JP2025087204A - Data reading method and device - Google Patents

Abstract

To provide a data reading method and device in which data is read from a storage layer without destroying the data represented by a spin state in a type of magnetic memory element that transmits information based on magnetic wall motion.SOLUTION: A method of reading data from a magnetic memory element includes: a step (S1) for flowing a pulse current to a layer structure 9 of a magnetic memory element 10 provided with a plurality of storage layers 1a to 1d, which can switch spin states, and a boundary layer 2 that is arranged between the plurality of storage layers 1a to 1d and composes a magnetic wall; a step (S2) for reading a response signal from the layer structure 9 to which the pulse current flows; and steps (S3 to S6) for specifying the spin state of each of the plurality of storage layers 1a to 1d according to the response signal.SELECTED DRAWING: Figure 9

Description

The present invention relates to a method and apparatus for reading data from a magnetic memory element that transmits information based on domain wall motion.

As the amount of information increases dramatically, there is a need for memory devices that can record information at high density. Currently, flash memory is widely used as such a memory device. However, due to the operating principle of flash memory, there are drawbacks in that the number of times it can be written is limited due to deterioration of the oxide film, and the writing speed slows down as information is repeatedly written. For this reason, various magnetic memories have been proposed in recent years as replacements for existing flash memory.

For example, Patent Document 1 discloses a racetrack memory, which is a three-dimensional magnetic memory. In the racetrack memory of Patent Document 1, ferromagnetic materials are separated by magnetic domains and arranged in stacks or lines. A bit is defined for each magnetic domain, and data is stored by the direction of magnetization in the magnetic domain. The magnetic domain walls are moved by passing a current through a thin wire of ferromagnetic material (magnetic nanowire). This causes the magnetization in the magnetic domains to move in one direction all at once, transmitting the data.

In the information transmission method based on domain wall motion as exemplified in Patent Document 1, there still remain problems such as a high driving current for domain wall motion and poor controllability of domain wall motion. In response to this, the present inventor has proposed magnetic memory elements as disclosed in Patent Documents 2 and 3 as magnetic memory elements that transmit information based on domain wall motion similar to that of Patent Document 1.

U.S. Pat. No. 6,834,005 International Publication No. WO 2022/014529 JP 2023-094193 A

The layer structure of the magnetic memory element disclosed in Patent Documents 2 and 3 improves the drive current required for domain wall motion and the controllability of domain wall motion compared to conventional magnetic memory elements. On the other hand, Patent Documents 2 and 3 also give an example of a destructive readout method as an example of a method for reading data from the memory layer. In a destructive readout method, the data in the memory layer is destroyed when data is read from the memory layer. There is a demand for a more refined method of reading data in magnetic memory elements that transmit information based on domain wall motion.

The present invention aims to read data from a storage layer in a magnetic memory element that transmits information based on domain wall motion without destroying the data.

The inventors of the present invention have been conducting intensive research to solve the above problems, and have found that there is a correspondence relationship between the bit strings represented by multiple memory cells and the waveforms of the response signals corresponding to those bit strings, as shown in Figures 5 and 6. Based on this correspondence relationship, it has been found that it is possible to identify the bit strings corresponding to the response signals read from a layer structure including multiple memory cells. In this case, even if the response signals are read from the layer structure, the bits of the memory cells included in the layer structure are not destroyed.

That is, in order to achieve the above object, the present invention includes, for example, the following aspects.
(Section 1)
A method for reading data represented by a spin state from a magnetic memory element that transmits information based on domain wall motion, comprising the steps of:
A step of passing a pulse current through a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and boundary layers disposed between the plurality of memory layers and constituting a domain wall;
reading a response signal from the layer structure through which the pulse current has been passed, and identifying the spin state for each of the plurality of memory layers based on the response signal;
23. A method for reading data from a magnetic memory element, comprising:
(Section 2)
2. The method according to claim 1, wherein the step of identifying the spin state identifies the combination of spin states corresponding to the read response signal based on a correspondence relationship between the combination of spin states and a plurality of reference response signals.
(Section 3)
The step of identifying the spin state includes:
calculating an index of signal similarity between the read response signal and each of the plurality of reference response signals;
Identifying the combination of the spin states corresponding to the read response signal based on the calculated index;
Item 3. The method according to item 2, comprising:
(Section 4)
The step of identifying the spin state includes:
The method further includes converting the read response signal from a time domain signal to a frequency domain signal;
The step of calculating the index comprises:
calculating a first measure of similarity between the response signal in the frequency domain and each of the plurality of reference response signals in the frequency domain;
calculating a second measure of similarity between the response signal in the time domain and each of the plurality of reference response signals in the time domain;
Including,
4. The method according to claim 3, wherein the step of identifying a combination identifies a combination of the spin states corresponding to the read response signal based on the calculated first index and second index.
(Section 5)
Item 5. The method according to item 4, wherein the first index and the second index are correlation coefficients.
(Section 6)
Item 5. The method according to item 4, wherein the first index is a correlation coefficient and the second index is a phase difference.
(Section 7)
An apparatus for reading data represented by a spin state from a magnetic memory element that transmits information based on domain wall motion,
a current source that supplies a pulse current to a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and boundary layers that are disposed between the plurality of memory layers and form domain walls;
a sensor for reading a response signal from the layer structure to which the pulse current has been applied;
a spin state specifying unit that specifies the spin state for each of the plurality of memory layers based on the response signal;
1. An apparatus for reading data from a magnetic memory element, comprising:
(Section 8)
8. The device according to item 7, wherein the spin state specifying unit specifies the combination of spin states corresponding to the read response signal based on a correspondence relationship between the combination of spin states and a plurality of reference response signals.
(Section 9)
The spin state specifying unit is
an index calculation unit that calculates an index relating to a degree of similarity between the read response signal and each of the plurality of reference response signals;
a combination specifying unit that specifies a combination of the spin states corresponding to the read response signal based on the calculated index;
Item 9. The device according to item 8, comprising:
(Section 10)
The spin state specifying unit is
A signal conversion unit converts the read response signal from a time domain signal to a frequency domain signal,
The index calculation unit
a first index calculation unit that calculates a first index relating to a degree of similarity between the response signal in a frequency domain and each of the plurality of reference response signals in a frequency domain;
a second index calculation unit that calculates a second index relating to a degree of similarity between the response signal in the time domain and each of the plurality of reference response signals in the time domain;
Equipped with
10. The device according to claim 9, wherein the combination identifying unit identifies the combination of the spin states corresponding to the read response signal based on the calculated first index and second index.

