Method for reducing critical current density in sot driven magnetization switching

Abstract

The application discloses a method for reducing critical current density in SOT driven magnetization flipping, and relates to the technical field of spintronics, and comprises the following steps: establishing a structure model of a magnetic thin film, the magnetic thin film being a double-layer structure, two defect regions being symmetrically arranged along an x-axis direction of the magnetic layer, and physical parameter values of the defect regions being smaller than those of the magnetic layer; an interface between the magnetic layer and the defect regions being a domain wall; performing micromagnetic simulation on the structure model to obtain a magnetic domain and domain wall evolution process, and determining a critical current density in SOT driven magnetization flipping based on the evolution process, wherein the critical current density is a minimum current density for driving the domain wall to move, and an SOT effective field generated by the critical current density is a minimum SOT effective field required for de-pinning the domain wall from the defect region. The application can be compatible with existing micro-nano electronic processes and material systems.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of spintronics, and particularly relates to a method for reducing critical current density in SOT-driven magnetization switching. BACKGROUND

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

[0003] A key challenge of SOT-driven PMS is how to determine and reduce the critical current density to achieve low power consumption of SOT-based spintronic devices. The existing methods determine the critical current density by macro-spin model and domain wall (DW) model. According to the macro-spin model, the predicted critical current density is linearly related to the perpendicular magnetic anisotropy field and the applied in-plane magnetic field, while the domain wall (DW) model shows 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 prediction of the macro-spin model and the domain wall model. Therefore, to solve the difference between the theoretical model and the experimental measurement, a more in-depth study of the critical current of PMS is needed to obtain a method for reducing the critical current density.

[0004] At present, there are methods for reducing the critical current density of SOT-driven PMS by ion implantation or using topological insulators, but these methods have poor compatibility with existing micro-nano electronic processes and material systems, and are not suitable for integration into existing spintronic devices. SUMMARY

[0005] Based on the defects of the prior art, the present application provides a method for reducing the critical current density in SOT-driven magnetization switching, which solves the problem that the existing critical current density reduction method has poor compatibility with the existing micro-nano electronic processes and material systems, and is not suitable for integration into existing spintronic devices.

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

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

[0008] A structural model of a magnetic thin film is established, the magnetic thin film is a double-layer structure, wherein a lower layer is a heavy metal layer for providing a spin-orbit torque (SOT), and an upper layer is a magnetic layer for generating a magnetization flipping; two defect regions are symmetrically arranged in the magnetic layer along an x-axis direction, physical parameter values of the defect regions are less than physical parameter values of the magnetic layer; an interface between the magnetic layer and the defect regions is a domain wall;

[0009] Micro-magnetic simulation is performed on the structural model to obtain a magnetic domain and domain wall evolution process, and a critical current density in SOT-driven magnetization flipping is determined based on the evolution process, wherein the critical current density is a minimum current density for driving a domain wall movement, and an SOT effective field generated by the critical current density is a minimum SOT effective field required for de-pinning the domain wall from the defect region.

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

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

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

[0013] Preferably, the structural model has a length of 1024 nm and a width of 128 nm, an area of each defect region is 8 nm x 8 nm, and a distance between the defect positions and a center position is 64 nm.

[0014] Preferably, the micro-magnetic simulation on the structural model to obtain the magnetic domain and domain wall evolution process comprises the following steps:

[0015] A magnetic field H z = 1500 Oe is applied to the structural model, and an initial state of the upper layer is set as a magnetization saturation along a +z direction;

[0016] A fixed magnetic field H x = 500 Oe is applied to the structural model, and a current density j is scanned from a -x direction to an +x direction;

[0017] When the current density j is scanned from 1.50 x 10 5 A cm -2 to 4.95 x 10 5 A cm -2 along the +x direction, the magnetic domain and domain wall evolution process are obtained.

[0018] Preferably, after the micro-magnetic simulation on the structural model, the following steps are further included:

[0019] A domain wall dynamics calculation is performed on the structural model to obtain a domain wall movement speed under different external magnetic fields;

[0020] The value of the external magnetic field intensity at which DW1 and DW2 reach the same speed is determined according to the speed of the domain wall under the action of different external magnetic fields, wherein DW1 is a domain wall from top to bottom, and DW2 is a domain wall from bottom to top.

