Rare earth oxide ferromagnetic material, method for producing rare earth oxide ferromagnetic material

Abstract

The present invention provides a rare-earth oxide ferromagnetic material that possesses ferromagnetism and conductivity, and can be produced at low cost through a simple process, as well as a method for producing the rare-earth oxide ferromagnetic material. [Solution] The first layer and the second layer are stacked alternately, one or more of each, periodically, with the first layer consisting of a rare earth oxide and the second layer consisting of an oxygen-deficient rare earth oxide.

Description

[Technical Field]

[0001] This invention relates to rare earth oxide ferromagnetic materials and methods for producing rare earth oxide ferromagnetic materials. [Background technology]

[0002] Traditionally, rare earth oxides have been known as highly dielectric insulators, but by using thin-film processes, it is becoming possible to synthesize rare earth oxides that possess both ferromagnetic and semiconductor properties.

[0003] For example, Non-Patent Document 1 describes that CeO epitaxial thin films can be obtained as rare earth monooxides with a rock salt structure by pulsed laser deposition. Such CeO epitaxial thin films are said to exhibit p-type conductivity.

[0004] Furthermore, for example, Non-Patent Document 2 describes that epitaxial thin films of DyO and ErO can be obtained as rare earth single oxides with a rock salt structure by pulsed laser deposition. These epitaxial thin films of DyO and ErO are said to possess ferromagnetic and semiconducting electrical conductivity. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Proceedings of the 80th Autumn Meeting of the Japan Society of Applied Physics: "p-type electrical transport properties of rare earth monooxide CeO epitaxial thin films" Nobuhito Abe, Takashi Yamamoto, Daichi Saito, Kenichi Kaminaga, Daichi Oka, Tomoaki Fukumura [Non-Patent Document 2] Proceedings of the 83rd Autumn Meeting of the Japan Society of Applied Physics: "Heavy Rare Earth Single Oxide Ferromagnetic Semiconductors: DyO·ErO Epitaxial Thin Films" Tomomi Sasaki, Daichi Oka, Daichi Saito, Chuichi Shimizu, Tomoaki Fukumura [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the rare earth single oxides disclosed in Non-Patent Documents 1 and 2 are chemically unstable and are formed on substrates containing elements other than the target rare earth oxide, such as YAlO3 and CaF2. Therefore, there is a problem in that they are difficult to apply to devices and the like as ferromagnetic semiconductor materials.

[0007] This invention was proposed in view of the above-mentioned problems, and aims to provide a rare-earth oxide ferromagnetic material that possesses ferromagnetism and conductivity, and can be produced at low cost through a simple process, as well as a method for producing a rare-earth oxide ferromagnetic material. [Means for solving the problem]

[0008] The inventors have discovered that by using plate-like or foil-like materials of rare earth elements as raw materials and subjecting them to heat treatment in an atmospheric environment and in an atmosphere with controlled oxygen partial pressure, oxygen vacancies are introduced into the rare earth oxides, resulting in the emergence of conductivity and ferromagnetism.

[0009] To solve the above problems, the rare earth oxide ferromagnetic material and the method for producing the rare earth oxide ferromagnetic material according to one embodiment of the present invention are proposed to be the following means. (1) The rare earth oxide ferromagnetic material according to embodiment 1 of the present invention is characterized in that one or more first layers and second layers are alternately stacked periodically, the first layer is made of a rare earth oxide, and the second layer is made of an oxygen-deficient rare earth oxide.

[0010] (2) Embodiment 2 of the present invention is a rare earth oxide ferromagnetic material of Embodiment 1, wherein the rare earth oxide constituting the first layer is represented by R2O3 (where R is a rare earth element), and the oxygen-deficient rare earth oxide constituting the second layer is R2O 3-x (where R is a rare earth element and x is less than 3)

[0011] (3) Embodiment 3 of the present invention is a rare earth oxide ferromagnetic material according to Embodiment 2, wherein the rare earth elements constituting the first layer and the second layer are Y or Gd.