According to the present invention, in a magnetic memory element that transmits information based on domain wall motion, data can be read from the storage layer without destroying the data.

1 is a cross-sectional view showing an example of a schematic configuration of a magnetic memory element from which data is read; 1 is a diagram for explaining the principle of a data reading method according to an embodiment of the present invention; FIG. 13 is a diagram for explaining the results of a simulation performed for the case of four memory cells. FIG. 13 is a diagram for explaining the results of a simulation performed for the case of four memory cells. 13 is a diagram showing the correspondence between all combinations of bit strings represented by four memory cells and the corresponding response signals; FIG. 13 is a diagram showing the correspondence between all combinations of bit strings represented by four memory cells and the corresponding response signals; FIG. 13 is a diagram for explaining a method for distinguishing and identifying bit strings represented by four memory cells when response signals in the frequency domain are similar or roughly the same. FIG. 13 is a diagram for explaining a method for distinguishing and identifying bit strings represented by four memory cells when response signals in the frequency domain are similar or roughly the same. FIG. 4 is a flowchart illustrating a procedure of a data reading method according to an embodiment of the present invention. 1 is a diagram illustrating a schematic configuration of a data reading device according to an embodiment of the present invention; FIG. 11 is a diagram illustrating a schematic configuration of a data reading device according to another embodiment of the present invention. 11 is a cross-sectional view showing a schematic diagram of another example of the general configuration of a magnetic memory element from which data is read. FIG.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Note that in the following description and drawings, the same reference numerals indicate the same or similar components, and therefore, redundant descriptions of the same or similar components will be omitted.

In this specification, the term "layer" does not only mean a layer that is distinguished from other layers by the material used to form it or by physical and chemical properties such as magnetism and conductivity, but also means a film or region formed on the surface or inside a material, such as an elemental metal or alloy, by a method such as sputtering. The term "layer" in the claims is to be interpreted as including "film" and "region."

In this embodiment, the magnetic memory element from which data is read is a type of magnetic memory element that transmits information based on domain wall motion. In the following, an example of a method for non-destructively reading data from a memory layer will be described for the magnetic memory element disclosed in Patent Document 2 as an example of this type of magnetic memory element.

First, as a magnetic memory element from which data is to be read, the configuration of a magnetic memory element exemplified in Patent Document 2 will be described with reference to Fig. 1. Next, the principle and method of non-destructively reading data from a memory layer of this exemplified magnetic memory element will be described with reference to Figs.
[Configuration of the Target Magnetic Memory Element]

Figure 1 is a cross-sectional view showing an example of the general configuration of a magnetic memory element from which data is read.

In this embodiment, the magnetic memory element 10 from which data is read includes multiple memory layers 1 (1a-1d), multiple boundary layers 2 (2a-2d), a first ferromagnetic layer 3, a first electrode 4, an insulating film 5, a second ferromagnetic layer 6, and a second electrode 7. The magnetic memory element 10 shown has a three-dimensional structure in which the first ferromagnetic layer 3, the layer structure 9 of multiple boundary layers 2 and multiple memory layers 1, the insulating film 5, and the second ferromagnetic layer 6 are stacked between the first electrode 4 and the second electrode 7 in order from the bottom in the figure.

The multiple storage layers 1 (1a to 1d) are layers whose spin state can be switched. In the illustrated magnetic memory element 10, the storage layer 1 is a ferromagnetic layer. In the illustrated embodiment, the spin state can have two states, for example, the spin arrow pointing up or down, and one storage layer 1 functions as a memory cell that stores one bit of binary information. Exemplarily, the storage layer 1 can be formed using simple metals such as iron and cobalt, or alloys of these metals, such as Fe _1-x Ni _x , Fe _1-x Co _x , Co _1-x Pt _x , and CoFeB. Here, x is the composition ratio of the alloy and takes a value in the range of 0<x<1.

The boundary layer 2 is disposed between the multiple memory layers 1 to form a magnetic domain wall. In the illustrated embodiment, the spin state of the boundary layer 2 can have three states, for example, the spin arrow pointing upward, downward, and sideways. When a magnetic domain wall is formed in the boundary layer 2, the spin state of the boundary layer 2 is represented by a sideways arrow. For convenience of explanation, the sideways state of the spin arrow is only the rightward direction. In the illustrated magnetic memory element 10, the boundary layer 2 is formed using a non-magnetic material. For example, the non-magnetic material of the boundary layer 2 can be a single metal that is not a ferromagnetic material, such as copper or platinum, or an alloy of cobalt and platinum whose composition is controlled as described later. In the illustrated magnetic memory element 10, even if the boundary layer 2 is a non-magnetic material, its thickness is made thin, so that the ferromagnetic layer (memory layer 1 or first ferromagnetic layer 3) adjacent to the boundary layer 2 causes the proximity effect to cause the boundary layer 2 to become a ferromagnetic material with a weak exchange stiffness constant.

