Method for reducing critical current density in SOT driving magnetization overturning

Abstract

The invention discloses a method for reducing critical current density in SOT driving magnetization overturning, and relates to the technical field of spintronics, and the method comprises the following steps: building a structure model of a magnetic film which is of a double-layer structure, symmetrically arranging two defect areas on the magnetic layer along the x-axis direction, the physical parameter value of the defect area is smaller than that of the magnetic layer; the junction of the magnetic layer and the defect region is a domain wall; the structure model is subjected to micro-magnetic simulation, a magnetic domain and domain wall evolution process is obtained, the critical current density in SOT driving magnetization overturning is determined based on the evolution process, the critical current density is the minimum current density for driving domain wall movement, and the maximum current density is the minimum current density for driving domain wall movement. And the SOT effective field generated by the critical current density is the minimum SOT effective field required for unpinning the domain wall from the defect area. The method can be compatible with the existing micro-nano electronic process and material system.

Description

Technical Field

[0001] The present invention relates to the technical field of spintronics, and particularly to a method for reducing the critical current density in spin-orbit-torque (SOT)-driven magnetization switching. Background Art

[0002] The spin Hall (Hall) effect or spin-orbit torque (SOT) generated by Rashba spin-orbit coupling can effectively manipulate the magnetization in ferromagnets, magnetic tunnel junctions, antiferromagnetic metals, and compensated ferrimagnets. Therefore, SOT-driven perpendicular magnetization switching (PMS) has become an important topic in spintronics.

[0003] The key challenge in SOT-driven PMS is how to determine and reduce the critical current density to achieve low power consumption in SOT-based spintronic devices. Existing methods determine the critical current density through macrospin models and domain wall (DW) models. According to the macrospin model, the predicted critical current density is linearly related to both the perpendicular magnetic anisotropy field and the applied in-plane magnetic field, while the domain wall (DW) model indicates that the critical current density is mainly determined by the coercivity. In fact, the experimentally measured critical current density is usually much smaller than the theoretical predictions of the macrospin model and the DW model. Therefore, to address the discrepancy between the theoretical models and experimental measurements, a more in-depth study of the critical current in PMS is needed to obtain a method for reducing the critical current density.

[0004] Currently, methods such as ion implantation or using topological insulators have been used to reduce the critical current density in SOT-driven PMS, but these methods have poor compatibility with existing micro-nanoelectronic processes and material systems and are not suitable for integration into existing spintronic devices. Summary of the Invention

[0005] Based on the above-mentioned defects existing in the prior art, the present invention provides a method for reducing the critical current density in SOT-driven magnetization switching, which solves the problem that the existing methods for reducing the critical current density have poor compatibility with existing micro-nanoelectronic processes and material systems and are not suitable for integration into existing spintronic devices.

[0006] The present invention adopts the following technical solutions:

[0007] The present invention provides a method for reducing the critical current density in SOT-driven magnetization switching, including the following steps:

[0008] A structural model of a magnetic thin film is established. The magnetic thin film has a bilayer structure, where the lower layer is a heavy metal layer for providing spin-orbit torque (SOT), and the upper layer is a magnetic layer for generating magnetization reversal. Two defect regions are symmetrically arranged along the x-axis in the magnetic layer, and the physical parameter values of the defect regions are smaller than those of the magnetic layer. The domain walls are located at the junctions between the magnetic layer and the defect regions.

[0009] The micromagnetic simulation of the structural model is carried out to obtain the evolution process of magnetic domains and domain walls. Based on the evolution process, the critical current density in the SOT-driven magnetization reversal is determined. The critical current density is the minimum current density for driving the domain wall motion, and the SOT effective field generated by this critical current density is the minimum SOT effective field required for the domain wall to depin from the defect region.

[0010] Preferably, the cross-section of the magnetic layer in the y-z plane is a wedge structure, and the height is inclined along the y-axis.

[0011] Preferably, the physical parameters include saturation magnetization, exchange stiffness, perpendicular magnetic anisotropy, and interaction strength.

[0012] Preferably, the material of the magnetic layer is CoPt, and the material of the heavy metal layer is Pt.

