Magnetic recording medium and magnetic recording apparatus

Abstract

To provide a magnetic recording medium capable of improving magnetization inversion efficiency without giving new magnetization inversion energy with a simple structure, and to provide a magnetic recording device provided with the same.SOLUTION: According to an aspect of the present invention, there is provided a magnetic recording medium 10 capable of recording information by irradiation with circularly polarized light, the magnetic recording medium 10 including a substrate 11, an antiferromagnetic layer 12 provided on one surface side of the substrate 11, and a ferromagnetic layer 13 provided on a surface 12B of the antiferromagnetic layer 12 opposite to a surface 12A on the substrate 11 side.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a magnetic recording medium and a magnetic recording device. [Background technology]

[0002] Magnetic recording media that record information by utilizing magnetization reversal in a magnetic layer have been known for some time. Known methods for reversing the magnetization of a magnetic layer include electrical magnetization reversal, magnetization reversal by applying an external magnetic field, and magnetization reversal by irradiating the magnetic layer with circularly polarized light. The magnetization reversal method by irradiating the magnetic layer with circularly polarized light is called all-optical magnetization switching (AOS), and has attracted attention from the perspective of reducing power consumption.

[0003] Currently, various reports have been made on techniques for magnetization reversal by irradiating a magnetic layer with circularly polarized light (see, for example, Non-Patent Documents 1 to 3). Non-Patent Document 1 shows that magnetization reversal depending on the polarity (helicity) of circularly polarized light can be obtained by using a laminated film including a non-crystalline GdFeCo alloy amorphous film.

[0004] Non-Patent Document 2 provides comprehensive proposals regarding material systems and microstructures suitable for AOS. However, even in Non-Patent Document 2, the current situation is limited to a limited number of ferrimagnetic alloy systems and multilayer films, with rare earth transition metal alloys such as the above-mentioned GdFeCo at the forefront. Non-Patent Document 2 also shows that inserting an underlayer at the interface with the substrate optimizes thermal diffusion in the AOS, thereby improving the AOS efficiency.

[0005] In Non-Patent Document 3, AOS was tested on an actual FePt granular magnetic recording medium, and it was shown that helicity-dependent magnetization reversal gradually occurred following multiple irradiations of pulsed circularly polarized light. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] “All-Optical Magnetic Recording with Circularly Polarized Light”, Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007) [Non-patent document 2] “Engineered materials for all-optical helicity-dependent magnetic switching”, Mangin et al., Nat. Mater. 13, 286 (2014) [Non-patent document 3] “Accumulative Magnetic Switching of Ultrahigh-Density Recording Media by Circularly Polarized Light”, Takahashi et al., Phys. Rev. Appl. 6, 054004 (2016) Summary of the Invention [Problem to be solved by the invention]

[0007] However, magnetization reversal by AOS has not yet been put to practical use in magnetic recording media such as hard disk drives. This is thought to be because the magnetization reversal efficiency of AOS is inferior to that of electrical magnetization reversal or external magnetic field magnetization reversal. Furthermore, FePt granular magnetic recording media require the formation of a special film called an FePt granular film, which requires a great deal of effort.

[0008] The present invention has been made to solve the above problems, and aims to provide a magnetic recording medium that has a simple structure and is capable of improving magnetization reversal efficiency without providing additional magnetization reversal energy, and a magnetic recording device equipped with the same. [Means for solving the problem]

[0009] [1] A magnetic recording medium capable of recording information by irradiation with circularly polarized light, comprising: a substrate; an antiferromagnetic layer provided on one side of the substrate; and a ferromagnetic layer provided on the surface of the antiferromagnetic layer opposite to the surface facing the substrate.

[0010] [2] The magnetic recording medium according to [1] above, wherein the ferromagnetic layer contains an FePt-based ferromagnetic material.

[0011] [3] The magnetic recording medium according to [1] or [2] above, wherein the antiferromagnetic layer contains at least one of a Mn-based antiferromagnetic material and a Ni-based antiferromagnetic material.

[0012] [4] The magnetic recording medium according to [3] above, wherein the Mn-based antiferromagnetic material contains nitrogen.

[0013] [5] The magnetic recording medium according to [4] above, wherein the Mn-based antiferromagnetic material contains Mn3PtN.

[0014] [6] The magnetic recording medium according to any one of [1] to [5] above, wherein the thickness of the ferromagnetic layer is 5 nm or more and 15 nm or less.