In the layer structure 9, the description will focus on one boundary layer 2 and a pair of memory layers 1 (1a, 1b) sandwiching the boundary layer 2. In the layer structure 9 of the magnetic memory element 10 illustrated, the boundary layer 2 generates a ferromagnetic interaction (magnetic stiffness) Aex between the multiple memory layers 1. More specifically, the boundary layer 2 has a thickness or composition that generates a ferromagnetic interaction Aex between the multiple memory layers 1. The ferromagnetic interaction Aex is an interaction for aligning the direction of spin. Since the ferromagnetic interaction Aex occurs between the multiple memory layers 1, the drive current required for domain wall movement and the controllability of the domain wall movement are improved in the magnetic memory element 10. The ferromagnetic interaction Aex also occurs between the memory layer 1a and the first ferromagnetic layer 3 sandwiching the boundary layer 2.

When a simple metal is used as the non-magnetic material of the boundary layer 2, the boundary layer 2 is formed using a simple metal with a thickness that generates ferromagnetic interaction Aex between the multiple memory layers 1. For example, when the boundary layer 2 is formed using copper, the thickness of the boundary layer 2 is preferably within the range of 1 to 3 copper atoms. More preferably, the thickness of the boundary layer 2 is within the range of 1 to 2 copper atoms. For example, when the boundary layer 2 is formed using platinum, the thickness of the boundary layer 2 is preferably within the range of 1 to 4 platinum atoms. More preferably, the thickness of the boundary layer 2 is within the range of 1 to 3 platinum atoms.

When an alloy is used for the non-magnetic material of the boundary layer 2, the boundary layer 2 is formed using an alloy having a composition that generates a ferromagnetic interaction Aex between the multiple memory layers 1. The magnitude of the ferromagnetic interaction Aex generated between the multiple memory layers 1 is controlled by controlling the composition ratio of the alloy used to form the boundary layer 2. Since the Curie temperature Tc is the transition temperature at which a ferromagnetic material changes into a paramagnetic material, it is possible to control whether the alloy shows the properties of a ferromagnetic material or a paramagnetic material (i.e., a non-magnetic material) by controlling the Curie temperature Tc of the alloy. The Curie temperature Tc and the ferromagnetic interaction Aex are proportional. On the other hand, the Curie temperature Tc of the alloy can be controlled by controlling the composition ratio of the alloy. For example, SA Ahern, MJC Martin and Willie Sucksmith, “The spontaneous magnetization of nickel + copper alloys”, Proc. Math. Phys. Eng. Sci., United Kingdom, The Royal Society, 11 November 1958, Volume 248, Issue 1253, p.145-152, https://doi.org/10.1098/rspa.1958.0235, Fig. 3 shows the composition dependence of the Curie temperature Tc in Ni _1-x Cu _x alloys. The same control can be applied not only to the Ni _1-x Cu _x alloys exemplified in this document, but also to Co _1-x Pt _x alloys. Therefore, by controlling the composition ratio of the alloy used to form the boundary layer 2, it is possible to control the Curie temperature Tc of the alloy and to control whether the alloy exhibits ferromagnetic or paramagnetic properties, thereby controlling the magnitude of the ferromagnetic interaction Aex.

The first ferromagnetic layer 3 is a ferromagnetic layer whose spin state can be switched. The first ferromagnetic layer 3 is disposed on the side of the memory layer 1a located at the bottom of the layer structure 9 in the figure, with the boundary layer 2 sandwiched between them. The first ferromagnetic layer 3 functions as a layer for writing 1 bit of binary information to the memory layer 1a. Exemplarily, the first ferromagnetic layer 3 can be formed using an alloy of cobalt and platinum, or an alloy of iron and nickel. The material of the first ferromagnetic layer 3 can be any of various materials used for the magnetization fixed layer in magnetoresistive memory (MRAM: Magnetoresistive Random Access Memory).

The first ferromagnetic layer 3 has a higher coercive force than the memory layer 1. For example, when at least one of the following three conditions is satisfied, the first ferromagnetic layer 3 has a higher coercive force than the memory layer 1. The first condition is that the first ferromagnetic layer 3 is formed thicker than the memory layer 1 even if the first ferromagnetic layer 3 and the memory layer 1 are formed using the same material. The second condition is that the first ferromagnetic layer 3 is formed using a material having a higher magnetic anisotropy than the memory layer 1 even if the thickness of the first ferromagnetic layer 3 and the thickness of the memory layer 1 are the same. The third condition is that the first ferromagnetic layer 3 is formed using a material having a sufficiently higher magnetic anisotropy than the memory layer 1 even if the thickness of the first ferromagnetic layer 3 is thinner than the thickness of the memory layer 1, and as a result, the coercive force of the first ferromagnetic layer 3 is higher than the memory layer 1.

The first electrode 4 is disposed adjacent to the first ferromagnetic layer 3 and switches the spin state of the first ferromagnetic layer 3 by spin-orbit torque. The first electrode 4 includes a spin-orbit torque (SOT) layer 41 and two bottom electrodes 42 (42a, 42b) electrically connected to the spin-orbit torque layer 41.

By passing a drive current between the terminals 21 and 22 of the first electrode 4 to switch the spin state of the first ferromagnetic layer 3, the write current Iw shown by the dashed line in the figure flows through the spin orbit torque layer 41, and the spin orbit torque generated by the spin orbit interaction switches the spin state of the first ferromagnetic layer 3. The spin state of the first ferromagnetic layer 3 is determined according to the direction of the write current Iw. In the illustrated magnetic memory element 10, the write current Iw is pulsed. Exemplarily, the spin orbit torque layer 41 can be formed using a heavy metal such as platinum. The bottom electrode 42 can be formed using various conductive metals such as gold and copper.

The insulating film 5 and the second ferromagnetic layer 6 function as a magnetic tunnel junction (MTJ) in combination with the memory layer 1d located at the top of the layer structure 9 in the figure. The memory layer 1d functions as the free layer of the magnetic tunnel junction.

