Data reading method and device

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

Technical Field

[0001] The present invention relates to a method and an apparatus for reading data from a magnetic memory element that transmits information based on magnetic wall motion.

Background Art

[0002] With the dramatic increase in the amount of information, there is a need for a memory device capable of recording information at high density. As such a memory device, flash memory is currently widely used. However, due to its operating principle, flash memory has drawbacks such as a limited number of writeable times due to deterioration of the oxide film and a slow write speed during repeated writing of information. For this reason, in recent years, various magnetic memories have been proposed as alternatives to existing flash memories.

[0003] 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 divided into magnetic domains and arranged in a stack or linearly. Bits are defined for each magnetic domain, and data is stored by the magnetization direction in the magnetic domain. By passing an electric current through a thin wire of ferromagnetic material (magnetic nanowire), the magnetic wall is moved. As a result, the magnetization in the magnetic domain moves in one direction all at once and data is transmitted.

[0004] In the type of information transmission method based on magnetic wall motion exemplified in Patent Document 1, problems such as a high drive current for magnetic wall movement and poor controllability of magnetic wall movement still exist. In contrast, the present inventor has proposed a magnetic memory element that performs information transmission of the same type based on magnetic wall motion as in Patent Document 1, such as the magnetic memory elements disclosed in Patent Documents 2 and 3.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] According to the layer structures of the magnetic memory elements disclosed in Patent Documents 2 and 3, compared with conventional magnetic memory elements, the drive current required for magnetic wall movement and the controllability of magnetic wall movement are improved. On the other hand, in Patent Documents 2 and 3, destructive readout is exemplified as an example of a method for reading data from the memory layer. In the method of destructive readout, when data is read from the memory layer, the data in the memory layer is destroyed. In a magnetic memory element of a type that performs information transmission based on magnetic wall movement, it is required to refine the method of reading data.

[0007] An object of the present invention is to read data from a memory layer without destroying the data in a magnetic memory element of a type that performs information transmission based on magnetic wall movement. [Means for Solving the Problems]

[0008] The inventor of the present invention has conducted intensive studies to solve the above problems, and as a result, has found that there is a correspondence relationship as shown in FIGS. 5 and 6 between the bit string represented by a plurality of memory cells and the waveform of the response signal corresponding to the bit string. And based on this correspondence relationship, it has been found that the bit string corresponding to the response signal read from the layer structure including a plurality of memory cells can be specified. At this time, even when the response signal is read from the layer structure, the bits of the memory cells included in the layer structure are not destroyed.

[0009] That is, the present invention for achieving the above object includes, for example, the following aspects. (Item 1) A method for reading data represented in a spin state from a magnetic memory element that transmits information based on magnetic wall movement, comprising: applying a pulsed current to a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and a boundary layer disposed between the plurality of memory layers to form a magnetic wall; reading a response signal from the layer structure to which the pulsed current has been applied, and identifying the spin state for each of the plurality of memory layers based on the response signal; A method for reading data from a magnetic memory element, comprising the steps of: (Item 2) The step of identifying the spin state identifies a 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, according to the method of Item 1. (Item 3) The step of identifying the spin state includes: calculating an index regarding the similarity of signals between the read response signal and each of the plurality of reference response signals; identifying a combination of spin states corresponding to the read response signal based on the calculated index, according to the method of Item 2. The method according to Item 2, comprising the steps of: (Item 4) The step of identifying the spin state includes: further including a step of converting the read response signal from a signal in the time domain to a signal in the frequency domain; The step of calculating the index includes: calculating a first index regarding the 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 index regarding the similarity between the response signal in the time domain and each of the plurality of reference response signals in the time domain; and including: The step of identifying the combination identifies a combination of spin states corresponding to the read response signal based on the calculated first index and second index, according to the method of Item 3. (Item 5) The method according to item 4, wherein the first index and the second index are correlation coefficients. (Item 6) The method according to item 4, wherein the first index is a correlation coefficient and the second index is a phase difference. (Item 7) An apparatus for reading data represented in a spin state from a magnetic memory element that transmits information based on magnetic domain wall motion, a current source that passes a pulsed current through a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and a boundary layer disposed between the plurality of memory layers to form a magnetic domain wall, a sensor that reads a response signal from the layer structure through which the pulsed current has passed, a spin state specifying unit that specifies the spin state for each of the plurality of memory layers based on the response signal, An apparatus for reading data from a magnetic memory element, comprising: (Item 8) The apparatus according to item 7, wherein the spin state specifying unit specifies a combination of the spin states corresponding to the read response signal based on a correspondence relationship between the combination of the spin states and a plurality of reference response signals. (Item 9) The spin state specifying unit includes an index calculation unit that calculates an index regarding the similarity of signals between the read response signal and each of the plurality of reference response signals, and a combination specifying unit that specifies a combination of the spin states corresponding to the read response signal based on the calculated index. The apparatus according to item 8, comprising: (Item 10) The spin state specifying unit further includes a signal conversion unit that converts the read response signal from a signal in the time domain to a signal in the frequency domain, and the index calculation unit includes a first index calculation unit that calculates a first index regarding the similarity between the response signal in the frequency domain and each of the plurality of reference response signals in the frequency domain, A second index calculation unit that calculates a second index related to the similarity between the response signal in the time domain and each of the plurality of reference response signals in the time domain; comprising; The apparatus according to item 9, wherein the combination specifying unit specifies the combination of the spin states corresponding to the read response signal based on the calculated first index and the second index.

