A high-coercivity hard magnetic oxide semiconductor thin film having perpendicular magnetic anisotropy and a method of manufacturing the same

Abstract

This invention discloses a high-coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy and its preparation method. The invention employs a multi-target alternating gradient film preparation process, using NiFeO4 and NiCo2O4 targets for alternating sputtering to prepare NiCo2-xFexO4-NiCo2O4 gradient thin films. Experimental results show that the obtained films exhibit a coercivity as high as 1T and good conductivity, with an anomalous Hall resistivity reaching 52.5 μΩ·cm. This preparation method has the advantages of simplicity and low cost, providing new possibilities for the development of magnetic storage devices. This method has significant application potential for realizing high-density, high-speed, and low-power magnetic storage devices, contributing to the development and application of magnetic storage technology.

Description

Technical Field

[0001] This invention belongs to the field of hard magnetic oxide thin film materials, specifically relating to a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy and its preparation method. Background Technology

[0002] Both the magnetic storage and magnetic sensing fields have created a high demand for hard magnetic materials with perpendicular magnetic anisotropy. Magnetic storage devices, such as hard disk drives and magnetic random access memory (MRAM), require hard magnetic materials with high coercivity to achieve stable magnetic storage states. Meanwhile, the field of magnetic sensors, especially next-generation perpendicular TMR structures, requires highly stable perpendicular pinning layers, necessitating the development of advanced perpendicular hard magnetic materials. However, traditional perpendicular magnetic anisotropy hard magnetic materials, such as cobalt alloys and iron-cobalt-boron alloys, have limitations, including high cost, complex fabrication processes, and high magnetic exchange coupling fields. Compared to metallic hard magnetic materials, oxide magnetic materials offer advantages such as high spin polarization, high impedance, the ability to form better epitaxial structures with MgO tunneling layers, and theoretically high magnetoresistivity. Based on composition and strain control, the coercivity of oxide magnetic materials can be more easily designed to meet the needs of different applications.

[0003] However, current oxide hard magnetic materials still have certain limitations in terms of coercivity and conductivity. Conventional oxide hard magnetic materials tend to have low coercivity and high resistivity, which limits their application in high-density magnetic storage.

[0004] Therefore, there is an urgent need for a method to prepare oxide hard magnetic materials that can improve coercivity and conductivity. Improving the coercivity of oxide hard magnetic materials, especially in the vertical direction, can significantly improve the performance and stability of magnetic storage devices. Furthermore, improving the conductivity of oxide hard magnetic materials to achieve lower resistivity can reduce the power consumption of magnetic storage devices and increase data read / write speeds.

[0005] Therefore, developing a method for preparing oxide hard magnetic materials with perpendicular magnetic anisotropy, high coercivity and conductivity has important scientific and applied value. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0008] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy.

[0009] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, comprising,

[0010] The substrate is MgAl2O4 (101), the seed layer is NiCo2O4 (102), and the thin film gradient layer is NiCo2-xFexO4 (103);

[0011] In the NiCo2-xFexO4, the Fe doping amount x gradually transitions from a minimum value xmin to a maximum value xmax as the film thickness increases from the substrate to the surface, with xmin ranging from 0 to 0.1 and xmax ranging from 0.5 to 1.

[0012] The thin film has a crystal orientation of (001) and is based on a MgAl2O4(001) single crystal.

[0013] As a preferred embodiment of the thin film described in this invention, when the Fe doping amount x = 0, the thin film exhibits the lowest coercivity, weak perpendicular magnetic anisotropy and optimal conductivity, and the coercivity is generally not higher than 100 Gs.

[0014] As xmax increases, the saturation magnetization and coercivity of the thin film gradually increase. When xmax = 1, the coercivity exceeds 10000 Gs at an appropriate thickness. The rectangularity of the thin film is always higher than 100%.

[0015] When xmax = xmin = 1, that is, there is no gradient in the thin film, the conductivity of the thin film is poor and the coercivity is lower than that of the gradient thin film.

[0016] Another objective of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, comprising,

[0017] NiFe2O4 and NiCo2O4 targets were prepared using standard ceramic sintering processes.

[0018] Thin films were deposited on a MgAl2O4(001) single crystal substrate using alternating NiCo2O4 / NiFe2O4 targets by pulsed laser deposition or magnetron sputtering, and the thickness of the film was controlled by the pulse / sputtering time.

[0019] After the thin film deposition is complete, it is annealed in situ for 30-60 minutes to make its composition more uniform, and then cooled down.

