Spin storage devices based on artificial antiferromagnetic exchange-coupled field gradient field-gradient field-free reversal

Abstract

This invention discloses a spin memory device based on an artificial antiferromagnetic exchange-coupled field gradient with field-free switching. From bottom to top, it comprises a substrate layer, a buffer layer, a spin flow layer, a composite free layer, a barrier layer, a reference layer, and a top electrode layer. The composite free layer includes two ferromagnetic layers and a wedge-shaped coupling layer disposed between the ferromagnetic layers. The wedge-shaped coupling layer can adjust the interlayer exchange intensity and generate a large exchange field gradient, thereby providing an additional out-of-plane torque, helping the device achieve a high ratio of field-free magnetization switching under current drive. Compared to single ferromagnetic or antiferromagnetic materials, the spin memory device fabricated using artificial antiferromagnetic materials in this application has the characteristics of zero net magnetic moment, strong anti-interference ability, fast switching speed, and ease of reading and writing, making it highly competitive in the field of magnetic storage.

Description

Technical Field

[0001] This invention belongs to the field of spin memory technology, specifically relating to a spin memory device based on field-free flipping of an artificial antiferromagnetic exchange-coupled field gradient. Background Technology

[0002] Spin-orbit torque (SOT), as an emerging magnetization manipulation method, has proven to drive vertically magnetized ferromagnets to flip and move domain walls with higher efficiency and lower power consumption compared to spin-transfer torque. It holds broad application prospects in low-power, non-volatile memories and logic devices. A typical SOT device consists of a heavy metal / ferromagnetic / oxide structure. In this structure, when in-plane charge current flows through the heavy metal layer, a pure spin current is generated by the spin Hall effect or the interface Rashba effect and injected into the ferromagnetic layer, applying SOT to the ferromagnetic layer and thus pulling the magnetic moment of the ferromagnetic layer to flip. The ferromagnetic layer allows for better signal read / write operations in the device.

[0003] In traditional magnetic tunnel junctions, effective flipping can be ensured through spin-transfer torque. However, achieving high storage density and small size requires reducing stray fields and enhancing thermal stability. Antiferromagnetic materials possess advantages such as zero net magnetic moment, strong anti-interference ability, and high speed, making them a hot topic in information storage due to their stability and speed. However, writing and reading signals in antiferromagnetic storage layers remains challenging.

[0004] Artificial antiferromagnetic materials (SAFs) can effectively address the problems of poor thermal stability and difficulty in reading and writing information from antiferromagnetic materials. SAFs are formed by the interaction and exchange coupling between two ferromagnetic materials via Ruderman-Kittel–Kasuya–Yoshida (RKKY). They combine the advantages of zero stray field and high stability of antiferromagnetism with the easy readability and writeability of ferromagnetism, potentially significantly improving the data density and stability of spintronic devices. In the structure of an SAF, the spin dynamics driven by magnetic fields and currents depend on the interlayer exchange coupling. Ensuring the exchange coupling field gradient of the artificial antiferromagnetic material to achieve field-free SOT flipping is crucial for spintronic devices. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention proposes a field-free spin memory device based on artificial antiferromagnetic exchange-coupled field gradient flipping. It improves the composite free layer structure of the smallest memory cell to achieve field-free SOT flipping, thereby increasing the storage density, thermal stability, and reducing power consumption of the memory device.

[0006] A spin memory device based on an artificial antiferromagnetic exchange-coupled field gradient field-free flipping structure comprises multiple minimum memory cells. Each minimum memory cell, from bottom to top, consists of a substrate layer, a buffer layer, a spin flow layer, a composite free layer, a barrier layer, a reference layer, and a top electrode layer. The composite free layer, from bottom to top, consists of a first ferromagnetic layer, a coupling layer, and a second ferromagnetic layer.

[0007] The buffer layer is a Ta film with a thickness of 1 nm. The spin flow layer is a Pt film with a thickness of 5.5 nm. The first ferromagnetic layer is a Co film with a thickness of 0.6–1.4 nm; the coupling layer is a wedge-shaped Ir film with a center thickness of 0.6–0.7 nm; the second ferromagnetic layer consists of a Co film and a CoFeB film from bottom to top, wherein the Co film has a thickness of 0.1–0.2 nm.

[0008] Preferably, the first ferromagnetic layer consists of a Co film, a Pt film, and another Co film from bottom to top, wherein the Co film has a thickness of 0.4 nm and the Pt film has a thickness of 0.3 nm.