[0021] The range of the applied magnetic field change when the SOT driven magnetization is reversed is determined by the value of the external magnetic field intensity.

[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 a plurality of curves of the z direction magnetization intensity varying with the current density, and a plurality of critical current density values are obtained according to the plurality of curves.

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

[0024] The present application designs a magnetic thin film and establishes a corresponding structural model, and performs micromagnetic simulation on the structural model. Two defect regions are provided on the magnetic thin film, and the physical parameters of the defect regions are smaller than the physical parameters of the magnetic layer. The physical parameter values of the defect regions are smaller than the physical parameter values of the magnetic layer. The design makes the defect regions relative to the main body of the magnetic layer be soft magnetic materials. When an external current or magnetic field is applied, it is easy to form a reverse magnetic domain nucleation point in the defect region, thereby forming a domain wall required for magnetization reversal. At the same time, due to the symmetrical distribution of the two defect regions, the symmetrical defect design can form two domain walls. When performing micromagnetic simulation, the applied current only needs to drive the two domain walls to move relative to each other, so as to make the entire material system undergo magnetization reversal. The critical current density only needs to drive the domain wall to be de-pinned from the defect and move, and does not need to reverse the magnetization intensity of the entire material system. Therefore, the critical current density can be significantly reduced. The present application reduces the critical current density in SOT driven magnetization reversal through the magnetic thin film, and can be compatible with existing micro-nano electronic processes and material systems, thereby providing a new way for the research and development of low-power and high-integration of spintronic devices, and opening up a broader prospect for the application of spintronic devices in storage and logic devices. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0026] Figure 1 The structural model of the magnetic thin film of the present application is shown in the figure.

[0027] Figure 2SOT driven PMS at low current density for the micromagnetic simulation implementation of the present application;

[0028] Figure 3 Domain wall motion and magnetization switching for the micromagnetic simulation of the present application;

[0029] wherein, Figure 3 (a) of FIG. 1: the scanning current density j is 1.50×10 5 A cm -2 Domain wall motion diagram of (b) of FIG. 1, Figure 3 (a) of FIG. 1: the scanning current density j is 1.50×10 5 A cm -2 Domain wall motion diagram of (b) of FIG. 1, Figure 3 (a) of FIG. 1: the scanning current density j is 1.50×10 5 A cm -2 Domain wall motion diagram of (b) of FIG. 1, Figure 3 (a) of FIG. 1: the scanning current density j is 1.50×10 5 A cm -2 Domain wall motion diagram of (b) of FIG. 1;

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

[0031] wherein, Figure 4 (a) of FIG. 2: the "top-down" and "bottom-up" domain wall profile, Figure 4 (b) of FIG. 2: the spherical coordinate system describing the domain wall orientation and shape;

[0032] Figure 5 Domain wall motion speed change with external magnetic field for the domain wall design calculation of the present application;

[0033] Figure 6 Measurement result diagram of the comparative experiment;

[0034] Figure 7 Flowchart of the method for reducing the critical current density in SOT driven magnetization switching of the present application. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0036] Reference Figure 7This invention provides a method for reducing the critical current density in SOT-driven magnetization reversal, specifically a method for reducing the critical current required to drive a PMS using SOT and improving its robustness, so as to achieve SOT-driven PMS at low current densities while exhibiting strong resistance to the influence of external magnetic fields. The method includes the following steps:

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

[0038] This invention designs such as Figure 1 The structural model shown includes defect design, wedge design, and domain wall design. Its basic form is a Pt / CoPt bilayer structure. The lower Pt layer is a heavy metal layer that provides spin current and SOT to the upper CoPt layer through the spin Hall effect. The upper CoPt layer is a magnetic layer and is the host material for driving PMS with SOT. This structural model is 1024 nm long and 128 nm wide. The height slopes along the y-axis, with a minimum of 1.3 nm and a maximum of 10 nm. The CoPt layer has a wedge-shaped structure in the yz plane. Two defect regions with an area of ​​8 nm × 8 nm are symmetrically arranged relative to the center, with the height consistent with the host material, and the defect location is 64 nm from the center. The boundary between the host material and the defect region is the domain wall region. The domain walls are magnetic domain walls, serving as transition regions between two magnetic domains. The domain walls are designed with a rigid profile and a left-handed Néel-type structure.