Effect of the Invention

[0010] According to the present invention, in a magnetic memory element of a type that performs information transmission based on magnetic wall movement, data can be read from a storage layer without destroying the data.

Brief Description of the Drawings

[0011]

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DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description and drawings, the same reference numerals denote the same or similar components, and thus redundant descriptions regarding the same or similar components will be omitted.

[0013] In this specification, the term "layer" does not mean only a layer that is distinguished from other layers by the material used for formation and physical and chemical properties such as magnetism and conductivity, but also means a film or region formed on or inside the surface of a certain material such as a single metal or alloy by a method such as sputtering. The term "layer" described in the claims is interpreted to include "film" and "region".

[0014] In the present embodiment, the magnetic memory element to be a target of data reading is a type of magnetic memory element that performs information transmission based on magnetic wall motion. Hereinafter, as an example of such a type of magnetic memory element, an example of a method for non-destructively reading data from a storage layer will be described with reference to the magnetic memory element disclosed in Patent Document 2.

[0015] First, referring to FIG. 1, the configuration of the magnetic memory element to be the target of data reading will be described with reference to the magnetic memory element exemplified in Patent Document 2. Next, referring to FIGS. 2 to 9, the principle and method of non-destructively reading data from the memory layer will be described for this exemplified magnetic memory element. [Configuration of the Target Magnetic Memory Element]

[0016] FIG. 1 is a cross-sectional view schematically showing an example of the schematic configuration of the magnetic memory element to be the target of data reading.

[0017] In the present embodiment, the magnetic memory element 10 to be the target of data reading includes a plurality of memory layers 1 (1a to 1d), a plurality of boundary layers 2 (2a to 2d), a first ferromagnetic layer 3, a first electrode 4, an insulating film 5, a second ferromagnetic layer 6, and a second electrode 7. In the exemplified magnetic memory element 10, the first ferromagnetic layer 3, the layer structure 9 of the plurality of boundary layers 2 and the plurality of memory layers 1, the insulating film 5, and the second ferromagnetic layer 6 are stacked in order from the lower side in the figure between the first electrode 4 and the second electrode 7 to have a three-dimensional structure.

[0018] The plurality of memory layers 1 (1a to 1d) are layers in which the spin state can be switched. In the exemplified magnetic memory element 10, the memory layer 1 is a ferromagnetic layer. In the illustrated aspect, the spin state can have two states, for example, the spin arrow pointing upward or downward, and one memory layer 1 functions as a memory cell for storing 1-bit binary information. Exemplarily, the memory layer 1 can be formed using a single metal such as iron and cobalt, or an alloy of these metals, such as Fe 1-x Ni x ,Fe 1-x Co x ,Co 1-x Pt x ,CoFeB. Here, x is the composition ratio of the alloy and takes a value in the range of 0 < x < 1.

[0019] The boundary layer 2 is disposed between a plurality of memory layers 1 to form a magnetic wall. In the illustrated embodiment, the spin state of the boundary layer 2 can have three states, for example, the spin arrows are upward, downward, and horizontal. When a magnetic wall is formed in the boundary layer 2, the spin state of the boundary layer 2 is represented by a horizontal arrow. For convenience of explanation, the state where the spin arrow is horizontal is only the rightward direction. In the illustrated magnetic memory element 10, the boundary layer 2 is formed using a non-magnetic material. Exemplarily, as the non-magnetic material of the boundary layer 2, a single metal that is not a ferromagnetic material such as copper and platinum, or an alloy of cobalt and platinum with a controlled composition as described later can be used. In the illustrated magnetic memory element 10, even if the boundary layer 2 is a non-magnetic material, since its thickness is thin, due to the influence of the ferromagnetic layer (memory layer 1 or first ferromagnetic layer 3) adjacent to the boundary layer 2, it becomes a ferromagnetic material with a weak exchange stiffness constant due to the proximity effect.

[0020] In the layer structure 9, the explanation will be given focusing on one boundary layer 2 and a pair of memory layers 1 (1a, 1b) sandwiching this boundary layer 2. In the layer structure 9 of the illustrated magnetic memory element 10, the boundary layer 2 generates a ferromagnetic interaction (Magnetic Stiffness) Aex between a plurality of memory layers 1. More specifically, the boundary layer 2 has a thickness or composition that generates a ferromagnetic interaction Aex between a plurality of memory layers 1. The ferromagnetic interaction Aex is an interaction for aligning the spin directions. Since a ferromagnetic interaction Aex occurs between a plurality of memory layers 1, in the magnetic memory element 10, the drive current required for magnetic wall movement and the controllability of magnetic wall movement are improved. Note that the ferromagnetic interaction Aex also occurs between the memory layer 1a sandwiching the boundary layer 2 and the first ferromagnetic layer 3.

