Spin state controlled ultrahigh magnetoresistance film and preparation method thereof

Abstract

The application discloses a spin state controlled ultrahigh magnetoresistance film and a preparation method thereof, and belongs to the fields of magnetic films and magnetic sensing. The film material is a MgO / NiFe / MgO / Ta multilayer film structure grown on a (111) or (110) single crystal MgO substrate. The single crystal MgO substrate is used to control the atomic spacing and atomic arrangement mode of the NiFe, to promote the O atoms to enter the NiFe crystal lattice and to adjust the crystal field of the NiFe through the superexchange interaction, to further optimize the spin state of the NiFe, and to finally obtain the NiFe film with ultrahigh anisotropic magnetoresistance. The magnesium oxide MgO / palladium alloy NiFe / magnesium oxide MgO / tantalum Ta multilayer film structure is deposited on the single crystal MgO substrate which is subjected to the annealing in the oxygen environment and the ultrasonic cleaning treatment, and the film with ultrahigh anisotropic magnetoresistance is obtained. The crystal field of the material is controlled to optimize the spin state of the NiFe, to improve the anisotropic magnetoresistance, and to prepare the NiFe film with ultrahigh anisotropic magnetoresistance ratio. Meanwhile, the NiFe film has a small magnetic hysteresis phenomenon, and can meet the demand of the anisotropic magnetoresistance effect magnetic sensor with ultrahigh sensitivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of magnetic thin film materials and sensing, and relates to a spin state controlled ultrahigh magnetoresistance film and a preparation method thereof. BACKGROUND

[0002] Palladium alloy (NiFe) has a small saturation field, weak hysteresis and near-zero magnetostriction effect, and is one of the commonly used materials of anisotropic magnetoresistance sensors. With the development of society and the progress of science and technology, the requirements of people's daily life and industrial production for the sensitivity of anisotropic magnetoresistance sensors are getting higher and higher. The anisotropic magnetoresistance ratio of a NiFe film is a key factor affecting the sensitivity of a magnetic sensor. Therefore, how to improve the anisotropic magnetoresistance ratio and sensitivity of a NiFe-based sensor has become one of the bottleneck problems restricting the development of anisotropic magnetoresistance sensors. In view of this problem, a lot of work has been carried out internationally, mainly through the use of a buffer layer to improve the crystal structure [Rare Metals. 31, 22 (2012);Rare Metals. 22, 202 (2003)] or the introduction of a nano-oxide layer to control the electron scattering path [Appl. Phys. Lett. 94, 162506 (2009);Appl. Phys. Lett. 96, 052515 (2010)] and through the regulation of the electronic structure [ACS Appl. Mater. Interfaces, 14, 9917 (2022)] to improve the anisotropic magnetoresistance ratio of the NiFe film. We previously proposed a method for improving the magnetoresistance ratio [Patent No.: ZL202210009801.9], which is to optimize the electronic structure, use a strong oxygen-absorbing Ru interlayer to adjust the Fe-O orbital hybridization near the NiFe / NiO interface, thereby controlling the hole distribution of the 3d orbital in the NiFe layer, promoting the transition probability of s electrons to d state holes in the NiFe layer, and then improving the anisotropic magnetoresistance value of the film. Although these methods improve the anisotropic magnetoresistance value to some extent, it is generally not more than 5%, which cannot meet the needs of ultrahigh performance anisotropic magnetoresistance sensors, and therefore there is an urgent need to develop a new method for preparing an ultrahigh anisotropic magnetoresistance effect film. SUMMARY

[0003] The application provides a spin state controlled ultrahigh magnetoresistance MgO single crystal substrate / MgO / NiFe / MgO / Ta film.