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

[0040] In the micromagnetic simulation, different physical parameters were used for the CoPt layer bulk material and the defect region, including saturation magnetization M, exchange stiffness A, and perpendicular magnetic anisotropy K. p The magnetodynamic evolution of the entire structural model was simulated. When a current and a magnetic field were applied along the x-axis, the magnetization at the center of the structural model was probed to determine if a magnetization reversal occurred. The smallest unit size used for the simulation was 4 nm × 4 nm × 1.3 nm, and the steady state of each unit was obtained after 5 ns of current and magnetic field application.

[0041] This invention uses Figure 1 The structural model shown was used to perform micromagnetic simulations of the aforementioned magnetic thin film. The physical parameters used in the defect region are smaller than those used in the magnetic layer. The physical parameters used for the host material of the CoPt layer in the simulation calculation are: M host =570kA m -1 A host =26.8pJ m -1 , Dzyaloshinskii-Moriya interaction (DMI) strength D host=1.6mJ m -2 Damping factor α = 0.1, spin Hall angle θ SH =0.032. The physical parameter used for the defect region (defc) is: M defc =148kA m -1 A defc =10.2pJ m -1 , D defc =0.54mJ m -2 With other parameters remaining unchanged, the in-plane magnetic field H along the x-direction of fixed magnitude was calculated. x Below, the magnetization M in the z-direction z The curve showing the change with current density j, as shown below. Figure 2 As shown. An external magnetic field H in the positive direction. x Below, the magnetization M in the z-direction z The curve showing the change in current density j cycles clockwise. For the absence of an external magnetic field (H... x =0) and each applied magnetic field H x (=500, 100, 1500), deterministic SOT-driven PMS was observed, and the critical current density j for magnetization reversal was obtained. crit . Figure 2 This indicates that when H x When j increases from 0 to 1500Oe, crit From 5.17×10 5 A cm -2 Reduced to 2.43 × 10 5 A cm -2 This indicates that the structural model designed in this invention operates at low current density "10 5 A cm -2 At the scale of "", a SOT-driven PMS was implemented.

[0042] Figure 3 For the purposes of this invention Figure 1 The structural model shown is in a fixed H x Micromagnetic simulations of reverse domain nucleation and domain wall motion induced by current j at a current j = 500 Oe were performed. First, a magnetic field H was applied... z =1500Oe, the initial state of the CoPt magnetic layer is set to magnetization saturation along the +z direction. Then a fixed magnetic field H is applied. x =500Oe, current density j is scanned from -x to +x. When scanning the current density j along the +x direction from 1.50 × 10 5 A cm -2As the current density gradually increases, we can see the reverse magnetic domains gradually nucleating and growing. When the current density along the +x direction reaches the critical value of 4.95 × 10⁻⁶... 5 A cm -2 At this time, domain wall motion causes the reverse magnetic domains to grow rapidly, leading to the generation of PMS. This micromagnetic simulation of domain and domain wall evolution shows that in the structural model of this invention, PMS is caused by domain wall motion driven by SOT. The critical current density at which PMS occurs corresponds to an effective SOT field that is just sufficient to depin the domain walls from the defects, allowing the domain walls to move freely. Due to the non-uniformity of the defect region and the bulk material in the CoPt layer, the applied current only needs to overcome the domain wall pinning at the defects, without needing to reverse the magnetization of the entire bulk material; therefore, SOT-driven PMS can be achieved at low current densities.

[0043] Ideally, SOT-driven PMS should not require an external in-plane magnetic field, which greatly simplifies device design and avoids Joule heating and energy consumption caused by magnetic field requirements. However, SOT-driven PMS typically disappears under the influence of an external in-plane magnetic field. Therefore, research on SOT-driven PMS urgently needs to find methods to maintain stable PMS over a large range of external magnetic field variations, i.e., to improve the robustness of SOT-driven PMS.

