MAGNETO-OPTICAL MATERIAL AND METHOD FOR PRODUCING SAME

Abstract

An object of the present invention is to provide a magneto-optical material capable of exhibiting the Faraday effect even though no magnetic field is applied. The magneto-optical material of the present invention has a nanogranular structure in which magnetic nanoparticles are dispersed in a fluoride matrix, and can exhibit Faraday properties without requiring the application of a magnetic field because the magnetic nanoparticles are configured by a magnetic material that has residual magnetization and consists of any of a FePt alloy, a CoPt alloy, a FeCoNiAl alloy, a Co ferrite, or a Ba ferrite.

Claims

1. A magneto-optical material, comprising a nanogranular structure in which nanoparticles are dispersed in a matrix, wherein the matrix is a fluoride matrix including at least one element selected from the group consisting of Li, Be, Mg, Al, Si, Ca, Sr, Ba, Bi, and rare earth elements, and the nanoparticles are magnetic nanoparticles having at least one magnetization selected from the group consisting of a FePt alloy, a CoPt alloy, a FeCoNiAl alloy, a Co ferrite, and a Ba ferrite, and including a FePt ordered phase, a Co.sub.3Pt ordered phase, or a spinel phase.

2. The magnet-optical material according to claim 1, wherein a volume ratio of all of the magnetic nanoparticles included in the nanogranular structure to the matrix is 10% or more and 50% or less.

3. The magnet-optical material according to claim 1, wherein an absolute value of Faraday rotation angle accompanied by residual magnetization is 0.08 [deg./m] or more for light in at least a part of a wavelength band of 600 nm to 1600 nm in a state of no magnetic field applied.

4. The magnet-optical material according to claim 2, wherein an absolute value of Faraday rotation angle accompanied by residual magnetization is 0.08 [deg./m] or more for light in at least a part of a wavelength band of 600 nm to 1600 nm in a state of no magnetic field applied.

5. A method for producing the magneto-optical material according to claim 1, the method comprising: a step of producing a nanogranular material having a nanogranular structure in which the magnetic nanoparticles are dispersed in the fluoride matrix; and a step of heat-treating the nanogranular material in a temperature range in which the magnetic nanoparticles are ordered.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 shows a view of the Faraday loop of sample 1.

[0014] FIG. 2 shows a view of the relationship between the wavelength of light and the absolute value of the Faraday rotation angle at zero magnetic field for the sample 1.

[0015] FIG. 3 shows a view of an XRD pattern of the sample 1.

[0016] FIG. 4 shows a view of the Faraday loop of sample 2.

[0017] FIG. 5 shows a view of the relationship between the wavelength of light and the absolute value of the Faraday rotation angle at zero magnetic field for the sample 2.

[0018] FIG. 6 shows a view of an XRD pattern of the sample 2.

[0019] FIG. 7 shows a view of the Faraday loop of sample 3.

[0020] FIG. 8 shows a view of the relationship between the wavelength of light and the absolute value of the Faraday rotation angle at zero magnetic field for the sample 3.

[0021] FIG. 9 shows a view of an XRD pattern of the sample 3.

[0022] FIG. 10 shows a view of the Faraday loop of sample 4.

[0023] FIG. 11 shows a view of the relationship between the wavelength of light and the absolute value of the Faraday rotation angle at zero magnetic field for the sample 4.

[0024] FIG. 12 shows a view of an XRD pattern of the sample 4.

[0025] FIG. 13 shows a view of the Faraday loop of sample 5.

[0026] FIG. 14 shows a view of the relationship between the wavelength of light and the absolute value of the Faraday rotation angle at zero magnetic field for the sample 5.

[0027] FIG. 15 shows a view of an XRD pattern of the sample 5.

DESCRIPTION OF EMBODIMENTS

[0028] The present inventors have performed extensive investigations to obtain a magneto-optical material that has high magnetization properties and good translucency, exhibits excellent magneto-optical properties in the near-infrared region, and is applicable to various optical communication devices. As a result, researchers including some of the present inventors have found that among the nanogranular magnetic materials proposed as giant magnetoresistance (GMR) materials (refer to Non-Patent Literature 1), a material composed of a fluoride matrix and nm-sized metal granules has high magnetization properties and good translucency, and exhibits excellent magneto-optical properties in the near-infrared region due to the effect of fine magnetic metal granules present in a dispersed state in a translucent matrix.