[0021] When using a single metal for the non-magnetic material of the boundary layer 2, the boundary layer 2 is formed using a single metal with a thickness that generates an exchange interaction Aex between the plurality of memory layers 1. For example, when forming the boundary layer 2 using copper, preferably, the thickness of the boundary layer 2 is within the range of the thickness corresponding to 1 to 3 copper atoms. More preferably, the thickness of the boundary layer 2 is within the range of the thickness corresponding to 1 to 2 copper atoms. For example, when forming the boundary layer 2 using platinum, preferably, the thickness of the boundary layer 2 is within the range of the thickness corresponding to 1 to 4 platinum atoms. More preferably, the thickness of the boundary layer 2 is within the range of the thickness corresponding to 1 to 3 platinum atoms.

[0022] When using an alloy for the non-magnetic material of the boundary layer 2, the boundary layer 2 is formed using an alloy with a composition that generates an exchange interaction Aex between the plurality of memory layers 1. By controlling the composition ratio of the alloy used to form the boundary layer 2, the magnitude of the exchange interaction Aex generated between the plurality of memory layers 1 can be controlled. Since the Curie temperature Tc is the transition temperature at which a ferromagnetic material changes to a paramagnetic material, by controlling the Curie temperature Tc of the alloy, it is possible to control whether the alloy exhibits ferromagnetic properties or paramagnetic (i.e., non-magnetic) properties. The Curie temperature Tc and the exchange 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, in Fig. 3 of 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 November 1958, Volume 248, Issue 1253, p.145-152, https: / / doi.org / 10.1098 / rspa.1958.0235, 1-x Cu x the composition dependence of the Curie temperature Tc in the alloy is described. Not limited to the Ni 1-x Cu x alloy exemplified in this document, Co1-x Pt x The same control is applicable to the alloy as well. Therefore, by controlling the composition ratio of the alloy used for forming the boundary layer 2, the Curie temperature Tc of the alloy can be controlled, and it is possible to control whether the alloy exhibits the properties of a ferromagnetic material or a paramagnetic material, and thereby control the magnitude of the ferromagnetic interaction Aex.

[0023] The first ferromagnetic layer 3 is a ferromagnetic layer whose spin state can be switched. The first ferromagnetic layer 3 is disposed with the boundary layer 2 interposed therebetween on the side of the memory layer 1a located on the lower side in the figure in the layer structure 9. The first ferromagnetic layer 3 functions as a layer for writing 1-bit 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. For the material of the first ferromagnetic layer 3, various materials used for the magnetization fixing layer in a magnetoresistive random access memory (MRAM) can be used.

[0024] The first ferromagnetic layer 3 has a coercive force higher than that of the memory layer 1. For example, when at least any one of the following three conditions is satisfied, the first ferromagnetic layer 3 has a coercive force higher than that of the memory layer 1. The first condition is that even when the first ferromagnetic layer 3 and the memory layer 1 are formed using the same material, the first ferromagnetic layer 3 is formed thicker than the memory layer 1. The second condition is that even when the thickness of the first ferromagnetic layer 3 and the thickness of the memory layer 1 are the same, the first ferromagnetic layer 3 is formed using a material having a higher magnetic anisotropy than the memory layer 1. The third condition is that even when the thickness of the first ferromagnetic layer 3 is thinner than the thickness of the memory layer 1, the first ferromagnetic layer 3 is formed using a material having a sufficiently higher magnetic anisotropy than the memory layer 1, and as a result, the coercive force of the first ferromagnetic layer 3 is higher than that of the memory layer 1.

[0025] 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.

[0026] By passing a drive current for switching the spin state of the first ferromagnetic layer 3 between the terminals 21 and 22 of the first electrode 4, a write current Iw indicated by a dashed-dotted line in the figure flows through the spin-orbit torque layer 41, and the spin state of the first ferromagnetic layer 3 is switched by the spin-orbit torque generated by the spin-orbit interaction. 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 in a pulse shape. 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 conductor metals such as gold and copper.

[0027] The insulating film 5 and the second ferromagnetic layer 6 are combined with the memory layer 1d located on the upper side in the figure in the layer structure 9 and function as a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction). The memory layer 1d functions as a free layer of the magnetic tunnel junction.

[0028] 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 on the upper side in the figure 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, it is assumed that the spin state of the second ferromagnetic layer 6 is fixed in a state where the arrow of the spin is 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 using an oxide film such as magnesium oxide (MgO). The second ferromagnetic layer 6 can be formed using, for example, CoFeB which is an alloy of cobalt, iron, and boron. Various materials used for the magnetization fixed layer in MRAM can be used for the material of the second ferromagnetic layer 6.

[0029] The second electrode 7 is disposed adjacent to the second ferromagnetic layer 6 on the side of the memory layer 1d located on the upper side in the figure. By flowing a drive current for moving the magnetic wall between the terminal 23 of the second electrode 7 and either one of the terminals 21 and 22 of the first electrode 4, a magnetic wall drive current Id indicated by a two-dot chain line in the figure flows between the second electrode 7 and the first electrode 4. Thereby, the magnetic wall can be moved in a plurality of boundary layers 2 (2a to 2d) located between the second electrode 7 and the first electrode 4, and the respective spin states in the plurality of memory layers 1 (1a to 1d) can be shifted in a racetrack manner and sequentially shifted. In the illustrated magnetic memory element 10, the magnetic wall drive current Id is pulsed. Exemplarily, the second electrode 7 can be formed using various conductive metal such as gold and copper. [Principle of data reading]

[0030] FIG. 2 is a diagram for explaining the principle of a data reading method according to an embodiment of the present invention.