[0013] Preferably, the length of the structural model is 1024 nm, the width is 128 nm, the area of each defect region is 8 nm × 8 nm, and the defect position is 64 nm away from the center position.

[0014] Preferably, the micromagnetic simulation of the structural model to obtain the evolution process of magnetic domains and domain walls includes the following steps:

[0015] Apply a magnetic field H z = 1500 Oe to the structural model, and set the initial state of the upper layer to be magnetically saturated along the +z direction.

[0016] Apply a fixed magnetic field H x = 500 Oe to the structural model, and scan the current density j from -x to +x directions.

[0017] When scanning the current density j from 1.50×10 5 A cm -2 to 4.95×10 5 A cm -2 along the +x direction, the evolution process of magnetic domains and domain walls is obtained.

[0018] Preferably, after the micromagnetic simulation of the structural model, it further includes:

[0019] Performing domain wall dynamics calculation on the structural model to obtain the domain wall motion velocity under different external magnetic fields.

[0020] Determine the external magnetic field strength value when DW1 and DW2 reach the same speed according to the domain wall movement speed under different external magnetic fields, where DW1 is the domain wall from top to bottom and DW2 is the domain wall from bottom to top;

[0021] Determine the range of the applied magnetic field change during SOT-driven magnetization reversal through this external magnetic field strength value.

[0022] Preferably, during the micromagnetic simulation of the structural model, an in-plane magnetic field along the x direction is applied to the structural model to obtain multiple curves of the magnetization intensity in the z direction varying with the current density, and multiple critical current density values are obtained according to the multiple curves.

[0023] Compared with the prior art, the above at least one technical solution adopted by the present invention can achieve the following beneficial effects:

[0024] The present invention designs a magnetic thin film, establishes a corresponding structural model, and conducts micromagnetic simulation on the structural model. There are two defect regions on the magnetic thin film, and the physical parameters adopted in the defect regions are smaller than the physical parameter values of the magnetic layer. This design makes the defect regions soft magnetic materials relative to the main body of the magnetic layer. When an external current or magnetic field is applied, reverse magnetic domain nucleation points are easily formed in the defect regions, thereby forming the domain walls required for magnetization reversal. At the same time, due to the symmetric distribution of the two defect regions, the symmetric defect design can form two domain walls. During micromagnetic simulation, the applied current only needs to drive these two domain walls to make them move relative to each other, and the entire material system can undergo magnetization reversal. The critical current density only needs to drive the domain walls to depin from the defects and generate movement, rather than reversing the magnetization intensity of the entire material system. Therefore, the critical current density can be significantly reduced. The present invention realizes the reduction of the critical current density in SOT-driven magnetization reversal through the magnetic thin film, can be compatible with the existing micro-nano electronic processes and material systems, provides a new approach for the research and development of low-power and high-integration spin electronic devices, and at the same time creates a broader prospect for its application in storage and logic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0026] Figure 1 Schematic diagram of the structural model of the magnetic thin film of the present invention;

[0027] Figure 2Schematic diagram of SOT-driven PMS at low current density realized by micromagnetic simulation of the present invention;

[0028] Figure 3 Schematic diagram of domain wall motion and magnetization reversal presented by micromagnetic simulation of the present invention;

[0029] Among them, Figure 3 (a): Schematic diagram of domain wall motion with a scanning current density j of 1.50×10 5 A cm -2 , Figure 3 (b): Schematic diagram of domain wall motion with a scanning current density j of 2.50×10 5 A cm -2 , Figure 3 (c): Schematic diagram of domain wall motion with a scanning current density j of 4.00×10 5 A cm -2 , Figure 3 (d): Schematic diagram of domain wall motion with a scanning current density j of 4.95×10 5 A cm -2 ;

[0030] Figure 4 Domain wall design in the structural model of the present invention;

[0031] Among them, Figure 4 (a): "Top-down" and "bottom-up" domain wall profiles, Figure 4 (b): Spherical coordinate system describing the domain wall orientation and shape;

[0032] Figure 5 Schematic diagram of the change of domain wall motion speed with external magnetic field calculated based on domain wall design of the present invention;

