Domain wall skyrmion driver and memory device

Abstract

To provide a domain wall skyrmion driving element that can be applied to a racetrack type memory and the like.SOLUTION: The present invention provides a domain-wall skyrmion driving device (Q) including a thin-film magnetic material (f), a magnetic section (m) that applies a magnetic field in a direction substantially perpendicular to the surface of the magnetic material, and an electric portion (e) that applies a driving current in the in-plane direction of the magnetic material. When domain-wall skyrmions are individually excited within the magnetic material at the boundary of a domain wall, the energy of the entire system decreases when they approach or adjoin each other, generating an attractive force and reaching a stable state. Such domain-wall skyrmions move along the direction of the current while maintaining their stable state, preventing annihilation due to contact with the edge of the sample and making them suitable for use as information carriers in magnetic memories, etc.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a domain wall skyrmion drive element and the like.

Background Art

[0002] Instead of magnetization reversal of magnetic domains (domains), movement of magnetic walls (domain walls), etc., a racetrack memory that moves skyrmions to record and erase information is regarded as a promising candidate for next-generation magnetic memory. A skyrmion has a topological magnetic structure (topological number / skyrmion number = ±1) in which electron spins (magnetic moments) are arranged in a spiral shape, and unlike a ferromagnetic state with uniform magnetization, it exhibits stable quasiparticle-like behavior. Also, skyrmions are extremely tiny and can move with a weak electric current (density). Therefore, if skyrmions can be used as information carriers, a magnetic memory that is robust against noise (external magnetic field, temperature disturbance, etc.) and can achieve high density and power saving can be realized.

[0003] Many proposals have been made regarding such skyrmions. For example, there are descriptions related to the following documents.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

[0006] However, none of the documents describe that the skyrmions individually excited (generated) can be orbitally controlled by current (induced) drive without being extinguished at the material edge or the like. The orbital control of skyrmions serving as information carriers is important for eliminating the risk of information loss in racetrack memories and the like.

[0007] The present invention has been made in view of such circumstances, and an object thereof is to provide a domain wall skyrmion drive element or the like that is expected to be used for magnetic memories and the like.

Means for Solving the Problems

[0008] The present inventor has newly found through intensive research that the orbit of individually excited skyrmions can be controlled without deviating from (without extinguishing) the direction of the drive current. By developing this result, the present invention described below has been completed.

[0009] 《Domain Wall Skyrmion Drive Element》 (1) The present invention includes a thin-film magnetic body, a magnetic part that applies a magnetic field in a direction substantially perpendicular to the plane of the magnetic body, and an electrical part that passes a drive current in the in-plane direction of the magnetic body, and skyrmions bounded by domain walls are excited individually or in a crystalline state within the magnetic body, and the domain wall skyrmion drive element is capable of moving the skyrmions along the direction of the drive current.

[0010] (2) According to the domain wall skyrmion drive element (also simply referred to as "element") of the present invention, skyrmions bounded by individually excited domain walls can be moved along the direction of the drive current without deviating. For this reason, for example, it is possible to avoid the skyrmions (also simply referred to as "skyrmions") deviating from their orbits and disappearing at the magnetic body edge or the like. As a result, according to the element of the present invention, for example, it is possible to provide a memory or a storage device that has no defects such as information loss or a significant decrease in information transfer speed.

[0011] As used herein, the "domain wall skyrmion" means a type with a domain wall at the skyrmion boundary. More specifically, it is a skyrmion in a state where the background magnetization of the skyrmion is not aligned in the direction perpendicular to the sample surface (tilted in the in-plane direction), and there is a region (domain wall) aligned in the direction perpendicular to the sample surface at the skyrmion end.

[0012] Also, as used herein, the "domain wall" is the region where Sz = 1 appears and its periphery when the background magnetization of the skyrmion is Sz≠1 (Sz: the in-plane component of the spin), and its specific thickness is not limited.

[0013] The "individually excited skyrmion" means that it is not in a crystalline state where a large number of skyrmions are regularly generated (a state where skyrmions are arranged in a triangular lattice / a state where a large number of skyrmions are generated and fill the space). More specifically, it is a state where a number of skyrmions less than the filling number are arranged without regularity. Individual skyrmions are not limited to a single one, but may be plural (two (a pair) or even three or more).