[0004] The application discloses a spin state controlled ultrahigh magnetoresistance film, characterized in that the ultrahigh anisotropic magnetoresistance film is a MgO / NiFe / MgO / Ta multilayer film structure grown on a (111) or (110) single crystal MgO substrate, the single crystal MgO substrate is used to control the atomic spacing and atomic arrangement mode of the NiFe, to promote the O atoms to enter the NiFe crystal lattice and to adjust the crystal field of the NiFe through the superexchange interaction, so as to optimize the spin state of the NiFe, and finally to obtain the NiFe film with ultrahigh anisotropic magnetoresistance.

[0005] The application further discloses a preparation method of the ultrahigh anisotropic magnetoresistance film.

[0006] (1) annealing the MgO single crystal substrate in an oxygen environment; then ultrasonic cleaning the MgO substrate, the thickness of the MgO substrate is 0.6-1 mm, the ultrasonic cleaning process is 10-15 min for each of acetone solution and alcohol solution, and then the above cleaning process is repeated again;

[0007] (2) placing the MgO substrate treated in the step (1) on a sample chamber base of a magnetron sputtering instrument, adjusting the vacuum degree of a sputtering chamber, introducing argon with a purity of 99.99%, and sequentially depositing MgO, NiFe, MgO and Ta to form a multilayer film with a structure of MgO layer / NiFe layer / MgO layer / Ta protective layer.

[0008] Further, the oxygen pressure range in the step (1) is 100-200 Pa, the annealing temperature range is 300-500 DEG C, and the holding time is 20-40 min.

[0009] Further, the thickness of the bottom MgO layer in the step (2) is 20-50 A, the thickness of the NiFe layer is 100-400 A, the thickness of the upper MgO layer is 20-50 A, and the thickness of the Ta protective layer is 30-50 A.

[0010] Further, the base vacuum degree of the sputtering chamber in the deposition process of the multilayer film in the step (2) is 1x10 -5 ~4x10 -5 Pa, the argon pressure is 0.4-0.9 Pa when the MgO, NiFe and Ta layers are deposited; when the MgO layer is deposited, mixed gas of high-purity argon with a purity of 99.99% and high-purity oxygen with a purity of 99.99% is introduced, and the argon / oxygen pressure ratio range is 6:1-1:1, so that the stoichiometric ratio of Mg and O in the MgO layer is as close to 1:1 as possible; the sputtering chamber temperature is 25-500 DEG C when the MgO and NiFe layers are deposited, and the sputtering chamber temperature is 20-30 DEG C when the Ta layer is deposited.

[0011] The principle of the present application is different from the above-mentioned granted patent [Patent No. ZL202210009801.9]: the above-mentioned granted patent mainly obtains a high-performance magnetoresistance film by optimizing the electronic structure of the material, and the film structure is: Ta / Ru / NiO / NiFe / NiO / Ru / Ta. The strong oxygen-absorbing Ru interlayer is used to adjust the Fe-O orbital hybridization near the NiFe / NiO interface, thereby controlling the hole distribution of the 3d orbital in the NiFe layer, promoting the transition probability of s electrons to d-state holes in the NiFe layer, and further improving the anisotropic magnetoresistance value of the film. While the present application patent is to obtain a high-performance magnetoresistance film by optimizing the crystal field of the material, and the film structure is: MgO single crystal substrate / MgO / NiFe / MgO / Ta film. The single crystal MgO substrate is used to control the atomic spacing and atomic arrangement of NiFe, promote the O atoms to enter the NiFe lattice and adjust the crystal field of NiFe through superexchange interaction, and then optimize the spin state of NiFe, so as to obtain a NiFe film with super-high anisotropic magnetoresistance. Based on the above different principles, the selection principles of the oxide layer or the substrate material are also different from the above-mentioned granted patent [Patent No. ZL202210009801.9]: in the above-mentioned granted invention patent, in order to better adjust the hole distribution of the 3d orbital in the NiFe layer, the oxide adjacent to the NiFe layer is selected as NiO with high enthalpy of formation; the metal interlayer adjacent to the NiO layer is selected as Ru with strong oxygen absorption, so as to adjust the interface orbital hybridization and the hole distribution of the orbital in the NiFe layer, and there is no special requirement for the selection of the substrate material. In the present application patent, in order to better adjust the crystal field of NiFe, the substrate material is selected as MgO with single crystal orientation; the oxide adjacent to the NiFe layer is selected as MgO with moderate lattice mismatch and containing large electronegative ions, so as to adjust the crystal field of NiFe through superexchange interaction.