[0015] [7] The magnetic recording medium according to any one of [1] to [6] above, wherein the thickness of the antiferromagnetic layer is 10 nm or more and 20 nm or less.

[0016] [8] The magnetic recording medium according to any one of [1] to [7] above, wherein the magnetic recording medium comprises only one antiferromagnetic layer and one ferromagnetic layer.

[0017] [9] A magnetic recording device comprising: the magnetic recording medium according to any one of [1] to [8] above; and a circularly polarized light irradiation system that irradiates the ferromagnetic layer of the magnetic recording medium with circularly polarized light. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a magnetic recording medium that has a simple structure and is capable of improving magnetization reversal efficiency without providing new magnetization reversal energy, and a magnetic recording device equipped with the same. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic diagram of a magnetic recording medium according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram of the magnetic recording device according to the embodiment. [Figure 3] FIG. 3 is a flowchart of image processing before and after irradiation with circularly polarized light in the magnetic recording media according to Example 1 and Comparative Example 1. [Figure 4] Figure 4A is a schematic magnetic domain image of the magnetic recording medium before irradiation with circularly polarized light, Figure 4B is a schematic magnetic domain image of the magnetic recording medium after irradiation with circularly polarized light, and Figure 4C is a differential magnetic domain image of the magnetic recording medium before and after irradiation with circularly polarized light. [Figure 5] 5A and 5B are differential images of the magnetization state before and after irradiation with circularly polarized light in the magnetic recording medium according to Example 1. FIG. [Figure 6] 6A and 6B are differential images of the magnetization state before and after irradiation with circularly polarized light in the magnetic recording medium according to Comparative Example 1. FIG. [Figure 7] FIG. 7 is a graph plotting the grayscale values ​​on the dashed dotted lines shown in FIGS. 5A and 5B. [Figure 8] FIG. 8 is a graph plotting the grayscale values ​​on the dashed dotted lines shown in FIGS. 6A and 6B. [Figure 9] Figure 9 is a graph showing the change in the coercive force Hc of the ferromagnetic layer in the magnetic recording medium of Example 1 when the difference ΔT in the measured temperature between the fixed copper plate on the top surface of the ferromagnetic layer and the substrate holder on the bottom surface of the antiferromagnetic layer is set to -32K, 0K, and 57K. [Figure 10] FIG. 10 is a graph showing the coercive force change rates of the magnetic recording media according to Examples 1 to 4. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0020] The magnetic recording medium and magnetic recording device according to the embodiment of the present invention will be described below. Fig. 1 is a schematic diagram of the magnetic recording medium according to the embodiment, and Fig. 2 is a schematic diagram of the magnetic recording device according to the embodiment.

[0021] <<Magnetic Recording Media>> The magnetic recording medium 10 shown in FIG. 1 is capable of recording information by irradiating it with circularly polarized light. As shown in FIG. 1, the magnetic recording medium 10 includes a substrate 11, an antiferromagnetic layer 12 provided on one side of the substrate 11, a ferromagnetic layer 13 provided on a side 12B of the antiferromagnetic layer 12 opposite to a side 12A of the antiferromagnetic layer 12 facing the substrate 11, and a protective layer 14 provided on a side 13B of the ferromagnetic layer 13 opposite to a side 13A of the antiferromagnetic layer 12. The surface 10A of the magnetic recording medium 10 is the surface of the protective layer 14. Note that the magnetic recording medium 10 does not necessarily have to include the protective layer 14. If the magnetic recording medium 10 does not include a protective layer, the surface 10A of the magnetic recording medium 10 is the surface of the ferromagnetic layer 13.

[0022] <Substrate> The substrate 11 is not particularly limited, but examples thereof include a single crystal substrate and a glass substrate, among which a single crystal substrate that allows epitaxial growth of an antiferromagnetic layer is preferred. Examples of single crystal substrates include an MgO single crystal substrate, an SrTiO3 single crystal substrate, and an Al2O3 single crystal substrate.

[0023] The thickness of substrate 11 is preferably, for example, 250 μm or more. If the thickness of substrate 11 is 250 μm or more, sufficient strength against loads such as bending can be obtained, and substrate 11 can function as a heat sink that dissipates heat from antiferromagnetic layer 12 when circularly polarized light is irradiated onto ferromagnetic layer 13. The lower limit of the thickness of substrate 11 is more preferably 350 μm or more, more preferably 500 μm or more, and the upper limit is more preferably 1 mm or less, more preferably 1.5 mm or less.