[0014] Whether the skyrmion becomes a crystalline state or an individually excited state depends on the strength (B) of the external magnetic field applied to the magnetic material. For example, when the external magnetic field increases (B / D 2 ≧0.7 and further B / D 2 >0.7), the ground state switches to the ferromagnetic phase, and the skyrmion transitions from the crystalline state to the individually excited state. When the skyrmion appears in the individually excited state, the number of them can also be adjusted.

[0015] 《Memory Device》 The present invention can also be understood as a memory device. For example, the present invention may be a memory device including the above-described domain wall skyrmion drive element and using the skyrmion as an information carrier. The memory device may be a memory carrier or may include an accessory device for operating the memory. Further, the present invention may be understood as a system including a computer equipped with the memory device, a program for operating the computer, and the like.

[0016] Others Unless otherwise specified, "x to y" as used in this specification includes the lower limit value x and the upper limit value y. For any numerical value included in the various numerical values or numerical ranges described in this specification, a new range such as "a to b" can be newly established with the new numerical value as the lower limit value or the upper limit value. Unless otherwise specified, "x to y nm" as used in this specification means x nm to y nm. The same applies to other unit systems (such as mT). Brief Description of the Drawings

[0017]

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Figure 7A

Figure 7B

Figure 8A

Figure 8B

[0018] One or more components arbitrarily selected from the matters described in this specification can be added to the configuration of the present invention described above. A component related to a method can also become a component related to an object (element, device, etc.). Whether any embodiment is the best depends on the object, required performance, etc.

[0019] 《Features and Background of the Present Invention》 Many of the conventional skyrmions were in a state where a large number of generated skyrmions were regularly arranged (skyrmion crystal). Such skyrmions move together in parallel while maintaining a relative arrangement (triangular lattice) with the minimum energy between them (i.e., as a skyrmion crystal).

[0020] Also, even when skyrmions are generated individually, it has been difficult to move each skyrmion along a desired trajectory (i.e., trajectory control) due to the repulsive force (repulsive force) acting between adjacent skyrmions.

[0021] On the other hand, in the present invention, a structure (skyrmion structure) in which an attractive interaction occurs between individually excited skyrmions with domain walls as boundaries, and its current drive control make it possible to move skyrmions along a desired trajectory. According to the present invention, for example, a magnetic memory (such as a skyrmion racetrack memory) that transmits information using skyrmions as information carriers can be realized.

[0022] 《Conditions》 The above-described skyrmion structure and current drive control can be realized, for example, under the following conditions.

[0023] (1) Magnetic part The magnetic body (which may be a part of the magnetic part) that is generated (excited) individually (in a non-crystalline state) with the domain wall as the boundary part can have any form (shape, size). The magnetic body may be, for example, in the form of a thin wire, a square, a circle / ellipse, etc.

[0024] The magnetic body is preferably in the form of a thin film. Its thickness is, for example, 25 - 250 nm, 50 - 200 nm. The size of the skyrmion (2Rsk / R sk The (radius) / R is usually several nm to several tens of nm. The thin film preferably has a thickness that is more than the minimum required for stable existence as a material. If the magnetic material becomes too thick, it becomes difficult to generate and control skyrmions due to, for example, the structural modulation of skyrmions in the film thickness direction and the effect of the interaction between skyrmions in the film thickness direction.

[0025] Since skyrmions are bounded by domain walls, the magnetic material preferably consists of a material that can take a ferromagnetic phase that easily generates magnetic domains or magnetic walls as the ground state at any temperature and magnetic field value. Also, the magnetic material preferably consists of a cubic crystal having a B20-type crystal structure such as space groups P4132, P4332, P213, etc. As such a magnetic material, for example, there is a CoZnMn-based alloy. In addition, for example, B20-type alloys (MnSi-based, FeGe-based, Fe x Co 1-x Si-based, etc.) may be used as the magnetic material. In this specification, "alloy" includes compounds.

[0026] (2) Magnetic part To individually generate skyrmions bounded by domain walls, the magnetic material is preferably in the ground state of the ferromagnetic phase. For this reason, for example, a magnetic field (B) that satisfies B / D 2 being 0.7 to 1, 0.75 to 0.95, or 0.8 to 0.9 may be applied in the direction perpendicular to the plane of the magnetic material.