[0012] (4) Aspect 4 of the present invention provides the rare earth oxide ferromagnetic material according to any one of Aspects 1 to 3, wherein the rare earth oxide ferromagnetic material is in a form of a film, a plate, or a block.

[0013] (5) Aspect 5 of the present invention provides the rare earth oxide ferromagnetic material according to any one of Aspects 1 to 4, wherein the rare earth oxide ferromagnetic material has a surface diffuse reflectance of 20% or less.

[0014] (6) A method for producing a rare earth oxide ferromagnetic material according to Aspect 6 of the present invention comprises: a first heat treatment step of thermally oxidizing a raw material composed of a rare earth element in the atmosphere to form a first layer composed of a rare earth oxide on a surface thereof; and subjecting the raw material that has undergone the first heat treatment step to an oxygen partial pressure of 1.0×10 -20 atm or more and less than 1.0 atm in a low oxygen partial pressure environment for thermal oxidation, diffusing oxygen toward a region deeper than the first layer, and forming a second layer composed of oxygen-deficient rare earth oxide; a second heat treatment step.

[0015] (7) Aspect 7 of the present invention provides the method for producing a rare earth oxide ferromagnetic material according to Aspect 6, wherein the raw material has a plate shape or a film shape.

[0016] (8) Aspect 8 of the present invention provides the method for producing a rare earth oxide ferromagnetic material according to Aspect 6 or 7, wherein in the first heat treatment step, the heating temperature is maintained in a temperature range of 400°C or higher and 600°C or lower for 0.5 hour or more and 2 hours or less, and in the second heat treatment step, the heating temperature is maintained in a temperature range of 800°C or higher and 1000°C or lower for 0.5 hour or more and 16 hours or less.

[0017] >(9) Aspect 9 of the present invention provides the method for producing a rare earth oxide ferromagnetic material according to Aspect 6 or 7, wherein the first heat treatment step and the second heat treatment step are performed for at least 2 or more cycles. Effects of the Invention

[0018] According to the present invention, it is possible to provide a rare earth oxide ferromagnetic material that possesses ferromagnetism and conductivity, and can be produced at low cost through a simple process, as well as a method for producing a rare earth oxide ferromagnetic material. [Brief explanation of the drawing]

[0019] [Figure 1] This is a schematic cross-sectional view showing a rare-earth oxide ferromagnetic material according to one embodiment of the present invention. [Figure 2] This is a schematic diagram illustrating the behavior of oxygen during the first and second heat treatment processes. [Figure 3] These are the measurement results for Verification Example 1. [Figure 4] These are the measurement results for Verification Example 1. [Figure 5] These are the measurement results for Verification Example 1. [Figure 6] These are the measurement results for Verification Example 1. [Figure 7] These are the measurement results for Verification Example 1. [Figure 8] These are the measurement results for Verification Example 1. [Figure 9] These are the measurement results for Verification Example 1. [Figure 10] This is another measurement result from Verification Example 1. [Figure 11] These are the measurement results for Verification Example 2. [Figure 12] These are the measurement results for Verification Example 2. [Figure 13] These are the measurement results for Verification Example 2. [Figure 14] These are the measurement results for Verification Example 2. [Modes for carrying out the invention]

[0020] Hereinafter, with reference to the drawings, a rare earth oxide ferromagnetic material according to one embodiment of the present invention and a method for producing the rare earth oxide ferromagnetic material will be described. The embodiments shown below are provided specifically to better illustrate the spirit of the invention and do not limit the present invention unless otherwise specified. Furthermore, the drawings used in the following description may be enlarged for convenience to make the features of the present invention easier to understand, and the dimensional ratios of each component may not be the same as in reality.

