CN104064350B - A kind of preparation method of the thin magnetic film with positive magnetic anisotropy temperature coefficient - Google Patents

Abstract

The invention provides a preparation method of a magnetic thin film with positive magnetic anisotropy temperature coefficient. The method selects a substrate with an anisotropic thermal expansion coefficient, first applies a certain strain to the substrate in the direction in which the absolute value of the thermal expansion coefficient is larger, and the strain value is greater than that when the temperature drops from room temperature The strain value generated by the substrate when the minimum working temperature of the magnetic film is reached. Under the condition of maintaining the strain, the magnetic film is grown on the surface of the substrate. After the growth of the magnetic film is completed, the strain is removed to obtain a magnetically anisotropic magnetic film. As the temperature increases, the magnetic anisotropy of the magnetic thin film material increases, that is, it has a positive magnetic anisotropy temperature coefficient, which is of great significance for improving the thermal stability of the magnetic thin film device.

Description

A kind of preparation method of magnetic film with positive magnetic anisotropy temperature coefficient

technical field

The invention relates to the field of preparation of magnetic thin films in high-frequency electromagnetic devices, microwave devices and magnetic sensor devices, in particular to a method for preparing magnetic thin films with positive magnetic anisotropy temperature coefficients.

Background technique

Magnetic anisotropy is a basic parameter of magnetic materials, and its magnitude determines the energy barrier that needs to be overcome for magnetic moment switching. With the rapid development of spintronics, magnetic thin films are widely used in magnetic storage, magnetic sensing, and microwave devices, and play an irreplaceable role in the development of information technology. For these magnetic thin film devices, magnetic anisotropy plays an important role. For example, in the field of magnetic storage, with the increase of magnetic recording density, the size of each magnetic recording unit is gradually shrinking. In order to overcome the thermal disturbance The influence needs to increase the magnetic anisotropy of the magnetic recording unit; in the magnetic sensor, the size of the magnetic anisotropy directly determines the sensitivity and linear working range of the sensor; in the microwave device, the size of the magnetic anisotropy determines the resonance frequency The size of the device determines the operating point and operating frequency range of the device.

As we all know, thermal stability is the basic parameter of the above-mentioned magnetic thin film devices. In order to ensure that the device can work normally in a certain temperature range, the device must have good thermal stability. However, any physical parameter has certain temperature-dependent characteristics, and magnetic anisotropy is no exception. In general, an increase in temperature leads to a decrease in the magnetization of the magnetic thin film, which in turn leads to a decrease in magnetic anisotropy. In addition, an increase in temperature often leads to a decrease in the coercive force of the magnetic film. This phenomenon that the magnetic anisotropy decreases with the increase of temperature indicates that the magnetic thin film has a negative magnetic anisotropy temperature coefficient. The negative magnetic anisotropy temperature coefficient leads to the reduction of the energy barrier and coercive force required to overcome the magnetic moment reversal, which is not conducive to the thermal stability of the device.

Therefore, how to obtain a magnetic thin film whose magnetic anisotropy remains unchanged or even increased with the increase of temperature through the improvement of the structure or the preparation method, that is, the magnetic thin film has a positive temperature coefficient of magnetic anisotropy, which has a positive effect on the thermal conductivity of the magnetic thin film device. Stability is of great significance.

Contents of the invention

Aiming at the problem that the magnetic films obtained by the existing preparation methods often have negative magnetic anisotropy temperature coefficients, the present invention aims to provide a method for preparing magnetic films, by which magnetic films with positive magnetic anisotropy temperature coefficients can be obtained. Thin films are of great significance for improving the thermal stability of magnetic thin film devices.

