JP2015095632A - Magnetic memory - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact and high-speed magnetic memory which allows write and read by current.SOLUTION: A ferromagnetic structure 10 provided with magnetic domain wall-holding parts 14, 15 at two places is used, a current source 20 is connected to both end parts of the ferromagnetic structure 10, and a voltmeter 30 is connected across one end part and a central part 12 of the ferromagnetic structure 10. Information writing is performed by holding a magnetic domain wall in either of the two magnetic domain wall-holding parts according to a direction of a current flowed from the current source 20. Information reading is performed by flowing unidirectional reading current from the current source and detecting the magnetic domain wall-holding part where the magnetic domain wall is held by output of the voltmeter. A current path during information writing is also used as a current path during the information reading, only one switching element is used for current control, thereby the magnetic memory is made compact. Also, by reading information non-destructively, conduction for returning the magnetic domain wall to its original position after reading is unnecessary.

Description

  The present invention relates to a nonvolatile magnetic memory capable of writing and reading by current.

  Electrons have a spin responsible for magnetism in addition to a charge responsible for electrical conduction. In recent years, attention has been focused on the development of spintronic devices that actively utilize the properties of spin, in addition to electronics that utilize the charge of electrons. For example, a spin injection element has been proposed as a tunnel magnetoresistive element using spin injection. Further, it is known that when a domain wall inserted in a ferromagnetic material is moved, an electromotive force is generated in the domain wall portion (Non-patent Document 1).

  Patent Document 1 describes a magnetic memory in which a ferromagnetic structure is used as a memory cell and information is written and read by moving a domain wall by an electric current. This magnetic memory includes first and second electrodes connected to both ends of a ferromagnetic structure having two domain wall holding portions formed therein, and a central electrode connected to the central portion. In recording information, a current is passed from the first and second electrodes at both ends to the ferromagnetic structure, and the domain wall is held in one of the two domain wall holders depending on the direction of the current. Which of the two domain wall holders holds the domain wall is made to correspond to information “1” and “0”. In reading information, a current is passed between the first electrode and the center electrode, and the domain wall holding portion where the domain wall is held is detected by the presence or absence of the current flowing between the second electrode and the center electrode. Do.

Japanese Patent No. 4968289

Physical Review B 82, 054410 (2010)

  The magnetic memory described in Patent Document 1 passes a current between electrodes connected to both ends of a ferromagnetic structure during information storage. On the other hand, when reading information, a current is passed between the electrode connected to one end of the ferromagnetic structure and the electrode connected to the center. That is, since current paths are different between information storage and information reading, two switching elements (transistors) for controlling the current flowing through the ferromagnetic structure are required, and the size of the magnetic memory increases. Also, in one domain wall holder, the domain wall moves to the other domain wall holder by the reading operation, and the stored information is destroyed. Therefore, energization is required to return the domain wall position to the original domain wall holder after reading. become.

  An object of the present invention is to provide a magnetic memory that can be written and read by current and has a small size. It is another object of the present invention to provide a magnetic memory in which information is not destroyed by a read operation.

  In the present invention, in a magnetic memory using a ferromagnetic structure having two domain wall holders, one switching element (transistor) for current control is made by using the same current path for writing information and reading information. As a result, the size of the magnetic memory was reduced.

  In addition, non-destructive reading of information eliminates the need for energization to restore the domain wall position after reading.

  In order to reduce the size of the magnetic memory that holds information in the domain wall holder, it is preferable to use a ferromagnetic structure having a small domain wall width. The material selection for that was performed.

  A magnetic memory according to the present invention includes a ferromagnetic structure including a first domain wall holding unit and a second domain wall holding unit that can stably hold a domain wall, a first electrode connected to both ends of the ferromagnetic structure, and A second electrode; a third electrode connected to a position between the first domain wall holder and the second domain wall holder of the ferromagnetic structure; and the first electrode and the second electrode. And a voltmeter connected between the first electrode and the third electrode, the current flowing from the current source to the ferromagnetic structure through the first electrode and the second electrode Information is recorded by holding the domain wall of the ferromagnetic structure in either the first domain wall holding unit or the second domain wall holding unit according to the orientation of the first and second electrodes from the current source. A domain wall in which a reading current in a certain direction is passed through the ferromagnetic structure through the electrode and the domain wall is held by the output of the voltmeter Performing information read by identifying the lifting unit.