[0009] Preferably, the buffer layer and the spin current layer are a composite structure formed by a W film and a Ta film. The first ferromagnetic layer consists of a CoFeB film and a Co film from bottom to top, wherein the thickness of the Co film is 0.1–0.2 nm. The W film is used to provide spin current, and the Ta film, with a thickness of 0.2 nm, is used to improve the crystal structure of the CoFeB film in the first ferromagnetic layer and increase the tunneling magnetoresistance.

[0010] Preferably, the composite free layer further includes an isolation and coupling adjustment layer and a CoFeB layer located above the second ferromagnetic layer, wherein the isolation and coupling adjustment layer is selected as a Ta film, Ir film, Mo film, or Ti film with a thickness of 0.2–0.3 nm. The second ferromagnetic layer consists of a Co film, a Pt film, and a Co film from bottom to top, wherein the Co film has a thickness of 0.6 nm and the Pt film has a thickness of 0.3 nm. The coupling layer is a wedge-shaped Ru film with a center thickness of 0.6–0.7 nm.

[0011] Preferably, the substrate layer is Si / SiO2, wherein the SiO2 layer has a thickness of 500 nm, which is used to isolate Si and the buffer layer to prevent leakage.

[0012] Preferably, the barrier layer is made of MgO material with a thickness of 1 nm to provide potential energy for electron tunneling.

[0013] Preferably, the reference layer comprises, from bottom to top, a CoFeB film, a Co film, an Ir film, another Co film, and a Pt / Co multilayer film, wherein the CoFeB film has a thickness of 1 nm; the Co film has a thickness of 0.1–0.2 nm; and the Ir film has a thickness of 0.5–0.9 nm, to achieve antiferromagnetic coupling between the layers; the Pt / Co multilayer film comprises a 5-layer Pt / Co bilayer structure, wherein each Pt / Co bilayer structure comprises a Pt film with a thickness of 0.3 nm and a Co film with a thickness of 0.5 nm.

[0014] The spin memory device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping includes two write terminals and one read terminal. The read direction is the same as the magnetization direction of the reference layer, and the write direction is perpendicular to the read direction. Therefore, the read path and the write path are independent and do not interfere with each other. When the magnetization direction of the composite free layer is the same as the magnetization direction of the reference layer, the smallest memory cell is in a low tunnel magnetoresistance state; when the magnetization direction of the composite free layer is not the same as the magnetization direction of the reference layer, the smallest memory cell is in a high tunnel magnetoresistance state. A write current flows through the write terminal, generating a spin current in the spin current layer of the smallest memory cell and injecting it into the composite free layer, changing the magnetization direction of the composite free layer, thereby changing the high / low tunnel magnetoresistance state of the smallest memory cell, completing the data writing. A read current flows through the read terminal, measuring the high / low tunnel magnetoresistance state of the smallest memory cell, realizing data reading.

[0015] The present invention has the following beneficial effects:

[0016] 1. Compared to traditional storage devices, this invention improves the results of composite free layers by utilizing the exchange-coupled field gradient in an artificial antiferromagnetic environment. Without the assistance of an in-plane magnetic field, magnetization reversal can be achieved in the composite free layer of a SOT-MTJ through current control, thus storing information. This provides a new physical mechanism for field-free SOT reversal in SAFs and paves the way for high-density, low-spurious-field, and low-power vertically magnetized SAFs, making information storage methods in SOT-MTJs more diverse and of great significance to low-power non-volatile memories and logic devices.

[0017] 2. The wedge-shaped coupling layer in the composite free layer can play a dual role in regulating the interlayer exchange intensity and generating a huge exchange field gradient. It was measured that when the exchange field gradient reaches 10T / nm, the field-free SOT magnetization reversal ratio is observed to be 81%, and the maximum SOT polarity reversal point can reach 60Oe. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an artificial antiferromagnetic memory cell structure.