In order to achieve the above-mentioned technical purpose, the inventors found after a large number of experimental explorations that when growing a magnetic film on the surface of the substrate (the magnetic film has magnetostrictive properties, including positive magnetostrictive properties and negative magnetostrictive properties), the selected For substrates with anisotropic thermal expansion coefficients (including positive thermal expansion coefficients and negative thermal expansion coefficients), a certain strain opposite to the direction of thermal expansion is first applied to the substrate along the direction in which the absolute value of the thermal expansion coefficient is larger, that is, when When the substrate has a positive thermal expansion coefficient, a certain compressive strain is applied to the substrate along the direction with a larger thermal expansion coefficient; when the substrate has a negative thermal expansion coefficient, a certain compressive strain is applied to the substrate along the direction with a larger thermal expansion coefficient. Tensile strain, grow a magnetic film on the surface of the substrate under the condition of maintaining the strain, and remove the strain after the growth of the magnetic film is completed, the specific performance of the magnetic film is as follows:

(1) The magnetic film is a positive magnetostrictive material, and the substrate has a positive thermal expansion coefficient

When growing a magnetic thin film on the surface of the substrate, a compressive strain is applied to the substrate along the direction of the larger thermal expansion coefficient of the substrate. After the film growth is completed, the compressive strain is removed, and the magnetic thin film bears the tensile strain from the substrate, and it is easily magnetized. The axis is along the direction with a larger thermal expansion coefficient of the substrate. Correspondingly, the hard magnetization axis is perpendicular to the direction with a larger thermal expansion coefficient of the substrate, that is, along the direction with a smaller thermal expansion coefficient of the substrate, as shown in Table 1 below;

When the temperature rises, comparing the remanence of the magnetic thin film, it is found that the remanence along the direction of the easy magnetization axis (the direction with the larger thermal expansion coefficient) and the remanence along the direction of the hard axis (the direction with the smaller thermal expansion coefficient) The difference of increases gradually, that is, as the temperature increases, the magnetic anisotropy of the magnetic film increases, showing a positive temperature coefficient of magnetic anisotropy.

(2) The magnetic film is a positive magnetostrictive material, and the substrate has a negative thermal expansion coefficient

When growing a magnetic thin film on the surface of the substrate, a tensile strain is applied to the substrate along the direction in which the absolute value of the thermal expansion coefficient of the substrate is larger. The easy axis of magnetization is along the direction of the smaller absolute value of the thermal expansion coefficient of the substrate, that is, perpendicular to the direction of the larger absolute value of the thermal expansion coefficient of the substrate. Correspondingly, the hard axis of magnetization is along the direction of the larger absolute value of the thermal expansion coefficient of the substrate, as follows As shown in Table 1;

When the temperature rises, compared with the remanence of the magnetic film, the remanence along the direction of the easy magnetization axis (the direction with the smaller absolute value of the thermal expansion coefficient) and the direction along the hard axis (the direction with the larger absolute value of the thermal expansion coefficient) The difference of the remanence increases gradually, that is, as the temperature increases, the magnetic anisotropy of the magnetic thin film increases, showing a positive magnetic anisotropy temperature coefficient.

(3) The magnetic film is a negative magnetostrictive material, and the substrate has a positive thermal expansion coefficient

When growing a magnetic thin film on the surface of the substrate, a compressive strain is applied to the substrate along the direction of the larger thermal expansion coefficient of the substrate. After the film growth is completed, the compressive strain is removed, and the magnetic thin film bears the tensile strain from the substrate, and it is easily magnetized. The axis is along the direction with a smaller thermal expansion coefficient of the substrate, that is, perpendicular to the direction with a larger thermal expansion coefficient of the substrate. Correspondingly, its hard magnetization axis is along the direction with a larger thermal expansion coefficient of the substrate, as shown in Table 1 below;

When the temperature rises, comparing the remanence of the magnetic film, it is found that the remanence along the direction of the easy magnetization axis (the direction with the smaller thermal expansion coefficient) and the remanence along the direction of the hard axis (the direction with the larger thermal expansion coefficient) The difference of increases gradually, that is, as the temperature increases, the magnetic anisotropy of the magnetic film increases, showing a positive temperature coefficient of magnetic anisotropy.