  The domain wall held by one of the domain wall holders does not move to the other domain wall holder by the read current. In other words, the magnitude of the read current is such that information stored by the read current is not destroyed.

  The ferromagnetic structure may have a shape in which a cross-sectional area in a direction perpendicular to a direction in which a current flows becomes a minimum at the first domain wall holder and the second domain wall holder.

Further, the ferromagnetic structure may be formed by laminating magnetization fixed films having magnetizations in opposite directions at both ends sandwiching the first domain wall holding unit and the second domain wall holding unit. This ensures that one domain wall is inserted into the ferromagnetic structure.
As the material of the ferromagnetic structure, Co, Co / Ni multilayer film, and FePt are preferable.

  The magnetic random access memory according to the present invention includes a ferromagnetic structure including a first domain wall holding unit and a second domain wall holding unit that can stably hold a domain wall, and a first structure connected to both ends of the ferromagnetic structure. A first electrode, a second electrode, a third electrode connected to a position between the first domain wall holder and the second domain wall holder of the ferromagnetic structure, and a current flowing through the ferromagnetic structure. A memory cell group in which magnetic memory cells each having a switching element for on / off control are arranged in a two-dimensional array, a selection means for selecting a desired magnetic memory cell in the memory cell group, and a selection by the selection means The magnetic domain wall of the ferromagnetic structure is held by the first domain wall holding portion and the second domain wall holding according to the direction of the current flowing through the ferromagnetic structure through the first electrode and the second electrode of the magnetic memory cell formed. By holding any of the parts The domain wall held in one domain wall holder is fixed to the ferromagnetic structure through the first electrode and the second electrode of the magnetic memory cell selected by the selection means. To read information by supplying a read current that does not move to the magnetic domain wall holding part and specifying the magnetic domain wall holding part that holds the magnetic domain wall by the voltage generated between the first electrode and the third electrode It is.

According to the present invention, it is possible to provide a nonvolatile memory capable of high integration and operating at high speed.
Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

The schematic diagram which shows the structural example of the magnetic memory of this invention. The figure which shows the relationship between the position of the domain wall which exists in a ferromagnetic structure, and domain wall energy. The schematic diagram explaining the write-in operation | movement of a magnetic memory. The schematic diagram explaining the write-in operation | movement of a magnetic memory. The schematic diagram explaining the read-out operation | movement of a magnetic memory. The schematic diagram which shows the example of the waveform of read-out current, and the output waveform of a voltmeter. The schematic diagram explaining the read-out operation | movement of a magnetic memory. The schematic diagram which shows the example of the waveform of read-out current, and the output waveform of a voltmeter. Schematic which shows the structural example of the magnetic memory cell of this invention. The figure which shows the structural example of a magnetic random access memory.

  Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram showing a configuration example of a magnetic memory according to the present invention.
The magnetic memory of the present embodiment includes a ferromagnetic structure 10, a current source 20 for supplying a current for writing and reading information to and from the magnetic memory, and a voltmeter 30 used for reading information. The ferromagnetic structure 10 has a shape such as a plate shape, a rod shape, or a column shape, and includes three portions: a first end portion 11, a central portion 12, and a second end portion 13. A constricted region having a reduced cross-sectional area is provided between the first end portion 11 and the central portion 12, and this constricted region functions as the first domain wall holding portion 14 that can stably retain the domain wall. Further, a constricted region having a reduced cross-sectional area is also provided between the second end portion 13 and the central portion 12, and this constricted region functions as a second domain wall holding portion 15 that can stably maintain the domain wall. The current source 20 is connected to the first end 11 and the second end 13 of the ferromagnetic structure 10, and allows a write current or a read current to flow through the ferromagnetic structure 10. The voltmeter 30 is connected to the first end portion 11 and the central portion 12 of the ferromagnetic structure 10, and is based on the output of the voltmeter 30 when a read current is applied from the current source 20 to the ferromagnetic structure 10. To read the information. The voltmeter 30 may be connected between the second end portion 13 and the central portion 12 of the ferromagnetic structure 10.