[0019] Figure 2 This is a schematic diagram of the stacked structure of the storage unit portion in Example 1;

[0020] Figure 3 This is a schematic diagram of the stacked structure of the storage unit portion in Example 2;

[0021] Figure 4 This is a schematic diagram of the stacked structure of the storage unit portion in Example 3;

[0022] Figure 5 This is a schematic diagram of the stacked structure of the storage unit portion in Example 4;

[0023] Figure 6 This is a schematic diagram of the Hall Bar structure fabricated in the embodiment;

[0024] Figure 7 This is a schematic diagram of the device hysteresis loop measured in the embodiment;

[0025] Figure 8 A schematic diagram of the magnetic moment flipping without magnetic field driven by the gradient of the exchange coupling field. Detailed Implementation

[0026] The present invention will be further explained below with reference to the accompanying drawings;

[0027] Example 1

[0028] This embodiment provides a spin memory device based on an artificial antiferromagnetic exchange-coupled field gradient without field reversal. The memory device includes multiple minimum memory cells, such as... Figure 1 As shown, each memory cell, from bottom to top, consists of a substrate layer, a buffer layer, a spin current layer, a recombination free layer, a barrier layer, a reference layer, and a top electrode layer. The magnetization direction of the reference layer remains constant, while the magnetization direction of the recombination free layer is flipped by the write current. When the magnetization direction of the recombination free layer is aligned with that of the reference layer, it is called a parallel state, and the smallest memory cell is in a low tunnel magnetoresistance state. When the magnetization direction of the recombination free layer is not aligned with that of the reference layer, it is called a non-parallel state, and the smallest memory cell is in a high tunnel magnetoresistance state. By inputting a write current perpendicular to the magnetization direction of the reference layer, a spin current can be generated in the spin current layer and injected into the recombination free layer, changing the magnetization direction of the recombination free layer and thus changing the high / low tunnel magnetoresistance state of the smallest memory cell, thereby achieving data writing. When data reading is required, a read current with the same magnetization direction as the reference layer is input, and then the high / low tunnel magnetoresistance state of the smallest memory cell is measured based on the voltage to achieve data reading. The substrate layer is Si / SiO2, wherein the SiO2 layer is 500nm thick and is used to isolate Si and the buffer layer to prevent leakage.

[0029] like Figure 2As shown, the buffer layer is a 1 nm thick Ta film, used to better adhere the spin flow layer to the substrate and improve flatness. The spin flow layer is a 5.5 nm thick Pt film, which converts the input charge flow into a spin flow, which is then injected into the composite free layer through strong spin-orbit coupling, thereby applying torque. The composite free layer consists of a first ferromagnetic layer, a coupling layer, and a second ferromagnetic layer from bottom to top. The first ferromagnetic layer is a 0.6–1.4 nm thick Co film; the coupling layer is a wedge-shaped Ir film with a center thickness of 0.6–0.7 nm; and the second ferromagnetic layer consists of a Co film and a CoFeB film from bottom to top, with the Co film having a thickness of 0.1–0.2 nm.

[0030] The wedge-shaped coupling layer provides exchange coupling between the two ferromagnetic layers, while also regulating the interlayer exchange strength and generating a huge exchange field gradient. It also breaks the spatial inversion symmetry and generates an equivalent field in the z-axis direction, thereby achieving magnetization reversal without a magnetic field.

[0031] The barrier layer is made of MgO material with a thickness of 1 nm, which provides potential energy for electron tunneling.

[0032] The reference layer comprises, from bottom to top, a CoFeB film, a Co film, an Ir film, another Co film, and a Pt / Co multilayer film. The CoFeB film has a thickness of 1 nm; the Co film has a thickness of 0.1–0.2 nm; and the Ir film has a thickness of 0.5–0.9 nm, to achieve antiferromagnetic coupling between the layers. The Pt / Co multilayer film comprises five Pt / Co bilayer structures, one of which includes a Pt film with a thickness of 0.3 nm and a Co film with a thickness of 0.5 nm.

[0033] The substrate layer is Si / SiO2, wherein the SiO2 layer is 500nm thick and is used to isolate Si and the buffer layer to prevent leakage.

[0034] The barrier layer is made of MgO material with a thickness of 1 nm, which provides potential energy for electron tunneling.

[0035] The reference layer comprises, from bottom to top, a CoFeB film, a Co film, an Ir film, another Co film, and a Pt / Co multilayer film. The CoFeB film has a thickness of 1 nm; the Co film has a thickness of 0.1–0.2 nm; and the Ir film has a thickness of 0.5–0.9 nm, to achieve antiferromagnetic coupling between the layers. The Pt / Co multilayer film comprises five Pt / Co bilayer structures, one of which includes a Pt film with a thickness of 0.3 nm and a Co film with a thickness of 0.5 nm.