(4) The magnetic film is a negative magnetostrictive material, and the substrate has a negative thermal expansion coefficient

When growing a magnetic film on the surface of the substrate, a tensile strain is applied to the substrate along the direction with a larger thermal expansion coefficient of the substrate. After the film growth is completed, the tensile strain is removed, and the magnetic film bears the compressive strain from the substrate. The magnetization axis is along the direction of the larger absolute value of the thermal expansion coefficient of the substrate. Correspondingly, the hard magnetization axis is perpendicular to the direction of the larger absolute value of the thermal expansion coefficient of the substrate, that is, along the direction of the smaller thermal expansion coefficient of the substrate, as shown in Table 1 shown;

When the temperature rises, comparing the remanence of the magnetic film, it is found that the remanence along the direction of the easy axis (the direction with the larger absolute value of the thermal expansion coefficient) is different from that along the direction of the hard axis (the direction with the smaller absolute value of the thermal expansion coefficient). ) The difference in remanence increases gradually, that is, as the temperature increases, the magnetic anisotropy of the magnetic film increases, showing a positive magnetic anisotropy temperature coefficient.

Table 1: A list of strain types and directions of easy magnetization axes applied when different magnetostrictive materials are combined with substrates with different thermal expansion properties;

Therefore, when the magnetic film is grown according to the above method, no matter how the positive or negative magnetostrictive material is combined with the substrate with positive or negative thermal expansion coefficient, the uniaxial magnetic anisotropy of the magnetic film will increase when the temperature rises. That is, the composite magnetic film has a positive magnetic anisotropy temperature coefficient; in addition, when the temperature rises, the magnetic field is along the easy axis of magnetization, and the coercive force also increases, which is beneficial to the thermal stability of magnetic devices at high temperatures.

That is, the technical solution adopted in the present invention is: a method for preparing a magnetic thin film with a positive magnetic anisotropy temperature coefficient, wherein the magnetic thin film is grown on the surface of a substrate, comprising the following steps:

Step 1: The substrate has an anisotropic thermal expansion coefficient, as shown in Figure 1, that is, the thermal expansion coefficients of the substrate along a mutually perpendicular X direction and Y direction are different; in the X direction and the Y direction, along the thermal expansion The direction with the larger absolute value of the coefficient exerts strain on the substrate, as follows:

When the substrate has a positive coefficient of thermal expansion, compressive strain is applied to the substrate along the direction with a larger coefficient of thermal expansion. In order to make the prepared magnetic thin film exhibit a positive magnetic anisotropy temperature coefficient in the entire working temperature range, as a preference, The compressive strain value is greater than the reference strain value, and the test method of the reference strain value is: in the working temperature range of the magnetic film, select the lowest working temperature as the test temperature, when the temperature drops from room temperature to the test temperature temperature, the strain value generated by thermal expansion or thermal contraction of the substrate;

When the substrate has a negative thermal expansion coefficient, a tensile strain is applied to the substrate along the direction in which the absolute value of the thermal expansion coefficient is larger. In order to make the prepared magnetic film exhibit a positive magnetic anisotropy temperature coefficient in the entire working temperature range, as Preferably, the tensile strain value is greater than the reference strain value, and the test method of the reference strain value is: in the working temperature range of the magnetic film, select the lowest working temperature as the test temperature, when the temperature drops from room temperature to At the test temperature, the strain value generated by thermal expansion or thermal contraction of the substrate;

Step 2: At room temperature, keep applying the strain described in step (1) to the substrate, and grow the magnetic thin film on the surface of the substrate;

Step 3: After the growth of the magnetic thin film is completed, remove the strain (including the compressive strain and the tensile strain) in step (1) to obtain a magnetic thin film with uniaxial magnetic anisotropy.

Among the above technical solutions:

The substrate is selected to have an anisotropic thermal expansion coefficient, that is, the thermal expansion coefficients of the substrate along a mutually perpendicular X direction and Y direction are different. The substrate includes, but is not limited to, a single crystal substrate, a ceramic substrate, a metal substrate, an organic substrate, a plastic substrate, a ferroelectric substrate, and the like. Preferably, a flexible organic substrate is selected.

