An artificial spin ice structure that can be regulated in situ and a regulation method thereof

Abstract

The present invention discloses an artificial spin ice structure that can be regulated in situ and a regulation method thereof, and belongs to the research field of artificial spin ice technology. The structure of the present invention grows nanomagnets on the surface of a silicon wafer by EBL and EBE, and the nanomagnets include nanomagnets of two different shapes and sizes, a semicircular tip and an elliptical tip. The nanomagnets with semicircular tips are used as the background, and the nanomagnets with elliptical tips are arranged therein according to a certain rule, and then initial magnetization and demagnetization treatment are performed by external magnetic fields of different sizes and directions, thereby realizing the in-situ switching of the ground state on the same sample. The in-situ adjustable artificial spin ice structure of the present invention regulates a variety of magnetization distribution states of the system in situ by an external magnetic field, enriches the functionality of this structure in the future production of heterogeneous devices, and increases the application potential of the artificial spin ice structure. At the same time, there are rich physical phenomena in the structure, which provides an excellent platform for studying frustration phenomena.

Description

Technical Field

[0001] The present invention belongs to the research field of artificial spin ice technology, and specifically relates to an artificial spin ice structure that can be regulated in situ and a regulation method thereof. Background Art

[0002] Artificial spin ice is an array of nano-sized magnets with dipole interactions that is prepared by modern micro-nano processing technology. It can be artificially designed and controlled. It was originally introduced to simulate the complex geometric frustration effects in water ice and spin ice materials. The diverse magnetization states of artificial spin ice can be used to directly observe many interesting physical phenomena such as geometric frustration, magnetic monopoles, Coulomb phases, exotic phase transitions, etc. It has potential application value in the fields of low-power data storage, encoding, and advanced computing. Different artificial spin ice structures determine their unique physical properties. However, the existing structures of artificial spin ice cannot control the ground state of their systems in situ. Previous structures require the preparation of samples with different lattice constants or different small magnet interactions to change the ground state of the structure. This makes these structures limited in function when making complex heterogeneous devices. Summary of the invention

[0003] In response to the problems mentioned in the background technology, the present invention proposes an artificial spin ice structure that can be controlled in situ and a control method thereof, so that the ground state of the system can be switched in situ by demagnetization after an external magnetic field is applied, which solves the problem that the ground state of the artificial spin ice structure cannot be controlled in situ. There is no need to prepare samples multiple times, and the in situ switching of the ground state can be achieved on the same sample.

[0004] Technical solution: In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0005] An artificial spin ice structure that can be controlled in situ is used to grow nanomagnets on the surface of a silicon wafer through EBL and EBE. The nanomagnets include nanomagnets of two different shapes and sizes, namely, semicircular tips and elliptical tips. The nanomagnets with semicircular tips are used as the background, and the nanomagnets with elliptical tips are arranged in a regular pattern therein.

[0006] Preferably, the long axis size of the nanomagnet with an elliptical tip is in the range of 210nm-230nm, and the short axis size is in the range of 65nm-75nm; the long axis size of the nanomagnet with a semicircular tip is in the range of 210nm-230nm, and the short axis size is in the range of 75nm-85nm.

[0007] Preferably, a plurality of square regions are drawn in a square artificial spin ice background composed of nanomagnets with semicircular tips; the square regions in each row are closely connected and aligned vertically, and are staggered between the rows; in the horizontal direction of the artificial spin ice background, adjacent square regions in the same row are closely arranged; in the vertical direction of the artificial spin ice background, the square regions in adjacent rows are staggered by a lattice constant length in the horizontal direction and by three lattice constant lengths in the vertical direction.

[0008] Preferably, in each frame area, the nanomagnets with semicircular tips in the first row and first column, the second row and fourth column, the sixth row and first column, and the seventh row and third column are replaced with nanomagnets with elliptical tips.

[0009] Preferably, the size of each square area is a square of 4 lattice constant lengths; the lattice constant is 300nm-380nm, and the lattice constant size is the length from the center of one magnet to the center of another nearest neighboring magnet.

[0010] A method for an in-situ controllable artificial spin ice structure, using any of the above-mentioned in-situ controllable artificial spin ice structures, performing initial magnetization and demagnetization through an external magnetic field, in-situ controlling a variety of system states, and realizing in-situ switching of the ground state on the same sample.