[0021] In the following explanation, rare earth elements refer to the following 17 elements: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0022] (Rare earth oxide ferromagnetic material) Figure 1 is a schematic cross-sectional view showing a rare-earth oxide ferromagnetic material according to one embodiment of the present invention. The rare-earth oxide ferromagnetic material 10 of this embodiment is formed by periodically stacking a first layer 11 and a second layer 12 alternately. The number of layers of the first layer 11 and the second layer 12 can be, for example, 2 to several hundred layers.

[0023] The first layer 11 is made of a rare earth oxide represented, for example, R2O3 (where R is a rare earth element). The R2O3 constituting the first layer 11 has a cubic crystal structure, for example. In this embodiment, the first layer 11 is Y2O3 or Gd2O3.

[0024] The second layer 12 is, for example, R2O 3-x It consists of a rare earth oxide having oxygen vacancies (holes) in its crystal lattice, represented by (R is a rare earth element, x is less than 3). The R2O3 constituting the second layer 12 forms a cubic crystal, for example, and the oxygen coordination portion is arbitrarily vacant. The second layer 12 in this embodiment is Y2O 3-x or Gd2O 3-x That is the case.

[0025] The rare earth oxide ferromagnetic material 10, when viewed as a whole, is composed of these first layer 11 and second layer 12, R2O ((3+3-x) / 2)These rare earth oxides have fewer oxygen vacancies compared to R2O3, resulting in a lower oxygen content.

[0026] Thus, the first layer 11, which consists of a rare earth oxide without oxygen vacancies represented by R2O3, and R2O 3-x A rare earth oxide ferromagnetic material 10, comprising a second layer 12 made of a rare earth oxide having oxygen vacancies as shown, can be realized, for example, by using foil or plate bodies of rare earth elements as raw materials and performing at least one, for example multiple, two-stage heat treatment processes described later.

[0027] Furthermore, in this embodiment, the rare earth oxide ferromagnetic material 10 is formed by repeatedly stacking the first layer 11 and the second layer 12 alternately and periodically, for example, in tens to hundreds of layers, and it is permissible for a metal layer made of rare earth metals such as Y or Gd to remain in the central portion.

[0028] Even if a metal layer remains in some parts, for example in the central part, the effects of the rare earth oxide ferromagnetic material 10 of this embodiment can be obtained as long as the majority of the material consists of repeated formations of the first layer 11 and the second layer 12.

[0029] In this embodiment, the rare-earth oxide ferromagnetic material 10, consisting of a first layer 11 and a second layer 12, exhibits ferromagnetism and conductivity even at room temperature (around 25°C) by using a rare-earth oxide containing, for example, a cubic crystal with oxygen vacancies (vacancies). While ordinary rare-earth oxides with almost no oxygen vacancies (vacancies) are paramagnetic and highly dielectric, the formation of oxygen vacancies (vacancies) causes a phase transition from paramagnetism to ferromagnetism, and the improved electron mobility leads to improved conductivity.

[0030] Furthermore, the rare earth oxide ferromagnetic material 10 of this embodiment has a surface diffuse reflectance of 20% or less. Conventional fully oxidized rare earth oxides all have a diffuse reflectance exceeding 20%. For example, Y2O3 and Gd2O3 exhibit a white color with high light reflectivity, and their diffuse reflectance exceeds 50%. On the other hand, the rare earth oxide ferromagnetic material 10 of this embodiment exhibits a black color with low light reflectivity and high light absorption. By applying such a rare earth oxide ferromagnetic material 10 with a surface diffuse reflectance of 20% or less to optical semiconductor devices, such as photoelectric conversion elements, it becomes possible to achieve high photoelectric conversion efficiency.

[0031] As described above, by using the rare earth oxide ferromagnetic material 10 of this embodiment, it becomes possible to realize a ferromagnetic semiconductor device without doping with elements other than rare earth elements and oxygen.