The magnetic film is not limited, including magnetic metal Fe, Co, Ni, magnetic alloy Fe-Ni, Fe-Ga, Co-Fe-B, magnetic oxide Fe ₃ O ₄ , La-Sr-Mn-O ₃ film Wait.

In said step 1, the application of strain, including the application of tensile strain or compressive strain, is not limited. One of the application methods is to place the substrate on a curved convex mold or concave mold. The convex mold or concave mold The resulting strain is t/2r, where t is the thickness of the magnetic film and substrate, and r is the radius of curvature of the mold.

In step 2, the growth method of the magnetic film is not limited, including magnetron sputtering or pulsed laser deposition.

In summary, the present invention combines positive magnetostrictive materials or negative magnetostrictive materials with substrates with positive or negative thermal expansion coefficients, applies strain to the substrates when growing magnetic materials, and removes the substrates after the growth is completed. Strain, which can effectively control the initial magnetic anisotropy of the magnetic film; by controlling the strain value, it is greater than the thermal expansion of the substrate when the temperature drops from room temperature to the lowest working temperature of the magnetic film in the working temperature range. The strain value can effectively control the initial magnetic anisotropy of the magnetic thin film material in the working temperature range; furthermore, as the temperature increases, the magnetic anisotropy of the magnetic thin film material increases, that is, it has a positive magnetic anisotropy temperature In addition, the preparation method is simple and easy to control, so a preparation method with application prospects is of great significance for improving the thermal stability of magnetic thin film devices.

Description of drawings

Fig. 1 is the substrate schematic diagram with anisotropic coefficient of thermal expansion selected in the magnetic thin film preparation method of the present invention;

Fig. 2 is the variation relationship of normalized remanence with temperature of the CoFeB/PVDF magnetic thin film in Example 1 of the present invention.

Fig. 3 is the variation relationship of the normalized remanence with temperature of the Ni/PVDF magnetic thin film in Example 2 of the present invention.

detailed description

The present invention will be described in further detail below with reference to the embodiments of the accompanying drawings. It should be noted that the following embodiments are intended to facilitate the understanding of the present invention, but do not limit it in any way.

Example 1:

In this embodiment, the organic ferroelectric material PVDF is used as the anisotropic thermal expansion substrate, and its thickness is 30 μm. As shown in Figure 1, PVDF has a negative anisotropic thermal expansion coefficient, the thermal expansion coefficient along the X direction is -145×10 ^-6 K ^-1 , and the thermal expansion coefficient along the Y direction is -13×10 ^-6 K ^-1 , The maximum in-plane strain of the PVDF substrate due to heating is -145×10 ^-6 ×T, and the minimum strain is -13×10 ^-6 ×T, where T is the temperature.

A magnetic CoFeB material with a positive magnetostriction coefficient is grown on the surface of the substrate with a thickness of 60nm. The working temperature range of the magnetic material is -60 degrees to 60 degrees, and the room temperature is 27 degrees. Concrete preparation method comprises the following steps:

Step 1: In the working temperature range of the magnetic CoFeB material, select the lowest working temperature -60 degrees as the test temperature. When the temperature drops from room temperature (ie 27 ℃) to the test temperature, the PVDF substrate along the X direction produces The thermal expansion strain is 1.3%, and the strain value of 1.3% is set as the reference strain value in the X direction, and the thermal expansion strain generated along the Y direction is 0.11%, and the strain value of 0.11% is set as the reference strain value in the Y direction.

A tensile strain is applied to the substrate along the X direction, so that the strain value generated by the substrate is 1.3% greater than the reference strain value in the X direction. In this embodiment, the tensile strain value is selected to be 1.5%. This tensile strain is achieved by placing the substrate in a curved male mold.

Estimate the strain generated by the convex mold according to t/2r, where t is the thickness of the magnetic film and substrate, and r is the radius of curvature of the mold. Since the tensile strain value is 1.5%, the radius of curvature of the convex mold is 1 mm.