[0011] As a preference, the reversal magnetic field of the nanomagnet with the semicircular tip is H g , the flipping magnetic field of the nanomagnet at the tip of the ellipse is H r , the flipping magnetic field H of the nanomagnet with a semicircular tip g The reversal magnetic field H of the nanomagnet smaller than the tip of the ellipse r ; with a value greater than H r The whole system is initially magnetized by a magnetic field of H g But less than H r The demagnetization process of this system is carried out using a magnetic field.

[0012] Preferably, during the demagnetization process, the magnetic moment of the nanomagnet with an elliptical tip cannot be reversed, while the magnetic moment of the nanomagnet with a semicircular tip can be reversed.

[0013] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0014] (1) The artificial spin ice structure of the present invention can be regulated in situ to produce a variety of system states by applying an external magnetic field. This is a ground state structure that has never been realized in the prior art. It enriches the functionality of this structure in the manufacture of heterogeneous devices and increases the application potential of the structure. At the same time, there are rich physical phenomena in the structure, which provides an excellent platform for studying frustration phenomena.

[0015] (2) In the present invention, after initial magnetization in the magnetization direction of 0 degrees, demagnetization is performed to obtain a magnetic domain wall with uniform distribution in the form of horizontal stripes. This shows that the direction of the magnetic domain wall can be controlled in this structure, which provides a good platform for studying the mechanism of directional evolution of magnetic domains. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the in-situ controllable artificial spin ice structure of the present invention, wherein the red one is a nanomagnet with an elliptical tip, and the gray one is a nanomagnet with a semicircular tip;

[0017] Figure 2 Schematic diagram of the arrangement of the square frame area and the nanomagnets with elliptical tips of the present invention, wherein the red ones are nanomagnets with elliptical tips, and the gray ones are nanomagnets with semicircular tips;

[0018] Figure 3 It is a magnetic force microscope (MFM) imaging result diagram of a sample with an initial magnetization of 0 degrees after demagnetization and a result diagram of statistically identifying the direction of the magnetic moment in the MFM of the present invention, wherein Figure (a) is an MFM imaging result diagram of a sample with an initial magnetization of 0 degrees after demagnetization, and Figure (b) is a result diagram of statistically identifying the direction of the magnetic moment in the MFM; wherein the black arrow represents the direction of the magnetic moment of a magnet with a semicircular tip, the red arrow represents the direction of the magnetic moment of a magnet with an elliptical tip, and the green and yellow arrows represent the direction of the magnetic domain wall;

[0019] Figure 4 These are the MFM imaging result diagrams of the sample initially magnetized at 90 degrees after demagnetization and the result diagrams of the statistical identification of the magnetic moment direction in the MFM of the present invention, wherein Figure (a) is the MFM imaging result diagram of the sample initially magnetized at 90 degrees after demagnetization, and Figure (b) is the result diagram of the statistical identification of the magnetic moment direction in the MFM, wherein the black arrow represents the magnetic moment direction of the magnet with a semicircular tip, the red arrow represents the magnetic moment direction of the magnet with an elliptical tip, and the green and yellow arrows represent the direction of the magnetic domain wall. DETAILED DESCRIPTION

[0020] The present invention is further illustrated below in conjunction with specific examples. The examples are implemented based on the technical solutions of the present invention. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0021] The in-situ controllable artificial spin ice structure provided in this embodiment grows nanomagnets on the surface of a silicon wafer by EBL (electron-blocking layer) and EBE (electron beam evaporation). The magnet array includes nanomagnets of two different shapes and sizes, with semicircular tips and elliptical tips. The nanomagnets with semicircular tips are used as the background, and the nanomagnets with elliptical tips are arranged therein according to a certain pattern.

[0022] Electron beam exposure is a photolithography process that uses an electron high-voltage gun to emit an electron beam to expose a sample coated with electron beam photoresist. It has high precision and can reach a precision of less than 100 nanometers or even a few nanometers. However, because it is written in the form of electron beams, while pursuing precision, the exposure speed is slow. By spin-coating one or more layers of photoresist on the surface of the sample, the thickness of different photoresists ranges from tens of nanometers to tens of microns. After electron beam exposure, the properties of the photoresist in the exposed area will change, and then the part soluble in the developer will be removed by development, leaving the designed pattern on the sample, and then proceed to the next step of the process. That is, using EBE, or electron beam evaporation, to grow a film.