[0020] As a preferred embodiment of the preparation method described in this invention, wherein: thin films are deposited alternately on the MgAl2O4(001) single crystal substrate using NiCo2O4 / NiFe2O4 targets, including,

[0021] First, NiCo2O4 target material is used to deposit the final thin film layer. After a certain thickness, NiFe2O4 target material is switched to the thick film. The thickness is measured or calculated based on the results.

[0022] Then, the target material was switched to NiCo2O4 to control the compositional gradient of Co and Fe in the thin film, and the Fe doping amount gradually transitioned from 0 to xmax.

[0023] In a preferred embodiment of the preparation method described in this invention, the relative contents of Fe and Co in the film are controlled by adjusting the ratio of NiCo2-xFexO4, and the composition distribution gradually transitions from Co-dominant to Fe-dominant.

[0024] In a preferred embodiment of the preparation method described in this invention, during pulsed laser deposition or magnetron sputtering, the compositional uniformity and crystal quality of the thin film are optimized by adjusting the pulse / sputtering energy and frequency, as well as the appropriate temperature of the target material and the angle of the substrate.

[0025] In a preferred embodiment of the preparation method described in this invention, during pulsed laser deposition or magnetron sputtering, the oxidation degree and lattice defects of the thin film are controlled by controlling the oxygen pressure of the atmosphere.

[0026] The oxygen pressure is 1 Pa to 20 Pa.

[0027] As a preferred embodiment of the preparation method described in this invention, the in-situ annealing is performed for 30 to 60 minutes at a temperature of 350 to 500°C and an oxygen pressure of 1 Pa to 20 Pa.

[0028] As a preferred embodiment of the preparation method described in this invention, in addition to alternating sputtering growth using NiCo2O4 / NiFe2O4 targets, it also includes directly preparing NiCoFeO4 / NiCo2O4 targets to achieve a specific gradient or ratio.

[0029] As a preferred embodiment of the preparation method described in this invention, it further includes: achieving stress regulation of the thin film by changing the composition, surface state and structure of the substrate MgAl2O4, thereby optimizing the coercivity and conductivity.

[0030] Beneficial effects of this invention:

[0031] (1) The present invention provides a method for preparing a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, which can overcome the limitations of traditional oxide hard magnetic materials. By using a multi-target alternating pulse laser deposition or radio frequency magnetron sputtering process, NiFeO4 and NiCo2O4 targets are used to prepare a NiCoFeO4-NiCo2O4 gradient thin film. The obtained film has a coercivity of up to 1T and excellent conductivity, with an anomalous Hall resistivity of up to 52.5μΩ·cm.

[0032] (2) The preparation method of the present invention has important application potential in the field of magnetic storage, and can provide new possibilities for the research and development of high-density, high-speed and low-power magnetic storage devices; at the same time, the method has the advantage of low material cost, which is conducive to large-scale preparation and industrial production. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0034] Figure 1 This is a structural diagram of the NiCo2-xFexO4 gradient thin film provided in the embodiment;

[0035] Figure 2 The X-ray diffraction pattern of the NiCo2-xFexO4 gradient thin film provided in the embodiment;

[0036] Figure 3 The X-ray reciprocal space diffraction pattern of the NiCo2-xFexO4 gradient thin film provided in the embodiment;

[0037] Figure 4 The anomalous Hall curves of the 35nm NiCo2-xFexO4 gradient film and the pure NiCo2O4 film provided in the examples are shown.

[0038] Figure 5 The flowchart shows the preparation process of the NiCo2-xFexO4 gradient thin film provided in the example. Detailed Implementation

[0039] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0040] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0041] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0042] like Figure 1 As shown, the present invention provides a NiCo2-xFexO4 gradient thin film, comprising a substrate MgAl2O4101, a seed layer NiCo2O4102, and a thin film gradient layer NiCo2-xFexO4103.

[0043] The seed layer 102 is located on the substrate 101. The seed layer 102 is mainly used to improve epitaxial matching, thereby improving crystal quality. Specifically, it is a single crystal thin film layer of NiCo2O4 with a thickness of 2-10 nm and (001) orientation.

[0044] The thin film gradient layer 103 is located above the seed layer 102, and better crystal quality can be obtained based on the growth of the seed layer.

[0045] like Figure 5 As shown, a method for preparing a hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy is provided, including steps 501-503:

[0046] Step 501, provide a substrate layer.