[0031] In the following example, the magnetic memory element 10 includes four memory cells (Cell No. 1 to Cell No. 4) and stores a total of 4-bit binary information. For convenience of explanation, it is assumed that the state where the arrow of the spin is downward means the value "0", and the upward state means the value "1".

[0032] Also, a bit string means an arrangement of 1-bit binary information represented by each of a plurality of memory cells, and means a combination (or arrangement) of spin states (value "0" or value "1") represented by each of a plurality of storage layers 1 (1a to 1d) included in the layer structure 9 shown in FIG. 1.

[0033] Refer to FIG. 2. The bit string represented by the four memory cells is

[0010] in the examples shown in (A) and (B), and

[1010] in the example shown in (C). The bit strings represented by the four memory cells are different between those shown in (A) and (B) and those shown in (C).

[0034] A data reading method according to an embodiment is a method of non-destructively reading data from the magnetic memory element 10. In the data reading method according to an embodiment, as shown in FIG. 2(A), a pulse current is passed through the layer structure 9 of the magnetic memory element 10 in which the bit string represented by a plurality of memory cells is in a certain state (for example,

[0010] shown in the figure). 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 magnetic wall drive current Id described with reference to FIG. 1 and is strong enough not to move the magnetic walls formed in each of the plurality of boundary layers 2. Exemplarily, the waveform of the pulse current passed through the layer structure 9 is rectangular, and the length of the pulse is about 1 ns (nanosecond). When a pulse current is passed through the layer structure 9, as shown in FIG. 2(B), magnetization dynamics is induced in the layer structure 9 by the spin torque effect. When the magnetization dynamics is induced, the spins in some of the storage layers 1 and the boundary layers 2 included in the layer structure 9 perform, for example, precession motion.

[0035] On the other hand, as shown in (C) of FIG. 2, even when a pulse current is passed through the layer structure 9 of the magnetic memory element 10 in a state where the bit string represented by a plurality of memory cells is different from the state

[0010] shown in (A) of FIG. 2 (for example,

[1010] shown in the drawing), magnetization dynamics is induced in the layer structure 9 by the spin torque effect in the same manner as the example shown in (B) of FIG. 2. Here, attention is paid to the induced magnetization dynamics. Since the bit strings represented by the plurality of memory cells included in the layer structure 9 are different between those shown in (B) and those shown in (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.

[0036] That is, when a pulse current is passed through the layer structure 9 and the response signal from the layer structure 9 is read, the read response signal reflects the difference in magnetization dynamics due to the difference in the bit string within the layer structure 9. Thereby, based on the response signal from the layer structure 9, it is possible to identify the layer structure 9, that is, to identify the bit string represented by the plurality of memory cells included in the layer structure 9.

[0037] Next, several examples of the bit strings represented by four memory cells will be specifically illustrated to explain that it is possible to distinguish these different bit strings using the response signal from the layer structure 9.

[0038] FIGS. 3 and 4 are diagrams for explaining the results of simulations performed for the case where there are four memory cells. (A) is a diagram schematically showing 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.

[0039] As shown in (A) of FIG. 3 and (A) of FIG. 4, the bit strings represented by the four memory cells are different between 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.

[0040] (B) shows the response signal from the layer structure 9 when a pulsed current is passed through the layer structure 9. The response signal read from the layer structure 9 is a signal whose signal intensity changes with the passage of time, that is, a response signal in the time domain. The signal shown in (B) can be read using the 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 that constitute the magnetic tunnel junction are provided on the upper side in the figure of the layer structure 9. As shown in (B) of FIG. 3 and (B) of FIG. 4, due to the difference in the bit strings in the layer structure 9, the response signals in the time domain read from the layer structure 9 are different between the example shown in FIG. 3 and the example shown in FIG. 4.

[0041] (C) shows the response signal in the frequency domain obtained by converting the response signal in the time domain shown in (B) into the frequency domain by, for example, Fourier transform. As shown in (C) of FIG. 3 and (C) of FIG. 4, for the response signal in the frequency domain as well as for the response signal in the time domain, due to the difference in the bit strings in the layer structure 9, the response signals in the frequency domain are different between the example shown in FIG. 3 and the example shown in FIG. 4.

[0042] Thus, when comparing the response signals in the time domain between the bit string

[0001] and the bit string

[0010] , these response signals in the time domain show completely different time dependencies. Therefore, based on the difference in the response signals in the time domain, it is possible to distinguish different bit strings contained in the layer structure 9. Similarly, when comparing the response signals in the frequency domain between the bit string

[0001] and the bit string

[0010] , these response signals in the frequency domain also show completely different frequency dependencies. Therefore, based on the difference in the response signals in the frequency domain, it is also possible to distinguish different bit strings contained in the layer structure 9. Note that, as shown by the comparison between (B) and (C) of FIG. 3 and (B) and (C) of FIG. 4, it is possible to distinguish the bit strings with higher accuracy by using the response signal in the frequency domain rather than using the response signal in the time domain.

