L10-FePt THIN FILM EXHIBITING BOTH HIGH MAGNETIC ANISOTROPY AND LOW DAMPING CONSTANT, AND MANUFACTURING METHOD FOR THE SAME

Abstract

To provide an L10-FePt thin film exhibiting both high magnetic anisotropy and low damping constant.SOLUTION: The L10-FePt thin film includes a single-crystal substrate made of magnesium oxide (MgO) or strontium titanate (SrTiO3), and an FePt thin film laminated on the single-crystal substrate. The damping constant falls within a range of 0.018 or more and 0.044 or less, and the magnetic anisotropy falls within a range of 3.0 or more and 7.0 T or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an L10-FePt thin film that combines high magnetic anisotropy with a small damping constant and is suitable for use in a detector / oscillator in the terahertz wave band of 100 GHz or more, and a method for producing the same. [Background technology]

[0002] In the case of detector / oscillator elements using magnetic materials, research and development has focused on soft magnetic materials with small magnetic anisotropy, such as amorphous CoFeB and permalloy, from the perspective of large magnetization and small damping constant. On the other hand, because the operating frequency of an element is determined by the magnitude of magnetic anisotropy, the soft magnetic materials mentioned above have a limit of frequencies below a few GHz. For example, as described in Non-Patent Documents 1 and 2, research and development has been conducted on detector / oscillator elements using ferromagnetic metal thin films. Non-Patent Document 1 demonstrates that oscillations of several tens of MHz are achieved using permalloy, a soft magnetic material. Because soft magnetic materials have a relatively large magnetization and a small damping constant, research and development has been actively pursued, but the small magnetic anisotropy has meant that the operating frequency cannot be increased.

[0003] Furthermore, in Patent Document 1, magnetic materials have been researched on the assumption that they will be used in the quasi-microwave band (1 to 3 GHz, wavelength 10 to 33 cm, or in radio terminology, ultra-high frequency (UHF)), such as the 2.4 GHz band of the wireless LAN standard, and the microwave band (3 to 30 GHz, wavelength 1 to 10 cm). On the other hand, while Mn-based compounds are known to have large magnetic anisotropy, they have the problem of being impractical due to their small saturation magnetization. L10-FePd has been investigated as a material using highly magnetic elements such as Fe and Co (Non-Patent Document 2), but it does not have as large a magnetic anisotropy as Mn-based compounds, and therefore is not suitable as a material for terahertz wave detection and oscillation devices.

[0004] An ordered FePt alloy having an L10 type structure is known, for example, from Non-Patent Document 4. An ordered FePt alloy having an L10 type structure is well known as a magnetic material having large magnetic anisotropy, and the damping constant of this material is known to be very large, ranging from 0.05 to 0.10 (Non-Patent Document 3). It is known that the damping constant of ferromagnetic materials can be controlled by changing the composition ratio of Fe and Pt, as in Non-Patent Document 3, or can be significantly changed by controlling element substitution or the degree of ordering. However, the only report focusing on controlling the damping constant of L10-FePt is Non-Patent Document 3, and it is generally found to be very large. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-41599 [Non-patent literature]

[0006] [Non-Patent Document 1] M. Haidar, et al., Nat. Commun. 10, 2362 (2019). [Non-patent document 2] K. Mukaiyama, et al., 2015 International Conference on Solid State Devices and Materials, P-3-5, (2015) [Non-patent document 3] X. Ma, et al., Phys. Rev. B 91, 014438 (2015) [Non-patent document 4] Kazuhisa Sato, Yoshihiko Hirotsu, "Structure and long-range order of ferromagnetic L10 ordered alloy nanoparticles", Microscope 39_144 (2004) Summary of the Invention [Problem to be solved by the invention]

[0007] Attempts have been made to expand the frequency band by using Mn-based compounds, but these materials have the problem of having little interaction with electromagnetic waves due to their low saturation magnetization.Similar studies have also been conducted with the Fe-based ferromagnetic material L10-FePd, but the magnetic anisotropy is insufficient to realize terahertz waves in the 100 GHz and above range, making it impossible to excite frequencies above 100 GHz. Although there have been several reports on the damping constant of L10-FePt, only large damping constants have been reported, and a method to reduce the damping constant is desired.