  The material of the ferromagnetic structure 10 is preferably a material having a narrow domain wall width and easy movement characteristics. For example, a Co, Co / Ni multilayer film, FePt, or the like can be used. In this example, a Co / Ni multilayer film was used as the ferromagnetic structure 10. In this embodiment, the ferromagnetic structure is shaped like a plate, the size is 20 nm in width, 120 nm in length, and 2 nm in thickness. The two constrictions are located at 40 nm inside from both ends of the plate. The width was 4 nm.

FIG. 2 is a diagram illustrating the relationship between the position of the domain wall existing in the ferromagnetic structure and the domain wall energy. Since the energy stored in the domain wall is proportional to the cross-sectional area of the domain wall position of the ferromagnetic structure, the first domain wall holding unit 14 and the second domain wall holding unit 15 take a minimum value, and the first domain wall holding unit 14 A maximum value is taken at the central portion 12 between the first and second domain wall holders 15. That is, the domain wall becomes energetically stable in the first domain wall holder 14 or the second domain wall holder 15 and is held by one of the two domain wall holders. Further, an energy barrier _Eb exists between the two domain wall holders 14 and 15, and when the domain wall held by one domain wall holder is supplied with energy exceeding the energy barrier _Eb , the domain wall is moved to the other. It can be moved to the domain wall holding part. In the case of the ferromagnetic structure of the present embodiment, the energy barrier E _b has the magnetic anisotropy constant K _u = 4 × 10 ⁵ J / m ³ , the exchange stiffness A _ex = 10 ⁻¹¹ J / m, and the cross-sectional area S _cross. Since = 10 nm ² , it is estimated that E _b = (K _u A _ex ) ^1/2 S _cross = 7.2 × 10 ⁻²⁰ J.

3 and 4 are schematic diagrams for explaining the write operation of the magnetic memory of this embodiment. FIG. 3 is an explanatory diagram of a write operation for moving the domain wall 16 held by the first domain wall holder 14 to the second domain wall holder 15. The domain wall can be moved by passing a current through the ferromagnetic structure 10. When the domain wall 16 held by the first domain wall holding unit 14 is moved to the second domain wall holding unit 15, as shown in FIG. A write current I _W is caused to flow from the end 13 toward the first end 11. When the write current I _W is a pulse current, in order to exceed the energy barrier E _b (7.2 × 10 ⁻²⁰ J), for example, a current having a magnitude of 100 μA may be given with a pulse width of 1 ns.

FIG. 4 is an explanatory diagram of a write operation for moving the domain wall 16 held by the second domain wall holding unit 15 to the first domain wall holding unit 14. In this case, the write current I _W is supplied from the current source 20 to the ferromagnetic structure 10 in the direction from the first end 11 to the second end 13. For the write current, for example, a current of −100 μA may be applied with a pulse width of 1 ns.

  For example, if the state in which the domain wall exists in the first domain wall holder 14 is defined as “1” and the state in which the domain wall exists in the second domain wall holder 15 is defined as “0”, a binary value is obtained depending on the location where the domain wall exists. Digital magnetic recording can be performed. After the domain wall 16 is held by the first domain wall holder 14 or the second domain wall holder 15, the domain wall 16 is permanently held by the domain wall holder even if the write current is interrupted. Therefore, the magnetic memory of this embodiment is a non-volatile memory.

  In this embodiment, the magnetization fixed layers 41 and 42 whose magnetizations are opposite to each other are laminated on the first end portion 11 and the second end portion 13 of the ferromagnetic structure 10. By laminating the magnetization fixed layers 41 and 42, the magnetization of the first end portion 11 and the magnetization of the second end portion 13 which are ferromagnetically coupled thereto are fixed in opposite directions. With this structure, one domain wall is always inserted between the first end portion 11 and the second end portion 13 of the ferromagnetic structure 10, in other words, the first domain wall holding portion 14 in a stable state. Alternatively, it is guaranteed that the domain wall is always held in any of the second domain wall holding portions 15. As the magnetization fixed layers 41 and 42, a Co, Co / Ni multilayer film, FePt, or the like can be used. In the figure, the magnetic film is shown as a perpendicular magnetization film, but the same applies even if the magnetic film is an in-plane magnetization film.