[0044] Figure 4 This invention designs domain walls in a structural model to calculate their velocity under the influence of current and magnetic field applied along the x-axis. Due to the strong Dzyaloshinskii-Moriya interaction (DMI) at the Pt / CoPt interface, left-handed Néel-type domain walls form in the CoPt magnetic layer, and magnetization reversal of the CoPt layer is achieved through domain wall motion. This invention introduces a rigid profile, a DMI effective field, a damping-like SOT effective field, a field-like SOT effective field, and a domain wall demagnetizing field into the domain wall design, which can effectively describe the domain wall motion and magnetization reversal under the influence of current and magnetic field.

[0045] Magnetization vector along the x-axis in this domain wall design Represented in spherical coordinates Where M is the magnetization intensity The size of θ is relative to The polar angle of the direction, ψ is relative to Azimuth of direction, such as Figure 4As shown in (b). The domain walls "from top to bottom" and "from bottom to top" are called DW1 and DW2, respectively, and the outlines of DW1 and DW2 are as follows. Figure 4 As shown in (a). In this domain wall design, the domain wall velocity is used. The domain wall is represented by the azimuth angle ψ at any position x and any time t.

[0046] As a crucial foundation for this domain wall design, the rigid profiles of DW1 and DW2 are given by the following equation:

[0047]

[0048] Where θ i for Regarding the polar angle along the z-axis, x represents any position of the CoPt layer along this direction, and q i Let t be the center position of the domain wall at any time t, and Δ be the width of the domain wall (subscripts i = 1, 2 correspond to DW1 and DW2 respectively).

[0049] Based on this domain wall design, this invention uses the Landau-Lifshitz-Gilbert (LLG) equation to describe the domain wall motion in the CoPt magnetic layer, introducing the precession and damping of the magnetic moment into the equation. Damped SOT caused by directional spin polarization, and Field-like SOT caused by directional spin polarization:

[0050]

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

[0052]

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

[0054] By combining equations (1) and (2) describing the domain wall profile, the LLG equation (3), and the expression for the magnetic static energy density (4), this invention can obtain the velocities of DW1 and DW2. and azimuth ψ i Differential equations:

[0055]

[0056] in,

[0057]

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

[0059] Figure 5 This invention is based on Figure 4 The domain wall design shown is used to calculate the domain wall motion velocity to demonstrate the robustness of the SOT-driven PMS in the structural model of this invention. The physical parameters used are: Δ = 4.3 nm, u1 = u2 = 0, α = 0.1, β1 = β2 = 0, M1 = M2 = 570 emu cm. -3 ,

[0060]

[0061] In principle, when DW1 and DW2 have the same velocity, they exhibit energy degeneracy, meaning the motions of the two domain walls are completely identical, thus preventing PMS (partial magnetic field loss). This invention defines an external magnetic field H that causes DW1 and DW2 to reach the same velocity. x For degenerate field H deg This is used to describe the robustness of SOT-driven PMS: it means that even in the presence of an external magnetic field, when the external magnetic field reaches H deg Previously, PMS would not disappear; but when the external magnetic field H... x Achieving a degenerate field H that makes the velocities of DW1 and DW2 the same deg At that time, the SOT-driven PMS was corrupted. From Figure 5 As can be seen from H x =H deg At -3500 Oe, DW1 and DW2 have the same speed, and PMS no longer occurs. However, at H... xWithin a large magnetic field range of >-3500 Oe, the velocities of DW1 and DW2 are different, which can guarantee the deterministic SOT-driven PMS. Therefore, the structural model designed in this invention has strong robustness to the influence of external magnetic fields.

[0062] Example

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

[0064] Sample preparation and measurement methods

[0065] A ferromagnetic thin film with a CoPt magnetic layer was prepared on a Si substrate (surface covered with SiO2) using magnetron sputtering. The overall structure of the sample is Ta2 / Pt5 / [Co0.5 / Pt0.3 / Co0.5] / Ru2, where the numbers represent the thickness of each layer (in nm). The CoPt magnetic layer, composed of [Co0.5 / Pt0.3 / Co0.5], exhibits perpendicular magnetic anisotropy, and the bottom Ta / Pt / Co interface dominates the SOT effect and DMI at the interface. To perform Hall measurements on the sample, the Hall bar structure of the thin film was prepared using 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. The in-plane magnetic field H was measured... x The magnetization reversal caused by the lower SOT was achieved by applying pulsed currents of different amplitudes and durations of 0.1s to the Hall bar, with the current I and the in-plane magnetic field H... x Both are along the x-axis; after the pulse current has completed its action, a DC current I is used. dc =0.1~0.5mA for measuring Hall voltage V dc All measurements of magnetic and electrical transport were performed at room temperature.