[0008] The present invention has been made to solve the above-mentioned problems of the prior art, and has as its object to provide a method for producing an L10-FePt thin film that achieves both high magnetic anisotropy and a small damping constant. [Means for solving the problem]

[0009] L10-FePt thin films are already well-known as next-generation magnetic recording materials due to their large saturation magnetization and large magnetic anisotropy energy. However, previous reports have shown that L10-FePt thin films have a relatively large damping constant of approximately 0.05-0.1, and therefore have not been considered suitable as a material for detector / oscillator devices. Therefore, the present inventors believed that if a method could be found to reduce the large damping constant of 0.05-0.1 in L10-FePt thin films, they would become a promising magnetic material for detector / oscillator devices in the terahertz waveband above 100 GHz, leading to the invention of the present invention.

[0010] [1] As shown in FIG. 1, for example, the L10-FePt thin film of the present invention is an L10-FePt thin film comprising a single crystal substrate made of magnesium oxide (MgO) or strontium titanate (SrTiO3) and an FePt thin film laminated on the single crystal substrate, and has a damping constant in the range of 0.018 to 0.044, and a magnetic anisotropy in the range of 3.0 to 7.0 T.

[0011] If the damping constant exceeds 0.044, there is a technical disadvantage of excessive attenuation when applied to a detector element. A small damping constant is preferable for application to a detector element, and for CoFeB and permalloy used in the MHz band, the damping constant is approximately 0.007 to 0.010. However, in the case of FePt thin films, those with a damping constant below 0.018 are difficult to manufacture stably, resulting in reduced manufacturing yields and high costs when obtaining materials below this value. If the magnetic anisotropy exceeds 7 T, the regularity of the L10 ordered crystal is too strict, resulting in a decrease in manufacturing yield and the cost of obtaining L10 ordered crystals exceeding this value being high.If the magnetic anisotropy is less than 3 T, the regularity of the L10 ordered crystal is low.

[0012] [2] In the L10-FePt thin film [1] of the present invention, the number of misfit dislocations per unit length in the in-plane direction near the interface between the single crystal substrate and the FePt thin film is preferably in the range of 0.25 / nm to 0.44 / nm. If the number of misfit dislocations per unit length in the in-plane direction near the interface between the single crystal substrate and the FePt thin film exceeds 0.44 / nm, there is a technical disadvantage in that the regularity of the L10 ordered crystal is low. If the number of misfit dislocations is less than 0.25 / nm, there is a technical disadvantage in that the regularity of the L10 ordered crystal is too strict, resulting in a decrease in production yield and high costs for obtaining L10 ordered crystals exceeding this value. [3] In the L10-FePt thin film [2] of the present invention, the number of misfit dislocations is preferably measured by a cross-sectional TEM image. [4] The L10-FePt thin film [1] of the present invention preferably further comprises a cap layer covering the FePt thin film. [5] In the L10-FePt thin film [4] of the present invention, the cap layer is preferably any one of an MgO(001) cap layer, an SrTiO3 cap layer, a non-magnetic metal cap layer such as Cr, Ta, or Ru, a VN or C cap layer. [6] In the L10-FePt thin film [1] of the present invention, the thickness of the FePt thin film is preferably in the range of 10 nm to 30 nm. [7] In the L10-FePt thin film [1] of the present invention, preferably, the damping constant is H ) should be obtained. [8] The method for producing an L10-FePt thin film of the present invention comprises: A single crystal substrate made of strontium titanate (SrTiO3) is prepared. A thin FePt film is formed on this single crystal substrate, The deposition temperature of the single crystal substrate is within the range of 300-500°C. [9] The method for producing an L10-FePt thin film of the present invention comprises: A single crystal substrate made of magnesium oxide (MgO) is prepared. A thin FePt film is formed on this single crystal substrate, The deposition temperature of the single crystal substrate is within the range of 450-500°C.