That is, PVDF was bent in the X direction in a male mold with a radius of curvature of 1 mm.

Step 2: At room temperature, keep applying the tensile strain described in step 1 to the PVDF substrate, and use the magnetron sputtering method to uniformly grow a magnetic CoFeB film on the surface of the PVDF substrate. The specific process parameters are: the back of the magnetron sputtering The bottom vacuum is 7×10 ^-5 Pa, the sputtering atmosphere is Ar, the sputtering pressure is 0.5Pa, the DC sputtering is 35W, and the deposition time is 6 minutes;

Step 3: After the growth of the magnetic CoFeB film is completed, the convex mold is removed, and a magnetic CoFeB film with uniaxial magnetic anisotropy is obtained at room temperature, and its easy magnetization axis is along the Y direction.

The direction of the magnetic field is along the X direction and the Y direction, respectively, and the relationship between the remanence of the magnetic CoFeB thin film prepared above and the temperature change is tested, and the test results are shown in FIG. 2 . It can be seen from Figure 2 that when the magnetic field is along the Y direction, as the temperature increases, the remanence of the magnetic CoFeB film gradually increases; on the contrary, when the magnetic field is along the X direction, as the temperature increases, The remanence of the magnetic CoFeB film gradually decreases; that is, as the temperature increases, the difference between the remanence of the magnetic field along the Y direction and the remanence of the magnetic field along the X direction gradually increases, which shows that as the temperature increases, The magnetic anisotropy of the magnetic CoFeB thin film is increasing, has a positive temperature coefficient of magnetic anisotropy, and its easy magnetization axis is in the direction of the Y axis.

Example 2:

In this embodiment, the organic ferroelectric material PVDF is used as the anisotropic thermal expansion substrate, and its thickness is 30 μm. As shown in Figure 1, PVDF has a negative anisotropic thermal expansion coefficient, the thermal expansion coefficient along the X direction is -145×10 ^-6 K ^-1 , and the thermal expansion coefficient along the Y direction is -13×10 ^-6 K ^-1 , The maximum in-plane strain of the PVDF substrate due to heating is -145×10 ^-6 ×T, and the minimum strain is -13×10 ^-6 ×T, where T is the temperature.

A magnetic Ni material with a negative magnetostriction coefficient is grown on the surface of the substrate with a thickness of 60nm. The working temperature range of the magnetic material is -60 degrees to 60 degrees, and the room temperature is 27 degrees. Concrete preparation method comprises the following steps:

Step 1: In the working temperature range of the magnetic Ni material, select the lowest working temperature -60 degrees as the test temperature. When the temperature drops from room temperature (ie 27 ℃) to the test temperature, the PVDF substrate along the X direction produces The thermal expansion strain is 1.3%, and the strain value of 1.3% is set as the reference strain value in the X direction, and the thermal expansion strain generated along the Y direction is 0.11%, and the strain value of 0.11% is set as the reference strain value in the Y direction.

A tensile strain is applied to the substrate along the X direction, so that the strain value generated by the substrate is 1.3% greater than the reference strain value in the X direction. In this embodiment, the tensile strain value is selected to be 1.5%. This tensile strain is achieved by placing the substrate in a curved male mold.

Estimate the strain generated by the convex mold according to t/2r, where t is the thickness of the magnetic film and substrate, and r is the radius of curvature of the mold. Since the tensile strain value is 1.5%, the radius of curvature of the convex mold is 1 mm.

That is, PVDF was bent in the X direction in a male mold with a radius of curvature of 1 mm.

Step 2: At room temperature, keep applying the tensile strain described in step 1 to the PVDF substrate, and use the magnetron sputtering method to uniformly grow a magnetic Ni film on the surface of the PVDF substrate. The specific process parameters are: the back of the magnetron sputtering The bottom vacuum is 7×10 ^-5 Pa, the sputtering atmosphere is Ar, the sputtering pressure is 0.6Pa, the DC sputtering is 45W, and the deposition time is 5 minutes;

Step 3: After the growth of the magnetic Ni film is completed, the convex mold is removed, and a magnetic Ni film with uniaxial magnetic anisotropy is obtained at room temperature, and its easy magnetization axis is along the X direction.

