A device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction

Abstract

A device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction. A device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction. (1) A heterojunction is formed by covering an antiferromagnetic topological insulator with a low-dimensional ferromagnetic insulator; there are two configurations of the heterojunction, namely, the ferromagnetic insulator completely covering the antiferromagnetic topological insulator and the ferromagnetic insulator semi-covering the antiferromagnetic topological insulator; (2) A non-magnetic insulating protective layer is covered above the heterojunction; (3) In the heterojunction, an obvious exchange bias effect can be observed; (4) By changing the coverage range of the ferromagnetic insulator, the sign of the exchange bias can be regulated; (5) The material system used for the device is van der Waals layered materials; by changing the coverage range of the ferromagnetic insulator, the sign of the exchange bias can be regulated; below the Curie temperature and Néel temperature of the magnetic material, the anomalous Hall effect can be measured by using low temperature and magnetic field for magnetotransport measurement.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnetic storage, and particularly relates to a device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction. Background Art

[0002] The exchange bias effect was first observed in Co nanoparticles encapsulated by CoO. Subsequently, a large number of studies on the exchange bias effect have emerged. The exchange bias has important application values in magnetic storage and magnetic recording devices. The exchange bias effect is caused by the exchange interaction at the ferromagnetic / antiferromagnetic interface, so the interface quality plays an important role in the exchange bias effect. However, there are some inevitable problems in the ferromagnetic / antiferromagnetic heterojunctions prepared by traditional thin film deposition methods, such as element diffusion between different material layers, stress at the interface, lattice matching, etc., which limit the systems that can achieve exchange bias. While magnetic van der Waals layered materials (such as CrI3, CrBr3, Cr2Si2Te6, VSe2, Cr2Ge2Te6, Fe3GeTe2, etc.) provide a new material system platform for the study of exchange bias. The rich magnetic van der Waals layered materials can provide stacking possibilities with multiple degrees of freedom, broaden the material selection range for the study of exchange bias, and can avoid surface reconstruction and component changes at the heterojunction interface.

[0003] For exchange bias, generally field cooling is required first. Taking the upward direction perpendicular to the sample plane as the positive direction (the same direction is used hereinafter without special instructions), the magnitude of the exchange bias is defined as μ0H eb =(μ0H c_r -μ0H c_l ) / 2, where μ0H c_r is the absolute value of the coercive field on the right side, and μ0H c_l is the absolute value of the coercive field on the left side. When μ0H eb is positive, it is defined as positive exchange bias; when μ0H eb is negative, it is defined as negative exchange bias.

[0004] In recent years, some beneficial progress has been made in the study of exchange bias in magnetic van der Waals materials. The National University of Singapore measured the exchange bias effect in a device of CrCl3 / Fe3GeTe2 heterojunction, and its μ0H ebThe maximum can reach 50 mT. Researchers at the Korea Institute of Science and Technology observed exchange bias in oxidized Fe3GeTe2, where the oxidized Fe3GeTe2 layer about a few nanometers on the sample surface is in an antiferromagnetic state, forming a ferromagnetic / antiferromagnetic interface with the unoxidized ferromagnetic Fe3GeTe2 to generate exchange bias. Researchers at Huazhong University of Science and Technology also observed obvious exchange bias phenomena in the MnPS3 / Fe3GeTe2 heterojunction and the MnPSe3 / Fe3GeTe2 heterojunction.

[0005] In these previous heterojunction studies, the sign of the exchange bias μ0H in the magnetic heterojunction eb depends on the magnetic interaction therein. When the field-cooling direction is positive, a negative exchange bias corresponds to a negative sign of μ0H eb . Under the opposite field-cooling direction, the sign of the obtained μ0H eb is also opposite; that is, a negative exchange bias has a positive μ0H in the negative field-cooling direction eb . When other conditions such as the field-cooling direction and measurement conditions remain unchanged, the sign of the exchange bias μ0H in the heterojunction of the same material type eb is basically unchanged. Therefore, it is very important to change the sign of the exchange bias by changing the device structure. Summary of the Invention

[0006] To solve the problems in the prior art, the object of the present invention is to provide a device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction and a preparation method thereof, aiming to change the sign of the exchange bias μ0H by changing the device configuration. eb

[0007] To achieve the above object, the technical solution adopted by the present invention is: a device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction.

[0008] The preparation method of the present invention includes the following steps: Among them, the preparation of the antiferromagnetic topological insulator / ferromagnetic insulator heterojunction includes the following steps:

[0009] Step 1: Use the mechanical exfoliation method to exfoliate the MnBi2Te4 bulk onto the tape to obtain a thin-layer MnBi2Te4 mesoscopic sample.