The insulating film 5 functions as a tunnel layer of the magnetic tunnel junction. The insulating film 5 is disposed between the memory layer 1d located at the upper side in the layer structure 9 and the second electrode 7. The second ferromagnetic layer 6 is a layer in which the spin state is fixed (upward arrow in the illustrated embodiment) and functions as a fixed layer of the magnetic tunnel junction. In the illustrated magnetic memory element 10, the spin state of the second ferromagnetic layer 6 is fixed in a state in which the spin arrow points upward. The second ferromagnetic layer 6 is disposed between the insulating film 5 and the second electrode 7. The insulating film 5 can be formed, for example, using an oxide film such as magnesium oxide (MgO). The second ferromagnetic layer 6 can be formed, for example, using CoFeB, which is an alloy of cobalt, iron, and boron. Various materials used for the magnetization fixed layer in MRAM can be used as the material for the second ferromagnetic layer 6.

The second electrode 7 is disposed adjacent to the second ferromagnetic layer 6 on the side of the memory layer 1d located at the upper side in the figure. By passing a drive current for moving the domain wall between the terminal 23 of the second electrode 7 and either the terminal 21 or the terminal 22 of the first electrode 4, a domain wall drive current Id shown by a two-dot chain line in the figure flows between the second electrode 7 and the first electrode 4. This allows the domain wall to be moved in the multiple boundary layers 2 (2a to 2d) located between the second electrode 7 and the first electrode 4, and the spin states in the multiple memory layers 1 (1a to 1d) can be shifted in a racetrack manner and sequentially transferred. In the illustrated magnetic memory element 10, the domain wall drive current Id is pulsed. Exemplarily, the second electrode 7 can be formed using various conductive metals such as gold and copper.
[Principle of data reading]

Figure 2 is a diagram for explaining the principle of a data reading method according to one embodiment of the present invention.

In the following example, the magnetic memory element 10 has four memory cells (cell No. 1 to cell No. 4) and stores a total of four bits of binary information. For ease of explanation, the downward state of the spin arrow represents a value of "0," and the upward state represents a value of "1."

A bit string refers to a sequence of 1 bit of binary information represented by each of multiple memory cells, and refers to a combination (or sequence) of the spin states (value "0" or value "1") represented by each of the multiple memory layers 1 (1a to 1d) included in the layer structure 9 shown in Figure 1.

Refer to FIG. 2. The bit string represented by the four memory cells is [0010] in the example shown in (A) and (B), and [1010] in the example shown in (C). The bit string represented by the four memory cells is different between (A) and (B) and that shown in (C).

The data reading method according to one embodiment is a method for non-destructively reading data from a magnetic memory element 10. In the data reading method according to one embodiment, as shown in FIG. 2A, a pulse current is passed through the layer structure 9 of the magnetic memory element 10 in which the bit strings represented by the multiple memory cells are in a certain state (for example, [0010] as shown). The pulse current flows between the second electrode 7 and the first electrode 4 as shown by the two-dot chain line in the figure. The magnitude of the pulse current is weaker than the domain wall driving current Id described with reference to FIG. 1, and is strong enough not to move the domain walls formed in each of the multiple boundary layers 2. Illustratively, the waveform of the pulse current passed through the layer structure 9 is rectangular, and the pulse length is about 1 ns (nanosecond). When the pulse current is passed through the layer structure 9, as shown in FIG. 2B, magnetization dynamics is induced in the layer structure 9 by the spin torque effect. When magnetization dynamics are induced, the spins in some of the memory layers 1 and boundary layers 2 contained in the layer structure 9 undergo, for example, precession.

On the other hand, as shown in FIG. 2C, even if a pulse current is passed through the layer structure 9 of the magnetic memory element 10 in which the bit strings represented by the multiple memory cells are in a state different from the state [0010] shown in FIG. 2A (for example, the illustrated [1010]), magnetization dynamics is induced in the layer structure 9 by the spin torque effect, as in the example shown in FIG. 2B. Here, attention is focused on the induced magnetization dynamics. Since the bit strings represented by the multiple memory cells included in the layer structure 9 are different between those shown in (B) and (C), as a result, the magnetization dynamics induced in the layer structure 9 by the spin torque effect also differs between those shown in (B) and those shown in (C). The magnetization dynamics induced in the layer structure 9 changes and decays over time.

That is, when a pulse current is passed through the layer structure 9 and a response signal from the layer structure 9 is read, the read response signal reflects the difference in magnetization dynamics caused by the difference in the bit strings within the layer structure 9. This makes it possible to identify the layer structure 9, i.e., to identify the bit strings represented by the multiple memory cells contained in the layer structure 9, based on the response signal from the layer structure 9.

Next, we will provide some specific examples of bit strings represented by the four memory cells and explain how these different bit strings can be distinguished using the response signal from layer structure 9.

Figures 3 and 4 are diagrams for explaining the results of a simulation performed for the case of four memory cells. (A) is a diagram showing a schematic representation of the layer structure of the magnetic memory element used in the simulation. (B) is the simulation result of the response signal in the time domain, and (C) is the simulation result of the response signal in the frequency domain.

As shown in FIG. 3A and FIG. 4A, the bit strings represented by the four memory cells are different in the example shown in FIG. 3 and the example shown in FIG. 4. The bit string represented by the four memory cells is [0001] in the example shown in FIG. 3, and [0010] in the example shown in FIG. 4.

The signal shown in (B) is a response signal from the layer structure 9 when a pulse current is passed through the layer structure 9. The response signal read from the layer structure 9 is a signal whose signal strength changes over time, that is, a time-domain response signal. The signal shown in (B) can be read using a magnetic tunnel junction provided in the magnetic memory element 10. In the example of the layer structure 9 shown in FIG. 1, the insulating film 5 and the second ferromagnetic layer 6 constituting the magnetic tunnel junction are provided on the upper side of the layer structure 9 in the figure. As shown in (B) of FIG. 3 and (B) of FIG. 4, due to the difference in the bit string in the layer structure 9, the time-domain response signal read from the layer structure 9 is different between the example shown in FIG. 3 and the example shown in FIG. 4.