[0029] The following is a description of the composition and structure, characteristics, and production method of the magneto-optical material according to the present invention.

[0030] (Composition and Structure)

[0031] The magneto-optical material of the present invention has a nanogranular structure in which magnetic nanoparticles (nanogranules) are dispersed in a fluoride matrix. The magnetic nanoparticles are configured by any magnetic material having residual magnetization of a FePt alloy, a CoPt alloy, a FeCoNiAl alloy, a Co ferrite, and a Ba ferrite. The matrix is a fluoride containing at least one element selected from Li, Be, Mg, Al, Si, Ca, Sr, Ba, Bi, and rare earth elements.

[0032] The magneto-optical material of the present invention has a nanogranular structure in which magnetic granules having an average particle size of 50 nm or less are uniformly distributed in a fluoride matrix. The effect of the fine magnetic granules present in a dispersed state in the fluoride having excellent optical transparency provides a magneto-optical material having excellent properties. That is, magnetic nanoparticles having an average particle diameter of 50 nm or less are dispersed in fluoride, thereby allowing to simultaneously exhibit magnetism due to the magnetic granules and optical transparency due to the matrix. Such a nanogranular structure can be obtained by forming a thin film having the above composition by a thin deposition technique such as sputtering.

[0033] The volume ratio of all magnetic nanoparticles contained in the nanogranular structure to the matrix is preferably 10% or more and 50% or less. The magnetic nanoparticles having the volume ratio of less than 10% lose the magnetism due to the small amount of the magnetic component contained therein, and no longer function as a magnetic material. In contrast, when the volume ratio of magnetic nanoparticles is more than 50%, the amount of the magnetic component contained therein increases, the particle size of the magnetic nanoparticles exceeds 50 nm since adjacent magnetic nanoparticles contact and bond with each other, and incident light transmitted through the matrix fluoride is blocked by the magnetic nanoparticles, leading to loss of the optical transparency.

[0034] The magnetic nanoparticles are composed of a FePt alloy, a CoPt alloy, a FeNiAl alloy, a Co ferrite, and a Ba ferrite magnets having coercive force and residual magnetization, and desirably have as large residual magnetization as possible. By using a magnetic material, the magneto-optical material of the present invention can have magnetization even though no magnetic field is applied, and can exhibit the Faraday effect associated with residual magnetization. In order for magnetic nanoparticles to exhibit magnet properties with residual magnetization, they need to be crystals with ordered phases or spinel phases that have undergone optimal heat treatment. Heat treatment in a magnetic field (cooling in a magnetic field) is also effective for imparting magnetic anisotropy.

[0035] The magneto-optical material according to the present invention may be a thin film or a bulk material as long as it has the above composition, but a thin film is suitable for miniaturization of optical communication devices.

[0036] (Characteristics)

[0037] The following is a description of the properties of the magneto-optical material according to the present invention.

[0038] (1. Optical Transparency)

[0039] The magneto-optical material according to the present invention has a light transmittance of 1% or more per 1 m thickness, preferably 10% or more per 1 m thickness of the magnetic material for incident light of optional wavelength in the ultraviolet to near-infrared region, including the visible light region with a wavelength of 400 nm to 2000 nm. The magneto-optical material according to the present invention has good optical transparency over such a wide range, and thus becomes suitable for various optical communication devices.

[0040] (2. Magnetic Properties)

[0041] The magneto-optical material according to the present invention exhibits ferromagnetism with residual magnetization. The magneto-optical material according to the present invention exhibits the Faraday effect associated with spontaneous magnetization even though no magnetic field is applied, due to the presence of magnetic nanoparticles consisting of a magnetic material having residual magnetization. The use of the magneto-optical material according to the present invention eliminates a need for a magnetic field applying mechanism for exhibiting the Faraday effect, allowing the device structure to be simplified and miniaturized and allowing the device manufacturing process to be simplified. This also makes it possible to reduce the cost of the device.