[0012] The feature of the present application is that a MgO / NiFe / MgO / Ta multilayer film is deposited on a single crystal MgO substrate which has been annealed in an oxygen environment and ultrasonically cleaned, and then a super-high anisotropic magnetoresistance film material is prepared.

[0013] The principle of the present application is that: the single crystal MgO substrate is subjected to oxygen environment annealing treatment, so that the crystal structure of the MgO substrate can be optimized, the atomic percentage of Mg and O is close to the stoichiometric ratio of 1:1, and the surface roughness of the MgO substrate is reduced; the single crystal MgO substrate is subjected to ultrasonic cleaning treatment, so that the surface pollutants can be greatly reduced, and a good surface quality is provided for the growth of the MgO / NiFe / MgO / Ta multilayer film on the single crystal MgO substrate. The MgO / NiFe / MgO / Ta multilayer film is deposited on the single crystal MgO substrate subjected to the above treatment, and the mixed gas of high-purity argon and high-purity oxygen is introduced during the deposition of the MgO layer, so that the oxygen vacancy defects in the MgO can be reduced, and the stoichiometric ratio of Mg and O in the upper and lower two MgO layers is close to 1:1. The MgO / NiFe / MgO / Ta multilayer film is deposited on the single crystal MgO substrate at high temperature, so that the atomic spacing and atomic arrangement mode of the NiFe layer can be effectively controlled, thereby promoting the O atoms to enter the NiFe lattice and adjusting the crystal field of the NiFe through superexchange interaction, and then optimizing the spin state of the NiFe, and finally obtaining the NiFe film with super-high anisotropic magnetoresistance.

[0014] The present application has the beneficial effect that: the previous work is mainly to enhance the anisotropic magnetoresistance effect of the thin film material by modulating the two degrees of freedom of the lattice or the charge, and the magnetoresistance value is low or the thin film saturation field is high and the hysteresis is large. The present application provides a new method for improving the thin film magnetoresistance value by regulating the spin state of NiFe, and the atomic spacing and atomic arrangement mode of NiFe are controlled by using the single crystal MgO substrate, the O atoms are promoted to enter the NiFe lattice, and the crystal field of the NiFe is adjusted through superexchange interaction, and then the spin state of the NiFe is optimized, and finally the NiFe film with super-high anisotropic magnetoresistance is obtained. Moreover, the hysteresis of the thin film is small, and the demand of the high-performance anisotropic magnetoresistance sensor can be met. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The anisotropic magnetoresistance curve of the MgO(20 Å) / NiFe(100 Å) / MgO(20 Å) / Ta(30 Å) multilayer film deposited on the (111), (110) crystal direction single crystal MgO substrate subjected to oxygen environment annealing and ultrasonic cleaning treatment at 25 ℃, wherein the substrate thickness is 0.6 mm, the annealing temperature is 300 ℃, the time is 20 min, and the oxygen gas pressure is 100 Pa. The ultrasonic cleaning process is to clean with acetone and alcohol solution for 10 min respectively, and then clean with acetone solution and alcohol solution for 10 min respectively. The instrument background vacuum degree during the thin film deposition process is 1x10 -5The argon gas pressure was 0.4 Pa when depositing MgO, NiFe, and Ta layers. The argon / oxygen pressure ratio was 6:1 when depositing the MgO layer. The sputtering chamber temperature was 25 ℃ when depositing the MgO and NiFe layers, and 20 ℃ when depositing the Ta layer.