[0027] In this specification, for convenience of explanation, each parameter is appropriately shown after being normalized. The constants (physical property values) and parameters that are the basis thereof are, for example, D: Dzyaloshinskii-Moriya interaction (energy), J: spin-spin exchange interaction (energy), R sk : skyrmion radius, etc.

[0028] The magnetic field (B) applied to the magnetic material is preferably consistent with the magnetocrystalline anisotropy energy (A) of the magnetic material. For example, a magnetic field (B) that satisfies A / B cr2 being 0.5 to 1.5, 0.7 to 1.3, or 0.9 to 1.1 may be applied in the direction perpendicular to the plane of the magnetic material. Note that B cr2 = 0.7D2 It is.

[0029] In such a case, domains aligned in the easy magnetization axis direction determined by crystal magnetic anisotropy or the like can be formed in the relative angular direction of individually excited proximity skyrmions. Such skyrmions become in a stable state (a state where the energy of the whole system decreases), and an attractive force can act between adjacent (proximity) skyrmions.

[0030] Units of each parameter, physical property value, etc. are not particularly limited. For example, D, J, A: joule [J], R sk : [nm], B: [T], etc.

[0031] (3) Electrical part Skyrmions can be driven by a minute current (density). The current supplied in the in-plane direction of the magnetic body is, for example, 10 6 ~10 12 A / m 2 , 10 7 ~10 11 A / m 2 , 10 8 ~10 10 A / m 2 may be sufficient. The current direction can be arbitrarily set along the desired movement direction of the skyrmion as long as it is in the in-plane direction of the magnetic body.

[0032] 《Gravitational interaction》 When the relative distance between approaching skyrmions is within a predetermined range, the energy between them becomes smaller, and an attractive force acts between adjacent ones.

[0033] The relative distance (dx) between skyrmions in the direction (extending direction) along the domain wall is, for example, such that dx / R sk satisfies 1.5 to 3.5, 1.8 to 3, or 2 to 2.5. Also, the relative distance (dy) between skyrmions in the direction orthogonal to the domain wall is, for example, such that dy / R sk satisfies 0 to 2, 0.2 to 1.5, or 0.5 to 1. Such skyrmions are in a stable state where an attractive force acts between adjacent ones.

[0034] Note that the skyrmion radius (R sk ) is the shortest distance from the center of the skyrmion (the point where the spin inside the vortex points in the direction opposite to the magnetic field among the directions perpendicular to the sample surface) to the point where the spin direction coincides with the magnetization direction in the sample far enough from the skyrmion, and is obtained numerically by finding such two points and measuring the distance between them.

Example

[0035] The trajectory control of individually generated domain wall skyrmions was evaluated by numerical analysis (simulation). While giving such specific examples, the present invention will be described in more detail below.

[0036] 《Analysis Model》 Assuming a magnetic memory element (domain wall skyrmion drive element), simulations were performed based on Model Q as shown in FIG. 1.

[0037] Model Q has a flat thin-film magnetic body f, an electrical part e that passes a drive current such as a pulse current through the magnetic body f, and a magnetic part m that applies a magnetic field (B) to the magnetic body f. The electrical part e is composed of, for example, electrodes provided on the end faces of the magnetic body f. The magnetic part m is composed of, for example, a coil along the plane of the magnetic body f.

[0038] In this example, the x-axis, y-axis, and z-axis were set as shown in FIG. 1. Appropriately, the directions along the x-y plane (x direction, y direction, etc.) are called in-plane directions, and the z-axis direction is called the out-of-plane direction. In the case of Model Q, the drive current flows between the electrodes along the in-plane direction, and the magnetic field B is applied in the out-of-plane direction.

[0039] The magnetic body f is composed of a cubic crystal having a B20-type crystal structure (space group P4132, P4332, P213, etc.) of a crystal. In this example, for convenience, the magnetic body f was regarded as a two-dimensional model (a square lattice with zero thickness). Even with such a model, it can be effectively approximated as long as the thickness (film thickness) of the magnetic thin film is up to the order of the cubic crystal of 10×R sk (skyrmion radius).

[0040] 《Analysis Method》 Based on Model Q, the motion of the time-evolving spins was analyzed under the energy function (Hamiltonian H) shown in Equation (1) of Figure 2. The physical parameters J, D, A, etc. determined by the material of the magnetic body f are summarized in Table 1. Each parameter was calculated after being normalized by J (spin-spin exchange interaction energy) as appropriate.