[0042] (3. Magneto-Optical Properties: Faraday Rotation Angle)

[0043] In optical communication devices such as optical isolators and optical circulators, there are used magnetic materials exhibiting a magneto-optical effect (Faraday effect) in which the plane of polarization rotates when linearly polarized light parallel to a magnetic field is transmitted. However, the use of yttrium garnet or bismuth-substituted garnet conventionally used as such a magnetic material causes the Faraday rotation angle to become significantly small in the near-infrared wavelength region (1550 nm) used for optical communication. In contrast, the magneto-optical material of the present invention exhibits the absolute value of the Faraday rotation angle of 0.1 [deg./m] or more, or further 0.3 [deg./m] or more without depending on a wavelength region, allowing a larger Faraday rotation angle to be obtained as compared with a case where conventional yttrium garnet or bismuth-substituted garnet is used, in the near-infrared wavelength region (1550 nm) used for optical communication. Such a large Faraday rotation angle obtained is considered to be due to the magneto-optical effect of ferromagnetic metals by light transmission or reflection in fine magnetic granules, or the actions of electromagnetic effect and quantum effect at the interface between the matrix and magnetic granules.

[0044] (Production Method)

[0045] The magneto-optical material of the present invention can be deposited as a thin film by a conventional sputtering device or RF sputtering device. Using a sputtering method or a RF sputtering deposition device, disk targets of pure Fe, pure Co, pure Ni, and/or an alloy including any of Fe, Co, Ni, Pt, Al and Ba therewith, or even a composite target having Pt, Al and/or Ba chips evenly arranged on the metal disk targets is sputtered simultaneously with a fluoride target, thereby providing a nanogranular structure film in which nm-sized (50 nm or less) magnetic nanoparticles are dispersed in a fluoride matrix. In this case, as a base for thin film formation, in addition to various substrates such as semiconductor substrates and/or insulator substrates, substrates having a semiconductor and/or insulator layer formed on the surface thereof may be used.

[0046] Specifically, using a conventional type RF sputtering device, a RF magnetron sputtering device, or a DC facing target sputtering device, disk targets with a diameter of 70 to 100 mm of pure Fe, pure Co, pure Ni, and/or an alloy including any two or more of Fe, Co, Ni, Pt, Al, and Ba, or even an alloy target further including one or more elements selected from the group consisting of Li, Be, Mg, Al, Si, Ca, Sr, Ba, Bi, and rare earth elements is sputtered simultaneously with a fluoride target of one or more elements selected from the group consisting of Li, Be, Mg, Al, Si, Ca, Sr, Ba, Bi, and rare earth elements, thereby producing a nanogranular thin film having a nanogranular structure. Ar gas or a mixed gas of Ar and O.sub.2 (0.1 to 10%) is used for the sputtering deposition. The film thickness is controlled by adjusting the deposition time, and the film is formed to a thickness of about 0.3 to 5 [m]. The base (substrate) is indirectly water-cooled or controlled at an optional temperature within the temperature range of 100 to 800 [ C.], the sputtering pressure during deposition is controlled to 1 to 60 mTorr, and the sputter power is adjusted within the range of 50 to 350 [W].

[0047] (Heat Treatment)

[0048] In order for magnetic nanoparticles to exhibit magnetic properties accompanied by residual magnetization, magnetic nanoparticles need to have an ordered phase or a spinel phase. In order to obtain an ordered phase and a spinel phase, a heating step such as a predetermined heat treatment is required after deposition. The temperature required for this is 500 C. or more, and heat treatment is performed at a predetermined temperature within the temperature range of 500 C. to 800 C. in vacuum or in an oxygen atmosphere.

[0049] The magneto-optical material of the present invention obtained as a thin film by the above method is a nanogranular structure composed of a fluoride matrix and nm-sized magnetic metal nanoparticles, and has high magnetization properties and good translucency due to the effect of magnetic nanoparticles with fine residual magnetization present in a dispersed state in the translucent matrix. In addition, since the magneto-optical material of the present invention has a large Faraday rotation angle (for example, 0.08 deg./m or more) associated with residual magnetization in all or a part of the wavelength band of 600 to 1600 nm in a state of no magnetic field applied, it can enable various optical communication devices to be simplified and miniaturized.

[0050] The production by the sputtering method is suitable for producing a thin film material with a film thickness of about 10 m or less, whereas an electrochemical reaction deposition method using an aqueous solution containing elements to be synthesized is suitable for producing a thick film of more than 10 m. Furthermore, for the production of bulk, there is a method of pulverizing and mixing powdery raw materials containing the elements to be synthesized and then sintering them.