[0024] <Antiferromagnetic layer> The antiferromagnetic layer 12 is a layer having antiferromagnetic properties. In this specification, "antiferromagnetic" means the property of not having a magnetic moment regardless of an external magnetic field. A material having antiferromagnetic properties is called an antiferromagnetic body. The antiferromagnetic layer 12 can be formed by a sputtering method such as magnetron sputtering.

[0025] The antiferromagnetic layer 12 includes an antiferromagnetic material. The antiferromagnetic material is not particularly limited, but examples include Mn-based antiferromagnetic materials and Ni-based antiferromagnetic materials. Examples of Mn-based antiferromagnetic materials include Mn3XN, Mn3X, and MnX, where X is a metal element. Examples of metal elements include Ni, Cu, Ga, Ge, Ir, Sn, and Pt. Among these, Mn3XN is preferred because it contains nitrogen, resulting in high crystal stability. Mn3PtN is particularly preferred because it exhibits a high coercivity change rate when a temperature difference is applied between the ferromagnetic layer and the antiferromagnetic layer. Examples of Ni-based antiferromagnetic materials include NiO.

[0026] The thickness of the antiferromagnetic layer 12 is preferably 10 nm or more and 20 nm or less. If the thickness of the antiferromagnetic layer 12 is 10 nm or more, the non-collinear magnetic structure is easily stabilized, and if the thickness is 20 nm or less, roughness at the interface between the antiferromagnetic layer 12 and the ferromagnetic layer 13 can be suppressed. The lower limit of the thickness of the antiferromagnetic layer 12 is more preferably 12 nm or more or 14 nm or more, and the upper limit is more preferably 18 nm or less or 16 nm or less. The thickness of the antiferromagnetic layer 12 may be, for example, 12 nm or more and 18 nm or less, or 14 nm or more and 16 nm or less.

[0027] The thickness of the antiferromagnetic layer 12 is determined by measuring the film thickness at 10 points on an image of the cross section of the antiferromagnetic layer 12 taken using a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM), and calculating the arithmetic mean value of the 10 values.

[0028] <Ferromagnetic layer> The ferromagnetic layer 13 is a layer that has ferromagnetic properties. In this specification, "ferromagnetic" means the property of being strongly magnetized in the same direction as an external magnetic field when an external magnetic field is applied. A material that has ferromagnetic properties is called a ferromagnetic body. A ferromagnetic body has the property of retaining strong magnetism even when the external magnetic field is removed to zero.

[0029] The ferromagnetic layer 13 is preferably joined to the antiferromagnetic layer 12. By joining the ferromagnetic layer 13 to the antiferromagnetic layer 12, magnetic coupling can be induced at the interface. The ferromagnetic layer 13 can be joined to the antiferromagnetic layer 12 by forming it by a sputtering method such as magnetron sputtering. The ferromagnetic layer 13 is preferably joined to the antiferromagnetic layer 12 by epitaxially growing it on the antiferromagnetic layer 12.

[0030] The ferromagnetic layer 13 contains a ferromagnetic material. Examples of the ferromagnetic material include pure metals such as iron (Fe), cobalt (Co), and nickel (Ni), alloys containing at least one of these, and ferrimagnetic materials. Examples of such alloys include Fe-N, Fe-Pt, Fe-Co, Fe-Ni, Fe-Si, Fe-Dy-Tb, Fe-Ga, Fe-Si-Al, and Mn-N alloys. Among these, Fe-Pt alloys such as FePt are preferred because they have high uniaxial magnetic anisotropy, as described below, and can stabilize data retention.

[0031] The ferromagnetic layer 13 preferably has uniaxial magnetic anisotropy in a direction perpendicular to the surface of the ferromagnetic layer 13. By having the ferromagnetic layer 13 have uniaxial magnetic anisotropy in such a direction, the magnetization reversal efficiency can be further improved. Whether the ferromagnetic layer 13 has uniaxial magnetic anisotropy in such a direction can be confirmed by measuring the magnetization process of the ferromagnetic layer when an external magnetic field is swept in the direction perpendicular to the surface of the ferromagnetic layer using a vibrating sample magnetometer.