[0010] Step 2: Stick the tape with the MnBi2Te4 sample on the silicon oxide substrate, and then remove the tape to let the MnBi2Te4 sample remain on the silicon oxide substrate by van der Waals force.

[0011] Step 3: Fabricate gold electrodes on the MnBi2Te4 thin-layer sample through electron beam lithography, electron beam evaporation, and lift-off processes.

[0012] ​Step 4: Use the mechanical exfoliation method to exfoliate the CrI3 bulk onto the tape, then transfer the CrI3 flakes onto a polydimethylsiloxane (PDMS) film and attach it to a glass slide.

[0013] Step 5: Use the transfer platform and through the dry transfer process, transfer the CrI3 flakes onto the prepared MnBi2Te4 sample with electrodes. The CrI3 flakes can completely cover or partially cover the MnBi2Te4 sample.

[0014] Step 6: Use the mechanical exfoliation method to exfoliate the hexagonal boron nitride (h-BN) bulk onto the tape, transfer its flakes onto a polydimethylsiloxane film and attach it to a glass slide.

[0015] Step 7: Use the transfer platform and through the dry transfer process, transfer the h-BN flakes onto the prepared CrI3 / MnBi2Te4 heterojunction. The h-BN should cover the entire heterojunction to isolate it from the atmosphere.

[0016] As a preferred solution, the tape in Step 1, Step 4 and Step 6 is selected as the tape produced by Nitto of Japan.

[0017] As a preferred solution, the substrate in Step 2 is selected as a silicon wafer covered with 300 nanometers of silicon oxide.

[0018] As a preferred solution, the length and width dimensions of the MnBi2Te4 sample in Step 2 are on the order of ten micrometers, and the thickness is about ten nanometers.

[0019] As a preferred solution, the thickness of the gold electrode in Step 3 is 20 nanometers.

[0020] As a preferred solution, the thickness of CrI3 in Step 4 is dozens of nanometers.

[0021] As a preferred solution, the above steps need to be carried out in a glove box filled with inert gas of argon to protect the sample from air oxidation.

[0022] Thus, a heterojunction composed of a low-dimensional ferromagnetic insulator covering an antiferromagnetic topological insulator is obtained; here, the antiferromagnetic topological insulator is in the lower layer and the ferromagnetic insulator is in the upper layer (the positions of the two materials can be reversed); there are two configurations of the heterojunction, namely, the ferromagnetic insulator completely covers the antiferromagnetic topological insulator and the ferromagnetic insulator semi-covers the antiferromagnetic topological insulator; (2) a non-magnetic insulating protective layer is covered above the heterojunction; (3) an obvious exchange bias effect can be observed in the heterojunction; (4) by changing the coverage range of the ferromagnetic insulator, the sign of the exchange bias can be regulated; (5) the material system used in the device is a van der Waals layered material, which is beneficial to the preparation and integration of the device; the silicon oxide wafer, silicon wafer, glass, and ceramic can all be used as the substrate of the present invention.

[0023] The low-dimensional ferromagnetic insulators in the heterojunction device are CrI3 and CrBr3, and the low-dimensional antiferromagnetic topological insulator is MnBi2Te4.

[0024] The heterojunction device is field-cooled, and the positive direction of field-cooling is defined as the magnetic field perpendicular to the sample surface and upward (this direction is used unless otherwise specified); below the Curie temperature and Néel temperature of the magnetic material, magnetotransport measurements are carried out under low-temperature and magnetic field environments to measure the anomalous Hall effect; in the fully covered heterojunction device, a negative exchange bias effect can be observed; in the semi-covered heterojunction device, a positive exchange bias effect can be observed.

[0025] Preferably, the antiferromagnetic topological insulator is MnBi2Te4, the ferromagnetic insulator is CrI3, the thickness of MnBi2Te4 is on the order of ten nanometers, and no observable anomalous Hall effect can be found in overly thick MnBi2Te4 samples.

[0026] Preferably, a magnetic field is applied for field-cooling starting from a temperature above 100K. The starting temperature of field-cooling should exceed the magnetic critical temperatures of the two materials, and then the temperature is lowered below the magnetic critical temperatures of both to measure the exchange bias.