[0036] Example 2

[0037] like Figure 3As shown, this embodiment provides a spin memory device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping. Based on Embodiment 1, the first ferromagnetic layer is replaced by a multilayer film structure instead of a single-layer film. The first ferromagnetic layer consists of a Co film, a Pt film, and another Co film from bottom to top, wherein the Co film has a thickness of 0.4 nm and the Pt film has a thickness of 0.3 nm.

[0038] Example 3

[0039] like Figure 4 As shown, this embodiment provides a spin memory device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping. Based on Embodiment 1, the buffer layer and spin current layer are replaced with a composite structure formed by a W film and a Ta film, with the Ta film having a thickness of 0.2 nm. The first ferromagnetic layer is then replaced with a CoFeB / Co bilayer structure, where the Co film has a thickness of 0.1–0.2 nm. The W film provides spin current, and the Ta film improves the crystal structure of the CoFeB film in the first ferromagnetic layer, thereby increasing tunneling magnetoresistance.

[0040] Example 4

[0041] like Figure 5 As shown, this embodiment provides a spin memory device based on an artificial antiferromagnetic exchange-coupled field gradient field-free flipping. The memory device is based on Embodiment 2, but the coupling layer material is replaced with Ru. The second ferromagnetic layer is replaced with a Co / Pt / Co three-layer structure, where the Co thickness is 0.6 nm and the Pt thickness is 0.3 nm. Furthermore, a Ta, Ir, Mo, or Ti film with a thickness of 0.2–0.3 nm is added above the second ferromagnetic layer as an isolation and coupling adjustment layer. A CoFeB layer is added above the isolation and coupling adjustment layer. The isolation and coupling adjustment layer is used to adjust the coupling between the CoFeB layer and the second ferromagnetic layer.

[0042] Using magnetron sputtering, a buffer layer, spin flow layer, composite free layer, barrier layer, and reference layer structure as described in Example 4 are sequentially sputtered above the substrate. Then, micro / nano fabrication methods such as photolithography, etching, and overlay are used to fabricate the structure as shown in Example 4. Figure 6 The Hall Bar structure shown has a current input terminal and a voltage readout terminal, both measuring 10 × 50 μm. Ti / Au electrodes are fabricated using magnetron sputtering for connection to the wires. An alternating current is applied to the current input terminal of the Hall Bar. The direction of the magnetic field to be measured is kept aligned with the Z-axis of the device. A magnetic field with an intensity increasing from -1T to 1T and then decreasing back to -1T is applied out-of-plane. The voltage across the voltage output terminal is collected, and the hysteresis loop of the device is measured. The results are as follows: Figure 7As shown, when the external magnetic field is in the range of -0.36T to 0.36T, the magnetic moments of the two ferromagnetic layers are oriented in opposite directions, exhibiting antiferromagnetic coupling. Gradually increasing the external magnetic field to 1T, the magnetic moments of both ferromagnetic layers are oriented in the same direction as the external magnetic field. Exchange coupling field (H ex The Hall resistance (R) is defined as the distance from the zero field during the second-step flip, as shown by the arrow in the diagram. Hall The value is defined as the difference between the positive and negative maximum magnetic field ordinates. It can be clearly observed that the device exhibits very strong perpendicular magnetic anisotropy and stepwise reversal of antiferromagnetic coupling, indicating that the device is suitable as a magnetic storage device with an artificial antiferromagnetic matrix.

[0043] In artificial antiferromagnetic structures, interlayer exchange coupling plays a crucial role in the spin dynamics driven by magnetic fields and currents. The strength of this exchange coupling depends on the thickness of the spacer layer; even small changes in the spacer layer thickness can have a significant impact on the exchange coupling. Introducing a wedge-shaped coupling layer into a composite free layer creates a height difference on both sides, resulting in a spatial variation in the interlayer exchange coupling strength. This generates an exchange coupling field gradient, enabling magnetic field-free spin reversal. Figure 8 As shown, SOT reversal was investigated by applying different in-plane auxiliary fields when the coupling layer thickness was 0.6 nm. It can be observed that when the applied pulse current was ±29 mA, the Hall resistance changed drastically, and the magnetic storage state transitioned between head-to-head (↓↑) and tail-to-tail (↑↓). Furthermore, current-driven magnetization reversal achieved a reversal ratio of 81% even without an external in-plane auxiliary field, indicating that magnetic field-free reversal is very well achieved when the exchange coupling field gradient is large.