[0032] The thickness of the ferromagnetic layer 13 is preferably 2 nm or more and 10 nm or less. If the thickness of the ferromagnetic layer 13 is 2 nm or more, the uniaxial magnetic anisotropy in the perpendicular direction of the ferromagnetic layer 13 can be stabilized, and if the thickness is 10 nm or less, the magnetization reversal threshold can be reduced. The lower limit of the thickness of the ferromagnetic layer 13 is more preferably 3 nm or more or 4 nm or more, and the upper limit is more preferably 9 nm or less or 8 nm or less. The thickness of the ferromagnetic layer 13 is measured in the same manner as the thickness of the antiferromagnetic layer 12. The thickness of the ferromagnetic layer 13 may be, for example, 3 nm or more and 9 nm or less, or 4 nm or more and 8 nm or less.

[0033] <Protective layer> The protective layer 14 is intended to prevent rusting of the ferromagnetic material that constitutes the ferromagnetic layer 13. The protective layer 14 can be made of an oxide, such as MgO, Al—O, Si—O, or Ru—O.

[0034] The thickness of the protective layer 14 is preferably 1 nm or more and 5 nm or less. If the thickness of the protective layer 14 is 1 nm or more, the surface of the ferromagnetic layer 13 can be sufficiently protected from oxygen and moisture in the atmosphere, and if the thickness is 5 nm or less, the circular polarization state of the irradiated light can be maintained. The lower limit of the thickness of the protective layer is more preferably 2 nm or more or 3 nm or more, and the upper limit is more preferably 4 nm or less or 3.5 nm or less. The thickness of the protective layer 14 is measured in the same manner as the thickness of the antiferromagnetic layer 12. The thickness of the protective layer 14 may be, for example, 2 nm or more and 4 nm or less.

[0035] <<Magnetic Recording Devices>> The magnetic recording medium 10 is incorporated into a magnetic recording device for use. The magnetic recording device 20 shown in Fig. 2 includes the magnetic recording medium 10, a circularly polarized light irradiation system 30 that irradiates the ferromagnetic layer 13 of the magnetic recording medium 10 with circularly polarized light, and a detection system 40 that observes the state of the magnetic domains. Note that the magnetic recording device 20 does not necessarily have to include the detection system 40.

[0036] <Circularly polarized light irradiation system> The circularly polarized light irradiation system 30 includes a laser light source 31 for irradiating laser light, a neutral density filter 32, a polarizing plate 33, a λ / 4 plate 34, a shutter 35, and an objective lens 36. The circularly polarized light irradiation system 30 is not limited to the above configuration as long as it can irradiate the ferromagnetic layer 13 of the magnetic recording medium 10 with circularly polarized light. The laser light emitted from the laser light source 31 is attenuated by the neutral density filter 32 and then passes through the polarizing plate 33 and the λ / 4 plate 34 to become circularly polarized light (left-handed or right-handed circularly polarized light). The circularly polarized laser light then passes through the shutter 35 and the objective lens 36 to irradiate the ferromagnetic layer 13 of the magnetic recording medium 10. The laser light emitted from the laser light source 31 is unpolarized, and femtosecond ultrashort pulse laser light with a wavelength of 514 nm, for example, can be used as the laser light. The laser light is preferably irradiated once.

[0037] The components of the circularly polarized light irradiation system 30 are arranged in a substantially straight line, which makes it possible to omit a mirror and achieve an ellipticity of 0.90 or more, i.e., to make substantially circularly polarized light incident on the ferromagnetic layer 13.

[0038] When the ferromagnetic layer 13 of the magnetic recording medium 10 is irradiated with circularly polarized light, a temperature gradient (thermal gradient), that is, a temperature difference, is generated between the antiferromagnetic layer 12 and the ferromagnetic layer 13 due to the optical energy of the circularly polarized light.

[0039] <Detection system> The detection system 40 includes an LED light source 41, a condenser lens 42, an aperture 43, a mirror 44, a polarizing plate 45, a condenser lens 46, beam splitters 47 and 48, a polarizing plate 49, a tube lens 50, and an imaging device 51 such as a CMOS camera or a CCD camera. Light emitted from the LED light source 41 is condensed by the condenser lens 42, passes through the aperture 43, and is incident on the magnetic recording medium 10 via the mirror 44, the polarizing plate 45, the condenser lens 46, the beam splitters 47 and 48, and the objective lens 36. The light reflected by the magnetic recording medium 10 passes through the beam splitters 48 and 47 and the polarizing plate 49, is imaged by the tube lens 50, and is then captured by the imaging device 51.