[0033] Figure 6 Schematic diagram of the measurement results of the comparative experiment;

[0034] Figure 7 Flowchart of a method for reducing the critical current density in SOT-driven magnetization reversal of the present invention. Specific embodiments

[0035] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] Refer to Figure 7, the present invention provides a method for reducing the critical current density in SOT-driven magnetization reversal, specifically a method for reducing the critical current required for SOT-driven PMS and improving its robustness, so as to achieve SOT-driven PMS at low current density and at the same time show strong resistance to the influence of external magnetic fields. The specific steps are as follows:

[0037] S1: Design the structural model of the magnetic thin film.

[0038] The present invention designs a structural model as shown in Figure 1 , which includes defect design, wedge design and domain wall design. Its basic form is a Pt / CoPt bilayer structure. The lower layer Pt is a heavy metal layer, which provides spin current and SOT for the upper layer CoPt through the spin Hall effect therein. The upper layer CoPt is a magnetic layer, which is the main material for driving PMS with SOT. The structural model is 1024 nm long and 128 nm wide. The height is inclined along the y-axis direction, with a minimum value of 1.3 nm and a maximum value of 10 nm. The CoPt layer is a wedge-shaped structure in the y-z plane. Two defect regions with an area of 8 nm × 8 nm are symmetrically arranged relative to the central position, and the height is the same as that of the main material. The defect position is 64 nm away from the central position. The junction between the main material and the defect region is the domain wall region. The domain wall is a magnetic domain wall, and the domain wall is the transition region between two magnetic domains. The domain wall is designed to have a rigid profile and a left-handed Néel-type structure.

[0039] S2: Conduct micromagnetic simulation and domain wall dynamics calculation on the structural model.

[0040] During micromagnetic simulation, different physical parameters are used for the main material and defect region of the CoPt layer, including saturation magnetization M, exchange stiffness A and perpendicular magnetic anisotropy K p , and the magnetic dynamics evolution of the entire structural model is simulated and calculated. When a current and a magnetic field are applied along the x-axis direction, the magnetization intensity at the central position of the structural model is detected to determine whether magnetization reversal occurs. The minimum unit size for simulation is 4 nm × 4 nm × 1.3 nm, and the steady state of each unit is obtained after 5 ns under the action of current and magnetic field.

[0041] The present invention uses the structural model shown in Figure 1 to conduct micromagnetic simulation on the above magnetic thin film. The physical parameters used in the defect region are less than those used in the magnetic layer. The physical parameters used for the main material (host) of the CoPt layer in the simulation calculation are: M host = 570 kA m -1 , A host = 26.8 pJ m -1 , The strength D of the Dzyaloshinskii-Moriya interaction (DMI) host= 1.6 mJ m -2 , the damping factor α = 0.1, and the spin Hall angle θ SH = 0.032. For the defect region (defc), the physical parameters are: M defc = 148 kA m -1 , A defc = 10.2 pJ m -1 , D defc = 0.54 mJ m -2 , and other parameters remain unchanged. The curve of the magnetization M x in the z - direction varying with the current density j under a fixed - magnitude in - plane magnetic field H z in the x - direction is calculated and shown as Figure 2 . Under a positive - direction external magnetic field H x , the curve of the magnetization M z in the z - direction varying with the current density j shows a clockwise loop. For zero external magnetic field (H x = 0) and each applied magnetic field H x (= 500, 100, 1500), deterministic SOT - driven PMS is observed, and the critical current density j crit for magnetization reversal is obtained. Figure 2 It shows that when H x increases from 0 to 1500 Oe, j crit decreases from 5.17×10 5 A cm -2 to 2.43×10 5 A cm -2 , which indicates that the structural model designed in the present invention realizes SOT - driven PMS at the order of magnitude of low current density "10 5 A cm -2 ".