[10] The terahertz wave band detector / oscillator of the present invention uses the above L10-FePt thin film [1]. [Effects of the Invention]

[0013] It has become clear that with the L10-FePt thin film of the present invention, the damping constant can be largely controlled within the range of 0.018-0.044 by appropriately selecting the single crystal substrate as the base material and the film formation temperature. Furthermore, according to the method for producing an L10-FePt thin film of the present invention, the microstructure can be controlled by changing the film formation temperature or the base material in order to reduce the damping constant of the L10-FePt thin film, thereby obtaining an L10-FePt thin film with an optimal number of misfit dislocations. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram of an L10-FePt thin film showing one embodiment of the present invention. [Figure 2]The figures show the magnetization curves when a magnetic field is applied perpendicular to the surface of the prepared samples. (a) shows the magnetization curve when a magnetic field is applied perpendicular to the film surface of a sample deposited on an MgO substrate at a film deposition temperature of 400°C, (b) shows the magnetization curve when a magnetic field is applied perpendicular to the film surface of a sample deposited on an SrTiO3 substrate at a film deposition temperature of 400°C, and (c) shows the magnetization curve when a magnetic field is applied perpendicular to the film surface of a sample deposited on an MgO substrate at a film deposition temperature of 500°C. [Figure 3] This figure shows the magnetic field strength dependence of the effective damping constant. (a) shows data for a sample deposited on an MgO substrate at a deposition temperature of 400°C, (b) shows data for a sample deposited on an SrTiO3 substrate at a deposition temperature of 400°C, and (c) shows data for a sample deposited on an MgO substrate at a deposition temperature of 500°C. [Figure 4] Figure 4 shows cross-sectional TEM images of the deposited samples and the results of Fourier analysis of the cross-sectional TEM images to investigate misfit transitions: Figure 4(a) shows the case where the deposition temperature was 400°C on an MgO substrate, Figure 4(b) shows the case where the deposition temperature was 400°C on an SrTiO3 substrate, and Figure 4(c) shows the case where the deposition temperature was 500°C on an MgO substrate. Figures 4(d), 4(e), and 4(f) show the Fourier transform images of Figures 4(a), 4(b), and 4(c), respectively. [Figure 5] Fig. 5(a) shows the magnetic field angle when ferromagnetic resonance was measured for a sample deposited on a SrTiO3 substrate at a deposition temperature of 400°C, with Fig. 5(b) showing the frequency spectrum when θH = 80° and Fig. 5(b) showing the frequency spectrum when θH = 45°. Fig. 5(c) shows the magnetic field strength dependence of the frequency spectrum calculated using the magnetic anisotropy estimated from the ferromagnetic resonance measurement results. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of an L10-FePt thin film showing one embodiment of the present invention. The L10-FePt thin film is a laminate of a substrate 10, an FePt thin film layer 20, and a cap layer 30. The substrate 10 is a single crystal substrate of magnesium oxide (MgO) or a single crystal substrate of strontium titanate (SrTiO). The single crystal substrate of magnesium oxide (MgO) is available from, for example, Tateho Chemical Industry Co., Ltd. (Ako City, Hyogo Prefecture). The single crystal substrate of strontium titanate (SrTiO) is available from, for example, Furuuchi Chemical Co., Ltd. (Shinagawa Ward, Tokyo) or Shinkosha Co., Ltd. (Sakae Ward, Yokohama City). The FePt thin film layer 20 has a crystal structure of L10 type, which gives it large saturation magnetization and magnetic anisotropy, and its thickness is, for example, in the range of 10-30 nm. Here, the L10 type refers to a structure in which Fe (iron) and Pt (platinum) are arranged in a completely regular, repeated pattern toward the c-axis (upward). For example, an MgO(001) cap layer, an SrTiO3 cap layer, or a VN or VC cap layer can be used as the cap layer 30. VN and VC have the same NaCl structure (B1 structure) as MgO and have similar lattice constants, making them suitable for comparison with MgO.