[0027] Beneficial effects: Compared with the prior art, through the technical solutions described in the present invention, the present invention can achieve opposite exchange bias effects in different heterojunction configurations under the same field-cooling direction through the two configurations of full coverage and semi-coverage of the heterojunction, that is, the sign of μ0H eb is opposite. By fully covering and semi-covering the surface of the antiferromagnetic material with the ferromagnetic insulator, the exchange bias will show opposite signs. When field-cooling is carried out with a magnetic field in the positive direction (perpendicular to the sample surface and upward), a negative exchange bias effect can be observed in the device where the ferromagnetic insulator fully covers the antiferromagnetic material; while in the device where the ferromagnetic insulator semi-covers the antiferromagnetic material, a positive exchange bias effect can be observed. The present invention uses an antiferromagnetic material as a platform for measuring the exchange bias, which is a new exchange bias system. Description of the Drawings

[0028] Figure 1A and Figure 1B are two schematic structural diagrams of the present invention. Figure 1A The left figure is the fully covered heterojunction structure. Figure 1B The right figure is the semi-covered heterojunction structure.

[0029] Figure 2 is the measurement schematic diagram of the exchange bias of the present invention, where the sample is along the xy plane, the magnetic field is along the z direction perpendicular to the sample plane, and the current is along the x direction; the Hall resistance R xy = V xy / I.

[0030] Figure 3A and Figure 3B are the schematic diagrams of positive and negative exchange bias of the present invention, Figure 3A the left figure corresponds to negative exchange bias, Figure 3B the right figure corresponds to positive exchange bias, where the field cooling direction is upward for both.

[0031] Figure 4A and Figure 4B are the measurement result diagrams of the exchange bias of the present invention; corresponding to the fully covered heterojunction structure and the semi-covered heterojunction structure respectively. Detailed implementation manners

[0032] Next, in combination with the accompanying drawings and examples, a device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction of the present invention will be further described in terms of technical solutions and advantages.

[0033] Example 1

[0034] Step 1: Use the tape produced by Nitto to pick up an appropriate amount of magnetic van der Waals layered materials MnBi2Te4 and CrI3 respectively, and then stick the tapes together about three times to thin the samples.

[0035] Step 2: Stick the tape with the thin-layer MnBi2Te4 sample on the silicon wafer substrate with 300-nanometer-thick silicon oxide, and then remove the tape so that the MnBi2Te4 sample remains on the silicon wafer substrate by van der Waals force. The obtained MnBi2Te4 sample has a thickness on the order of ten nanometers and a length and width on the order of ten micrometers.

[0036] Step 3: Fabricate gold electrodes on the thin-layer MnBi2Te4 sample through electron beam lithography, electron beam evaporation, and lift-off processes. The thickness of the gold electrodes is about 20 nanometers.

[0037] Step 4: Transfer the mechanically exfoliated CrI3 flakes to the PDMS and stick them on the glass slide.

[0038] Step 5: Use the transfer platform to transfer the CrI3 flakes to the prepared MnBi2Te4 sample with electrodes through a dry transfer process; for the dry transfer, the stage needs to be heated to about 50 °C after bonding, and then the glass slide is lifted; the CrI3 flakes can completely cover or partially cover the MnBi2Te4 sample.

[0039] Step 6: Use the transfer platform to transfer the h-BN flakes to the prepared CrI3 / MnBi2Te4 heterojunction through the same dry transfer process as in Step 5. The h-BN should completely cover the heterojunction to obtain fully covered (Figure 1) and partially covered (Figure 1) heterojunction devices respectively.

[0040] Step 7: Connect the encapsulated heterojunction sample prepared in Step 6 to the sample stage and place it in a low-temperature superconducting magnet.

[0041] Step 8: First cool down to a temperature of 100 K, then apply a positive magnetic field of 0.1 T, and then cool down to 2 K. This process is the positive field cooling.

[0042] Step 9: At 2 K, perform magnetotransport measurements on the device and measure the Hall effect of the device using the lock-in technique ( Figure 2 ).

[0043] Step 10: It can be observed that in the device where CrI3 completely covers MnBi2Te4, there is a negative exchange bias effect (Figures 3 and 4).

[0044] Step 11: For the heterojunction device with partial coverage, repeat Steps 8 and 9; it can be observed that in the device where CrI3 semi-covers MnBi2Te4, there is a positive exchange bias effect (Figures 3 and 4).

[0045] The above description is only the preferred embodiment of the present invention. It should also be pointed out that for those skilled in the art of this technology, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements are also within the protection scope of the present invention.