[0032] (Method for producing rare earth oxide ferromagnetic materials) Next, a method for producing the rare earth oxide ferromagnetic material described above, according to one embodiment of the present invention, will be explained. The production of rare earth oxide ferromagnetic materials can be achieved by performing two stages of heat treatment on raw materials consisting of rare earth elements under different heat treatment conditions.

[0033] As raw materials, for example, when manufacturing yttrium oxide ferromagnetic materials, yttrium foil (99% purity) with a thickness of about 10 μm to 90 μm or yttrium plates (99% purity) with a thickness of about 0.1 mm to 1.0 mm can be used. When manufacturing gadolinium oxide ferromagnetic materials, gadolinium foil (99% purity) with a thickness of about 10 μm to 100 μm or gadolinium (99% purity) with a thickness of about 0.1 mm to 1.0 mm can be used.

[0034] Furthermore, such foils and plate-like bodies are easy to mold (deform), and by bending them into an arbitrary shape, a rare earth oxide ferromagnetic body having a three-dimensional shape can be produced. Such formation of a three-dimensional shape can be performed either on the raw material before the heat treatment step described below, or on the rare earth oxide ferromagnetic body formed by performing the heat treatment step.

[0035] First, a first heat treatment step is performed on such a raw material made of a rare earth element in the form of a foil or a plate-like body. The first heat treatment step is performed by thermally oxidizing the raw material in the atmosphere. Through the first heat treatment step of heating the raw material in such an oxygen-rich atmosphere, for example, yttrium becomes Y2O3 with few oxygen defects, and gadolinium becomes Gd2O3 with few oxygen defects.

[0036] As for the conditions of the first heat treatment step, the heating temperature is, for example, in a temperature range of 400°C or higher and 600°C or lower, and in the present embodiment, it is 500°C. The holding time after raising the temperature to 500°C is, for example, in a range of 0.5 hours or more and 2 hours or less, and in the present embodiment, it is 1 hour. For heating, for example, a high-frequency heating device or an infrared heating device can be used.

[0037] Next, a second heat treatment step is performed on the raw material on which the first layer 11 has been formed by the first heat treatment step. The second heat treatment step is performed on the raw material on which the first layer 11 has been formed, with an oxygen partial pressure of 1.0×10 -20 atm or more and less than 1.0 atm in a low oxygen partial pressure environment by thermal oxidation. Through the second heat treatment step of heating the raw material on which the first layer 11 is formed in such a low oxygen partial pressure environment with little oxygen, for example, yttrium becomes Y2O with many oxygen defects 3-x is formed, and gadolinium becomes Gd2O with many oxygen defects 3-x is formed.

[0038] As for the conditions of the second heat treatment step, the heating temperature is, for example, in a temperature range of 800°C or higher and 1000°C or lower, and in the present embodiment, it is 900°C. The holding time after raising the temperature to 900°C is, for example, in a range of 0.5 hours or more and 16 hours or less, and in the present embodiment, it is 10 hours.

[0039] Adjusting the oxygen partial pressure involves, for example, supplying oxygen gas from an oxygen supply means so that the oxygen partial pressure is 1.0 × 10⁻⁶. -20 A range of atm or more and less than 1.0 atm, preferably 1.0 × 10 -20 atm or more 1.0×10 -10 This can be done by supplying a low-oxygen gas with an adjusted oxygen partial pressure below atm.

[0040] The second layer 12, formed by heat treatment in such a low oxygen partial pressure environment, is, for example, an oxygen-defect rare earth oxide (R2O) in which the coordination position of oxygen becomes a vacancy (hole) while maintaining a cubic crystal structure. 3-x ) This second layer 12, composed of oxygen-depleted rare earth oxides, exhibits ferromagnetism and conductivity relative to the rare earth oxides.