[0016] The L10-FePt thin film structure thus configured is manufactured as follows. First, a FePt thin film is formed on an MgO or SrTiO3 substrate using magnetron sputtering. Next, the MgO or SrTiO3 substrate on which the FePt thin film has been deposited is heat-treated at a temperature in the range of 300-500°C, whereby the FePt thin film becomes an L10-FePt thin film in which atoms are regularly arranged. [Example]

[0017] Example 1 is a sample in which a film was formed on a SrTiO3 substrate at a film formation temperature of 400°C. 2(b) shows the magnetization curve when a magnetic field is applied perpendicular to the film surface of Example 1, with the horizontal axis representing the magnetic field H and the vertical axis representing the magnetization M. It can be seen that the sample of Example 1 exhibits perpendicular magnetic anisotropy. FIG. 3(b) shows the damping constant of the sample of Example 1 at different magnetic field angles θ HThe horizontal axis shows the magnetic field H, and the vertical axis shows the damping constant α eff represents. magnetic field angle θ H The magnetic field angle θ is plotted at 80° and 45° from the direction perpendicular to the film surface, with the symbol ◯ representing the angle of 80° from the direction perpendicular to the film surface and the symbol △ representing the angle of 45° from the direction perpendicular to the film surface. H is the damping constant α when the angle is 80° with respect to the direction normal to the membrane surface. eff is 0.018. Magnetic field angle θ H is the damping constant α when the angle is 45° with respect to the direction normal to the membrane surface. eff is 0.027. Figure 4(b) shows a cross-sectional TEM image of the sample prepared in Example 1, and Figure 4(f) shows the results of Fourier analysis of the cross-sectional TEM image to investigate misfit dislocations. In Example 1, the cross-sectional TEM image shows 9 misfit dislocations in an area of ​​50 nm × 50 nm.

[0018] In addition, Figure 5 shows the magnetic field angle when ferromagnetic resonance was investigated for a sample formed on a SrTiO3 substrate at a film formation temperature of 400°C. Figure 5(a) shows the magnetic field angle when θ H =80° and Fig. 5(b) shows θ H = 45°. Figures 5(a) and 5(b) show the ferromagnetic resonance spectra of the sample with the smallest damping constant. It can be seen that a precession frequency of 100 GHz or more was achieved in both applied magnetic fields. Figure 5(c) shows the results of numerical calculations of the magnetic field strength dependence of the magnetization precession frequency at various magnetic field angles, based on the magnitude of the magnetic anisotropy estimated from ferromagnetic resonance. These calculation results reveal that a frequency spectrum of 100 GHz or more can be obtained even at the magnetization precession frequency when no magnetic field or a low magnetic field is applied, which is important for practical purposes. [Comparative Example 1]

[0019] Comparative Example 1 is a sample in which a film was formed on an MgO substrate at a film formation temperature of 400°C. 2(a) shows the magnetization curve when a magnetic field is applied perpendicular to the film surface of Comparative Example 1, with the horizontal axis representing the magnetic field H and the vertical axis representing the magnetization M. It can be seen that the sample of Comparative Example 1 exhibits perpendicular magnetic anisotropy. FIG. 3(a) shows the damping constant of the sample of Comparative Example 1 at different magnetic field angles θ H The horizontal axis shows the magnetic field H, and the vertical axis shows the damping constant α eff represents. magnetic field angle θ H The magnetic field angle θ is plotted at 80° and 45° from the direction perpendicular to the film surface, with the symbol ◯ representing the angle of 80° from the direction perpendicular to the film surface and the symbol △ representing the angle of 45° from the direction perpendicular to the film surface. H is the damping constant α when the angle is 80° with respect to the direction normal to the membrane surface. eff is 0.058. Magnetic field angle θ H is the damping constant α when the angle is 45° with respect to the direction normal to the membrane surface. eff is 0.046. Figure 4(a) is a cross-sectional TEM image of the film-formed sample of Comparative Example 1, and Figure 4(d) shows the results of Fourier analysis of the cross-sectional TEM image to investigate misfit dislocations. In Comparative Example 1, the cross-sectional TEM image shows 13 misfit dislocations in an area of ​​50 nm × 50 nm. [Example]