[0016] Figure 2 Anisotropic magnetoresistance curves were obtained for a MgO(35 Å) / NiFe(250 Å) / MgO(35 Å) / Ta(40 Å) multilayer film deposited at 250 ℃ on a (111) and (110) oriented single-crystal MgO substrate after annealing and ultrasonic cleaning in an oxygen environment. The substrate thickness was 0.8 mm, the annealing temperature was 400 ℃, the annealing time was 30 min, and the oxygen pressure was 150 Pa. The ultrasonic cleaning process involved cleaning with acetone and alcohol solutions for 10 min each, followed by cleaning with acetone and alcohol solutions for 10 min each. The instrument's base vacuum during the film deposition process was 2 × 10⁻⁶. -5 The argon gas pressure was 0.6 Pa when depositing MgO, NiFe, and Ta layers. The argon / oxygen pressure ratio was 3:1 when depositing the MgO layer. The sputtering chamber temperature was 250 ℃ when depositing the MgO and NiFe layers, and 25 ℃ when depositing the Ta layer.

[0017] Figure 3 Anisotropic magnetoresistance curves of a MgO(50 Å) / NiFe(400 Å) / MgO(50 Å) / Ta(50 Å) multilayer film deposited at 250 ℃ on a (111) and (110) oriented single-crystal MgO substrate after annealing and ultrasonic cleaning in an oxygen environment were obtained. The substrate thickness was 1 mm, the annealing temperature was 500 ℃, the annealing time was 40 min, and the oxygen pressure was 200 Pa. The ultrasonic cleaning process involved cleaning with acetone and alcohol solutions for 10 min each, followed by cleaning with acetone and alcohol solutions for 10 min each. The instrument's base vacuum during the film deposition process was 4 × 10⁻⁶. -5 The argon gas pressure was 0.9 Pa when depositing MgO, NiFe, and Ta layers. The argon / oxygen pressure ratio was 1:1 when depositing the MgO layer. The sputtering chamber temperature was 500 ℃ when depositing the MgO and NiFe layers, and 30 ℃ when depositing the Ta layer.

[0018] Figure 4 This is a graph showing the anisotropic magnetoresistance as a function of deposition temperature for a typical sample structure: single-crystal MgO substrate / MgO (40 Å) / NiFe (200 Å) / MgO (30 Å) / Ta (50 Å). The sample preparation conditions and process are the same as above. Figure 3 The samples were identical, with a deposition temperature range of 25–500 °C. Detailed Implementation

[0019] Example 1, see Figure 1 , the sample preparation conditions are as follows: first, cut the single crystal MgO substrate with a thickness of 0.6 mm and a (111) or (110) crystal direction into a square of 10 mm x 10 mm, and then perform oxygen environment annealing and ultrasonic cleaning treatment. The annealing temperature is 300 °C, the time is 20 min, and the oxygen gas pressure is 100 Pa. The ultrasonic cleaning process is to clean with acetone and alcohol solution for 10 min respectively, and then clean with acetone solution and alcohol solution for 10 min respectively. Then, a MgO substrate / MgO (20 Å) / NiFe (100 Å) / MgO (20 Å) / Ta (30 Å) multilayer film is prepared by sequentially depositing MgO atoms (thickness of 20 Å), NiFe atoms (thickness of 100 Å), MgO atoms (thickness of 20 Å), and Ta atoms (thickness of 30 Å) on the above cleaned substrate by using a magnetron sputtering method. The base vacuum degree of the sputtering chamber is 1 x 10 -5 Pa, the argon gas pressure is 0.4 Pa when depositing MgO, NiFe, and Ta layers, the argon / oxygen pressure ratio is 6:1 when depositing the MgO layer, the sputtering chamber temperature is 25 °C when depositing the MgO and NiFe layers, and the sputtering chamber temperature is 20 °C when depositing the Ta layer. Then, the anisotropic magnetoresistance value of the sample is measured at room temperature by using a four-probe method, and a 1 mA current is applied along the easy magnetization direction during the test, and the magnetic field is scanned perpendicular to the easy magnetization direction of the sample.