[0040] The present inventors have conducted extensive research into the magnetization reversal of ferromagnetic layers and have found that by providing a ferromagnetic layer on an antiferromagnetic layer and irradiating the ferromagnetic layer with circularly polarized light, the magnetization reversal efficiency can be improved without providing any additional magnetization reversal energy other than the circularly polarized light. This is believed to be due to the following mechanism. First, when the ferromagnetic layer of a magnetic recording medium is irradiated with left-handed and right-handed circularly polarized light, the circular polarization-induced magnetization reversal phenomenon occurs when the ferromagnetic layer is irradiated with left-handed and right-handed circularly polarized light. However, when only a ferromagnetic layer is provided, the magnetization reversal efficiency is poor. On the other hand, when a ferromagnetic layer is provided on an antiferromagnetic layer, interface uncompensated spins are generated in the antiferromagnetic layer near the interface with the ferromagnetic layer, with the direction opposite (e.g., downward) to the magnetization direction (e.g., upward) of the ferromagnetic layer. As a result, conduction electrons with spins in the same direction (e.g., downward) as the interface uncompensated spins are present. In this state, when the ferromagnetic layer is irradiated with left-handed and right-handed circularly polarized light, a temperature gradient is generated between the antiferromagnetic layer and the ferromagnetic layer, such that the temperature of the ferromagnetic layer is higher than that of the antiferromagnetic layer. This generates a spin current of conduction electrons (e.g., a spin current with a downward spin) in the antiferromagnetic layer due to the spin Seebeck effect, and the spin current flows into the ferromagnetic layer along the temperature gradient perpendicular to the surface of the magnetic recording medium. As a result, a spin torque is applied to the magnetization of the ferromagnetic layer, assisting the circularly polarized light-induced magnetization reversal phenomenon and reducing the coercivity of the ferromagnetic layer (e.g., the coercivity of the upward magnetization), thereby facilitating magnetization reversal of the ferromagnetic layer. According to this embodiment, the antiferromagnetic layer 12 is provided on the ferromagnetic layer 13. Therefore, when the ferromagnetic layer 13 is irradiated with circularly polarized light, the magnetization reversal efficiency can be improved with a simple structure without providing additional magnetization reversal energy other than the circularly polarized light. Since the effect of reducing the coercive force can be obtained by having one ferromagnetic layer 13 and one antiferromagnetic layer 12, two or more ferromagnetic layers 13 and two or more antiferromagnetic layers 12 may be used. However, it is preferable that there is only one ferromagnetic layer 13 and one antiferromagnetic layer 12, as shown in FIG. 1. [Example]

[0041] To explain the present invention in detail, the following examples are provided. However, the present invention is not limited to these examples. FIG. 3 is a flowchart of image processing for magnetic recording media according to Example 1 and Comparative Example 1 before and after irradiation with circularly polarized light. FIG. 4A is a schematic magnetic domain image of the magnetic recording medium before irradiation with circularly polarized light. FIG. 4B is a schematic magnetic domain image of the magnetic recording medium after irradiation with circularly polarized light. FIG. 4C is a differential magnetic domain image of the magnetic recording medium before and after irradiation with circularly polarized light. FIGS. 5A and 5B are differential images of the magnetization state of the magnetic recording medium according to Example 1 before and after irradiation with circularly polarized light. FIGS. 6A and 6B are differential images of the magnetization state of the magnetic recording medium according to Comparative Example 1 before and after irradiation with circularly polarized light. FIG. 7 is a graph plotting the grayscale values ​​on the dashed-dotted lines shown in FIGS. 5A and 5B. FIG. 8 is a graph plotting the grayscale values ​​on the dashed-dotted lines shown in FIGS. 6A and 6B. Figure 9 is a graph showing the change in the coercive force Hc of the ferromagnetic layer in the magnetic recording medium of Example 1 when the difference ΔT in the measured temperature between the fixed copper plate on the top surface of the ferromagnetic layer and the substrate holder on the bottom surface of the antiferromagnetic layer is set to -32K, 0K, and 57K, and Figure 10 is a graph showing the coercive force change rate in the magnetic recording media of Examples 1 to 4.