The signal shown in (C) is a frequency domain response signal obtained by converting the time domain response signal shown in (B) into the frequency domain, for example by a Fourier transform. As shown in (C) of FIG. 3 and (C) of FIG. 4, the frequency domain response signal is different between the example shown in FIG. 3 and the example shown in FIG. 4, as with the time domain response signal, due to differences in the bit sequence within layer structure 9.

In this way, when the response signals in the time domain of the bit string [0001] and the bit string [0010] are compared, these time domain response signals show completely different time dependencies. Therefore, it is possible to distinguish different bit strings contained in the layer structure 9 based on the difference in the response signals in the time domain. Similarly, when the response signals in the frequency domain of the bit string [0001] and the bit string [0010] are compared, these frequency domain response signals also show completely different frequency dependencies. Therefore, it is possible to distinguish different bit strings contained in the layer structure 9 based on the difference in the response signals in the frequency domain. Note that, as shown by comparing (B) and (C) of FIG. 3 with (B) and (C) of FIG. 4, it is possible to distinguish bit strings more accurately by using the response signals in the frequency domain than by using the response signals in the time domain.

Next, we will explain how it is possible to distinguish between all combinations of bit strings represented by the four memory cells using the response signal from layer structure 9.

5 and 6 are diagrams showing the correspondence between all combinations of bit strings represented by four memory cells and the corresponding response signals. In both diagrams, the correspondence between the bit strings and the waveforms of the response signals corresponding to the bit strings is shown for each of the total of 2 ⁴ = 16 combinations of bit strings. In addition, in order to facilitate visual comparison and understanding of the invention, the correspondence between the bit strings represented by the memory cells and the response signals is shown as an example in the case where the response signals are frequency domain response signals.

Figure 5 shows eight correspondences between bit strings and response signals, and the waveforms of the eight response signals for these eight correspondences are different from one another. Therefore, if each waveform of the eight response signals shown in Figure 5 is used as a reference response signal and the correspondences between these reference response signals and bit strings are prepared in advance, it is possible to identify the bit string corresponding to the response signal read from the layer structure 9 based on the correspondences between the reference response signals and bit strings. Matching between the response signal read from the layer structure 9 and the reference response signal can be performed, for example, by calculating the similarity of the signals. As an index of the similarity of the signals, for example, the cross-correlation coefficient or the magnitude of the phase difference of the signals can be used.

As with the example shown in FIG. 5, the eight correspondence relationships shown in FIG. 6 have different waveforms for the eight response signals. Therefore, based on the previously prepared correspondence relationships between the reference response signals and the bit strings, it is possible to identify the bit strings that correspond to the response signals read from the layer structure 9.

In this way, correspondences between reference response signals and bit strings (16 in the illustrated example) are prepared in advance, and the signal similarity is calculated between the prepared reference response signals and the response signals read from the layer structure 9. Then, based on the calculated similarity, one reference response signal corresponding to the response signal read from the layer structure 9 is identified, and the bit string corresponding to the identified reference response signal is identified. This makes it possible to identify the bit string corresponding to the response signal from the layer structure 9.

On the other hand, when referring to the correspondence relationship shown in FIG. 5 and the correspondence relationship shown in FIG. 6, there are cases where the waveforms of the response signals in the frequency domain are similar or roughly the same, even though the bit strings are different. For example, when referring to the bit string [0001] shown in FIG. 5 and the bit string [1110] shown in FIG. 6, the waveforms of the response signals in the frequency domain are roughly the same, even though the bit strings are different. In such cases, it is not possible to uniquely identify the bit string corresponding to the read response signal by simply converting the time domain response signal read from the layer structure 9 into the frequency domain and comparing the converted frequency domain response signal with a reference response signal in the frequency domain that has been prepared in advance.

In such a case, as described below, by using not only the frequency domain response signal but also the time domain response signal to identify the bit string, it is possible to uniquely identify the bit string corresponding to the response signal read from the layer structure 9.

Figures 7 and 8 are diagrams for explaining a method for distinguishing and identifying bit strings represented by four memory cells when the response signals in the frequency domain are similar or roughly the same. (A) is a diagram showing a schematic representation of the layer structure of a magnetic memory element used in the simulation. (B) is a simulation result of the response signal in the time domain, and (C) is a simulation result of the response signal in the frequency domain. (D) is a simulation result of the resistance value of a memory cell (memory layer 1d of cell No. 4) that constitutes a magnetic tunnel junction.

The bit string represented by the four memory cells is [0001] in the example shown in FIG. 7, and [1110] in the example shown in FIG. 8. Although the bit strings represented by the four memory cells are different in the example shown in FIG. 7 and the example shown in FIG. 8, as shown in (C) of FIG. 7 and (C) of FIG. 8, the waveforms of the response signals in the frequency domain are similar or roughly the same. In such a case, simply using the response signal in the frequency domain to identify the bit string cannot uniquely identify the bit string corresponding to the response signal read from layer structure 9.

In such a case, in one embodiment of the data reading method, not only the response signal in the frequency domain but also the response signal in the time domain is used to identify the bit string. This makes it possible to uniquely identify the bit string corresponding to the response signal read from the layer structure 9.

As described above, the waveforms of the response signals in the frequency domain are similar or almost the same in the example shown in FIG. 7 and the example shown in FIG. 8, but there is a difference in the waveforms of the response signals in the time domain as shown in FIG. 7B and FIG. 8B. Specifically, a phase difference occurs in the response signals in the time domain between the example shown in FIG. 7 and the example shown in FIG. 8. More specifically, the phase of the response signals in the time domain is inverted between the example shown in FIG. 7 and the example shown in FIG. 8. The reason for the phase inversion is that the difference in the spin state in the memory cell (memory layer 1d of cell No. 4) located at the upper side in the figure in the layer structure 9 constituting the magnetic tunnel junction is reflected in the response signal in the time domain read from the layer structure 9. This can also be understood from the fact that the simulation result of the resistance value of the memory cell constituting the magnetic tunnel junction shown in FIG. 7D corresponds to the simulation result of the response signal in the time domain shown in FIG. 7B. As in FIG. 7, the simulation result shown in FIG. 8 corresponds to the simulation result shown in FIG. 7B.
[Data reading procedure]

Figure 9 is a flowchart illustrating the steps of a data reading method according to one embodiment of the present invention.