[0043] Next, it will be explained that for all combinations of bit strings represented by four memory cells, it is possible to distinguish these different bit strings using the response signals from the layer structure 9.

[0044] FIGS. 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 FIGS. 5 and 6, for each of a total of 2 4 = 16 combinations of bit strings in both figures, the correspondence between the bit string and the waveform of the response signal corresponding to that bit string is shown. In FIGS. 5 and 6, in order to enable visual comparison and facilitate understanding of the invention, the correspondence between the bit string represented by the memory cell and the response signal is illustrated for the case where the response signal is a response signal in the frequency domain.

[0045] FIG. 5 shows eight correspondences between the bit string and the response signal, and for these eight correspondences, the waveforms of the eight response signals are different from each other. Therefore, if the waveform of each of the eight response signals shown in FIG. 5 is used as a reference response signal and the correspondence between this reference response signal and the bit string is prepared in advance, based on the correspondence between such a reference response signal and the bit string, the bit string corresponding to the response signal read from the layer structure 9 can be specified. The 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 regarding the similarity of the signals, for example, the cross-correlation coefficient or the magnitude of the phase difference of the signals can be used.

[0046] Regarding the eight correspondences shown in FIG. 6, similar to the example shown in FIG. 5, the waveforms of the eight response signals are different from each other. Therefore, based on the correspondence between the reference response signal and the bit string prepared in advance, the bit string corresponding to the response signal read from the layer structure 9 can be specified.

[0047] In this way, a correspondence relationship between the reference response signal and the bit string (16 cases in the illustrated example) is prepared in advance, and the similarity of the signals is calculated between the prepared reference response signal and the response signal 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 specified, and the bit string corresponding to the specified reference response signal is specified. Thereby, it is possible to specify the bit string corresponding to the response signal from the layer structure 9.

[0048] On the other hand, referring to the correspondence relationship shown in FIG. 5 and the correspondence relationship shown in FIG. 6, there are cases where although the bit strings are different, the waveforms of the response signals in the frequency domain are similar or generally coincide. For example, referring to the bit string

[0001] shown in FIG. 5 and the bit string

[1110] shown in FIG. 6, although the bit strings are different, the waveforms of the response signals in the frequency domain are generally the same. In such a case, simply converting the response signal in the time domain read from the layer structure 9 into the frequency domain and comparing the converted response signal in the frequency domain with the reference response signal in the frequency domain prepared in advance cannot uniquely specify the bit string corresponding to the read response signal.

[0049] In such a case, as will be described below, by using not only the response signal in the frequency domain but also the response signal in the time domain for specifying the bit string, it is possible to uniquely specify the bit string corresponding to the response signal read from the layer structure 9.

[0050] FIGS. 7 and 8 are diagrams for explaining a method of distinguishing and specifying the bit strings represented by four memory cells when the response signals in the frequency domain are similar or generally coincide. (A) is a diagram schematically showing 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. (D) is the simulation result of the resistance value of the memory cell (storage layer 1d of cell No. 4) constituting the magnetic tunnel junction.

[0051] The bit sequences represented by the four memory cells are

[0001] in the example shown in FIG. 7 and

[1110] in the example shown in FIG. 8. Although the bit sequences represented by the four memory cells are different between 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 substantially the same. In such a case, using only the response signal in the frequency domain to specify the bit sequence cannot uniquely specify the bit sequence corresponding to the response signal read from the layer structure 9.

[0052] In such a case, in the data reading method according to one embodiment, not only the response signal in the frequency domain but also the response signal in the time domain is used to specify the bit sequence. Thereby, it is possible to uniquely specify the bit sequence corresponding to the response signal read from the layer structure 9.

[0053] As described above, although the waveforms of the response signals in the frequency domain are similar or substantially the same between the example shown in FIG. 7 and the example shown in FIG. 8, as shown in (B) of FIG. 7 and (B) of FIG. 8, there are differences in the waveforms of the response signals in the time domain. Specifically, a phase difference occurs in the response signal in the time domain between the example shown in FIG. 7 and the example shown in FIG. 8. More specifically, the phase is inverted in the response signal in the time domain 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 states in the memory cell (the storage layer 1d of cell No. 4) located on the upper side in the figure in the layer structure 9 that constitutes 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 (D) of FIG. 7 corresponds to the simulation result of the response signal in the time domain shown in (B) of FIG. 7. Similarly for FIG. 8 as for FIG. 7, the simulation result shown in (D) corresponds to the simulation result shown in (B). [Procedure for Data Reading]

[0054] FIG. 9 is a flowchart for explaining the procedure of the data reading method according to one embodiment of the present invention.

[0055] In this embodiment, the magnetic memory element targeted for data reading is the magnetic memory element 10 having the configuration described with reference to FIG. 1. The magnetic memory element 10 includes a plurality of storage layers 1 whose spin states are switchable, and a boundary layer 2 disposed between the plurality of storage layers 1 and constituting a magnetic wall.