[0042] Example 1 An antiferromagnetic layer made of Mn3PtN crystals with a thickness of 10 nm was formed on a substrate made of MgO single crystals with a thickness of 500 μm by nitrogen-reactive ultra-high vacuum magnetron sputtering. Next, a ferromagnetic layer made of FePt crystals with a thickness of 10 nm was formed on the antiferromagnetic layer by magnetron sputtering. The ferromagnetic layer was bonded to the antiferromagnetic layer. The antiferromagnetic layer and ferromagnetic layer were then patterned into disks with diameters of 40 μm, 20 μm, 10 μm, and 5 μm, respectively, by photolithography to facilitate alignment of the magnetic recording medium before and after irradiation with a pulsed laser beam, as described below. This resulted in the magnetic recording medium of Example 1. The magnetic recording medium of Example 1 did not have a protective layer, and the surface of the magnetic recording medium was the surface of the ferromagnetic layer.

[0043] <Comparative Example 1> A ferromagnetic layer made of FePt crystals and having a thickness of 10 nm was formed by magnetron sputtering on a substrate made of MgO single crystals and having a thickness of 500 μm. The ferromagnetic layer was then patterned into disk-shaped shapes with diameters of 40 μm, 20 μm, 10 μm, and 5 μm by photolithography. This resulted in a magnetic recording medium according to Comparative Example 1. The magnetic recording medium according to Comparative Example 1 did not have a protective layer, and the surface of the magnetic recording medium was the surface of the ferromagnetic layer.

[0044] <Magnetization reversal evaluation under circularly polarized light irradiation> We evaluated whether magnetization reversal dependent on the helicity of circularly polarized light was achieved when left-handed and right-handed circularly polarized light was irradiated onto multiple ferromagnetic materials in the magnetic recording media of Example 1 and Comparative Example 1. Specifically, we first prepared a magnetic recording device equipped with the circularly polarized light irradiation system and detection system shown in Figure 2. Femtosecond ultrashort pulse laser light with a wavelength of 514 nm generated from a laser light source was converted into circularly polarized light or left-handed circularly polarized light using a polarizer and a λ / 4 plate. The left-handed and right-handed circularly polarized light was then irradiated onto the ferromagnetic layer of the magnetic recording medium via a shutter, beam splitter, and objective lens at a laser output of 0.036 mW and for an irradiation time of 0.3 seconds. Furthermore, before and after irradiation of the ferromagnetic layer of the magnetic recording medium with left-handed and right-handed circularly polarized light, light with a wavelength of 630 nm generated from an LED light source was irradiated onto the ferromagnetic layer of the magnetic recording medium via a condenser lens, aperture, mirror, polarizer, condenser lens, and beam splitter. Then, the light reflected by the surface of the ferromagnetic layer was passed through a beam splitter and a polarizing plate, and was imaged by a tube lens, and the image was photographed by a CMOS camera ("Zyla" manufactured by OXFORD INSTRUMENTS).

[0045] Next, the magnetic domain images captured with the CMOS camera were processed to determine whether magnetization reversal had occurred. Image processing was performed using the image editing and processing software "GIMP (version 2.10)" (free software) and a self-written program. First, the magnetic domain images before and after irradiation with circularly polarized light (see Figure 4A) and after irradiation (see Figure 4B) were loaded into GIMP as separate layers (step S1), and the position of each magnetic domain image was adjusted (step S2). Specifically, the magnetic domain image after irradiation with circularly polarized light was placed on the top layer, and the standard mode was switched to subtraction mode. The magnetic domain image before irradiation with circularly polarized light was placed on the bottom layer, and the pixel values ​​of the magnetic domain image on the top layer were subtracted from those of the magnetic domain image on the bottom layer. If the result was negative, a zero was entered. The two magnetic domain images were adjusted to be as close to the same position as possible and then photographed. Subtraction yielded the difference between the two magnetic domain images, and the position was manually adjusted using surface dirt and magnetic domains that showed no change as a guide. In each adjusted magnetic domain image, there were areas that changed before and after irradiation with circularly polarized light. After that, each magnetic domain image was trimmed for each light intensity condition of the circularly polarized light irradiation (step S3). Then, the layer mode was returned to standard, and each magnetic domain image before and after irradiation with circularly polarized light was output (step S4).