In this embodiment, the magnetic memory element from which data is read is a magnetic memory element 10 having the configuration described with reference to FIG. 1. The magnetic memory element 10 includes multiple memory layers 1 whose spin states are switchable, and boundary layers 2 disposed between the multiple memory layers 1 and constituting a domain wall.

In one embodiment of the data read method, the bit strings represented by the multiple memory cells are identified using a response signal in the frequency domain and a response signal in the time domain. The correspondence between the reference response signal and the bit strings described with reference to Figures 5 and 6 is prepared in advance before implementing the data read method according to this embodiment.

In this embodiment, as described above, the bit strings represented by the multiple memory cells are identified using the response signal in the frequency domain and the response signal in the time domain, so that the above-mentioned correspondence relationships prepared in advance for the reference response signal are prepared in total of two sets, a first set relating to the correspondence relationship between the response signal in the frequency domain and the bit string, and a second set relating to the correspondence relationship between the response signal in the time domain and the bit string. For example, when the magnetic memory element 10 stores a total of 4 bits of binary information, the number of memory cells required is 4 as shown in the figure, and in this case, the number of first sets prepared in advance is 2 ⁴ = 16, and the number of second sets prepared in advance is 2 ⁴ = 16.

In step S1, a pulse current is passed through the layer structure of the magnetic memory element from which data is to be read. When a pulse current is passed through the layer structure 9 of the magnetic memory element 10, magnetization dynamics based on the spin torque effect is induced in the layer structure 9.

In step S2, a response signal is read from the layer structure through which the pulse current has been passed. In this embodiment, a time domain response signal is read from the layer structure 9. The response signal reflects the difference in magnetization dynamics caused by the difference in the bit strings in the layer structure 9.

In step S3, the response signal read in S2 is converted from a time domain signal to a frequency domain signal. In this embodiment, the time domain response signal read from the layer structure 9 is converted to a frequency domain response signal by Fourier transform.

In step S4, a first index of signal similarity is calculated between the frequency domain response signal converted in S3 and each of a plurality of reference frequency domain response signals prepared in advance. In this embodiment, the first index is a cross-correlation coefficient. Since a method for performing cross-correlation analysis on two different signals is well known, further detailed explanation is omitted in this specification.

In step S5, a second index of signal similarity is calculated between the time-domain response signal read in S2 and each of a plurality of time-domain reference response signals prepared in advance. In this embodiment, the second index is a phase difference. There are various methods for calculating the phase difference between two different signals, and all of these methods are publicly known, so further detailed explanation is omitted in this specification.

In step S6, a bit string corresponding to the time domain response signal read in S2 is identified based on the first and second indices of signal similarity calculated in steps S4 and S5. First, as described with reference to Figures 3 to 8, one reference response signal that most certainly corresponds to the time domain response signal read in S2 is identified based on the indices of signal similarity. Next, a bit string corresponding to the identified reference response signal is identified based on the correspondence between the reference response signal and the bit string that has been prepared in advance. This uniquely identifies the bit string corresponding to the response signal read from the layer structure 9. The identified bit string is a bit string represented by multiple memory cells included in the layer structure 9, and is a combination of the spin states represented by these multiple memory cells.

As described above, according to the data reading method according to one embodiment of the present invention, data can be read from the memory layer (memory cell) without destroying the data in the memory layer.
[Data Readout Device]

Figure 10 is a diagram showing a schematic configuration of a data reading device according to one embodiment of the present invention.

The data reading device 20 according to one embodiment is a device that reads data represented by a spin state from a magnetic memory element 10, and includes a current source 24, a sensor 25, and a spin state determination unit 29. The spin state determination unit 29 can be implemented using, for example, various semiconductor integrated circuits and electric circuits.

The current source 24 passes a pulse current for reading data from the second electrode 7 to the first electrode 4 of the magnetic memory element 10. The sensor 25 measures the current value of the current flowing through the layer structure 9 of the magnetic memory element 10. The current source 24, the sensor 25, and the spin state determination unit 29 are connected to the memory controller 26. The memory controller 26 controls the operation of the current source 24, the sensor 25, and the spin state determination unit 29 to control the data read operation from the magnetic memory element 10 described with reference to Figures 2 to 9. The data read via the spin state determination unit 29 is transmitted and received through the data bus 27. The connection from the current source 24 to the first electrode 4 and the second electrode 7 of the magnetic memory element 10 is switched using, for example, switches 28a and 28b. The operation of the switches 28a and 28b is controlled, for example, by the memory controller 26. A plurality of magnetic memory elements 10 can be arranged in an array to form a memory array.

The current source 24 applies a pulse current for data reading to the layer structure 9 of the magnetic memory element 10 from which data is to be read, inducing magnetization dynamics based on the spin torque effect in the layer structure 9. The sensor 25 reads a response signal from the layer structure 9 through which the pulse current is applied.

The spin state determination unit 29 determines the bit strings represented by the multiple memory cells based on the response signal read by the sensor 25. In this embodiment, the spin state determination unit 29 (29a) includes a signal conversion unit 291, a first index calculation unit 292, a bandpass filter 293, a second index calculation unit 294, a combination determination unit 295, and a reference data storage unit 299. The first index calculation unit 292 and the second index calculation unit 294 may be integrated to form an index calculation unit. As will be described later with reference to FIG. 11, in other embodiments, the spin state determination unit 29 (29b) may be configured without the bandpass filter 293 and the second index calculation unit 294.