[0041] Each individual spin follows the Landau-Lifshitz-Gilbert equation shown in Equation (2) of Figure 2. B shown in that equation eff represents the effective magnetic field felt by each individual spin, and each spin tries to align in the direction of that magnetic field. The third term on the right side of Equation (2) represents the interaction that a skyrmion receives from the current j when the interaction acting between skyrmions is not considered, and the skyrmion moves parallel in the direction of that current.

[0042] The current direction can be arbitrarily set if it is in the in-plane direction. In this embodiment, for convenience, the current direction was set as the x-axis direction or the y-axis direction, and Equation (2) was numerically integrated by the second-order Runge-Kutta method to calculate the time evolution of the skyrmion.

[0043] 《Analysis Example》 (1) Generation The magnetic field (B) and the magnitude of the magnetocrystalline anisotropy (A) were simulated within the ranges shown in Figure 3 (0.7 ≤ B / D 2 and 0.5 ≤ A / B cr2 ). At this time, domain wall skyrmions were individually excited. Note that D: Dzyaloshinskii-Moriya interaction (DMI), B cr1 = 0.3D 2 , B cr2 = 0.7D 2 .

[0044] The state of the generated skyrmions (one and a pair) is shown in Fig. 4. In the figure, the white line drawn above the skyrmion indicates the domain wall (region with Sz = 1). All of them were accompanied by domain walls and were in a stable state (thermal equilibrium state). The "stable state" means a state in which the skyrmion remains without disappearing from the magnetic body f. For example, if there is one skyrmion (alone), the skyrmion remains at an arbitrary position of the magnetic body f. Even when there are multiple skyrmions, if the skyrmions are sufficiently separated from each other, even when a repulsive force acts, there is no substantial repulsive force acting between them, and each skyrmion remains on the magnetic body f.

[0045] In the case of this embodiment, Fig. 5 shows that even when multiple skyrmions are close or adjacent to each other, under predetermined conditions, the skyrmions are in a stable state and remain on the magnetic body f. Fig. 5 is a simulation result showing the relative position dependence of the interaction acting between domain wall skyrmions. The x-axis direction indicates the direction along the domain wall, and the y-axis direction indicates the direction intersecting (orthogonal) to the domain wall.

[0046] Specifically, the simulation according to Fig. 5 was performed as follows. Numerically generate a plurality of skyrmions generated based on physical property values such as A and B. The energy E when there is one skyrmion 1Sk and the energy E when there is a pair (two) of skyrmions 2Sk are obtained. The energy difference V(R) = E 2Sk - 2E 1Sk regarding the relative distance R between the skyrmions is obtained. Fig. 5 is a plot of the interaction V(R) with signs.

[0047] As is clear from Fig. 5, the relative distance (dx) between the skyrmions along the domain wall (along the x-axis direction) is about 2R sk (1.5R sk ≦ dx ≦ 3.5R skIn the vicinity where it becomes (near the circled area in Fig. 5), the interaction between skyrmions was minimized (maximum on the negative side). That is, it was found that such skyrmions are under a large attractive force, and the energy required to stay near each other is small, resulting in a stable state.

[0048] When observing in the direction intersecting the domain wall (y-axis direction), the relative distance (dy) between skyrmions is 0 to 2R sk or so (0 ≦ dx ≦ 2R sk ), it was also found that an attractive force acts between skyrmions and the skyrmions reach a stable state.

[0049] (2) Driving The velocity (drift velocity) when a pair of domain wall skyrmions in a stable state in the ferromagnetic phase move due to a driving current was simulated. The current dependence of the obtained drift velocity is shown in Fig. 6. As is clear from Fig. 6, it was confirmed that the skyrmions move at a velocity proportional to the current density regardless of the current direction. For example, with a current of about 10 10 A / m 2 , it was found that the skyrmions move at a sufficient velocity.

[0050] (3) Trajectory The state of movement (trajectory / orbit) of a pair of stable skyrmions generated in a magnetic body f with a magnetic field applied by a magnetic part m on the magnetic body f due to energization from an electric part e was simulated.