[0023] Electron beam evaporation is a thin film growth device that works in a relatively high background vacuum. It is a physical vapor deposition technology. The material used to grow the thin film is generally placed in a high-temperature resistant crucible in the form of particles or small pieces, and the crucible is placed at the bottom of a specific chamber. When growing the thin film, a high voltage of more than several thousand volts is applied to the electron gun, and the electron gun will emit a high-energy electron beam. The electron beam carrying the high-energy electron beam is hit on the material in the crucible under the control of a specific magnetic field, so that the material is quickly heated to an extremely high temperature above its melting point, vaporized, and deposited on the surface of the sample. The advantage of electron beam evaporation is that the directionality of the grown film is better, and the grown film is easier to peel off the parts outside the pattern than magnetron sputtering. Because artificial spin ice is generally composed of a relatively complex periodic two-dimensional array, it is not easy to peel off the sample after the thin film is grown. Therefore, in this embodiment, electron beam evaporation is selected when growing Permalloy to facilitate subsequent peeling to ensure the preparation of large-area artificial spin ice samples with few defects.

[0024] In the experiment of this embodiment, first, the sample substrate is spin-coated with 495 PMMA A2 glue at a speed of 2000 rpm / min, and the sample is baked at 180 ° C for 4 minutes using a drying table. The thickness of the 495 PMMA A2 glue is about 80 nm. After the sample is cooled, a layer of 950 PMMA A2 glue is spin-coated on the sample at a speed of 3000 rpm / min, and the sample is baked at 180 ° C for 4 minutes using a drying table. The thickness of the 950 PMMA A2 glue is about 100 nm. The sample is then transferred to the sample stage of the electron beam exposure, and the level and height of the sample are adjusted. The sample is sent into the electron beam exposure chamber for exposure. The electron beam exposure voltage is 100 keV, and the selected exposure dose is 825 μC / cm 2, the electron beam current is 0.2 nA, and after finding the selected sample mark, the previously imported and processed pattern is exposed under the set parameters. After the exposure is completed, the sample is taken out and placed in a solution of MIBK (methyl isobutyl ketone) / IPA (isopropyl alcohol) = 1:3 for development for 60 s, and finally fixed in IPA for 60 seconds, and the sample surface is blown dry with a nitrogen gun.

[0025] Then electron beam evaporation was used to grow the permalloy. The sample was transferred to the electron beam exposure chamber, the permalloy crucible position was selected, and the permalloy crucible was heated for 10 min. -7 The film was grown under a background vacuum of 2000 Å. The electron beam high voltage was 7 kV, the growth rate was 0.03 nm / s, and the growth thickness was 20 nm. After the growth was completed, the sample was taken out and the equipment was turned off. Finally, the PMMA (polymethyl methacrylate) glue (lift-off) was removed, and the sample was placed in an N-methyl-pyrrolidone solvent, heated in a 75-degree Celsius water bath for 30 minutes, and then ultrasonicated for 3 minutes, followed by ultrasonic treatment in acetone and alcohol for 3 minutes to ensure that all PMMA glue was removed. Finally, the sample was blown dry with a nitrogen gun to obtain the final sample of artificial spin ice.

[0026] In this embodiment, two shapes of nanomagnets are designed by utilizing the different hysteresis loops of different magnet shapes. The different hysteresis loops of the two magnets indicate that they have different switching critical magnetic fields.

[0027] like Figure 1 As shown, the red magnet is a nanomagnet with an elliptical tip shape, whose long axis can have a positive or negative deviation of 10nm, that is, the long axis size range is 210nm-230nm, and the short axis can have a positive or negative deviation of 5nm, that is, the short axis size range is 65nm-75nm; the gray magnet is a nanomagnet with a semicircular tip, whose long axis can have a positive or negative deviation of 10nm, that is, the long axis size range is 210nm-230nm, and the short axis can have a positive or negative deviation of 5nm, that is, the short axis size range is 75nm-85nm.