  FIG. 5 is a schematic diagram for explaining the read operation of the magnetic memory of this embodiment. FIG. 5 shows an operation when the domain wall 16 is held by the first domain wall holder 14 of the ferromagnetic structure 10 constituting the magnetic memory, that is, when information “1” is read. FIG. 6 is a schematic diagram showing an example of the waveform of the read current and the output waveform of the voltmeter at that time.

A read current pulse is applied from the current source 20 to the ferromagnetic structure 10. In the illustrated example, a read current I _R is applied to the ferromagnetic structure 10 in the direction from the second end portion 13 toward the first end portion 11. By this read current I _R , the domain wall 16 held by the first domain wall holder 14 moves in the direction of the second domain wall holder 15. However, at this time, the energy given to the domain wall by the read current I _R is adjusted so that the domain wall can not exceed the energy barrier E _b shown in FIG. In the present embodiment, the energy barrier E _b is 7.2 × 10 ⁻²⁰ J. Therefore, for example, a read current I _R may be given a current of 50 μA with a pulse width of 1 ns.

  At this time, the domain wall 16 held by the first domain wall holder 14 moves in the direction of the second domain wall holder 15 by the application of the read current pulse, and the first domain wall holder 14 and the central portion 12 are wide. It reaches the position 17 between the parts but cannot exceed the wide part of the central part 12 due to the energy barrier. When the application of the read current pulse is stopped, the domain wall located at the position 17 between the first domain wall holding unit 14 and the wide part of the central portion 12 moves toward the first domain wall holding unit 14 having a low domain wall energy. Returning, it reaches the first domain wall holder 14 and is stably held there. At this time, an electromotive force is generated in the central portion 12 of the ferromagnetic structure 10 due to the domain wall movement. Since the direction of the electromotive force depends on the direction of domain wall motion, the voltmeter 30 connected to the first end portion 11 and the central portion 12 of the ferromagnetic structure 10 has an output waveform as shown in FIG. It is done.

  FIG. 7 is another schematic diagram for explaining the read operation of the magnetic memory of this embodiment. FIG. 7 shows an operation when the domain wall 16 is present in the second domain wall holder 15 of the ferromagnetic structure 10 constituting the magnetic memory, that is, when information “0” is read. FIG. 8 is a schematic diagram showing an example of the waveform of the read current and the output waveform of the voltmeter at that time.

Similar to the case of FIG. 5, the read current I _{R in the} direction from the second end portion 13 toward the first end portion 11 is passed from the current source 20 to the ferromagnetic structure 10. The read current I _R is the same as that at the time of reading the information “1”. Due to this read current, the domain wall held in the second domain wall holder 15 of the ferromagnetic structure 10 moves in the direction of the second end 13. However, the energy given to the domain wall by the read current at this time is such an energy that the domain wall 16 does not exceed the energy barrier E _b shown in FIG. 2 and reaches the wide part of the second end part 13. Since it is smaller than the required energy, the domain wall 16 cannot reach the wide portion of the second end 13 of the ferromagnetic structure 10.

  When the application of the read current pulse is stopped, the domain wall located between the second domain wall holding unit 15 and the wide end portion of the second end 13 has the domain wall energy of the second domain wall holding unit 15 having a low domain wall energy. Returning to the direction, the second domain wall holder 15 is reached and stably held there. At this time, an electromotive force is generated in the portion of the second end portion 13 of the ferromagnetic structure 10 due to the domain wall movement. However, since the voltmeter 30 is connected to the first end portion 11 and the central portion 12 of the ferromagnetic structure 10, the voltmeter 30 cannot detect an electromotive force due to this domain wall movement, resulting in a voltage There is no change in the output of the total 30.