[0056] In a data reading method according to an embodiment, a bit string represented by a plurality of memory cells is specified 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 string described with reference to FIGS. 5 and 6 shall be prepared in advance before implementing the data reading method according to this embodiment.

[0057] Note that in this embodiment, as described above, since a bit string represented by a plurality of memory cells is specified using a response signal in the frequency domain and a response signal in the time domain, the above-described correspondence prepared in advance regarding the reference response signal also includes a total of two sets: a first set regarding the correspondence between the response signal in the frequency domain and the bit string, and a second set regarding the correspondence between the response signal in the time domain and the bit string. For example, when the magnetic memory element 10 stores binary information of a total of 4 bits, the number of memory cells required is 4 as shown in the figure. In this case, the number of the first set prepared in advance is 2 4 = 16, and the number of the second set prepared in advance is 2 4 = 16.

[0058] In step S1, a pulse current is passed through the layer structure of the magnetic memory element targeted for data reading. 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 are induced in the layer structure 9.

[0059] In step S2, a response signal from the layer structure through which the pulse current has been passed is read. In this embodiment, a response signal in the time domain is read from the layer structure 9. The response signal reflects the difference in magnetization dynamics due to the difference in the bit string within the layer structure 9.

[0060] In step S3, the response signal read in S2 is converted from a time-domain signal to a frequency-domain signal. In the present embodiment, the response signal in the time domain read from the layer structure 9 is converted into a response signal in the frequency domain by performing a Fourier transform.

[0061] In step S4, a first index regarding the similarity of signals is calculated between the response signal in the frequency domain converted in S3 and each of a plurality of reference response signals in the frequency domain prepared in advance. In the present embodiment, the first index is a cross-correlation coefficient. Since the method of performing cross-correlation analysis on two different signals is known, further detailed description in this specification is omitted.

[0062] In step S5, a second index regarding the similarity of signals is calculated between the response signal in the time domain read in S2 and each of a plurality of reference response signals in the time domain prepared in advance. In the present embodiment, the second index is a phase difference. Since there are various methods for calculating the phase difference between two different signals and all of them are known, further detailed description in this specification is omitted.

[0063] In step S6, based on the first index and the second index regarding the similarity of signals calculated in steps S4 and S5, the bit string corresponding to the response signal in the time domain read in S2 is specified. First, as described with reference to FIGS. 3 to 8, based on the index regarding the similarity of signals, one reference response signal that most surely corresponds to the response signal in the time domain read in S2 is specified. Next, based on the correspondence relationship between the reference response signal prepared in advance and the bit string, the bit string corresponding to the specified reference response signal is specified. Thereby, the bit string corresponding to the response signal read from the layer structure 9 is uniquely specified. The specified bit string is the bit string represented by a plurality of memory cells included in the layer structure 9, and is a combination of spin states represented by these plurality of memory cells.

[0064] According to the data reading method according to an embodiment of the present invention described above, data can be read from the memory layer (memory cell) without destroying the data in the memory layer. [Data Reading Device]

[0065] FIG. 10 is a diagram schematically showing a schematic configuration of a data reading device according to an embodiment of the present invention.

[0066] A data reading device 20 according to an 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 specifying unit 29. The spin state specifying unit 29 can be implemented using, for example, various semiconductor integrated circuits and electric circuits.

[0067] The current source 24 passes a pulse current for data reading 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 specifying unit 29 are connected to a memory controller 26. The memory controller 26 controls the operations of the current source 24, the sensor 25, and the spin state specifying unit 29 to control the data reading operation from the magnetic memory element 10 described with reference to FIGS. 2 to 9. The data read through the spin state specifying unit 29 is transmitted and received through a data bus 27. The connections from the current source 24 to the first electrode 4 and the second electrode 7 of the magnetic memory element 10 are switched using, for example, switches 28a and 28b. The operations of the switches 28a and 28b are controlled by, for example, the memory controller 26. A plurality of magnetic memory elements 10 can be arranged in an array to form a memory array.

[0068] The current source 24 passes a pulse current for data reading through the layer structure 9 of the magnetic memory element 10 to induce 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 has flowed.

[0069] Based on the response signal read by the sensor 25, the spin state identification unit 29 identifies the bit string represented by a plurality of memory cells. In the present embodiment, the spin state identification unit 29 (29a) includes a signal conversion unit 291, a first index calculation unit 292, a band-pass filter 293, a second index calculation unit 294, a combination identification unit 295, and a reference data storage unit 299. Note that 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 identification unit 29 (29b) can be configured not to include the band-pass filter 293 and the second index calculation unit 294.

[0070] 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 the present embodiment, the response signal in the time domain read from the layer structure 9 is subjected to Fourier transform. In this case, the signal conversion unit 291 can be referred to as a Fourier transform unit 291.

[0071] The first index calculation unit 292 calculates a first index regarding the similarity of signals between the response signal in the frequency domain converted by the signal conversion unit 291 and each of a plurality of reference response signals in the frequency domain prepared in advance. In the present 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-stage circuit.

[0072] Also, in the present embodiment, the response signal read by the sensor 25 is input to the second index calculation unit 294 via the band-pass filter 293.