The direction of the magnetic field is along the X direction and the Y direction, respectively, and the relationship between the remanence of the magnetic Ni thin film prepared above and the temperature is tested. The test results are shown in FIG. 3 . It can be seen from Figure 3 that when the magnetic field is along the X direction, as the temperature increases, the remanence of the magnetic Ni film gradually increases; on the contrary, when the magnetic field is along the Y direction, as the temperature increases, The remanence of the magnetic Ni film gradually decreases; that is, as the temperature increases, the difference between the remanence of the magnetic field along the X direction and the remanence of the magnetic field along the Y direction increases gradually, which shows that as the temperature increases, The magnetic anisotropy of the magnetic Ni thin film is increasing, has a positive temperature coefficient of magnetic anisotropy, and its easy magnetization axis is in the direction of the X axis.

The embodiments described above have described the technical solutions of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. All done within the principle scope of the present invention Any modification, supplement or substitution in a similar manner shall be included within the protection scope of the present invention.

Claims (4)

1. a kind of preparation method with the magnetic film of positive magnetic anisotropy temperature coefficient, described magnetic film is to grow and make on substrate surface, it is characterized in that: comprise the steps: Step (1): The substrate has an anisotropic thermal expansion coefficient, and the thermal expansion coefficients are different along a mutually perpendicular X direction and Y direction; in the X direction and the Y direction, the absolute value of the thermal expansion coefficient along the X direction is larger The direction exerts strain on the substrate as follows: When the substrate has a positive coefficient of thermal expansion, compressive strain is applied to the substrate along the direction with a larger coefficient of thermal expansion, and the compressive strain value is greater than the reference strain value. The test method for the reference strain value is: in the The working temperature range of the magnetic thin film, select the lowest working temperature as the test temperature, when the temperature drops from room temperature to the test temperature, test the strain value generated by thermal expansion or thermal contraction of the substrate; When the substrate has a negative coefficient of thermal expansion, a tensile strain is applied to the substrate along the direction in which the absolute value of the coefficient of thermal expansion is larger, and the tensile strain value is greater than the reference strain value. The test method for the reference strain value is: For the working temperature range of the magnetic thin film, select the lowest working temperature as the test temperature, and when the temperature drops from room temperature to the test temperature, test the strain value generated by thermal expansion or thermal contraction of the substrate; Step (2): At room temperature, keep applying the strain described in step (1) to the substrate, and grow the magnetic thin film on the surface of the substrate; Step (3): After the growth of the magnetic thin film is completed, the strain described in step (1) is removed to obtain a magnetic thin film with uniaxial magnetic anisotropy. 2. A kind of preparation method of the magnetic thin film with positive magnetic anisotropy temperature coefficient according to claim 1, it is characterized in that: described substrate comprises single crystal substrate, ceramic substrate, metal substrate, organic matter substrate, plastic substrate or ferroelectric substrate. 3. a kind of preparation method with the magnetic thin film of positive magnetic anisotropy temperature coefficient according to claim 1 is characterized in that: described magnetic thin film comprises magnetic metal, magnetic alloy or magnetic oxide; Described magnetic The metal includes Fe, Co or Ni, the magnetic alloy includes Fe-Ni, Fe-Ga or Co-Fe-B, and the magnetic oxide includes Fe ₃ O ₄ or La-Sr-Mn-O ₃ . 4. A kind of preparation method of the magnetic thin film with positive magnetic anisotropy temperature coefficient according to claim 1, it is characterized in that: in described step (1), by placing substrate on convex mold or concave mold Applying strain to the substrate, the strain produced by the male mold or the concave mold is t/2r, where t is the thickness of the magnetic film and the substrate, and r is the radius of curvature of the mold.