[0042] Figure 3 This is the micromagnetic simulation of the reverse magnetic domain nucleation and domain - wall motion caused by the current j for the structural model shown in the present invention at a fixed H Figure 1 = 500 Oe. First, by applying a magnetic field H x = 1500 Oe, the initial state of the CoPt magnetic layer is set to be magnetically saturated along the +z direction. Then, with a fixed magnetic field H z = 500 Oe, the current density j is scanned from -x to +x direction. When scanning the current density j in the +x direction from 1.50×10 x = 500 Oe, the current density j is scanned from -x to +x direction. When scanning the current density j in the +x direction from 1.50×10 5 A cm -2When it starts to gradually increase, it can be seen that the reverse magnetic domains gradually nucleate and grow. When the current density j in the +x direction reaches the critical value of 4.95×10 5 A cm -2 , the domain wall motion causes the rapid growth of the reverse magnetic domains, thereby leading to the generation of PMS. The evolution process of magnetic domains and domain walls in this micromagnetic simulation shows that in the structural model of the present invention, PMS is caused by the SOT-driven domain wall motion. The critical current density at which PMS occurs, and the corresponding SOT effective field can just make the domain wall depin from the defect, so that the domain wall can move freely. Due to the inhomogeneity of the defect region and the main material in the CoPt layer, the applied current only needs to overcome the domain wall pinning at the defect, rather than flipping the magnetization of the entire main material, so SOT-driven PMS can be achieved at a low current density.

[0043] The ideal condition for using SOT to drive PMS is that there is no need to apply an in-plane magnetic field externally, which can greatly simplify the device design and avoid the Joule heat and energy consumption caused by the need for a magnetic field. However, usually, the SOT-driven PMS will disappear under the influence of an externally applied in-plane magnetic field. Therefore, in the research of SOT-driven PMS, it is urgent to find a method that can maintain stable PMS within a large range of externally applied magnetic field changes, that is, to improve the robustness of SOT-driven PMS.

[0044] Figure 4 For the domain wall design in the structural model, it is used to calculate the motion speed of the domain wall when a current and a magnetic field are applied along the x-axis direction. Due to the strong Dzyaloshinskii-Moriya interaction (DMI) at the Pt / CoPt interface, a Néel-type domain wall with left-handed chirality will be formed in the CoPt magnetic layer, and the magnetization reversal of the CoPt layer is achieved through the domain wall motion. The present invention introduces a rigid profile, DMI effective field, damping-like SOT effective field, field-like SOT effective field, and domain wall demagnetizing field in the domain wall design, which can well describe the domain wall motion and magnetization reversal under the action of current and magnetic field.

[0045] In this domain wall design, the magnetization intensity vector along the x-axis is represented in spherical coordinates as where M is the magnitude of the magnetization , θ is the polar angle relative to the direction, ψ is the azimuthal angle relative to the direction, as Figure 4As shown in (b). The "top-down" domain wall and the "bottom-up" domain wall are respectively referred to as DW1 and DW2, and the profiles of DW1 and DW2 are as shown in Figure 4 (a). In this domain wall design, the state of the domain wall at any position x and any time t is represented by the velocity and azimuth angle ψ of the domain wall.

[0046] As an important basis for this domain wall design, the rigid profiles of DW1 and DW2 are given by the following formula:

[0047]

[0048] where θ i is the polar angle with respect to the z-axis, x is an arbitrary position of the CoPt layer in this direction, q i is the center position of the domain wall at any time t, and Δ is the domain wall width (subscripts i = 1, 2 correspond to DW1 and DW2 respectively).

[0049] Based on this domain wall design, the present invention uses the Landau-Lifshitz-Gilbert (LLG) equation to describe the domain wall motion in the CoPt magnetic layer. The precession and damping of the magnetic moment are introduced into the equation, the damping-like SOT caused by the in-plane spin polarization, and the field-like SOT caused by the out-of-plane spin polarization:

[0050]

[0051] The static effective field in equation (3) is determined by the corresponding static magnetic energy density w: For the magnetic layer studied in the present invention, w includes the internal exchange energy, the perpendicular magnetic anisotropy energy, the demagnetization energy, and the Zeeman energy:

[0052]

[0053] In formula (4), A is the exchange stiffness, K p is the perpendicular magnetic anisotropy constant, is the domain wall width, (d: magnetic layer thickness) is the amplitude of the in-plane demagnetizing field of the domain wall, H lg = H x + H DM is the longitudinal field, which includes the applied magnetic field H x and the DMI effective field H DM .