[0073] The second index calculation unit 294 calculates a second index regarding the similarity of signals between the response signal read by the sensor 25 and each of a plurality of reference response signals in the time domain prepared in advance. In the present embodiment, the second index is a phase difference. The calculated second index is input to the combination identification unit 295, which is a subsequent-stage circuit.

[0074] The combination specifying unit 295 specifies a bit string corresponding to the response signal in the time domain read in S2 based on the first index and the second index regarding the similarity of the signal, which are calculated by the first index calculating unit 292 and the second index calculating unit 294.

[0075] The reference data holding unit 299 holds data regarding the correspondence relationship between the reference response signal and the bit strings represented by a plurality of memory cells as data used as a reference when specifying the spin state. In the present embodiment, the reference data holding unit 299 holds a first data set regarding the correspondence relationship between the response signal in the frequency domain and the bit string, which is prepared in advance, and a second data set regarding the correspondence relationship between the response signal in the time domain and the bit string. The reference data holding unit 299 can be implemented using a non-volatile memory circuit such as an EEPROM.

[0076] As described above, according to the data reading device according to an embodiment of the present invention, data can be read from the storage layer (memory cell) without destroying the data in the storage layer. [Other Forms]

[0077] Although the present invention has been described with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0078] The number of memory cells included in the illustrated magnetic memory element 10 is not limited to four, and the magnetic memory element 10 can include a larger number of memory cells.

[0079] The structure of the illustrated magnetic memory element 10 is a three-dimensional structure in which various layers are stacked in the vertical direction. However, the structure of the magnetic memory element can also be a structure in which regions corresponding to various layers are arranged horizontally and mounted on a plane. The layer structure described in the claims does not mean only a three-dimensional structure in which various layers are stacked in the vertical direction, but also means a structure in which various regions are arranged horizontally in this way and mounted on a plane.

[0080] In the above-described embodiment, a Fourier transform is performed to convert a signal in the time domain into a signal in the frequency domain. However, the calculation method used for signal conversion is not limited to the Fourier transform. For example, instead of the Fourier transform, a fast Fourier transform or a discrete Fourier transform may be used to convert the signal in the time domain into a signal in the frequency domain.

[0081] In the above-described embodiment, in order to enable visual comparison and facilitate understanding of the invention, the correspondence relationship between the bit string represented by the memory cell and the response signal is illustrated for the case where the response signal is a response signal in the frequency domain. However, the illustrated correspondence relationship between the bit string and the response signal is not limited to the case where the response signal is a response signal in the frequency domain. For the case where the response signal is a response signal in the time domain as well, a correspondence relationship between the bit string represented by the memory cell and the response signal can be shown in the same manner as in FIGS. 5 and 6.

[0082] In the above-described embodiment, in step S5, a phase difference is calculated as a second index regarding the similarity of signals between the response signal in the time domain and each of a plurality of reference response signals in the time domain prepared in advance. However, the second index calculated in step S5 is not limited to the phase difference. The index calculated in step S5 may be any index regarding the similarity of signals and reflecting the phase difference of the signals. For example, a cross-correlation coefficient may be calculated. In this case, in step S4, a cross-correlation coefficient is calculated as the first index between the response signal in the frequency domain and each of a plurality of reference response signals in the frequency domain prepared in advance, and in step S5, a cross-correlation coefficient is calculated as the second index between the response signal in the time domain and each of a plurality of reference response signals in the time domain prepared in advance.

[0083] FIG. 11 is a diagram schematically showing a schematic configuration of a data reading device according to another embodiment of the present invention. In the above-described embodiment, a bit string represented by a plurality of memory cells is specified using a response signal in the frequency domain and a response signal in the time domain. However, it is not necessary to use both the response signal in the frequency domain and the response signal in the time domain for specifying the bit string. In the above-described embodiment, as shown by comparing (B) and (C) of FIG. 3 with (B) and (C) of FIG. 4, although it is possible to distinguish the bit string more accurately by using the response signal in the frequency domain than by using the response signal in the time domain, for example, it is also possible to specify the bit string represented by a plurality of memory cells by using only the response signal in the time domain. In this case, for example, a cross-correlation coefficient may be calculated as an index regarding the similarity of signals between the response signal in the time domain and each of a plurality of reference response signals in the time domain prepared in advance. In this case, as illustrated in FIG. 11, the spin state specifying unit 29 (29b) can be configured not to include the signal conversion unit 291 and the second index calculation unit 294.

[0084] In the above-described embodiment, the magnetic memory element 10 illustrated in FIG. 1 is targeted for data reading. However, the magnetic memory element targeted for data reading is not limited to the magnetic memory element 10 illustrated in FIG. 1. In the present invention, the magnetic memory element targeted for data reading may be any magnetic memory element that performs information transmission based on magnetic wall motion, or any magnetic memory element in which magnetization dynamics based on the spin torque effect is induced in a layer structure. Another example of the magnetic memory element targeted for data reading in the present invention is shown in FIG. 12.

