A method for realizing polymorphic storage based on CrOCl material

Abstract

The present invention discloses a method for realizing multi-state storage based on CrOCl material. The magnetoelectric coupling effect in the two-dimensional antiferromagnetic insulator CrOCl is utilized, and a voltage pulse is applied to the tunnel junction to control the metastable state in the magnetic phase change process of CrOCl, so that the resistance state of the tunnel junction can be controlled and switched arbitrarily within a certain range. The present invention determines the resistance state of the tunnel junction by changing the speed and amplitude of the voltage scan, and can erase the information stored in the tunnel junction by scanning the magnetic field. The storage of one decimal data can be realized in a single device, and it has the advantages of ultra-small size and ultra-high performance, and has the potential for large-scale application.

Description

Technical Field

[0001] The present invention relates to the field of spin electronics, and in particular to a method for applying a specific electric field in a two-dimensional magnetic material CrOCl tunnel junction to change the magnetization state and tunnel resistance of the tunnel junction. Background Art

[0002] Traditional spin electronics is based on ferromagnetic materials and their giant magnetoresistance effect. It measures and controls the direction of spin polarization of ferromagnetic materials through external magnetic fields and electric fields, thereby achieving data storage, computing and transmission. For most ferromagnetic materials, the polarization states in the two directions along the easy magnetization axis are energy-degenerate, and the conversion between these two states requires overcoming the magnetic anisotropy, so a certain electric or magnetic field energy is required to drive the flip, which constitutes the two states of "0" and "1" from the perspective of information science.

[0003] A series of recent research results show that compared with ferromagnetic materials, antiferromagnetic materials have more excellent properties in the application of spin electronics and are a material system with great potential for the development of a new generation of spin electronics devices. For example, antiferromagnetic materials are stable to external interference, that is, the spin information they store is non-volatile; the net magnetic moment is zero, so there is no leakage, which can realize smaller size and higher density array devices; the exchange interaction is strong, which can achieve ultra-fast operation; and the complex phase change behavior can construct more information storage states. However, the zero magnetic moment of antiferromagnetic materials also creates great difficulties for writing information. The spin transfer torque and spin-orbit torque commonly used in ferromagnetic materials are difficult to achieve effective magnetic moment reversal in antiferromagnetic materials. Therefore, how to control the magnetic phase change of antiferromagnetic materials with electrical signals is one of the most important and challenging problems in the field of spin electronics. Summary of the invention

[0004] In view of the problems existing in the regulation of antiferromagnetic materials mentioned above, the present invention proposes a method for realizing multi-state information storage by stably and controllably utilizing the magnetic phase change of the two-dimensional antiferromagnetic material CrOCl. The different resistance states used in the present invention are derived from the magnetic phase change of the CrOCl material at low temperature, that is, the CrOCl material undergoes an antiferromagnetic phase change at 14K, the magnetic ground state is a ↑↑↓↓ banded antiferromagnetic state, and its tunneling resistance exhibits a high resistance state; under an external magnetic field, CrOCl will undergo a phase change to a ↑↑↑↓↓ state, and its tunneling current exhibits a low resistance state.

[0005] In order to achieve the purpose of the present invention, the present invention provides a method for realizing polymorphic storage based on CrOCl material, and the specific steps include:

[0006] 1) preparing hBN-graphite-CrOCl-graphite-hBN tunnel junction devices;

[0007] 2) Cool the device to below 14K, measure the change of tunneling current with magnetic field, and find the magnetic field B0 corresponding to the maximum hysteresis in the magnetic phase transition and the magnetic field B corresponding to the disappearance of hysteresis. s ;

[0008] 3) Change the external magnetic field from B s Scan to B0 and fix the magnetic field at B0, the tunnel junction will be in the ↑↑↑↓↓ high current state, and by applying a voltage pulse, the tunnel junction can be adjusted to any resistance state between the ↑↑↑↓↓ high current state and the ↑↑↓↓ low current state. The specific resistance state is determined by the amplitude of the voltage pulse and the scanning speed, that is, information writing is realized; when a sufficiently large voltage pulse is applied, the tunnel junction resistance state will reach the ↑↑↓↓ low current state; by applying a magnetic field pulse to B s Then return to B0 and adjust the tunnel junction back to the ↑↑↑↓↓ high current state, thus erasing the information. Conversely, the magnetic field can be fixed at B0, the ↑↑↓↓ state can be used as the ground state, and the resistance state can be adjusted to any resistance state between the ↑↑↓↓ low current state and the ↑↑↑↓↓ high current state by applying different magnetic field pulses. The resistance value is determined only by the amplitude of the magnetic field pulse, thus realizing information writing. When the peak value of the applied magnetic field pulse exceeds B0, the resistance value can be adjusted to any resistance state between the ↑↑↓↓ low current state and the ↑↑↑↓↓ high current state. s , the tunnel junction resistance state will reach the ↑↑↑↓↓ state; applying a sufficiently large voltage pulse to erase the tunnel junction information will return it to the ↑↑↓↓ state, thus achieving information erasure.