[0041] Figure 2 is a schematic diagram illustrating the behavior of oxygen in the first and second heat treatment processes. In Figure 2, the thermal oxidation of yttrium is used as an example of a rare earth element. In the aforementioned first heat treatment process, heating is carried out in an atmosphere with an abundant supply of oxygen, so oxygen diffuses from the surface of the yttrium (raw material) into the interior, forming Y2O3 with almost no oxygen vacancies.

[0042] Next, in the second heat treatment process, the oxygen partial pressure is low (1.0 × 10⁻⁶). -20 Because the oxygen pressure is between atm and 1.0 atm, the rate of oxygen diffusion within the yttrium is greater than the rate of oxygen supply. As a result, the oxygen constituting the Y2O3 formed in the first heat treatment process diffuses in the depth direction, causing the Y2O3 to appear reduced, and the oxygen coordination position becomes a vacant Y2O 3-x A second layer 12 is formed, consisting of oxygen-deficient yttrium oxide as shown.

[0043] While the first and second heat treatment processes can be performed alternately for at least one cycle to form a rare-earth oxide ferromagnetic material 10 with one first layer 11 and one second layer 12, in practice, it is preferable to repeat the first and second heat treatment processes for tens to hundreds of cycles to form a rare-earth oxide ferromagnetic material 10 in which tens to hundreds of layers of the first layer 11 and the second layer 12 are periodically stacked. In this case, it is also permissible for a metal layer made of rare-earth metals such as Y or Gd to remain in the central portion.

[0044] As described above, the method for producing a rare earth oxide ferromagnetic material of this embodiment makes it possible to realize a rare earth oxide ferromagnetic material exhibiting ferromagnetism and conductivity, in which the first and second layers are periodically stacked, through a two-stage heat treatment process consisting of a first heat treatment step performed in an atmosphere with sufficient oxygen and a second heat treatment step performed in a low-oxygen atmosphere with a low oxygen partial pressure.

[0045] In the embodiments described above, examples are given of cases where yttrium and gadolinium are used as rare earth elements, but the same method can be applied to other rare earth elements as well, and a rare earth oxide ferromagnetic material exhibiting both ferromagnetism and conductivity can be realized.

[0046] Although embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be carried out in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Examples]

[0047] The following verification was performed on the rare earth oxide ferromagnetic material of the present invention. (Verification Example 1: Gd) For Verification Example 1, a circular Gd foil (3 mm in diameter) with a purity of 99.9% was prepared. As an example of the present invention, this Gd foil was heat-treated in air at 500°C for 10 minutes as the first heat treatment step. Subsequently, the holding temperatures were 800°C, 900°C, and 1000°C, and the oxygen partial pressure was 1.0 × 10⁻⁶. -5 atm, 1.0 × 10 -10 atm, 1.0 × 10 -20 At an ATM, heat treatment was performed for one hour in each case as the second heat treatment process. As a comparative example, a Gd foil was heated in air at a heating rate of 10°C / min to 1000°C.

[0048] Figure 3 shows the peak intensity of the sample of the present invention obtained by X-ray diffraction (XRD). Figure 4 shows the peak intensity of the comparative example sample obtained by XRD. According to the XRD measurement results, the peak intensity of metallic Gd is increased in the two-stage heat treatment example of the present invention, where the oxygen partial pressure is controlled to a low-oxygen atmosphere, compared to the comparative example of a one-stage heat treatment. Furthermore, the peak intensity of metallic Gd increases as the heat treatment temperature of the second heat treatment step increases.

[0049] These XRD results suggest that the second heat treatment process, controlled in a low-oxygen atmosphere, causes oxygen atoms to diffuse deep into the Gd foil in the thickness direction, and reduction causes oxygen to detach to the surface, creating vacancies.

[0050] Next, the first heat treatment step described above, and a holding temperature of 800°C and an oxygen partial pressure of 1.0 × 10⁻⁶ -10 The sample of this embodiment, which underwent a two-stage heat treatment process including a second heat treatment step using an atm (automated thermometer), was observed using a transmission electron microscope (TEM). The results are shown in Figures 5 and 6.