[0051] Using the time evolution equation [Equation (2) in Fig. 2], the time change of the skyrmion position was calculated, and the result is shown in Fig. 7A. Fig. 7A shows the cases where current is supplied in the x-axis direction (direction along the domain wall) and the y-axis direction (direction intersecting the domain wall), respectively.

[0052] The left figure in Fig. 7A shows two adjacent skyrmions (stable state) at the initial time (t = 0 s). The right figure shows the positions of those skyrmions at a predetermined time (t = 0.225 ns, 0.375 ns). The middle figure shows the displacement (trajectory) of the skyrmions over a certain period of time (t = 0 to 0.4 ns). The trajectory consists of the central positions of the skyrmions for each time step. The central position is the coordinate where the spin vector is closest to (0, 0, -1).

[0053] As is clear from Fig. 7A, the stable-state skyrmions moved in a substantially straight line along the direction of the driving current while maintaining their state without disruption.

[0054] As shown in Equation (3) of Fig. 2, the stable-state skyrmions move at a speed V proportional to the current j in the steady state after the start of energization. sk in the direction of movement.

[0055] As a comparative example, Fig. 7B shows the trajectory (in the direction of the current: x-axis direction) of skyrmions (unstable state) in which a repulsive force rather than an attractive force acts between adjacent skyrmions, like conventional skyrmions. The trajectories of the unstable-state skyrmions interfered with each other and did not move along Equation (3).

[0056] Note that the simulation of the comparative example was performed under the following conditions. The crystal magnetic anisotropy constant: A = 0, the external magnetic field: B / D 2 = 0.8, and the other parameters were the same as those in the example.

[0057] 《Discussion》 As is clear from Fig. 7A, for stable-state skyrmions, trajectory control by the driving current becomes possible. Such skyrmions can avoid disappearance due to contact with the magnetic body end (sample end) and thus can serve as information carriers. A schematic example of the application of such skyrmions to a racetrack memory is shown in Fig. 8A.

[0058] On the one hand, as shown in FIG. 7B, since the unstable skyrmion cannot control its trajectory, it disappears when it contacts or collides with the magnetic end, resulting in the loss of information. Such a skyrmion cannot be an information carrier in a racetrack memory as shown in FIG. 8B.

[0059]

Table 1

Explanation of Symbols

[0060] Q analysis model (domain wall skyrmion drive element) f magnetic body e electrical part m magnetic part

Claims

1. A thin-film magnetic body, a magnetic part that applies a magnetic field in a direction substantially perpendicular to the plane of the magnetic body, and an electrical part that passes a drive current in the in-plane direction of the magnetic body, wherein domain wall skyrmions bounded by domain walls are individually or in a crystalline state excited within the magnetic body, and the skyrmions are moved along the direction of the drive current, a domain wall skyrmion drive element.

2. The domain wall skyrmion drive element according to claim 1, wherein an attractive force acts between adjacent skyrmions.

3. The magnetic field (B) satisfies 0.7 ≦ B / D ２ (D: Dzyaloshinskii-Moriya interaction), and The magnetocrystalline anisotropy energy (A) of the magnetic material satisfies 0.5 ≦ A / B ｃｒ２ (B ｃｒ２ = 0.7D ２ ) The domain wall skyrmion drive element according to claim 1

4. The drive current has a current density of 10 ６ to 10 １２ A / m ２ The domain wall skyrmion drive element according to claim 1, wherein the domain wall skyrmion drive element is such that

5. The skyrmions, 1.5 ≤ dx / R ｓｋ ≤ 3.5 and 0 ≤ dy / R ｓｋ ≤ 2 (dx: adjacent interval along the extending direction of the domain wall, dy: adjacent interval along the direction orthogonal to the domain wall, R ｓｋ : Skillion radius) satisfy the domain wall skyrmion drive element according to claim 2.

6. The domain wall skyrmion drive element according to claim 1, wherein the magnetic body has a thickness of 25 to 250 nm.

7. The domain wall skyrmion drive element according to claim 1, wherein the magnetic body is composed of a cubic crystal having a B20-type crystal structure.

8. The domain wall skyrmion drive element according to claim 1, wherein the magnetic body is made of a ferromagnetic material.

9. A storage device comprising the domain wall skyrmion drive element according to any one of claims 1 to 8, wherein the skyrmions are used as information carriers.

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