  Thus, by observing the output of the voltmeter by applying the same read current to the ferromagnetic structure, it is specified which of the two domain wall holders of the ferromagnetic structure constituting the magnetic memory holds the domain wall. be able to. That is, when the same read current is passed through the ferromagnetic structure 10, the output of the voltmeter 30 differs depending on whether the information stored in the magnetic memory is “1” or “0”. Information can be read based on the output.

  Here, the read current is passed from the second end 13 to the first end 11 of the ferromagnetic structure 10, but conversely, from the first end 11 toward the second end 13. The read current may be supplied. Further, the voltmeter 30 may be connected to the second end 13 and the central portion 12 instead of the first end 11 and the central portion 12 of the ferromagnetic structure 10.

  According to the present embodiment, the domain wall held by one of the domain wall holders does not move to the other domain wall holder by the read current. That is, since the reading method of the present embodiment is non-destructive reading that does not destroy information stored in the magnetic memory, the information destroyed after the information reading required in the case of destructive reading is restored. No operation is required to Therefore, according to the present embodiment, information can be read at a higher speed than in the case of destructive reading.

FIG. 9 is a schematic diagram showing a configuration example of the magnetic memory cell of the present invention.
The MOS transistor 50 includes two n-type semiconductors 51 and 52 and one p-type semiconductor 53. The drain electrode 54 connected to the n-type semiconductor 52 is grounded via the electrode 55. A source electrode 56 is connected to the n-type semiconductor 51. The ON / OFF of the current flowing between the source electrode 56 and the drain electrode 54 is controlled by ON / OFF of the gate electrode 57. The source electrode 56 is connected to the second end 13 of the ferromagnetic structure 10 through several layers of electrodes stacked thereon. The bit line 58 is connected to the first end 11 of the ferromagnetic structure 10. The magnetic memory cell of this embodiment further has an output line 59 connected to the central portion 12 of the ferromagnetic structure 10.

The magnetic memory cell of this embodiment can be written and read by one switching element 50. As can be seen from FIG. 9, the area occupied by this magnetic memory cell is as high as 2F × 4F = 8F ² . In order to reduce the size of the magnetic memory cell and achieve high integration, it is necessary to keep the size of the ferromagnetic structure 10 small. To that end, it is necessary to select a ferromagnetic material that can reduce the domain wall width. There is. Even if it is a ferromagnetic material, for example, since permalloy has a domain wall width of about 100 nm, the size of the ferromagnetic structure cannot be increased and the size of the magnetic memory cell cannot be reduced. From the viewpoint of the small domain wall width, the material of the ferromagnetic structure is preferably Co, Co / Ni multilayer film, FePt or the like. Since the domain wall width dw is expressed as dw = (A _ex / K _u ) ^−1/2 , it is estimated that Co is 5.8 nm, Co / Ni multilayer film is 5 nm, and FePt is 14 nm.

  FIG. 10 is a diagram showing a configuration example of a magnetic random access memory configured by arranging a plurality of magnetic memory cells shown in FIG. 9 in an array. The gate electrode 57, the bit line 58, and the output line 59 are electrically connected to the magnetic memory cell. A bit line driver 61 is installed as a driver for controlling the bit line 58, and a gate driver 62 is installed as a driver for controlling the gate electrode 57. Further, the signal of the output line 59 is supplied to the readout circuit 63, and the signal is read out. The bit line driver 61 and the gate line driver 62 select a magnetic memory cell on which information is written or read. Information is written to the magnetic random access memory by applying a positive or negative write current pulse to the selected magnetic memory cell. In addition, information is read from the magnetic random access memory by applying, for example, a read current pulse in the positive direction to the selected magnetic memory cell and detecting the voltage of the output line at that time.

  The information writing time and reading time of the magnetic memory of this embodiment are determined by (the distance traveled by the domain wall) / (moving speed of the domain wall). In the case of the present embodiment, (the distance traveled by the domain wall) is about 30 nm, and the moving speed of the domain wall is about 15 m / s, so the writing time and the reading time are about 10 nsec. Therefore, a high-speed magnetic memory that operates in the GHz band can be realized.