[0054] Combined with Equation (1) and (2) that describe the domain wall profile, the LLG equation (3), and the expression of the magnetostatic energy density (4), the present invention can obtain the velocities of DW1 and DW2 and the azimuth angle ψ i of the differential equations:

[0055]

[0056] where,

[0057]

[0058] The solutions of Equation (5) and Equation (6) are the domain wall dynamics solutions based on this domain wall design. In the formula, "±" and the upper and lower signs respectively correspond to DW1 (i = 1) and DW2 (i = 2). Using this solution, the present invention can study the domain wall dynamics characteristics of the magnetic layer under the action of different external currents and external magnetic fields, that is, the velocity of the domain wall at any time and the azimuth angle ψ of the domain wall i .

[0059] Figure 5 This is the calculation of the domain wall movement speed by the present invention based on the domain wall design shown in Figure 4 to demonstrate the robustness of the SOT-driven PMS in the structural model of the present invention. The physical parameters used are: Δ = 4.3 nm, u 1 = u 2 = 0, α = 0.1, β 1 = β 2 = 0, M 1 = M 2 = 570 emu cm -3 .

[0060]

[0061] In principle, when the velocities of DW1 and DW2 are exactly the same, the energies of the two are degenerate. At this time, the movements of the two domain walls are exactly the same, so PMS will not occur. The present invention defines the external magnetic field H x that makes DW1 and DW2 reach the same velocity as the degenerate field H deg , which is used to describe the robustness of the SOT-driven PMS: it means that even if there is an external magnetic field, PMS will not disappear before the external magnetic field reaches H deg ; and when the external magnetic field H x reaches the degenerate field H deg that makes DW1 and DW2 have the same velocity, the SOT-driven PMS is destroyed. As can be seen from Figure 5 when Hx = H deg = -3500 Oe, the velocities of DW1 and DW2 are the same, and PMS no longer occurs. While in the larger magnetic field range of H x > -3500 Oe, the velocities of DW1 and DW2 are different, which can ensure deterministic SOT-driven PMS. Therefore, the structural model designed in the present invention has strong robustness to the influence of an external magnetic field.

[0062] Embodiment

[0063] The experimental measurement of the critical current density before improvement was used as a control experiment, and the measurement results of the present invention were compared with those of the control experiment.

[0064] Sample preparation and measurement method

[0065] A ferromagnetic thin film with a magnetic layer of CoPt was prepared on a Si substrate (covered with SiO 2 ) by magnetron sputtering. The overall structure of the sample is Ta2 / Pt5 / [Co0.5 / Pt0.3 / Co0.5] / Ru2, where the numbers are the thicknesses of each layer (unit: nm). The CoPt magnetic layer is composed of [Co0.5 / Pt0.3 / Co0.5], has perpendicular magnetic anisotropy, and the Ta / Pt / Co interface at the bottom dominates the SOT effect of the entire material system and the DMI at the interface. To perform Hall measurement on the sample, a Hall bar structure of the thin film was prepared by photolithography and Ar ion beam etching. The Hall bar has a width of 5 μm in the y-axis direction and a length of 70 μm in the x-axis direction. To measure the magnetization reversal caused by SOT under an in-plane magnetic field H x , pulsed currents with different amplitudes and a duration of 0.1 s were applied to the Hall bar, and the current I and the in-plane magnetic field H x both are along the x-axis direction; after the pulsed current action is completed, a direct current I dc = 0.1 - 0.5 mA is used to measure the Hall voltage V dc . All measurements of magnetism and electrical transport were carried out at room temperature.