[0051] As shown in Figure 5, it was confirmed that a moiré-like periodic structure (a periodic structure between the first and second layers) was formed. Furthermore, according to Figure 6, the moiré-like periodic structure shown in Figure 5 consists of a first layer made of cubic Gd2O3 and Gd2O 3-X It was confirmed that the second layer consisted of an oxygen-deficient material.

[0052] Furthermore, according to the HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) image shown in Figure 7, the sample of this embodiment has a periodic structure of several nanometers, consisting of a first layer made of cubic Gd2O3 and Gd2O 3-X It was confirmed that a regular formation of a second layer, which contains oxygen deficiencies, was observed.

[0053] Next, Figure 8 shows a graph of the results of evaluating the magnetic properties of the metal Gd foil and the sample of this embodiment obtained by the two-step heat treatment described above at room temperature (27°C) using a superconducting quantum interference device (SQUID). As shown in Figure 8, it was confirmed that the sample of this embodiment exhibited saturation magnetization and remanent magnetization comparable to that of metallic Gd foil.

[0054] Next, the diffuse reflectance at the surface of the sample of this embodiment, which underwent the two-stage heat treatment described above, and the comparative example sample, which underwent a one-stage heat treatment, was measured between incident wavelengths of 200 nm and 2000 nm. For reference, the diffuse reflectance of commercially available Gd2O3 powder was also measured. These measurement results are shown in Figure 9.

[0055] As shown in Figure 9, the sample of this embodiment, subjected to a two-stage heat treatment, exhibited a lower diffuse reflectance at wavelengths above 300 nm compared to the comparative example, due to increased oxygen deficiency. This confirmed that the sample of this embodiment had a higher light absorption rate.

[0056] Furthermore, according to the XPS measurement results of the Gd foil after the first heat treatment process at 500°C for 10 minutes, as shown in Figure 10, in addition to the peaks within the Gd2O3 lattice and the peaks of O due to surface adsorption, oxygen vacancies within the lattice were also generated in the Gd2O3 lattice during the first heat treatment process. 3-x A peak was also observed. Based on these XPS measurement results, it can be inferred that oxygen has already begun to detach and vacancies have started to form even in the first heat treatment process.

[0057] (Verification Example 2: Y) For Verification Example 2, a circular Y-shaped plate (3 mm in diameter, 100 μm thick) with a purity of 99.9% was prepared. As an example of the present invention, this Y plate was heat-treated in air at 500°C for 1 hour as the first heat treatment step. Subsequently, it was held at temperatures of 800°C, 900°C, and 1000°C, with an oxygen partial pressure of 1.0 × 10⁻⁶. -5 At an ATM, heat treatment was performed for one hour in each case as the second heat treatment process. As a comparative example, Gd foil was heated in air at a heating rate of 10°C / min to 600°C, 700°C, and 1000°C.

[0058] Figure 11 shows the peak intensity of the sample of the present invention obtained by grazing incidence X-ray diffraction (GI-XRD). Figure 12 shows the peak intensity of the comparative example sample obtained by GI-XRD. According to the GI-XRD measurement results, the peak intensity of metal Y is increased in the two-stage heat treatment example of the present invention, where the oxygen partial pressure is controlled to a low-oxygen atmosphere, compared to the comparative example of a one-stage heat treatment. Furthermore, the peak intensity of metal Y increases as the heat treatment temperature of the second heat treatment step increases.

[0059] These GI-XRD results suggest that the second heat treatment process, controlled in a low-oxygen atmosphere, causes oxygen atoms to diffuse deep into the Y-plate in the thickness direction, and reduction causes oxygen to detach to the surface, creating voids.