EXAMPLES

Examples of the Present Invention Will be Described Below

[0051] (1. Preliminary Experiment)

[0052] The substrates used were about 0.5 mm thick #7059, #2000, #XG (trade name of Corning Incorporated) glasses manufactured by Corning Incorporated, a 0.5 mm thick Si wafer with the surface thermally oxidized, and a 0.5 mm thick quartz glass.

[0053] (2. Production and Evaluation of Thin Film)

[0054] Table 1 summarizes an example of conditions for producing thin film samples.

TABLE-US-00001 TABLE 1 deposition apparatus RF magnetron sputtering apparatus, DC facing target sputtering apparatus Substrate #2000 glass, quartz glass, Si wafer Film thickness of thin-film 0.1-1.0 m sample Substrate temperature Water cooled to 800 C. Sputtering pressure 0.3-20 mTorr Sputtering power 50-200 W Heat treatment 500 to 800 C., in vacuum, or in the atmosphere of a mixed gas of 0.1 to 1.0% O.sub.2 + Ar at 30 mTorr

[0055] For the sample of the magneto-optical material (magneto-optical thin film) produced as described above under the above conditions, the film composition was analyzed by energy dispersive spectroscopy (EDS) or wavelength dispersive spectroscopy (WDS).

[0056] Light transmittance was measured for each sample by a Fourier transform infrared spectrophotometer (FTIR). In addition, the Faraday effect (Faraday rotation angle) of each sample was measured in an optional magnetic field up to a maximum magnetic field of 800 [kA/m] for light in the wavelength region of 405 to 1550 [nm]. Furthermore, the magnetization of each sample was measured by a vibrating sample magnetization measuring device (VSM). In addition, the structure of each sample was analyzed by X-ray diffraction (XRD).

[0057] Table 2 shows the measurement result of the Faraday rotation angle in the heat treatment condition and in zero magnetic field at a wavelength of 1550 [nm] for each sample.

TABLE-US-00002 TABLE 2 Sample Heat treatment Absolute value of Faraday number Sample composition condition rotation angle (1550 nm) 1 Co.sub.20Pt.sub.5Mg.sub.27F.sub.48 600 C. 1 h (in vacuum) 0.26 (deg./m) 2 Co.sub.11Pt.sub.4Ca.sub.25F.sub.60 600 C. 1 h (in vacuum) 0.21 (deg./m) 3 Fe.sub.6Co.sub.4Pt.sub.11Mg.sub.25F.sub.54 650 C. 1 h (in vacuum) 0.17 (deg./m) 4 Fe.sub.8Co.sub.5Pt.sub.14Ba.sub.22F.sub.52 600 C. 1 h (in vacuum) 0.11 (deg./m) 5 Fe.sub.8Co.sub.5Ba.sub.13F.sub.46O.sub.28 600 C. 1 h (Ar + 1% O.sub.2 0.3 (deg./m) (30 mTorr)) 6 Fe.sub.10Pt.sub.10Ba.sub.24Ca.sub.8F.sub.48 550 C. 1 h (in vacuum) 0.2 (deg./m) 7 Co.sub.19Pt.sub.8Mg.sub.15Sr.sub.10F.sub.48 500 C. 1 h (in vacuum) 0.2 (deg./m) 8 F.sub.18Co.sub.13Ca.sub.22F.sub.25O.sub.22 700 C. 1 h (Ar + 0.1% O.sub.2 0.1 (deg./m) (30 mTorr)) 9 Fe.sub.15Co.sub.9Ni.sub.6Al.sub.3Ca.sub.17Y.sub.5F.sub.45 800 C. 1 h (in vacuum) 0.1 (deg./m) 10 Fe.sub.14Pt.sub.9Mg.sub.20Be.sub.5Si.sub.7F.sub.45 650 C. 1 h (in vacuum) 0.12 (deg./m) 11 Co.sub.10Pt.sub.5Ca.sub.18Li.sub.4Bi.sub.3F.sub.60 600 C. 1 h (in vacuum) 0.08 (deg./m)

[0058] FIG. 1 shows Faraday loops for incident light of wavelengths 600, 800, 1310, and 1550 nm for sample 1. The Faraday loop corresponds to the magnetization curve and has hysteresis with residual magnetization and coercivity. As can be seen from FIG. 1, even though the magnetic field (H) is 0, the Faraday rotation angle has an absolute value of 0.57 [deg./m] at 600 nm, an absolute value of 0.59 [deg./m] at 800 nm, an absolute value of 0.33 [deg./m] at 1310 nm, and an absolute value of 0.26 [deg./m] at 1550 nm.