[0028] like Figure 2 As shown, first, a square artificial spin ice background composed of semicircular tip nanomagnets is generated, and multiple square areas are drawn in the artificial spin ice background. The square areas in each row are closely connected and aligned up and down, and the rows are staggered; in the horizontal direction of the artificial spin ice background, adjacent square areas in the same row are closely arranged; in the vertical direction of the artificial spin ice background, the square areas in adjacent rows are staggered by a lattice constant length in the horizontal direction and by three lattice constant lengths in the vertical direction.

[0029] In this embodiment, the size of each square frame area is a square of 4 lattice constant lengths, that is, the horizontal direction of the square frame is 4 lattice constant lengths, and the vertical direction is also 4 lattice constant lengths. The lattice constant is 300nm-380nm, and the lattice constant size is the length from the center of one magnet to the center of the nearest neighboring magnet.

[0030] In each box, 4 magnets are replaced with oval magnets (red magnets). In this embodiment, the rows are marked from top to bottom and the columns are marked from left to right. The positions of the replaced magnets are: the first row and the first column, the second row and the fourth column, the sixth row and the first column, and the seventh row and the third column.

[0031] The method for in-situ controllable artificial spin ice structure provided in this embodiment adopts the above-mentioned in-situ controllable artificial spin ice structure, performs initial magnetization and demagnetization through external magnetic fields of different sizes and directions, in-situ controls a variety of system states, and realizes in-situ switching of the ground state on the same sample.

[0032] In this embodiment, the switching magnetic field H of the red magnet r (i.e. H red , H represents the magnetic field) is greater than the flipping magnetic field H of the gray magnet g (i.e. H grey , H represents magnetic field).

[0033] By using a value greater than H r The whole system is initially magnetized by a large magnetic field of H g But less than H r The demagnetization process of this system is carried out in a magnetic field. In this demagnetization process, the magnetic moment of the red magnet can be regarded as non-reversible, and only the magnetic moment of the gray magnet can be reversed.

[0034] For different initial magnetization states, the system exhibits different demagnetization states after demagnetization. Specifically, the initial magnetization of the sample with an angle of 45 degrees to the upper left and the initial magnetization of the sample with an angle of 45 degrees to the upper right are completely different after demagnetization. In this embodiment, the magnetization direction of 0 degrees is defined as the angle of 45 degrees to the upper left, and the corresponding magnetization direction of 90 degrees is defined as the angle of 45 degrees to the upper right.

[0035] Specifically, for a sample with an initial magnetization of 0 degrees, a magnetic field of 1500 Gs is first applied along the sample at a 45-degree angle to the upper left. This magnetic field can flip all the nanomagnets on the sample, so that the magnetic moment of the vertical magnets is upward and the magnetic moment of the horizontal magnets is to the left. After that, the sample is demagnetized with an in-plane magnetic field that slowly decreases from 500 Gs to 0 and oscillates sinusoidally. In this process, the magnetic moment of the elliptical magnet will not flip, while the magnetic moments of the remaining magnets can flip. The sample is mounted on a motor with a rotation speed of 2000 rpm. The change in the oscillating in-plane magnetic field applied to the test sample is a sine wave with a period of 40 s. During the demagnetization process, the magnetic moment of the magnet gradually distributes in the direction of the lowest energy of the overall system.

[0036] For the sample with an initial magnetization of 90 degrees, a magnetic field of 1500 Gs is first applied along the sample at a 45-degree angle to the upper right. This magnetic field can flip all the nanomagnets on the sample, so that the magnetic moment of the vertical magnets is upward and the magnetic moment of the horizontal magnets is to the right. After that, the sample is demagnetized with a sinusoidal oscillating in-plane magnetic field that slowly decreases from 500 Gs to 0. Also in this process, the magnetic moment of the elliptical magnet will not flip, while the magnetic moments of the remaining magnets can flip. The sample is also mounted on a motor with a rotation speed of 2000 rpm. The change in the oscillating in-plane magnetic field applied to the test sample is a sine wave with a period of 40 s. During the demagnetization process, the magnetic moment of the magnet is also distributed in the direction with the lowest energy of the overall system.

[0037] After demagnetization, the samples with two different initial magnetization states correspond to different demagnetization results. The 0-degree magnetization sample corresponds to a continuous horizontal domain wall distribution, such as Figure 3 As shown; another one is the free distribution of domain walls, corresponding to the sample with 90 degrees initial magnetization, as shown Figure 4 shown.