[0060] Next, the first heat treatment step described above, and a holding temperature of 900°C and an oxygen partial pressure of 1.0 × 10⁻⁶ -5 The sample of this embodiment, subjected to a two-stage heat treatment process consisting of a 12-hour atm treatment and a second heat treatment step, was observed using a transmission electron microscope (TEM). The results are shown in Figure 13.

[0061] As shown in Figure 13, the two-stage heat treatment reduces some of the oxygen in Y2O3 on the surface of the Y plate to Y2O 3-x This resulted in an oxygen deficiency. This part is YO 1.428 It was presumed to be a phase.

[0062] Next, the diffuse reflectance at the surface of the sample of this embodiment, which underwent the two-stage heat treatment described above, and the comparative example sample, which underwent a one-stage heat treatment, was measured between incident wavelengths of 200 nm and 2000 nm. For reference, the diffuse reflectance of commercially available Y2O3 powder was also measured. These measurement results are shown in Figure 14.

[0063] As shown in Figure 14, the sample of this embodiment, subjected to a two-stage heat treatment, exhibited a decrease in diffuse reflectance compared to the comparative example at wavelengths of 300 nm and above due to increased oxygen deficiency, and this difference increased with increasing wavelength. This confirmed that the sample of this embodiment had a higher light absorption rate. [Industrial applicability]

[0064] According to the rare-earth oxide ferromagnetic material and rare-earth oxide ferromagnetic material manufacturing apparatus of the present invention, it becomes possible to realize semiconductor devices with a small energy gap of approximately 1 eV or less and that can utilize visible light, using a rare-earth oxide ferromagnetic material that is ferromagnetic and conductive at room temperature and has high visible light absorption. Therefore, it has industrial applicability. [Explanation of symbols]

[0065] 10…Rare earth oxide ferromagnetic materials 11…1st layer 12…Second layer

Claims

1. The first and second layers are stacked alternately, one or more of each, periodically. A rare earth oxide ferromagnetic material comprising a first layer of rare earth oxide and a second layer of oxygen-deficient rare earth oxide.

2. The rare earth oxide constituting the first layer is R ２ O ３ (where R is a rare earth element) is represented as, and the oxygen-deficient rare earth oxide constituting the second layer is R ２ O ３－ｘ A rare earth oxide ferromagnetic material according to claim 1, wherein R is a rare earth element and x is less than 3.

3. The rare earth oxide ferromagnetic material according to claim 2, wherein the rare earth elements constituting the first and second layers are Y or Gd.

4. The rare earth oxide ferromagnetic material is in the form of a film, a plate, or a lump, according to any one of claims 1 to 3.

5. The rare earth oxide ferromagnetic material according to any one of claims 1 to 3, wherein the surface diffuse reflectance of the rare earth oxide ferromagnetic material is 20% or less.

6. A first heat treatment step involves thermally oxidizing a raw material consisting of rare earth elements in the atmosphere to form a first layer consisting of rare earth oxides on the surface, The raw material that has undergone the first heat treatment process is subjected to an oxygen partial pressure of 1.0 × 10 －２０ A method for producing a rare earth oxide ferromagnetic material, comprising: a second heat treatment step of thermal oxidation in a low oxygen partial pressure environment in the range of atm or more and less than 1.0 atm, to diffuse oxygen into a region deeper than the first layer, thereby forming a second layer consisting of an oxygen-deficient rare earth oxide.

7. The method for producing a rare earth oxide ferromagnetic material according to claim 6, wherein the raw material is in the form of a plate or a film.

8. A method for producing a rare earth oxide ferromagnetic material according to claim 6 or 7, wherein the first heat treatment step involves holding the heating temperature in a temperature range of 400°C to 600°C for 0.5 hours to 2 hours, and the second heat treatment step involves holding the heating temperature in a temperature range of 800°C to 1000°C for 0.5 hours to 16 hours.

9. A method for producing a rare earth oxide ferromagnetic material according to claim 6 or 7, wherein the first heat treatment step and the second heat treatment step are performed for at least two cycles or more.

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