[0009] In the above step 1), a two-dimensional layered boron nitride (hBN) material, a graphite material, and a CrOCl material are obtained by a mechanical exfoliation method, and the hBN material, the graphite material, the CrOCl material, the graphite material, and the hBN material are placed on a Si-SiO2 substrate in sequence by dry transfer, and a tunnel junction device is prepared by electron beam exposure, reactive ion etching, electron beam evaporation, and other steps.

[0010] The growth method of the CrOCl material in the present invention is a chemical vapor transport method, that is, CrCl3 and Cr2O3 are sealed in a quartz tube in an equal molar ratio for reaction. The quartz tube is placed in a double-temperature zone tube furnace, the source end is set to 940°C-950°C, and the low temperature end is set to 800°C-850°C. After two weeks of reaction, it is slowly cooled to room temperature. After cutting the quartz tube, a bulk CrOCl material can be obtained, and then a two-dimensional CrOCl material with a thickness of less than 20nm is obtained by mechanical stripping.

[0011] The two-dimensional hBN material in the present invention is obtained by mechanically exfoliating directly purchased hBN crystals, and the thickness can be between 10 and 50 nm.

[0012] The graphite material in the present invention is obtained by mechanically exfoliating directly purchased graphite crystals, and has a thickness of 1 to 3 nm. Too thin graphene exhibits obvious SdH oscillation phenomenon under a magnetic field, which has a greater impact on phase change measurement; too thick graphite is easy to crush the two-dimensional CrOCl during the transfer process, thereby causing device leakage.

[0013] The above step 2) requires measuring the tunneling current with increasing magnetic field (IB up ) and reduce (IB down ) change curve. The difference between the two |I(B up )-I(B down )| is at its maximum (i.e., the point where the hysteresis is maximum) and the magnetic field is B0 (generally between 3.0 and 3.8 T), where the metastable state has the largest range of continuous control. The magnetic field at the point where the two overlap (i.e., the point where the hysteresis disappears) is B s (generally between 5.0 and 6.0 T), this magnetic field is the minimum magnetic field required to erase the electric field to write information. The magnetic field can be along the out-of-plane (CrOCl material lattice c-axis) or in-plane (CrOCl material lattice a-axis), but not along the CrOCl material lattice b-axis, because the b-axis is the most difficult magnetization axis, and no phase change occurs along the b-axis. The phase change magnetic field along the c-axis is the smallest. During the measurement process, the tunneling current should be controlled within 50-100nA and the total power should be controlled within 1μW to avoid excessive tunneling junction current causing thermal effects. In addition, an ammeter with an accuracy of less than pA should be used for measurement and the measurement system should be properly electrostatically shielded to control the electromagnetic noise in the environment within 1 / 100 of the tunneling junction bias to avoid electromagnetic pulses in the environmental noise from writing information to the tunneling junction.

[0014] The physical principle used in the present invention is as follows: CrOCl undergoes an antiferromagnetic phase transition at 14K, and the magnetic ground state at low temperature is an ↑↑↓↓ banded antiferromagnetic state. The magnetic ground state leads to the breaking of rotational symmetry and translational symmetry, so that the magnetoelectric coupling effect can exist in the same period. Under the influence of the magnetoelectric coupling effect, the atoms of CrOCl are dislocated at low temperature to form a corresponding antiferroelectric structure. In the two-dimensional tunneling junction device structure, the huge electric field in the atomic-thickness insulating layer can cause the electric dipole moment to flip, thereby changing the resistance state of the tunneling junction; and in the magnetic phase transition process, due to the metastable state caused by a large amount of magnetoelectric coupling, the magnetic phase transition of CrOCl shows a huge hysteresis. Therefore, the metastable state in the phase transition process can be regulated by scanning the voltage, thereby changing the resistance state of the CrOCl tunneling junction.