[0047] The substrate used in this invention is a MgAl2O4 substrate commonly used in commercial microwave devices. The MgAl2O4 substrate is a (001) oriented single-crystal MgAl2O4 substrate with a size of 1x1cm and a thickness of 500 micrometers. After selecting the substrate, the MgAl2O4 substrate is immersed in alcohol and ultrasonically cleaned for 3-5 minutes. After cleaning, the MgAl2O4 substrate is dried with nitrogen gas and immediately placed on the deposition stage of the magnetron sputtering system.

[0048] Step 502: A NiCo2O4 seed layer is formed on the substrate.

[0049] Specifically, the NiCo2O4 seed layer can be prepared by magnetron sputtering. Preferably, the thickness of the seed layer is 2–10 nm.

[0050] Step 503: A gradient thin film layer is formed on the seed layer.

[0051] Thin films were deposited on a MgAl2O4(001) single crystal substrate using alternating NiCo2O4 / NiFe2O4 targets by pulsed laser deposition or magnetron sputtering, and the thickness of the film was controlled by the pulse / sputtering time.

[0052] First, NiCo2O4 target material is used to deposit the final thin film layer. After a certain thickness, NiFe2O4 target material is switched to. Based on the thickness detection or calculation results, NiCo2O4 target material is then switched to control the composition gradient of Co and Fe in the thin film. The Fe doping amount gradually transitions from 0 to xmax.

[0053] The thickness of the gradient thin film layer is 10-30 nm. The maximum value of Fe doping in the NiCo2-xFexO4 thin film gradient layer, xmax, is generally not higher than 1. If it is higher than 1, the conductivity of the thin film will decrease sharply. The minimum value of doping, xmin, is generally not lower than 0.1.

[0054] The seed layer and the thin film gradient layer were prepared sequentially using radio frequency magnetron sputtering:

[0055] Before sputtering each thin film layer, the ceramic target material required for each layer is prepared by solid-state sintering. Specifically, the ceramic target material used for growing the ferrimagnetic free layer and the ferrimagnetic reference layer NiCo2O4 thin film by radio frequency magnetron sputtering is prepared from NiO powder and Co2O3 powder with a purity of 4N.

[0056] The ceramic target used to grow the MgAl2O4 thin film of the tunneling layer is prepared from MgO powder and Al2O3 powder with a purity of 4N.

[0057] The preparation steps of the ceramic target material are as follows: powder is mixed according to chemical ratio, binder is added, it is shaped in a mold, pressed into a sheet using a press, and finally sintered into a ceramic target material. During the sintering of the ceramic target material, the sintering temperature is 100–200°C lower than the phase formation temperature of each system.

[0058] After the prepared ceramic target is polished, it is installed in the magnetron sputtering system. The distance between the ceramic target and the substrate is adjusted to 40 mm. A 1:1 argon / oxygen (Ar / O2) mixture is introduced and pre-sputtered for 10 to 12 hours at room temperature with the gas pressure adjusted to 0.1 to 20 Pa to remove impurities from the target surface.

[0059] A multi-stage pumping system, consisting of a combination of mechanical and molecular pumps, is used to evacuate the deposition chamber of the magnetron sputtering system to a high vacuum state, ensuring a vacuum level of no less than 10⁻⁶. -6mTorr is used to ensure a clean growth environment; then the substrate temperature in the deposition chamber is raised to 350℃~500℃, so that the substrate layer is kept at a temperature of 350℃~500℃ for 10 minutes; finally, a mixture of argon and oxygen with a volume ratio of 1:1 is slowly introduced into the deposition chamber, and the mass flow meter is adjusted to make the required growth gas pressure in the deposition chamber 100mTorr.

[0060] After the gas pressure in the deposition chamber stabilizes, the growth time is adjusted to sequentially grow the subferromagnetic free layer and the tunneling layer on the substrate; then the growth gas pressure in the deposition chamber is adjusted to 200 mTorr to grow the subferromagnetic reference layer.

[0061] In this embodiment of the invention, during sputtering, the growth thickness of each thin film does not exceed 100 nm, and the consumption of sputtering target material per growth is minimal. The nanoscale thin film meets the requirements for miniaturization and integration. The fabrication method, radio frequency magnetron sputtering, is an industrial mass production method. The growth temperature of 350℃~500℃ has good process compatibility and meets the requirements of today's industrial mass production.

[0062] In this embodiment of the invention, the oxidation degree and lattice defects of the thin film can be controlled by controlling the oxygen pressure of the atmosphere; typically, the oxygen pressure is between 1 Pa and 20 Pa, and the higher the pressure, the greater the coercivity of the thin film. Annealing can be carried out for half an hour at an oxygen pressure of 1 Pa to 20 Pa within the range of 350 to 500 °C to promote the optimization of the thin film crystal structure and the improvement of magnetic properties. The higher the oxygen pressure, the higher the coercivity; the higher the temperature, the higher the resistivity.