[0020] In Example 2, a sample was formed on an MgO substrate at a film formation temperature of 500°C, and the magnetic field angle θ H is at 45° to the direction normal to the film surface. 2(c) shows the magnetization curve when a magnetic field is applied perpendicular to the film surface of Example 2, with the horizontal axis representing the magnetic field H and the vertical axis representing the magnetization M. It can be seen that the sample of Example 2 exhibits perpendicular magnetic anisotropy. FIG. 3(c) shows the damping constant of the sample of Example 2 at different magnetic field angles θ H The horizontal axis shows the magnetic field H, and the vertical axis shows the damping constant α eff represents. magnetic field angle θ HThe magnetic field angle θ is plotted at 80° and 45° from the direction perpendicular to the film surface, with the symbol ◯ representing the angle of 80° from the direction perpendicular to the film surface and the symbol △ representing the angle of 45° from the direction perpendicular to the film surface. H is the damping constant α when the angle is 45° with respect to the direction normal to the membrane surface. eff is 0.029. Figure 4(c) is a cross-sectional TEM image of the sample film formed in Example 2, and Figure 4(d) shows the results of Fourier analysis of the cross-sectional TEM image to investigate misfit dislocations. In Example 2, the cross-sectional TEM image shows 16 misfit dislocations in an area of ​​50 nm × 50 nm. Comparative Example 2

[0021] Comparative Example 2 is a sample in which a film was formed on an MgO substrate at a film formation temperature of 500°C, and the magnetic field angle θ H That is, in the comparison with Example 2, Comparative Example 2 has in common the fact that the sample was formed on an MgO substrate at a film formation temperature of 500° C., but the magnetic field angle θ H are different. FIG. 3(c) shows the damping constant of the sample of Comparative Example 2 at different magnetic field angles θ H The horizontal axis shows the magnetic field H, and the vertical axis shows the damping constant α eff represents. magnetic field angle θ H The magnetic field angle θ is plotted at 80° and 45° relative to the direction perpendicular to the film surface, and the symbol ◯ indicates the case where the magnetic field angle θ is 80° relative to the direction perpendicular to the film surface. H is the damping constant α when the angle is 80° with respect to the direction normal to the membrane surface. eff is 0.057.

[0022] <Method for measuring misfit transitions> Misfit dislocations can be measured by obtaining TEM lattice images of the deposited samples using a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM).

[0023] <Method of measuring damping constant> Regarding the measurement method of the damping constant, common methods for measuring the damping constant for materials include cavity ferromagnetic resonance (FMR) using microwaves, high frequency FMR using microstrip lines, and spin torque FMR using microfabricated structures. However, because the ferromagnetic resonance frequency of highly anisotropic materials like FePt is sub-THz, measuring the magnetization dynamics for damping evaluation requires the time-resolved magneto-optical Kerr effect (TRMOKE). TRMOKE requires the sample's magnetization state to be initialized every few milliseconds for repeated measurements. Because the magnetic field required for initialization is very high for highly anisotropic samples like FePt, introducing a superconducting magnet into the TRMOKE optical path enables measurements in a strong magnetic field. Understanding the magnetization behavior during thermally assisted magnetic recording in FePt magnetic recording media also necessitates the design of materials that achieve highly efficient magnetization reversal with minimal energy assistance. Therefore, when applying FePt magnetic recording media, such as STT-MRAM, measuring the damping constant is important because the magnetization reversal current depends on damping (see Yukiko Takahashi, "Research on Nanostructure Control of High-Performance Magnetic Thin Films," Proceedings of the 46th Annual Meeting of the Magnetics Society of Japan (2022) 07FL-3).