Further, the area occupied by the magnetic memory cell is as high as 8F ² and can be integrated as much as a DRAM, and can be expected to be used as a highly integrated SRAM.

  An MRAM using a magnetic tunnel junction is also being developed as a magnetic memory. However, an oxide film such as MgO is used for the magnetic tunnel junction, and the oxide film has a problem in terms of rewrite resistance because of deterioration due to current. In this respect, since the magnetic memory of the present invention does not use an oxide film, there is no fear of deterioration due to current, and there is an advantage that it has excellent rewrite durability.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

DESCRIPTION OF SYMBOLS 10 Ferromagnetic structure 11 1st edge part 12 Center part 13 2nd edge part 14 1st domain wall holding part 15 2nd domain wall holding part 16 Domain wall 20 Current source 30 Voltmeter 41 Magnetization fixed layer 42 Magnetization fixed layer 50 MOS transistor 54 Drain electrode 56 Source electrode 57 Gate electrode 58 Bit line 59 Output line 61 Bit line driver 62 Gate driver 63 Read circuit

Claims (7)

A ferromagnetic structure comprising a first domain wall holder and a second domain wall holder capable of stably holding a domain wall;
A first electrode and a second electrode connected to both ends of the ferromagnetic structure;
A third electrode connected to a position between the first domain wall holder and the second domain wall holder of the ferromagnetic structure;
A current source connected to the first electrode and the second electrode;
A voltmeter connected between the first electrode and the third electrode,
The magnetic domain wall of the ferromagnetic structure is moved from the current source through the first electrode and the second electrode to the ferromagnetic structure in accordance with the direction of the current, Record information by holding it in one of the domain wall holders,
A reading current in a certain direction is passed from the current source to the ferromagnetic structure through the first electrode and the second electrode, and the domain wall holding unit that holds the domain wall is specified by the output of the voltmeter. Thus, a magnetic memory is characterized in that information is read out.   2. The magnetic memory according to claim 1, wherein the domain wall held by one of the domain wall holders does not move to the other domain wall holder by the read current.   2. The magnetic memory according to claim 1, wherein the ferromagnetic structure has a shape in which a cross-sectional area in a direction perpendicular to a direction in which a current flows is minimized in the first domain wall holding unit and the second domain wall holding unit. A magnetic memory.   4. The magnetic memory according to claim 3, wherein the ferromagnetic structure is formed by stacking magnetization fixed films having magnetizations opposite to each other at both ends sandwiching the first domain wall holding unit and the second domain wall holding unit. Magnetic memory characterized by that.   2. The magnetic memory according to claim 1, wherein the ferromagnetic structure is made of a Co, Co / Ni multilayer film and FePt. A ferromagnetic structure including a first domain wall holder and a second domain wall holder capable of stably holding a domain wall; a first electrode and a second electrode connected to both ends of the ferromagnetic structure; A third electrode connected to a position between the first domain wall holder and the second domain wall holder of the ferromagnetic structure, and switching for controlling on / off of a current flowing through the ferromagnetic structure A memory cell group in which magnetic memory cells each including an element are arranged in a two-dimensional array;
Selecting means for selecting a desired magnetic memory cell in the memory cell group;
The domain wall of the ferromagnetic structure is changed to the first domain wall according to the direction of the current flowing through the ferromagnetic structure via the first electrode and the second electrode of the magnetic memory cell selected by the selection means. Recording information by holding either the holding unit or the second domain wall holding unit,
A domain wall held in one domain wall holding portion in a certain direction and the other domain wall by the ferromagnetic structure through the first electrode and the second electrode of the magnetic memory cell selected by the selection means A read current of a magnitude that does not move to the holding unit is passed, and information is read by specifying the domain wall holding unit that holds the domain wall based on the voltage generated between the first electrode and the third electrode. Magnetic random access memory characterized by that.   7. The magnetic random access memory according to claim 6, wherein the domain wall held in one of the domain wall holders does not move to the other domain wall holder by the read current.

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