[0015] The technical advantages of the present invention are mainly reflected in the following points:

[0016] First, compared with other two-dimensional magnetic materials, CrOCl has good air stability, low cleavage energy, and is easy to obtain thin layer samples.

[0017] Secondly, the magnetism and magnetoelectric coupling effects of CrOCl are independent of thickness and still exist in a single layer. As an antiferromagnetic material, it has the characteristics of no magnetic leakage and non-volatile information, so it has the potential to prepare large-scale high-performance nanoscale devices.

[0018] Finally, the continuous metastable state caused by magnetoelectric coupling in CrOCl allows its resistance state to be switched arbitrarily over a wide range. Its ability and efficiency to store information far exceeds that of traditional ferromagnetic materials, and it can be used for the storage and calculation of analog signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 .Optical photograph of the tunnel junction in a specific embodiment of the present invention.

[0020] Figure 2 . The curve of the tunnel junction current changing with the external magnetic field under the bias voltage of 5.5V in the specific embodiment of the present invention, through which the maximum hysteresis field B0 and the hysteresis vanishing field B can be obtained s .

[0021] Figure 3 .In a specific embodiment of the present invention, under a bias voltage of 5.5V, the change of the current of the tunnel junction with time after passing through different voltage pulses.

[0022] Figure 4 .In a specific embodiment of the present invention, under a bias voltage of 5.5V, the metastable state reached by the tunnel junction after passing through different magnetic field pulses. DETAILED DESCRIPTION

[0023] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0024] This example uses a 10nm CrOCl tunneling device to demonstrate the advantages of the storage device, which is stable, efficient, and can continuously adjust the resistance state:

[0025] (1) Device preparation. A two-dimensional layered hBN material with a thickness between 10 and 50 nm, a graphite material with a thickness of 1 to 3 nm, and a CrOCl material with a thickness of less than 20 nm were obtained by mechanical exfoliation. hBN, graphite, CrOCl, graphite, and hBN were placed on a Si-SiO2 substrate in sequence by dry transfer. A tunneling junction device was prepared by electron beam exposure, reactive ion etching, electron beam evaporation, and other steps. The optical photograph of the CrOCl tunneling junction device is shown in Figure 2. Figure 1 shown.

[0026] (2) Measure the change of tunneling current with magnetic field, such as Figure 2 As shown, the magnetic phase transition occurs at about 4T, and by reading |I(B up )-I(B down)|The maximum value can be obtained when the hysteresis is maximum. The magnetic field B0=3.33T, and the hysteresis disappears at B s =5.63T.

[0027] (3) The external magnetic field is increased to 8 T and then reduced back to 3.33 T. At this time, the tunnel junction is in a high current state of ↑↑↑↓↓. Subsequently, four voltage pulses of δV = 0.85 V, 1.01 V, 1.13 V and 2 V are applied respectively (i.e., the voltage is scanned from 5.5 V to 5.5 V + δV, and then scanned back to 5.5 V). The scanning speed is 0.005 V / s. After each voltage pulse, the tunnel junction will reach a new resistance state, such as Figure 3 As shown. The larger the voltage pulse, the closer the resistance state is to the ↑↑↓↓ low current state; the faster the scanning speed, the closer the resistance state is to the ↑↑↓↓ low current state. These resistance states are very stable and hardly change over time, which means that information is written; after a 2V voltage pulse, the tunnel junction reaches the ↑↑↓↓ low current state. If the magnetic field is swept to 8T and then swept back to 3.33T, the information stored by the electrical signal is erased, and the tunnel junction returns to the ↑↑↑↓↓ high current state, which means that information is erased.

[0028] (4) In contrast to (3), the magnetic field is fixed at B0, and the ↑↑↓↓ low current state is used as the ground state of the tunnel junction. By applying four magnetic field pulses of δB = 0.2T, 0.3T, 0.6T, and 0.7T (i.e., scanning the magnetic field from B0 to B0+δB and then back to B0), the tunnel junction reaches different resistance states between the ↑↑↓↓ low current state and the ↑↑↑↓↓ high current state, thus realizing information writing, as shown in FIG. Figure 4 As shown; when B0+δB exceeds B s When the tunnel junction reaches the ↑↑↑↓↓ high current state, the tunnel junction will reach the ↑↑↑↓↓ high current state. Finally, the tunnel junction information can be erased by a 2V voltage pulse, so that the tunnel junction resistance state returns to the ground state ↑↑↓↓ low current state, that is, information erasure is achieved. The current difference between the ↑↑↓↓ low current state and the ↑↑↑↓↓ high current state is 2nA, which is about 10% of the current value; and its current noise δI<0.02nA, so if |I n -I n+1 |>10δI, that is, the current difference between adjacent states is greater than ten times the noise, then each device can store at least one decimal data bit.

