory controller17Data bus18(18a,18b) Switches20Magnetic memory device41Spin-orbit torque layer42(42a,42b) Bottom electrodesAex Ferromagnetic interactionId Domain wall drive currentIw Write currentIr Readout current