[0020] Example 2, see Figure 2 , the sample preparation conditions are as follows: first, cut the single crystal MgO substrate with a thickness of 0.8 mm and a (111) or (110) crystal direction into a square of 10 mm x 10 mm, and then perform oxygen environment annealing and ultrasonic cleaning treatment. The annealing temperature is 400 °C, the time is 30 min, and the oxygen gas pressure is 150 Pa. The ultrasonic cleaning process is to clean with acetone and alcohol solution for 10 min respectively, and then clean with acetone solution and alcohol solution for 10 min respectively. Then, a MgO substrate / MgO (35 Å) / NiFe (250 Å) / MgO (35 Å) / Ta (40 Å) multilayer film is prepared by sequentially depositing MgO atoms (thickness of 35 Å), NiFe atoms (thickness of 250 Å), MgO atoms (thickness of 35 Å), and Ta atoms (thickness of 40 Å) on the above cleaned substrate by using a magnetron sputtering method. The base vacuum degree of the sputtering chamber is 2 x 10 -5Pa, the argon pressure was 0.6 Pa when depositing the MgO, NiFe, Ta layers, the argon / oxygen pressure ratio was 3:1 when depositing the MgO layer, the sputtering chamber temperature was 250 ℃ when depositing the MgO, NiFe layers, and the sputtering chamber temperature was 25 ℃ when depositing the Ta layer. Then, the anisotropic magnetoresistance value of the sample was measured at room temperature by using the four-probe method, and a 1 mA current was applied along the easy magnetization direction, and the magnetic field was scanned perpendicular to the easy magnetization direction of the sample during the test.

[0021] Example 3, see Figure 3 , the sample preparation conditions were as follows: first, a single-crystal MgO substrate with a thickness of 1 mm and a (111), (110) crystal direction was cut into a square with a side length of 10 mm, and then the substrate was subjected to oxygen environment annealing and ultrasonic cleaning. The annealing temperature was 500 ℃, the time was 40 min, and the oxygen pressure was 200 Pa. The ultrasonic cleaning process was as follows: first, the substrate was cleaned with acetone and alcohol solutions for 10 min, respectively, and then the substrate was cleaned with acetone and alcohol solutions for 10 min, respectively. Then, a MgO substrate / MgO (50 Å) / NiFe (400 Å) / MgO (50 Å) / Ta (50 Å) multilayer film was prepared on the above cleaned substrate by using a magnetron sputtering method to sequentially deposit MgO atoms (with a thickness of 50 Å), NiFe atoms (with a thickness of 400 Å), MgO atoms (with a thickness of 50 Å), and Ta atoms (with a thickness of 50 Å). The base vacuum degree of the sputtering chamber was 4×10 -5 Pa, the argon pressure was 0.6 Pa when depositing the MgO, NiFe, Ta layers, the argon / oxygen pressure ratio was 3:1 when depositing the MgO layer, the sputtering chamber temperature was 250 ℃ when depositing the MgO, NiFe layers, and the sputtering chamber temperature was 25 ℃ when depositing the Ta layer. Then, the anisotropic magnetoresistance value of the sample was measured at room temperature by using the four-probe method, and a 1 mA current was applied along the easy magnetization direction, and the magnetic field was scanned perpendicular to the easy magnetization direction of the sample during the test.

[0022] From Figures 1 to 3 it can be seen that by using the (111), (110) oriented single-crystal MgO substrate, the anisotropic magnetoresistance value of the sample grown thereon at a high temperature is significantly enhanced. Taking the sample in Figure 3 as an example, by changing the growth temperature, a graph showing the relationship between the anisotropic magnetoresistance value of the sample and the growth temperature can be obtained, as shown in Figure 4 With the increase of the growth temperature, the anisotropic magnetoresistance value of the (110) sample increased from 3% to 8%, and the anisotropic magnetoresistance value of the (111) sample increased from 2.4% to 8.7%. This shows that by using the single-crystal MgO substrate to control the crystal of NiFe and using O atoms to control the crystal field, the anisotropic magnetoresistance value of the MgO / NiFe / MgO / Ta film can be effectively enhanced.