[0029] Finally, it should be noted that the purpose of publishing the embodiments is to help further understand the present invention, but those skilled in the art can understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection claimed by the present invention shall be subject to the scope defined in the claims.

Claims

1. A method for realizing polymorphic storage based on CrOCl material, characterized in that the steps include: 1) preparing hBN-graphite-CrOCl-graphite-hBN tunnel junction devices; 2) Cool the device to below 14K, measure the change of tunneling current with magnetic field, and find the magnetic field B0 corresponding to the maximum hysteresis in the magnetic phase transition and the magnetic field B corresponding to the disappearance of hysteresis. s ; 3) Change the magnetic field from B s Scan to B0 and fix the magnetic field at B0. The tunnel junction is in the ↑↑↑↓↓ high current state. By applying a voltage pulse, the tunnel junction is adjusted to any resistance state between the ↑↑↑↓↓ high current state and the ↑↑↓↓ low current state. The specific resistance state is determined by the amplitude of the voltage pulse and the scanning speed to achieve information writing. Apply a sufficiently large voltage pulse to make the tunnel junction resistance state reach the ↑↑↓↓ low current state. Then apply a magnetic field pulse to B s Then go back to B0 and adjust the tunnel junction back to the ↑↑↑↓↓ high current state to achieve information erasure; Alternatively, the magnetic field is fixed at B0, the ↑↑↓↓ low current state is taken as the base state, and the resistance state is adjusted to any resistance state between the ↑↑↓↓ low current state and the ↑↑↑↓↓ high current state by applying different magnetic field pulses. The resistance value is determined by the amplitude of the magnetic field pulse, and information writing is achieved; the peak value of the applied magnetic field pulse exceeds B0. s , the tunnel junction resistance state reaches the ↑↑↑↓↓ high current state; then a sufficiently large voltage pulse is applied to erase the tunnel junction information, causing it to return to the ↑↑↓↓ low current state, thereby achieving information erasure.

2. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: In step 1), two-dimensional layered hBN material, graphite material, and CrOCl material are first prepared, and then the hBN material, graphite material, CrOCl material, graphite material, and hBN material are placed on a Si-SiO2 substrate in sequence by a dry transfer method to prepare a tunnel junction device.

3. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: The thickness of the CrOCl material is within 20 nm.

4. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: The thickness of the hBN material is between 10 and 50 nm, and the thickness of the graphite material is between 1 and 3 nm.

5. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: The growth method of CrOCl material is chemical vapor transport, that is, CrCl3 and Cr2O3 are sealed in a quartz tube in an equal molar ratio, the quartz tube is placed in a double-temperature zone tubular furnace, the source end is set to 940℃-950℃, and the low-temperature end is set to 800℃-850℃. After two weeks of reaction, it is slowly cooled to room temperature. After cutting the quartz tube, bulk CrOCl is obtained, and then two-dimensional CrOCl material is obtained by mechanical exfoliation.

6. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: Step 2) Setting the magnetic field along the c-axis of the CrOCl material lattice or the a-axis of the CrOCl material lattice, but not along the b-axis of the CrOCl material lattice, measuring the curve of the tunneling current changing with the increase of the magnetic field and the curve of the tunneling current changing with the decrease of the magnetic field, the magnetic field at the maximum difference between the two changing curves is B0, and the magnetic field at the overlap of the two changing curves is B s .

7. The method for realizing polymorphic storage based on CrOCl material according to claim 6, characterized in that: During the measurement process, the tunneling current is controlled within 50-100 nA and the total power is controlled within 1 μW.

8. The method for realizing polymorphic storage based on CrOCl material according to claim 6, characterized in that: The measurement is performed using an ammeter with an accuracy within pA and the measurement system is electrostatically shielded to control the electromagnetic noise in the environment to within 1 / 100 of the tunnel junction bias.

9. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: In step 3), the amplitude of the voltage pulse is set according to the resistance of the device, wherein the tunnel junction power does not exceed 10 μW at the peak value of the voltage pulse.

10. The method for realizing polymorphic storage based on CrOCl material according to claim 1, characterized in that: In step 3), the scanning speed of the applied voltage pulse does not exceed 0.01 V / s.

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