[0024] <Method for measuring magnetic anisotropy> Methods for measuring magnetic anisotropy are described, for example, in Kazushi Ishiyama et al., "Evaluation of Magnetic Properties of Soft Magnetic Thin Films with Anisotropic Dispersion," Journal of the Magnetic Society of Japan, Vol. 20, p. 493 (1996). A known measuring device is the magnetic anisotropy torque meter (Tamagawa Seisakusho, Sendai City). The magnetic anisotropy torque test involves suspending a ferromagnetic sample on an elastic thread in a magnetic field. When a magnetic field is applied, the sample experiences a force due to magnetic internal energy, causing the sample to rotate in the direction of easy magnetization. The torque generated by this force is measured from the twist of the elastic thread. The measurement is performed as a function of the magnetic field direction, and the resulting measurement curve is called a torque curve. [Industrial Applicability]

[0025] The L10-FePt thin film of the present invention achieves a small damping constant in the magnetic material L10-FePt, which has large saturation magnetization and magnetic anisotropy, and will dramatically advance the development of terahertz wave detector / oscillator elements. In particular, detector / oscillator elements using magnetic materials operate with a simple structure, leading to the realization of inexpensive terahertz elements. This will enable the widespread use of high-capacity, high-speed communication technologies, which are expected to continue to develop in the future. According to the method for producing an L10-FePt thin film of the present invention, an L10-FePt thin film with a small damping constant suitable for use in a terahertz wave band detector / oscillator can be produced efficiently. [Explanation of symbols]

[0026] 10 Substrate 20 FePt thin film layer 30 cap layers

Claims

1. Magnesium oxide (MgO) or strontium titanate (SrTiO ３ a single crystal substrate made of a thin FePt film laminated on the single crystal substrate; L1 comprising ０ - FePt thin films, The damping constant is in the range of 0.018 to 0.044, The magnetic anisotropy is in the range of 3.0 to 7.0 T. L1 ０ -FePt film.

2. 2. The method according to claim 1, wherein the number of misfit dislocations per unit length in the in-plane direction of the film near the interface between the single crystal substrate and the FePt thin film is in the range of 0.25 / nm to 0.44 / nm. ０ - FePt thin film.

3. The number of misfit dislocations is measured by a cross-sectional TEM image. ０ - FePt thin film.

4. 10. The L1 of claim 1, further comprising a cap layer covering the FePt thin film. ０ - FePt thin film.

5. The cap layer may be a MgO(001) cap layer, a SrTiO ３ 5. The L1 of claim 4, which is either a cap layer, a VN or a VC cap layer. ０ - FePt thin film.

6. 2. The method according to claim 1, wherein the thickness of the FePt thin film is in the range of 10 nm to 30 nm. ０ - FePt thin film.

7. The damping constant is set at a magnetic field angle of 80° or 45° relative to the direction perpendicular to the film surface (θ Ｈ ) L1 according to claim 1 ０ - FePt thin film.

8. Strontium titanate (SrTiO ３ a single crystal substrate made of A thin FePt film is formed on this single crystal substrate, The deposition temperature of the single crystal substrate is within the range of 300-500°C; L1 ０ -Method for producing FePt thin films.

9. A single crystal substrate made of magnesium oxide (MgO) is prepared. A thin FePt film is formed on this single crystal substrate, The deposition temperature of the single crystal substrate is within the range of 450-500°C; L1 ０ -Method for producing FePt thin films.

10. L1 according to claim 1 ０ - Terahertz wave band detector / oscillator using FePt thin film.

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