[0066] Experimental measurement results

[0067] The PMS of the CoPt thin film with the structure of Ta2 / Pt5 / [Co0.5 / Pt0.3 / Co0.5] / Ru2 was measured, and the corresponding critical current intensity j x under a fixed H crit was obtained, as shown in Figure 6 . Under a fixed in-plane positive magnetic field H x , the Hall resistance R H curve shows a clockwise cycle with the change of the current j. For each applied magnetic field Hx , deterministic magnetization reversal was observed, and the critical current density j for magnetization reversal was obtained crit . Figure 6 It is shown that when H x increases from 100 Oe to 9000 Oe, j crit decreases from 5.11×10 7 A / cm -2 to 2.65×10 7 A / cm -2 . The measurement results of the present invention are as Figure 2 shown. When H x increases from 0 to 1500 Oe, j crit decreases from 5.17×10 5 A / cm -2 to 2.43×10 5 A / cm -2 , which indicates that the structural model designed in the present invention realizes a reduction in the critical current density at the order of low current density "10 5 A / cm -2 ".

[0068] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications once they learn the basic creative concepts. Therefore, the appended claims are intended to be construed to include the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0069] Obviously, those skilled in the art can make various changes and deformations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and deformations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and deformations.

Claims

1. A method for reducing critical current density in SOT driven magnetization reversal, characterized in that: The following steps are involved: A structural model of a magnetic film is established, wherein the magnetic film is a double-layer structure, wherein the lower layer is a heavy metal layer for providing a spin-orbit torque (SOT), and the upper layer is a magnetic layer for generating a magnetization reversal; two defect regions are symmetrically arranged in the magnetic layer along the x-axis direction, and the physical parameter values ​​of the defect regions are smaller than the physical parameter values ​​of the magnetic layer; and the boundary between the magnetic layer and the defect region is a domain wall; Micromagnetic simulation is performed on the structural model to obtain the evolution process of magnetic domains and domain walls. The critical current density in SOT-driven magnetization reversal is determined based on the evolution process, where the critical current density is the minimum current density that drives the motion of domain walls, and the SOT effective field generated by this critical current density is the minimum SOT effective field required to depin the domain walls from the defect area.

2. A method for reducing critical current density in SOT driven magnetization reversal according to claim 1, characterized in that: The cross section of the magnetic layer in the yz plane is a wedge-shaped structure, and the height is inclined along the y-axis direction.

3. A method for reducing critical current density in SOT driven magnetization reversal according to claim 1, characterized in that: The physical parameters include saturation magnetization, exchange stiffness, perpendicular magnetic anisotropy and interaction strength.

4. A method for reducing critical current density in SOT driven magnetization reversal according to claim 1, characterized in that: The material of the magnetic layer is CoPt, and the material of the heavy metal layer is Pt.

5. A method for reducing critical current density in SOT driven magnetization reversal as claimed in claim 1, characterized in that: The length of the structural model is 1024 nm, the width is 128 nm, the area of ​​each defect region is 8 nm×8 nm, and the defect position is 64 nm away from the center position.

6. A method for reducing critical current density in SOT driven magnetization switching according to claim 1, characterized in that: The micromagnetic simulation of the structural model is performed to obtain the evolution process of magnetic domains and domain walls, comprising the following steps: Apply a magnetic field H to the structural model z = 1500Oe, the initial state of the upper layer is set to be magnetized saturation along the +z direction; Apply a fixed magnetic field H to the structural model. x =500Oe, scan current density j from -x to +x direction; When the current density j is scanned from 1.50×10 5 A cm -2 to 4.95×10 5 A cm -2 The evolution process of magnetic domains and domain walls is obtained.

7. A method for reducing critical current density in SOT driven magnetization switching according to claim 1, characterized in that: After the micromagnetic simulation of the structural model is performed, the method further includes: The domain wall dynamics of the structural model were calculated, and the domain wall movement speed under different external magnetic fields was obtained. According to the domain wall movement speed under different external magnetic fields, the external magnetic field strength value when DW1 and DW2 reach the same speed is determined, where DW1 is the domain wall from top to bottom and DW2 is the domain wall from bottom to top; The external magnetic field intensity value is used to determine the variation range of the external magnetic field when the SOT drives the magnetization switching.

8. A method for reducing critical current density in SOT driven magnetization switching according to claim 1, characterized in that: During the micromagnetic simulation of the structural model, an in-plane magnetic field along the x-direction is applied to the structural model to obtain multiple curves of magnetization intensity in the z-direction varying with current density, and multiple critical current density values ​​are obtained based on the multiple curves.

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