Claims

1. A spin storage device based on an artificial antiferromagnetic exchange-coupled field gradient field without field reversal, characterized in that: It includes multiple minimum storage cells, each of which, from bottom to top, consists of a substrate layer, a buffer layer, a spin flow layer, a composite free layer, a barrier layer, a reference layer, and a top electrode layer; the composite free layer, from bottom to top, consists of a first ferromagnetic layer, a coupling layer, and a second ferromagnetic layer. The buffer layer is a Ta film with a thickness of 1 nm; the spin flow layer is a Pt film with a thickness of 5.5 nm; the first ferromagnetic layer is a Co film with a thickness of 0.6–1.4 nm; the coupling layer is a wedge-shaped Ir film with a center thickness of 0.6–0.7 nm; the second ferromagnetic layer consists of a Co film and a CoFeB film from bottom to top, wherein the Co film has a thickness of 0.1–0.2 nm.

2. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in claim 1, characterized in that: The first ferromagnetic layer consists of a Co film, a Pt film, and another Co film from bottom to top, wherein the Co film has a thickness of 0.4 nm and the Pt film has a thickness of 0.3 nm.

3. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in claim 2, characterized in that: The composite free layer further includes an isolation and coupling adjustment layer and a CoFeB layer located above the second ferromagnetic layer. The isolation and coupling adjustment layer is selected as a Ta film, Ir film, Mo film or Ti film with a thickness of 0.2 to 0.3 nm. The second ferromagnetic layer consists of a Co film, a Pt film and a Co film from bottom to top, wherein the Co film has a thickness of 0.6 nm and the Pt film has a thickness of 0.3 nm. The coupling layer is a wedge-shaped Ru film with a center thickness of 0.6 to 0.7 nm.

4. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in claim 1, characterized in that: The buffer layer and the spin flow layer are a composite structure formed by a W film and a Ta film, with the Ta film having a thickness of 0.2 nm; the first ferromagnetic layer consists of a CoFeB film and a Co film from bottom to top, with the Co film having a thickness of 0.1–0.2 nm.

5. The spin storage device based on field-free flipping of an artificial antiferromagnetic exchange-coupled field gradient as described in any one of claims 1 to 4, characterized in that: The substrate is Si / SiO2, wherein the thickness of SiO2 is 500 nm.

6. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in any one of claims 1 to 4, characterized in that: The barrier layer is made of MgO material with a thickness of 1 nm.

7. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in any one of claims 1 to 4, characterized in that: The reference layer comprises, from bottom to top, a CoFeB film, a Co film, an Ir film, a Co film, and a Pt / Co multilayer film, wherein the CoFeB film has a thickness of 1 nm; the Co film has a thickness of 0.1–0.2 nm; the Ir film has a thickness of 0.5–0.9 nm; and the Pt / Co multilayer film comprises 5 Pt / Co bilayer structures, one of which comprises a Pt film with a thickness of 0.3 nm and a Co film with a thickness of 0.5 nm.

8. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in any one of claims 1 to 4, characterized in that: It includes two write terminals and one read terminal, where the read direction is the same as the magnetization direction of the reference layer, and the write direction is perpendicular to the read direction. When the magnetization direction of the composite free layer is the same as the magnetization direction of the reference layer, the minimum storage cell is in a low tunnel magnetoresistance state; when the magnetization direction of the composite free layer is not the same as the magnetization direction of the reference layer, the minimum storage cell is in a high tunnel magnetoresistance state. The write current flows through the write terminal, generating a spin current in the spin current layer of the minimum storage cell and injecting it into the composite free layer, changing the magnetization direction of the composite free layer, thereby changing the high / low tunnel magnetoresistance state of the minimum storage cell and completing the data writing. Data reading is achieved by measuring the high / low state of the tunnel magnetoresistance of the smallest storage cell by passing a read current through the read terminal.

9. The spin storage device based on artificial antiferromagnetic exchange-coupled field gradient field-free flipping as described in any one of claims 1 to 4, characterized in that: A buffer layer, a spin flow layer, a composite free layer, a barrier layer, and a reference layer are sequentially sputtered on a substrate using magnetron sputtering. The desired shape is then fabricated using micro-nano fabrication methods such as photolithography, etching, and overlay. Finally, a top electrode layer is fabricated using magnetron sputtering to obtain the spin memory device.

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