[0038] Figure 3 (a) is the MFM (magnetic force microscope) imaging result after demagnetization of the sample with initial magnetization of 0 degrees. In order to better illustrate the situation, Figure 3 As shown in (b), the direction of the magnetic moment in the statistical diagram of this embodiment is indicated by black and red arrows in the figure. Among them, the red arrow represents the elliptical nanomagnet, and the black arrow represents the semicircular tip nanomagnet. In addition, the position and direction of the magnetic domain wall are indicated in the figure with green and yellow arrows. It can be seen that in the case of 0 degree initial magnetization, the magnetic domain wall (yellow arrow) in the sample after the demagnetization process is horizontally distributed.

[0039] Likewise, Figure 4 (a) is the MFM (magnetic force microscope) imaging result after the sample with initial magnetization of 90 degrees was demagnetized, and the map was made in the same way, such as Figure 4 As shown in (b), the red arrow indicates the elliptical nanomagnet, the black arrow indicates the semicircular tip nanomagnet, and the green and yellow arrows indicate the position and direction of the magnetic domain wall in the figure. It can be seen that in the case of 90-degree initial magnetization, the direction of the domain wall in the sample after the demagnetization process is free.

[0040] The structure of the present invention is in-situ adjustable, and can be in-situ regulated to produce a variety of system states by applying an external magnetic field, which has never been achieved in the prior art, and it enriches the functionality of the structure in the manufacture of heterogeneous devices.

[0041] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An artificial spin ice structure that can be regulated in situ, characterized in that: Nanomagnets are grown on the surface of silicon wafers by EBL and EBE. The nanomagnets include nanomagnets of two different shapes and sizes, semicircular tip and elliptical tip. The nanomagnets with semicircular tip are used as the background, and the nanomagnets with elliptical tip are arranged in a regular pattern. A plurality of square regions are drawn in a square artificial spin ice background composed of nanomagnets with semicircular tips; the square regions in each row are closely connected and aligned vertically, and the rows are staggered; in the horizontal direction of the artificial spin ice background, adjacent square regions in the same row are closely arranged; in the vertical direction of the artificial spin ice background, the square regions in adjacent rows are staggered by a lattice constant length in the horizontal direction and by three lattice constant lengths in the vertical direction; In each frame area, the nanomagnets with semicircular tips in the first row and first column, the second row and fourth column, the sixth row and first column, and the seventh row and third column are replaced with nanomagnets with elliptical tips.

2. The in-situ controllable artificial spin ice structure according to claim 1, characterized in that: The long axis size of the nanomagnet with an elliptical tip is in the range of 210nm-230nm, and the short axis size is in the range of 65nm-75nm; the long axis size of the nanomagnet with a semicircular tip is in the range of 210nm-230nm, and the short axis size is in the range of 75nm-85nm.

3. The in-situ controllable artificial spin ice structure according to claim 1, characterized in that: The size of each box area is a square of 4 lattice constant lengths; the lattice constant is 300nm-380nm, and the lattice constant size is the length from the center of one magnet to the center of the nearest neighboring magnet in the same direction.

4. A method for controlling an in-situ controllable artificial spin ice structure, according to the in-situ controllable artificial spin ice structure according to any one of claims 1 to 3, characterized in that: By performing initial magnetization and demagnetization using external magnetic fields of different magnitudes, the system's various magnetization distribution states can be in-situ controlled, and in-situ switching of the ground state can be achieved on the same sample.

5. The method for controlling an in-situ controllable artificial spin ice structure according to claim 4, characterized in that: The flipping magnetic field of the nanomagnet with a semicircular tip is H g , the flipping magnetic field of the nanomagnet at the tip of the ellipse is H r , the flipping magnetic field H of the nanomagnet with a semicircular tip g The reversal magnetic field H of the nanomagnet smaller than the tip of the ellipse r ; with a value greater than H r The whole system is initially magnetized by a magnetic field of H g But less than H r The demagnetization process of this system is carried out using a magnetic field.

6. The method for controlling an in-situ controllable artificial spin ice structure according to claim 5, characterized in that: During the demagnetization process, the magnetic moment of the nanomagnet with an elliptical tip cannot be reversed, while the magnetic moment of the nanomagnet with a semicircular tip can be reversed.

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