[0023] Table 1 shows the comparison of the magnetoresistance ratio of the thin film prepared in the present application and in the patent ZL202210009801.9. It can be found that the performance of the magnetoresistance thin film prepared by optimizing the crystal field to control the NiFe spin state in the present application is far superior to that of the magnetoresistance thin film obtained by optimizing the electronic structure in the patent ZL202210009801.9.

[0024] Table 1 Comparison of the magnetoresistance ratio of the thin film prepared in the present application and in the patent ZL202210009801.9

[0025]

Claims

1. A spin state controlled super-high magneto-resistive thin film, characterized in that, The super-high anisotropic magnetoresistance film is a MgO / Ni Fe / MgO / Ta multilayer film structure grown on a (111) or (110) single crystal MgO substrate, the single crystal MgO substrate is used to control the atomic spacing and atomic arrangement mode of the Ni Fe, to promote the O atoms to enter the Ni Fe crystal lattice and to adjust the crystal field of the Ni Fe through the super-exchange interaction, and then to optimize the spin state of the Ni Fe, and finally to obtain the super-high anisotropic magnetoresistance Ni Fe film; the thickness of each layer in the super-high anisotropic magnetoresistance film MgO / Ni Fe / MgO / Ta structure is in turn The orientation of the MgO single crystal substrate is (111) or (110). The preparation method of the super-high anisotropic magnetoresistance film comprises the following steps: depositing MgO / NiFe / MgO / Ta multi-layer film on a single-crystal MgO substrate which is annealed in oxygen environment and ultrasonically cleaned, and then preparing the super-high anisotropic magnetoresistance film material.

2. The method of claim 1, wherein the spin state controlled superhigh magnetoresistance thin film is prepared by a process comprising: The specific preparation steps are as follows: ​ (1) annealing the MgO single-crystal substrate in oxygen environment; then ultrasonically cleaning the MgO substrate, wherein the thickness of the MgO substrate is 0.6-1 mm, and the ultrasonic cleaning process comprises the following steps: ultrasonically cleaning in acetone solution for 10-15 min, ultrasonically cleaning in alcohol solution for 10-15 min, and repeating the above cleaning process again; (2) placing the MgO substrate treated in step (1) on the sample chamber base of a magnetron sputtering instrument, adjusting the vacuum degree of the sputtering chamber, introducing argon with a purity of 99.99%, and sequentially depositing MgO, NiFe, MgO and Ta to form a multi-layer film with the structure of MgO layer / NiFe layer / MgO layer / Ta protective layer.

3. The method for preparing a spin-controlled ultra-high magnetoresistive thin film according to claim 2, characterized in that: The oxygen pressure range in step (1) is 100-200 Pa, the annealing temperature range is 300-500℃, and the holding time is 20-40 min.

4. The method of claim 2, wherein the spin state controlled superhigh magnetoresistance thin film is prepared by the steps of: The bottom MgO layer in step (2) has a thickness of The Ni Fe layer has a thickness of The upper MgO layer has a thickness of The Ta protective layer has a thickness of ​ 5. The method of claim 2 or 4, wherein: The base vacuum of the sputtering chamber during the deposition of the multilayer film in step (2) is 1 x 10 -5 ~ 4 x 10 -5 Pa, and the argon gas pressure during the deposition of Ta, MgO, and NiFe layers is 0.4 ~ 0.9 Pa. When depositing the MgO layer, mixed gas of high-purity argon with a purity of 99.99% and high-purity oxygen with a purity of 99.99% is introduced, wherein the argon / oxygen pressure ratio range is 6:1-1:1, so that the stoichiometric ratio of Mg and O in the MgO layer is 1:1; when depositing the MgO and NiFe layers, the sputtering chamber temperature is 25-500℃, and when depositing the Ta layer, the sputtering chamber temperature is 20-30℃.

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