The signal conversion unit 291 converts the response signal read by the sensor 25 from a time domain signal to a frequency domain signal. In this embodiment, the time domain response signal read from the layer structure 9 is Fourier transformed. In this case, the signal conversion unit 291 can be called a Fourier transform unit 291.

The first index calculation unit 292 calculates a first index relating to the signal similarity between the frequency domain response signal converted by the signal conversion unit 291 and each of a plurality of reference response signals in the frequency domain that have been prepared in advance. In this embodiment, the first index is a cross-correlation coefficient. The calculated first index is input to the combination identification unit 295, which is a subsequent circuit.

In addition, in this embodiment, the response signal read by the sensor 25 is input to the second index calculation unit 294 via a bandpass filter 293.

The second index calculation unit 294 calculates a second index relating to the signal similarity between the response signal read by the sensor 25 and each of a plurality of reference response signals in a time domain prepared in advance. In this embodiment, the second index is a phase difference. The calculated second index is input to the combination identification unit 295, which is a subsequent circuit.

The combination identification unit 295 identifies a bit string corresponding to the time-domain response signal read in S2 based on the first index and second index related to the signal similarity calculated by the first index calculation unit 292 and the second index calculation unit 294.

The reference data storage unit 299 stores data regarding the correspondence between the reference response signal and the bit strings represented by multiple memory cells as data used as a reference when identifying the spin state. In this embodiment, the reference data storage unit 299 stores a first data set regarding the correspondence between the response signal in the frequency domain and the bit string, and a second data set regarding the correspondence between the response signal in the time domain and the bit string, which are prepared in advance. The reference data storage unit 299 can be implemented using a non-volatile storage circuit such as an EEPROM.

As described above, the data reading device according to one embodiment of the present invention can read data from a memory layer (memory cell) without destroying the data in the memory layer.
[Other forms]

Although the present invention has been described above using specific embodiments, the present invention is not limited to the above embodiments.

The number of memory cells in the illustrated magnetic memory element 10 is not limited to four, and the magnetic memory element 10 can have a greater number of memory cells.

The structure of the magnetic memory element 10 shown as an example is a three-dimensional structure in which various layers are stacked vertically, but the structure of the magnetic memory element can also be a structure in which regions corresponding to the various layers are arranged horizontally and mounted on a plane. The layer structure described in the claims does not only mean a three-dimensional structure in which various layers are stacked vertically, but also means a structure in which various regions are arranged horizontally in this way and mounted on a plane.

In the above embodiment, a Fourier transform is used to convert a time domain signal into a frequency domain signal, but the calculation method used to convert the signal is not limited to a Fourier transform. Instead of a Fourier transform, a time domain signal may be converted into a frequency domain signal by, for example, a fast Fourier transform or a discrete Fourier transform.

In the above embodiment, in order to facilitate visual comparison and understanding of the invention, the correspondence between the bit strings represented by the memory cells and the response signals is illustrated as an example in which the response signals are frequency domain response signals, but the illustrated correspondence between the bit strings and the response signals is not limited to the case in which the response signals are frequency domain response signals. Similarly to Figures 5 and 6, the correspondence between the bit strings represented by the memory cells and the response signals can also be shown in the case in which the response signals are time domain response signals.

In the above embodiment, in step S5, a phase difference is calculated as a second index of signal similarity between the time domain response signal and each of the multiple reference response signals in the time domain that have been prepared in advance, but the second index calculated in step S5 is not limited to a phase difference. The index calculated in step S5 may be an index of signal similarity that reflects the phase difference of the signals, and may be, for example, a cross-correlation coefficient. In this case, in step S4, a cross-correlation coefficient is calculated as a first index between the frequency domain response signal and each of the multiple reference response signals in the frequency domain that have been prepared in advance, and in step S5, a cross-correlation coefficient is calculated as a second index between the time domain response signal and each of the multiple reference response signals in the time domain that have been prepared in advance.

FIG. 11 is a diagram showing a schematic configuration of a data reading device according to another embodiment of the present invention. In the above embodiment, the bit strings represented by the multiple memory cells are identified using a frequency domain response signal and a time domain response signal, but the response signal used to identify the bit string does not need to use both the frequency domain response signal and the time domain response signal. In the above embodiment, as shown by comparing (B) and (C) of FIG. 3 with (B) and (C) of FIG. 4, it is possible to distinguish the bit strings more accurately by using the frequency domain response signal than by using the time domain response signal, but it is also possible to identify the bit strings represented by the multiple memory cells using, for example, only the time domain response signal. In this case, for example, a cross-correlation coefficient may be calculated as an index of the similarity of the signals between the time domain response signal and each of the multiple reference response signals in the time domain prepared in advance. In this case, the spin state identification unit 29 (29b) may be configured without the signal conversion unit 291 and the second index calculation unit 294, as exemplified in FIG. 11.

In the above embodiment, the magnetic memory element 10 illustrated in FIG. 1 is the target of data reading, but the magnetic memory element from which data is read is not limited to the magnetic memory element 10 illustrated in FIG. 1. The magnetic memory element from which data is read in the present invention may be any type of magnetic memory element that transmits information based on domain wall motion, and may be any magnetic memory element in which magnetization dynamics based on the spin torque effect is induced in the layer structure. Another example of a magnetic memory element from which data is read in the present invention is shown in FIG. 12.

Figure 12 is a cross-sectional view showing another example of the general configuration of a magnetic memory element from which data is read. The magnetic memory element shown in Figure 12 is the magnetic memory element shown in Patent Document 3. Unless otherwise specified, the configurations of the other examples of magnetic memory elements described below are similar to the configuration of the magnetic memory element 10 exemplified as the target of data read in the above embodiment, and therefore repeated explanations will be omitted.