[0066] Experimental measurement results

[0067] PMS was measured on a CoPt film with the structure Ta2 / Pt5 / [Co0.5 / Pt0.3 / Co0.5] / Ru2, and fixed H was obtained. x The corresponding critical current intensity j crit ,like Figure 6 As shown. A positive magnetic field H in a fixed plane. x Below, Hall resistor R H The curve cycles clockwise as the current j changes. For each applied magnetic field H... x A deterministic magnetization reversal was observed, and the critical current density j for the magnetization reversal was obtained. crit . Figure 6 This indicates that when H xj increased from 0 to 1500 Oe crit from 5.11 x 10 7 A cm -2 decreased to 2.65 x 10 7 A cm -2 The measured results of the present application are shown in Figure 2 x j increased from 0 to 1500 Oe crit from 5.17 x 10 5 A cm -2 decreased to 2.43 x 10 5 A cm -2 This indicates that the structure model designed in the present application has achieved the reduction of critical current density at the order of low current density "10 5 A cm -2 ".

[0068] While the preferred embodiments of the application have been described, additional modifications and changes can occur to those skilled in the art once they learn of the basic creative principles disclosed herein. Accordingly, it is intended that the claims be construed as including all such modifications and changes as fall within the scope of the present application.

[0069] Obviously, many modifications and variations of the present application are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims and their equivalents, the application can be practiced otherwise than as specifically described.​

Claims

1. A method for reducing the critical current density during SOT-driven magnetization reversal, characterized in that, Includes the following steps: A structural model of a magnetic thin film is established. The magnetic thin film has a two-layer structure, wherein 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. Along the magnetic layer... x Two defect regions are symmetrically arranged along the axial direction, and the physical parameter values ​​of the defect regions are lower than those of the magnetic layer; the boundary between the magnetic layer and the defect regions is a domain wall; the magnetic layer is... - The cross-section of the surface is a wedge-shaped structure, with a height along... The axis is tilted; the material of the magnetic layer is CoPt, and the material of the heavy metal layer is Pt; 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 location is 64 nm away from the center. Micromagnetic simulations were performed on the structural model 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 was determined. The critical current density is the minimum current density that drives the domain wall motion. The SOT effective field generated by this critical current density is the minimum SOT effective field required to depin the domain wall from the defect region.

2. The method for reducing the critical current density during SOT-driven magnetization reversal as described in claim 1, characterized in that, The physical parameters include saturation magnetization, exchange stiffness, perpendicular magnetic anisotropy, and interaction strength.

3. The method for reducing the critical current density during SOT-driven magnetization reversal as described in claim 1, characterized in that, The process of performing micromagnetic simulation on the structural model to obtain the evolution of magnetic domains and domain walls includes the following steps: Applying a magnetic field to the structural model Set the initial state of the upper layer to along Directional magnetization saturation; Apply a fixed magnetic field to the structural model ,from arrive Directional scanning current density ; When along Directional scanning current density from arrive At that time, the evolution process of magnetic domains and domain walls is obtained.

4. The method for reducing the critical current density during SOT-driven magnetization reversal as described in claim 1, characterized in that, After performing micromagnetic simulation on the structural model, the process also includes: Domain wall dynamics calculations were performed on the structural model to obtain the domain wall velocity under different external magnetic fields. The external magnetic field strength value that makes DW1 and DW2 reach the same speed is determined based on the domain wall movement velocity 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. The range of external magnetic field variation during SOT-driven magnetization reversal is determined by this external magnetic field strength value.

5. The method for reducing the critical current density during SOT-driven magnetization reversal as described in claim 1, characterized in that, During the micromagnetic simulation of the structural model, an along-... The in-plane magnetic field in the direction is obtained. Multiple curves showing the change of directional magnetization with current density are used to obtain multiple critical current density values.

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