[0063] In this embodiment of the invention, in addition to using the above-mentioned NiCo2O4 / NiFe2O4 target material for alternating sputtering growth, a specific gradient or ratio can also be achieved by directly preparing a NiCoFeO4 / NiCo2O4 target material. By changing the composition, surface state and structure of the substrate MgAl2O4, the stress of the thin film can be controlled, thereby optimizing the coercivity and conductivity to meet the needs of different application fields.

[0064] The aforementioned materials can achieve customized coercivity from 100Gs to over 10000Gs, without the need for magnetic annealing, requiring only stress modulation of the substrate.

[0065] Example 1

[0066] A method for preparing highly coercive hard magnetic oxide semiconductor thin films with perpendicular magnetic anisotropy, the process steps of which are as follows:

[0067] Step 1: Prepare NiFe2O4 and NiCo2O4 targets using standard ceramic sintering processes. Clean the substrate, place it in the cavity, evacuate, and heat to complete the preparations. After reaching the required vacuum level, maintain an oxygen pressure atmosphere of 13 Pa and heat the substrate to 350°C.

[0068] The preparation process of NiFe2O4 target material is as follows:

[0069] Ni2O3 and Fe2O3 powders with a purity of over 99.9% were mixed in a 1:2 ratio, ball-milled, granulated, pressed into a target material under a pressure of 15 kPa, and sintered into ceramic at 1000 degrees Celsius.

[0070] The preparation process of NiCo2O4 target material is as follows:

[0071] Ni2O3 and Co2O3 powders with a purity of over 99.9% were mixed in a 1:2 ratio, ball-milled, granulated, pressed into a target material under a pressure of 15 kPa, and sintered into ceramic at 1000℃.

[0072] Step 2: Deposit thin films on a MgAl2O4(001) single crystal substrate using alternating NiCo2O4 / NiFe2O4 targets by pulsed laser deposition or magnetron sputtering, and control the film thickness by pulse / sputtering time;

[0073] Specifically, the energy density of pulsed laser deposition is approximately 2 J / cm². 2 The sputtering rate is 120 pulses / nm, the magnetron sputtering power is 50W, and the sputtering rate is 50nm / hour;

[0074] First, a NiCo2O4 target is used to deposit the initial thin film layer. After the first thickness is reached, the target is switched to NiFe2O4. Based on the thickness detection or calculation results, the target is then switched back to NiCo2O4 to control the compositional gradient of Co and Fe in the film. The Fe doping amount gradually transitions from 0 to xmax. Typically, the first thickness is 5 nm as a seed layer for growth, and the total thickness of the film is optimal at 22–30 nm.

[0075] Step 3: After the thin film deposition is completed, anneal in situ at 350-500℃ for 30 minutes to make its composition more uniform, and then cool down to room temperature.

[0076] Specifically, in step 1, the relative contents of Fe and Co in the film are controlled by adjusting the ratio of NiCo2-xFexO4, and the composition distribution is gradually transitioned from Co-dominant to Fe-dominant. The composition gradient is achieved by continuously increasing the thickness of the NiCoFeO4 layer and adding annealing to improve the composition.

[0077] In one embodiment, the maximum doping amount of Fe is x = 1, and its growth sequence is as follows: NiCo2O4 (3nm) → NiCoFeO4 (1nm) → NiCo2O4 (1nm) → NiCoFeO4 (2nm) → NiCo2O4 (1nm) → NiCoFeO4 (3nm) → NiCo2O4 (1nm) → NiCoFeO4 (4nm) → NiCo2O4 (1nm) → NiCo2O4 (5nm).

[0078] In step 2, during pulsed laser deposition or magnetron sputtering, the compositional uniformity and crystal quality of the thin film can be optimized by adjusting the pulse / sputtering energy and frequency, as well as the appropriate temperature of the target and the angle of the substrate.

[0079] Figure 2 The X-ray diffraction results of the above thin film structure are given. The diffraction peak of the (004) film exists only at about 43 degrees and the diffraction peak of the substrate exists at about 44 degrees, which proves its single orientation.

[0080] Figure 3 The X-ray reciprocal space diffraction results of the above thin film structure are presented. It can be seen that the diffraction peak 104 of the thin film is collinear with the diffraction peak 105 of the substrate, exhibiting a coherent growth mode. This is a prerequisite for high coercivity.