As shown in FIG. 12, in another example of a magnetic memory element 10 exemplified as a target for data reading, the memory layer 1 is an antiferromagnetic layer. The boundary layer 2 has a thickness that allows a domain wall to be formed inside and operates as a domain wall layer. This antiferromagnetic layer 1 that functions as the memory layer 1 does not have magnetization as a whole, but has a magnetic order at the micro level, and one bit of binary information, "0" or "1", can be assigned to this state. In other words, the antiferromagnetic layer 1 (memory layer 1) can be described as a layer to which a spin state corresponding to one bit of binary information is assigned, when the spin state is switchable.

The antiferromagnetic layer 1 (memory layer 1) comprises a plurality of stacked ferromagnetic layers 11 and 12 and a non-magnetic layer (not shown) disposed between the plurality of ferromagnetic layers 11 and 12. The non-magnetic layer in the antiferromagnetic layer 1 may be of any configuration, and in another example, the non-magnetic layer may be omitted. In the antiferromagnetic layer 1, an interaction Jex that maintains the spin directions in opposite directions occurs between the stacked ferromagnetic layers 11 and 12. As a result, in one antiferromagnetic layer 1, the spin direction in one ferromagnetic layer 11 and the spin direction in the other ferromagnetic layer 12 are maintained in opposite directions. For example, in the antiferromagnetic layer 1, the binary information "0" can be assigned to a state in which the spin arrow in the ferromagnetic layer 11 is pointing upward and the spin arrow in the ferromagnetic layer 12 is pointing downward, and the binary information "1" can be assigned to a state in which the spin arrow in the ferromagnetic layer 11 is pointing downward and the spin arrow in the ferromagnetic layer 12 is pointing upward. The allocation of binary information in the antiferromagnetic layer 1 is not limited to the illustrated embodiment, and the binary information to be allocated may have the opposite logic to that illustrated.

Exemplarily, the antiferromagnetic layer 1 can be formed using a Co/Pt/Gd laminated structure in which cobalt, platinum, and gadolinium are laminated, or a (Co/Pt) ₆ /Ir/(Co/Pt) ₆ laminated structure in which iridium is laminated between two laminated layers of cobalt and platinum. The symbol "/" means a laminate of layer structures. These laminated structures can be formed, for example, by a sputtering method. In the Co/Pt/Gd laminated structure constituting the antiferromagnetic layer 1, the Co layer and the Gd layer correspond to the ferromagnetic layers 11 and 12, and the Pt layer corresponds to the nonmagnetic layer. Exemplarily, the thickness of the Co layer is about 1 nm, the thickness of the Pt layer is about 0.1 nm, and the thickness of the Gd layer is about 1 nm. In the (Co/Pt) ₆ /Ir/(Co/Pt) ₆ layer structure, the two (Co/Pt) layers correspond to the ferromagnetic layers 11 and 12, and the Ir layer corresponds to the nonmagnetic layer. Exemplarily, the thickness of the (Co/Pt) ₆ layer is about 2.4 nm, and the thickness of the Ir layer is about 0.5 nm.

In another example of the magnetic memory element 10 shown in FIG. 12, the boundary layer 2 is formed using an oxide film such as magnesium oxide (MgO). Illustratively, the thickness of the MgO layer used for the boundary layer 2 is about 1 nm. The boundary layer 2 can be formed, for example, by a sputtering method.

1 (1a to 1d) Memory layer (ferromagnetic layer or antiferromagnetic layer)
2 (2a to 2d) Boundary layer 3 First ferromagnetic layer 4 First electrode 5 Insulating film 6 Second ferromagnetic layer 7 Second electrode 9 Layer structure 10 Magnetic memory element 11 Ferromagnetic layer 12 Ferromagnetic layer 20 Data reading device 21 to 23 Terminal 24 Current source 25 Sensor 26 Memory controller 27 Data bus 28 (28a, 28b) Switch 29 (29a, 29b) Spin state determination unit 291 Signal conversion unit 292 First index calculation unit 293 Bandpass filter 294 Second index calculation unit 295 Combination determination unit 299 Reference data storage unit 41 Spin-orbit torque layer 42 (42a, 42b) Bottom electrode Aex Ferromagnetic interaction Id Domain wall driving current Iw Write current

Claims (7)

A method for reading data represented by a spin state from a magnetic memory element that transmits information based on domain wall motion, comprising the steps of:
A step of passing a pulse current through a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and boundary layers disposed between the plurality of memory layers and constituting a domain wall;
reading a response signal from the layer structure through which the pulse current has been passed, and identifying the spin state for each of the plurality of memory layers based on the response signal;
23. A method for reading data from a magnetic memory element, comprising: The method according to claim 1, wherein the step of identifying the spin state identifies the combination of spin states corresponding to the read response signal based on a correspondence between the combination of spin states and a plurality of reference response signals. The step of identifying the spin state includes:
calculating an index of signal similarity between the read response signal and each of the plurality of reference response signals;
Identifying the combination of the spin states corresponding to the read response signal based on the calculated index;
The method of claim 2 , comprising: The step of identifying the spin state includes:
The method further includes converting the read response signal from a time domain signal to a frequency domain signal;
The step of calculating the index comprises:
calculating a first measure of similarity between the response signal in the frequency domain and each of the plurality of reference response signals in the frequency domain;
calculating a second measure of similarity between the response signal in the time domain and each of the plurality of reference response signals in the time domain;
Including,
The method according to claim 3 , wherein the step of identifying a combination includes identifying a combination of the spin states corresponding to the read response signal based on the calculated first index and second index. The method of claim 4, wherein the first index and the second index are correlation coefficients. The method of claim 4, wherein the first index is a correlation coefficient and the second index is a phase difference. An apparatus for reading data represented by a spin state from a magnetic memory element that transmits information based on domain wall motion,
a current source that supplies a pulse current to a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and boundary layers that are disposed between the plurality of memory layers and form domain walls;
a sensor for reading a response signal from the layer structure to which the pulse current has been applied;
a spin state specifying unit that specifies the spin state for each of the plurality of memory layers based on the response signal;
1. An apparatus for reading data from a magnetic memory element, comprising:

Images