[0046] Next, I used a homemade program written in the programming language Python to perform the following steps. First, I preprocessed each magnetic domain image output from GIMP (step S5). Specifically, because the magnetic domain image is a 16-bit PNG image, subtraction is not possible as is due to data type limitations. This is because, since it is an unsigned 16-bit integer, if calculations are performed as is, all negative values ​​will be treated as 0, and the darker areas cannot be processed. For this reason, as preprocessing, each pixel value was divided by 65535 to convert it to a range of 1.0 to 0.0.

[0047] After preprocessing, the brightness of each magnetic domain image was adjusted (step S6). Specifically, since the brightness of the magnetic domain images before and after irradiation with circularly polarized light differed slightly, the brightness was compared using the median pixel value of each image as a reference. Then, the median value of the magnetic domain image before irradiation with circularly polarized light was subtracted from the median value of the magnetic domain image after irradiation with circularly polarized light, and the resulting value was added to the pixel value of the magnetic domain image before irradiation with circularly polarized light.

[0048] After adjusting the brightness, the magnetic domain image after irradiation with circularly polarized light was subtracted from the image before irradiation with circularly polarized light after brightness adjustment to obtain a differential magnetic domain image (step S7). The differential magnetic domain image was then color-coded by value using Matplotlib, a Python diagramming library, and formatted for output as an image (see Figure 4C, step S8). The contrast of the differential magnetic domain image was adjusted so that the magnetization-reversed and non-reversed regions could be distinguished.

[0049] Image processing was performed using this procedure to obtain the differential magnetic domain images shown in FIGS. 5A, 5B, 6A, and 6B. In FIGS. 5A, 5B, 6A, and 6B, the circled areas indicate the size of the area irradiated with circularly polarized light, and the shades of gray indicate the direction of magnetization perpendicular to the surface of the ferromagnetic layer. Dark gray indicates magnetic domains with upward magnetization (direction from the back surface of the ferromagnetic layer to the front surface), and light gray indicates magnetic domains with downward magnetization (direction from the front surface of the ferromagnetic layer to the back surface). From FIGS. 6A and 6B, it was confirmed that the images of the magnetic recording medium according to Comparative Example 1 showed similar numbers of upward and downward magnetic domains when irradiated with left-handed circularly polarized light and right-handed circularly polarized light. In contrast, from FIGS. 5A and 5B, it was confirmed that the images of the magnetic recording medium according to Example 1 showed that many downward magnetic domains were generated when irradiated with left-handed circularly polarized light, and many upward magnetic domains were generated when irradiated with right-handed circularly polarized light.

[0050] This is also evident from the results of examining the magnetization directions of the areas indicated by the dashed-dotted lines in Figures 5A, 5B, 6A, and 6B. The grayscale values ​​indicated by the dashed-dotted lines in Figures 5A, 5B, 6A, and 6B were plotted to obtain the graphs shown in Figures 7 and 8. The grayscale values ​​indicate the magnetization directions. As shown in Figure 8, in the magnetic recording medium of Comparative Example 1, the magnetization direction was random in both the cases of irradiation with left-handed circularly polarized light and right-handed circularly polarized light, confirming that it was not dependent on the helicity of the circularly polarized light. In contrast, as shown in Figure 7, in the magnetic recording medium of Example 1, the magnetization direction was mostly in the range of 0 to 1 when irradiated with left-handed circularly polarized light, and mostly in the range of -1 to 0 when irradiated with right-handed circularly polarized light, confirming that it was dependent on the helicity of the circularly polarized light.

[0051] <Example 2> In Example 2, a magnetic recording medium was obtained in the same manner as the magnetic recording medium of Example 1, except that Mn3Pt was used as the antiferromagnetic material in the antiferromagnetic layer instead of Mn3PtN.

[0052] Example 3 In Example 3, a magnetic recording medium was obtained in the same manner as the magnetic recording medium of Example 1, except that Mn3Ir was used as the antiferromagnetic material in the antiferromagnetic layer instead of Mn3PtN.

[0053] Example 4 In Example 4, a magnetic recording medium was obtained in the same manner as the magnetic recording medium of Example 1, except that NiO was used as the antiferromagnetic material in the antiferromagnetic layer instead of Mn3PtN.