[0059] FIG. 2 shows the relationship between the absolute value of the Faraday rotation angle and the wavelength of the incident light when no magnetic field is applied to the sample 1 (H=0). As can be seen from FIG. 2, the sample 1 has a value of 0.2 [deg./m] or more in the wavelength region of 500 nm or more.

[0060] FIG. 3 shows the XRD diffraction pattern of the sample 1. Heat treatment at 600 C. in vacuum after deposition promotes ordering of the CoPt alloy, allowing the respective peaks indicating the existence of the ordered phase of Co.sub.3Pt to be confirmed at approximately 2=43[deg.], 50 [deg.], 73 [deg.], and 88 [deg.]. In addition, the respective peaks indicating the existence of the MgF.sub.2 phase can be confirmed at approximately 2=27 [deg.] and 57 [deg.]. It is found from the confirmation results that the sample 1 has a nanogranular structure in which magnet particles formed of the CoPt alloy of the Co.sub.3Pt ordered phase are dispersed in a matrix formed of Mg fluoride. The existence of the CoPt magnet particles in the sample 1 causes residual magnetization, and the magneto-optical material of the sample 1 exhibits the Faraday effect even though H=0.

[0061] FIG. 4 shows Faraday loops for incident light of wavelengths 800, 1310, and 1550 [nm] for sample 2. As can be seen from FIG. 4, even though H=0, the Faraday rotation angle has an absolute value of 0.08 [deg./m] at 800 nm, an absolute value of 0.21 [deg./m] at 1310 nm, and an absolute value of 0.21 [deg./m] at 1550 nm.

[0062] FIG. 5 shows the relationship between the absolute value of the Faraday rotation angle and the wavelength of the incident light at H=0 for the sample 2. As can be seen from FIG. 5, the sample 2 has a value of 0.08 [deg./m] or more in the wavelength region of 800 nm or more.

[0063] FIG. 6 shows the XRD diffraction pattern of the sample 2. Heat treatment at 600 C. in vacuum after deposition promotes ordering of the CoPt alloy, allowing the respective peaks indicating the existence of the ordered phase of Co.sub.3Pt to be confirmed at approximately 2=43 [deg.], 50 [deg.], 73 [deg.], and 88 [deg.]. In addition, the respective peaks indicating the existence of the CaF.sub.2 phase can be confirmed at approximately 2=28 [deg.], 47 [deg.], and 76 [deg.]. It is found from the confirmation results that the sample 2 has a nanogranular structure in which magnet particles formed of the CoPt alloy of the Co.sub.3Pt ordered phase are dispersed in a matrix formed of Ca fluoride. The existence of the CoPt magnet particles in the sample 2 causes residual magnetization, and the magneto-optical material of the sample 2 exhibits the Faraday effect even though H=0.

[0064] FIG. 7 shows Faraday loops for incident light of wavelengths 800, 1310, and 1550 [nm] for sample 3. As can be seen from FIG. 7, even though H=0, the Faraday rotation angle has an absolute value of 0.19 [deg./m] at 800 nm, an absolute value of 0.23 [deg./m] at 1310 nm, and an absolute value of 0.17 [deg./m] at 1550 nm.

[0065] FIG. 8 shows the relationship between the absolute value of the Faraday rotation angle and the wavelength of the incident light at H=0 for the sample 3. As can be seen from FIG. 8, the sample 3 has a value of 0.08 [deg./m] or more in the wavelength region of 500 to 600 nm and 720 nm or more.