[0081] The maximum value of Fe doping in the NiCo2-xFexO4 thin film gradient layer, xmax, is generally no higher than 1. A value higher than 1 will cause a sharp decrease in the conductivity of the film, while a value lower than 0.5 will result in a coercivity generally lower than 2000 Gs. The minimum value of Fe doping, xmax, is generally 0 or no higher than 0.1 to ensure good lattice matching and transition.

[0082] The anomalous Hall resistance of gradient thin films can be measured using a patterned Hall bar, such as... Figure 4 As shown, in this embodiment, the NiCo2O4 seed layer thickness is 5 nm and the gradient layer thickness is 22 nm. The resulting film has an anomalous Hall resistance of 15 Ω and an anomalous Hall resistivity of 52.5 μΩ·cm, which is significantly higher than that of many metal composite multilayer film materials, including CoFeB multilayer films (typically not exceeding 10). It also shows a substantial improvement compared to pure NiCo2O4 films of the same thickness. Pure NiCoFeO4 films are difficult to measure for anomalous Hall signals due to their extremely high resistance.

[0083] This invention aims to provide a hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, high coercivity, and certain conductivity. The fabrication method requires, on the one hand, a good epitaxial matching relationship between the thin film and the substrate to achieve a highly crystalline multilayer film material; on the other hand, it requires control over the film composition to achieve a high coercivity structure. The growth process and structural design of the highly crystalline, high-quality multilayer film are crucial. Furthermore, traditional ferromagnetic free-layer materials exhibit ferromagnetic resonance in the GHz band, thus limiting the device's response speed, which further affects the sensor's operating frequency band.

[0084] Therefore, there is an urgent need to study new oxide semiconductor thin film material structures with high spin polarization, high magnetoresistive density, and high response speed to meet the ever-growing demand for magnetic sensitive elements.

[0085] This invention provides a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, the material composition of which is NiCo2-xFexO4, wherein the Fe doping amount x gradually transitions from a minimum value xmin to a maximum value xmax as the film thickness increases from the substrate to the surface. The crystal orientation of the film is (001), and a MgAl2O4 (001) single crystal is used as the substrate. When the Fe doping amount x = 0, the film exhibits the lowest coercivity, weak perpendicular magnetic anisotropy, and optimal conductivity, with the coercivity generally not exceeding 100 Gs. As xmax increases, the saturation magnetization and coercivity of the film gradually increase, and when xmax = 1, the coercivity can exceed 10000 Gs at an appropriate thickness. The rectangularity of the film is always higher than 100%. When xmax = xmin = 1, that is, when there is no gradient in the film, the conductivity of the film is poor and the coercivity is lower than that of the gradient film.

[0086] In summary, this invention provides a method for preparing a high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, which overcomes the limitations of traditional oxide hard magnetic materials. By employing a multi-target alternating pulsed laser deposition or radio frequency magnetron sputtering process, and using NiFeO4 and NiCo2O4 targets, a NiCoFeO4-NiCo2O4 gradient thin film is prepared. The resulting film has a coercivity of up to 1T and excellent conductivity, with an anomalous Hall resistivity as high as 52.5 μΩ·cm.

[0087] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A high coercivity hard magnetic oxide semiconductor thin film with perpendicular magnetic anisotropy, characterized in that: include, Substrate MgAl2O4, seed layer NiCo2O4, and thin film gradient layer NiCo 2-x Fe x O4; The thin film gradient layer NiCo 2-x Fe x Methods for preparing O4 include preparing NiFe2O4 and NiCo2O4 targets using standard ceramic sintering processes; Thin films were deposited on a MgAl2O4(001) single crystal substrate using alternating NiCo2O4 / NiFe2O4 targets by pulsed laser deposition or magnetron sputtering, and the thickness of the film was controlled by the pulse / sputtering time. After the thin film deposition is complete, it is annealed in situ for 30-60 minutes to make its composition more uniform, and then cooled down. In this process, NiCo2O4 target material is used for thin film deposition. After a certain thickness, NiFe2O4 target material is used. Based on the thickness detection or calculation results, NiCo2O4 target material is used again to control the composition change of Co and Fe in the thin film. The Fe doping amount gradually transitions from 0 to xmax, and the value of xmax ranges from 0.5 to 1. The thin film has a crystal orientation of (001) and is based on a MgAl2O4(001) single crystal.

2. The thin film as described in claim 1, characterized in that: The in-situ annealing lasts for 30-60 minutes at a temperature of 350-500°C and an oxygen pressure of 1-20 Pa.

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