[0054] <Evaluation of coercive force change due to temperature difference and calculation of coercive force change rate> The magnetic recording media according to Examples 1 to 4 were used to evaluate the rate of change in coercivity per unit temperature difference when a temperature gradient was applied to the magnetic recording media. Specifically, a 1 mm-thick copper plate was fixed to the ferromagnetic layer of the magnetic recording media with thermally conductive grease ("TG-K-030-R" manufactured by Thermal Grizzly Corporation), and a 3 mm-thick ceramic substrate holder was fixed to the MgO substrate of the magnetic recording media with thermally conductive grease ("TG-K-030-R" manufactured by Thermal Grizzly Corporation). K-type thermocouples ("CT-TC-CHAL-003" manufactured by Omega Engineering Co., Ltd.) were attached to the copper plate and the substrate holder. Resistance heaters ("Micro Ceramic Heater MC Series" manufactured by Sakaguchi Electric Heating Co., Ltd.) were used to heat the copper plate and the substrate holder, respectively, to apply a temperature difference perpendicular to the surface of the ferromagnetic layer. Since it is not possible to measure the actual temperature difference applied in the direction perpendicular to the surface of the ferromagnetic layer, the temperature difference was taken as the temperature difference (ΔT = T2 - T1) between the measurement value (T1 = 500 K) of the K-type thermocouple connected to the substrate holder and the measurement value (T2) of the K-type thermocouple connected to the fixed copper plate.

[0055] Then, in the magnetic recording medium according to Example 1, the magnetization process of the ferromagnetic layer made of FePt was measured in a thermal equilibrium state when ΔT was -32 K, 0 K, and 57 K. The coercive force (Hc) per unit temperature difference was evaluated from the magnetic hysteresis loop (anomalous Hall hysteresis loop) of the in-plane transverse electromotive force when a perpendicular external magnetic field was swept. From Figure 9, it can be seen that the coercive force Hc decreases when ΔT > 0, i.e., when the ferromagnetic layer is in a higher temperature state than the antiferromagnetic layer.

[0056] Similarly, the magnetization process of the ferromagnetic layer was measured to determine the coercivity (Hc) per unit temperature difference in the magnetic recording media of Examples 2 to 4. The coercivity change rates of the magnetic recording media of Examples 2 to 4 when ΔT was 57 K were compared using the coercivity change rate of the magnetic recording media of Example 1 when ΔT was 57 K as a reference, and it was confirmed that, as shown in Fig. 10, when a temperature difference was applied to the magnetic recording media of Examples 1 to 4, the coercivity change rate increased in all cases, but the magnetic recording medium of Example 1 in which the antiferromagnetic layer was Mn3PtN had the highest rate. [Explanation of symbols]

[0057] 10...Magnetic recording medium 11... Circuit board 12...antiferromagnetic layer 13...Ferromagnetic layer 14...Protective layer 20...Magnetic recording device 30...Circularly polarized light irradiation system 31...Laser light source 33...Polarizing plate 34…λ / 4 plate

Claims

1. A magnetic recording medium capable of recording information by irradiation with circularly polarized light, A substrate; an antiferromagnetic layer provided on one surface of the substrate; a ferromagnetic layer provided on a surface of the antiferromagnetic layer opposite to the surface facing the substrate; A magnetic recording medium comprising:

2. 2. The magnetic recording medium according to claim 1, wherein the ferromagnetic layer contains an Fe--Pt ferromagnetic material.

3. 3. The magnetic recording medium according to claim 1, wherein the antiferromagnetic layer contains at least one of a Mn-based antiferromagnetic material and a Ni-based antiferromagnetic material.

4. 4. The magnetic recording medium according to claim 3, wherein the Mn-based antiferromagnetic material contains nitrogen.

5. The Mn-based antiferromagnetic material is Mn ３ The magnetic recording medium of claim 4 comprising PtN.

6. 6. The magnetic recording medium according to claim 1, wherein the thickness of the ferromagnetic layer is 5 nm or more and 15 nm or less.

7. 7. The magnetic recording medium according to claim 1, wherein the antiferromagnetic layer has a thickness of 10 nm or more and 20 nm or less.

8. 8. The magnetic recording medium according to claim 1, wherein the magnetic recording medium comprises only one antiferromagnetic layer and one ferromagnetic layer.

9. The magnetic recording medium according to any one of claims 1 to 8, a circularly polarized light irradiation system that irradiates the ferromagnetic layer of the magnetic recording medium with circularly polarized light; A magnetic recording device comprising:

Images