[0066] FIG. 9 shows the XRD diffraction pattern of the sample 3. Heat treatment at 650 C. in vacuum after deposition promotes ordering of the FePt alloy, allowing the respective peaks indicating the existence of the ordered phase of FePt to be confirmed at approximately 2=24 [deg.], 33 [deg.], 42 [deg.], 47 [deg.], 49 [deg.], 68 [deg.], and 71 [deg.]. In addition, the respective peaks indicating the existence of the MgF.sub.2 phase can be confirmed at approximately 2=28 [deg.] and 57 [deg.]. It is found from the confirmation results that the sample 3 has a nanogranular structure in which magnet particles formed of the FeCoPt alloy of the FePt ordered phase are dispersed in a matrix formed of Mg fluoride. The existence of the FeCoPt magnet particles in the sample 3 causes residual magnetization, and the magneto-optical material of the sample 3 exhibits the Faraday effect even though H=0.

[0067] FIG. 10 shows Faraday loops for incident light of wavelengths 600, 800, 1310, and 1550 [nm] for sample 4. As can be seen from FIG. 10, even though H=0, the Faraday rotation angle has an absolute value of 0.09 [deg./m] at 600 nm, an absolute value of 0.24 [deg./m] at 800 nm, an absolute value of 0.14 [deg./m] at 1310 nm, and an absolute value of 0.11 [deg./m] at 1550 nm.

[0068] FIG. 11 shows the relationship between the absolute value of the Faraday rotation angle and the wavelength of the incident light at H=0 for the sample 4. As can be seen from FIG. 11, the sample 4 has a value of 0.08 [deg./m] or more in the range of 600 nm or more.

[0069] FIG. 12 shows the XRD diffraction pattern of the sample 4. Heat treatment at 600 C. in vacuum after deposition promotes ordering of the FePt alloy, allowing the respective peaks indicating the existence of the ordered phase of FePt to be confirmed at approximately 2=24 [deg.], 33 [deg.], 42 [deg.], 47 [deg.], 58 [deg.], and 84 [deg.]. In addition, the respective peaks indicating the existence of the BaF.sub.2 phase can be confirmed at approximately 2=25 [deg.], 42 [deg.], 48 [deg.], 67 [deg.], 75 [deg.], and 81 [deg.]. It is found from the confirmation results that the sample 4 has a nanogranular structure in which magnet particles formed of the FeCoPt alloy of the FePt ordered phase are dispersed in a matrix formed of Ba fluoride. The existence of the FeCoPt magnet particles in the sample 4 causes residual magnetization, and the magneto-optical material of the sample 4 exhibits the Faraday effect even though H=0.

[0070] FIG. 13 shows Faraday loops for incident light of wavelengths 550, 760, and 1550 [nm] for sample 5. As can be seen from FIG. 13, even though H=0, the Faraday rotation angle has an absolute value of 1.2 [deg./m] at 550 nm, an absolute value of 0.5 [deg./m] at 760 nm, and an absolute value of 0.3 [deg./m] at 1550 nm.

[0071] FIG. 14 shows the relationship between the absolute value of the Faraday rotation angle and the wavelength of the incident light at H=0 for the sample 5. As can be seen from FIG. 14, the sample 5 has a value of 0.08 [deg./m] or more in the wavelength region of 410 to 690 nm, 710 to 1180 nm, and 1330 nm or more.

[0072] FIG. 15 shows the XRD diffraction pattern of the sample 5. Nanogranules consisting of Co-ferrite magnets with spinel structures are synthesized by heat treatment in an oxygen atmosphere, and the respective peaks indicating the existence of the spinel phase of CoFe.sub.2O.sub.4 can be confirmed at approximately 2=31 [deg.], 35 [deg.], 37 [deg.], 43 [deg.], 53 [deg.], and 63 [deg.]. In addition, the respective peaks indicating the existence of the BaF.sub.2 phase can be confirmed at approximately 20=25 [deg.] and 29 [deg.]. It is found from the confirmation results that the sample 5 has a nanogranular structure in which magnet particles formed of a CoFe.sub.2O.sub.4 spinel ferrite are dispersed in a matrix formed of Ba fluoride. The existence of the CoFe.sub.2O.sub.4 magnet particles in the sample 5 causes residual magnetization, and the magneto-optical material of the sample 5 exhibits the Faraday effect even though H=0.

[0073] The magneto-optical material of the present invention exhibits the Faraday effect even though no magnetic field is applied, and therefore a magnetic field application mechanism is unnecessary in device design, and simplification and/or miniaturization of the device becomes possible.