[0085] FIG. 12 is a cross-sectional view schematically showing another example of the schematic configuration of the magnetic memory element targeted for data reading. The magnetic memory element illustrated in FIG. 12 is the magnetic memory element illustrated in Patent Document 3. The configuration of another example of the magnetic memory element described below is the same as the configuration of the magnetic memory element 10 illustrated as the target for data reading in the above-described embodiment unless otherwise specified, and thus redundant explanations are omitted.

[0086] As shown in FIG. 12, in another example of the magnetic memory element 10 exemplified as the target of data reading, the memory layer 1 is an antiferromagnetic layer. The thickness of the boundary layer 2 is such that a magnetic wall can be formed therein and it can operate as a magnetic wall layer. Although this antiferromagnetic layer 1 functioning as the memory layer 1 does not have magnetization as a whole, it has magnetic order microscopically, and 1-bit binary information of "0" or "1" can be assigned to this state. That is, the fact that the spin state can be switched in the antiferromagnetic layer 1 (memory layer 1) can be expressed as a layer to which a spin state corresponding to 1-bit binary information is assigned.

[0087] The antiferromagnetic layer 1 (memory layer 1) includes 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. In the antiferromagnetic layer 1, the non-magnetic layer can have an arbitrary configuration, and in another example, the non-magnetic layer can be omitted. In the antiferromagnetic layer 1, an interaction Jex that maintains the spin directions in opposite directions to each other occurs between the plurality of stacked ferromagnetic layers 11 and 12. As a result, within 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 to each other. For example, in the antiferromagnetic layer 1, binary information "0" can be assigned to a state where the spin arrow in the ferromagnetic layer 11 is upward and the spin arrow in the ferromagnetic layer 12 is downward, and binary information "1" can be assigned to a state where the spin arrow in the ferromagnetic layer 11 is downward and the spin arrow in the ferromagnetic layer 12 is upward. Note that the assignment of binary information in the antiferromagnetic layer 1 is not limited to this exemplified mode, and the logic opposite to the exemplified mode of the binary information to be assigned may also be used.

[0088] Exemplarily, the antiferromagnetic layer 1 has a stacked structure of Co / Pt / Gd in which cobalt, platinum, and gadolinium are stacked, or (Co / Pt) 6 / Ir / (Co / Pt) 6It can be formed using the laminated structure. The symbol " / " means the lamination of the layer structure. These laminated structures can be formed, for example, by sputtering. 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. (Co / Pt) 6 / Ir / (Co / Pt) 6 In the layer structure of, two (Co / Pt) layers correspond to the ferromagnetic layers 11 and 12, and the Ir layer corresponds to the nonmagnetic layer. Exemplarily, (Co / Pt) 6 The thickness of the layer is about 2.4 nm, and the thickness of the Ir layer is about 0.5 nm.

[0089] 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). Exemplarily, 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 sputtering.

Explanation of symbols

[0090] 1(1a~1d) Memory layer (ferromagnetic layer or antiferromagnetic layer) 2(2a~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 21~23 Terminals 24 Current source 25 Sensor 26 Memory controller 27 Data bus 28(28a,28b) Switch 29(29a,29b) Spin state specifying section 291 Signal conversion unit 292 First index calculation unit 293 Band-pass filter 294 Second index calculation unit 295 Combination identification unit 299 Reference data holding unit 41 Spin-orbit torque layer 42(42a, 42b) Bottom electrode Aex Exchange interaction Id Magnetic wall drive current Iw Write current

Claims

1. A method for reading data represented in a spin state from a magnetic memory element that transmits information based on magnetic wall movement, comprising: applying a pulsed current to a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and a boundary layer disposed between the plurality of memory layers to form a magnetic wall; reading a response signal from the layer structure to which the pulsed current has been applied, and identifying the spin state for each of the plurality of memory layers based on the response signal; A method for reading data from a magnetic memory element, including the above steps.

2. The method according to claim 1, wherein the step of identifying the spin state identifies a 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.

3. The step of identifying the spin state includes: calculating an index regarding the similarity of signals between the read response signal and each of the plurality of reference response signals; identifying a combination of spin states corresponding to the read response signal based on the calculated index. The method according to claim 2, including the above steps.

4. The step of identifying the spin state further includes: converting the read response signal from a signal in the time domain to a signal in the frequency domain; The step of calculating the index includes: calculating a first index regarding the 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 index regarding the similarity between the response signal in the time domain and each of the plurality of reference response signals in the time domain; including; The step of identifying the combination identifies a combination of spin states corresponding to the read response signal based on the calculated first index and second index. The method according to claim 3.

5. The method according to claim 4, wherein the first index and the second index are correlation coefficients.

6. The method according to claim 4, wherein the first index is a correlation coefficient and the second index is a phase difference.

7. An apparatus for reading data represented in a spin state from a magnetic memory element that transmits information based on magnetic wall movement, comprising: a current source that applies a pulsed current to a layer structure of a magnetic memory element including a plurality of memory layers whose spin states are switchable and a boundary layer disposed between the plurality of memory layers to form a magnetic wall; A sensor that reads a response signal from the layer structure through which the pulsed current has flowed; A spin state specifying unit that specifies the spin state for each of the plurality of memory layers based on the response signal; An apparatus for reading data from a magnetic memory element, comprising the above.

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