Claims

1. A device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction, characterized in that: (1) A heterojunction is formed by covering an antiferromagnetic topological insulator with a low-dimensional ferromagnetic insulator; there are two configurations of the heterojunction, namely, the ferromagnetic insulator completely covers the antiferromagnetic topological insulator and the ferromagnetic insulator semi-covers the antiferromagnetic topological insulator; (2) A non-magnetic insulating protective layer is covered on top of the heterojunction; (3) In the heterojunction, an obvious exchange bias effect can be observed; (4) By changing the coverage range of the ferromagnetic insulator, the sign of the exchange bias can be regulated; (5) The material system used for the device is van der Waals layered materials, which is conducive to the preparation and integration of the device; by changing the coverage range of the ferromagnetic insulator, the sign of the exchange bias can be regulated; Field cooling is performed on the heterojunction device, and the positive direction of field cooling is taken as the magnetic field perpendicular to the sample surface upward; below the Curie temperature and Néel temperature of the magnetic material, magnetic transport measurements are carried out using a low-temperature and magnetic field environment, and the anomalous Hall effect can be measured; in the fully covered heterojunction device, a negative exchange bias effect can be observed; in the semi-covered heterojunction device, a positive exchange bias effect can be observed; A magnetic field is applied for field cooling starting from a temperature above 100K, and the starting temperature of field cooling should exceed the magnetic critical temperatures of the two materials, and then the temperature is lowered below the magnetic critical temperatures of the two to measure the exchange bias; The low-dimensional ferromagnetic insulators in the heterojunction device are CrI3 and CrBr3, and the low-dimensional antiferromagnetic topological insulator is selected as MnBi2Te4.

2. The device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction according to claim 1, characterized in that: The thickness of the antiferromagnetic topological insulator material in the heterojunction device is on the order of ten nanometers.

3. The device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction according to claim 1, characterized in that: A non-magnetic insulating protective layer is covered on the heterojunction device, and hexagonal boron nitride or mica is selected.

4. The device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction according to claim 1, characterized in that: The upper and lower positions of the antiferromagnetic material and the ferromagnetic insulator material can be reversed or interchanged with each other.

5. The device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction according to claim 1, characterized in that: After field cooling with the positive direction being the magnetic field perpendicular to the sample surface upward, an exchange bias effect is observed in the heterojunction device, and the magnitude of the exchange bias is μ0H eb =(μ0H c_r - μ0H c_l ) / 2, where μ0H c_r is the absolute value of the coercive field on the right side, and μ0H c_l is the absolute value of the coercive field on the left side.

6. The device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction according to claim 1, characterized in that: A negative exchange bias appears in the device where the ferromagnetic insulator completely covers the antiferromagnetic material; A positive exchange bias appears in the device where the ferromagnetic insulator semi-covers the antiferromagnetic material.

7. The device with adjustable exchange bias based on a low-dimensional antiferromagnetic / ferromagnetic insulator heterojunction according to claim 1, characterized in that: The described exchange bias device is integrated for constructing a magnetic memory.

8. A method for preparing the device according to any one of claims 1-7, characterized in that, It includes the following steps: Step 1 Use the mechanical exfoliation method to exfoliate the MnBi2Te4 bulk onto the tape to obtain a thin-layer MnBi2Te4 mesoscopic sample; Step 2 Stick the tape with the MnBi2Te4 sample on a substrate such as a silicon oxide wafer, and then remove the tape so that the MnBi2Te4 sample remains on the substrate by van der Waals force; Step 3 Fabricate gold electrodes on the MnBi2Te4 thin-layer sample through electron beam lithography, electron beam evaporation, and lift-off processes; Step 4 Use the mechanical exfoliation method to exfoliate the CrI3 bulk onto the tape, and then transfer the CrI3 thin sheet to a polydimethylsiloxane (PDMS) film and stick it on a glass slide; Step 5 Use a transfer platform and through a dry transfer process, transfer the CrI3 thin sheet to the prepared MnBi2Te4 sample with electrodes, and the CrI3 thin sheet can completely cover or partially cover the MnBi2Te4 sample; Step 6 Use the mechanical exfoliation method to exfoliate the hexagonal boron nitride (h-BN) bulk onto the tape, transfer its thin sheet to a polydimethylsiloxane film, and stick it on a glass slide; Step 7: Using a transfer platform, transfer the h-BN flakes onto the prepared CrI3 / MnBi2Te4 heterojunction through a dry transfer process. The h-BN covers the entire heterojunction to isolate air.

9. The preparation method according to claim 8, characterized in that, The above operations are carried out in a glove box for isolating air, and the substrate is a silicon wafer covered with 300 nm of silicon oxide.

10. The preparation method according to claim 8, characterized in that, The length and width dimensions of the MnBi2Te4 sample in Step 2 are on the order of ten micrometers, and the thickness is about ten nanometers; the thickness of CrI3 in Step 4 is dozens of nanometers.

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