JP2014160043A - Magnetic photonic crystal and magnetooptic imaging device, opto-magnetic recording medium, arithmetic element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-cavity magnetic photonic crystal and a magnetooptic imaging device which measure three-dimensional information of a magnetic field.SOLUTION: Attention is focused on the generation of different leakage magnetic field distributions, and a leakage magnetic field distribution generated from a target portion is obtained in positions each having a liftoff from an analyte different in some μm scale. A multi-cavity magnetic photonic crystal to be used comprises: a plurality of magnetic material layers laminated with appropriate intervals in an optical axis direction of irradiation light; and a plurality of dielectric layers laminated with the magnetic material layers interposed therebetween. The appropriate intervals are maintained by the dielectric layers laminated between the adjacent magnetic material layers. The use of matrix approach method allows light localization of design wavelength within the crystal and an increase of Faraday rotation angle, thus detecting a state in a depth direction by only shifting the wavelength.

Description

The present invention relates to a magnetophotonic crystal for measuring three-dimensional information of a magnetic field in a magnetized object. In particular, it is used in a magneto-optical imaging apparatus that measures the depth and crack shape of minute scratches.

Currently, many non-destructive tests are being conducted to improve the safety of our lives. Since fatigue fracture of a structure generally starts from a minute scratch (hereinafter referred to as a microcrack), it is important to detect the microcrack at an early stage. For microcrack flaw detection, a high spatial resolution of the order of 10 μm is required.

  In general, an eddy current test is often used for fatigue fracture inspection. Since the spatial resolution of the eddy current test depends on the physical size of the detection coil, it is on the order of mm, and the spatial resolution is insufficient for the microcrack test described above. Furthermore, when testing an object having a large surface area, it takes time to sweep the probe, and it is necessary to devise so as not to damage the object.

In addition, the fatigue fracture inspection includes an imaging technique (hereinafter referred to as MO imaging) using a magneto-optic (MO) effect that can inspect defects in a magnetized state (for example, Patent Document 1, Non-Patent Document). Reference 1). The MO imaging is a technique in which Faraday rotation generated by magnetizing a magneto-optical material is converted into light intensity and inspected by a leakage magnetic field generated from a defect. Flaw detection can be performed by magnetizing the magnetic garnet film with the leakage magnetic field, converting the rotation angle of the polarization plane according to the magnetization into light intensity with an analyzer and a CCD camera, and acquiring a magnetic image according to the defect shape. Generally, the spatial resolution of a single crystal magnetic garnet used for MO imaging is about 100 μm due to its magnetic domain structure.

JP 2012-47645 A

Z. Zeng, X. Liu, Y. Deng, L. Udpa, L.C. Xuan, W. C. L. Shih, G. L. Fitzpatrick, IEEE Trans. Magn, 42, p. 3737 (2006)

However, while MO imaging can evaluate the opening state of defects such as scratches with high spatial resolution, it is difficult to evaluate the inside of the structure such as defect depth because it is based on a two-dimensional image. In order to ensure the security of the structure, not only the opening state but also the defect internal information such as the dimension in the defect depth direction and the shape of the crack tip are required.

Therefore, an object of the present invention is to use a multi-cavity magnetic photonic crystal (MPC) that detects the three-dimensional size and shape of a microcrack, which has been difficult with the conventional MO imaging technique, and the MPC. It is to provide an MO imaging apparatus.

A first multi-cavity magnetophotonic crystal according to the present invention is formed by laminating a plurality of magnetic layers laminated with appropriate intervals in the optical axis direction of irradiated light, and sandwiching each of the magnetic layers. A plurality of dielectric layers, and the appropriate distance is maintained by a dielectric layer laminated between the adjacent magnetic layers.

The second multi-cavity magnetic photonic crystal according to the present invention has a thickness of the magnetic layer and dielectric layer of the first multi-cavity magnetic photonic crystal according to the present invention determined by a matrix approach method. The calculated magnetic layers have arbitrarily different localized wavelengths depending on the calculated film thickness.

The third multi-cavity magnetic photonic crystal according to the present invention is such that the magnetic layer of the first and second multi-cavity magnetic photonic crystals according to the present invention comprises a polycrystalline magnetic garnet thin film. is there.

Furthermore, a fourth multi-cavity magnetic photonic crystal according to the present invention is laminated on at least the outermost layer of the dielectric layers of the first to third multi-cavity magnetic photonic crystals according to the present invention. Is a dielectric multilayer film in which two or more layers of silicon oxide film and tantalum pentoxide film are laminated.

On the other hand, a magneto-optical imaging apparatus according to the present invention is a magneto-optical imaging apparatus using the first to fourth multi-cavity magnetic photonic crystals according to the present invention, wherein the multi-cavity magneto-photonic crystal, A light source that irradiates light having an optical axis parallel to the stacking direction of the multi-cavity magnetophotonic crystal, an excitation unit that applies a magnetic field perpendicular to the optical axis direction of the light by the light source to the inspection object, and multi-cavity magnetism A detection unit that detects light of a specific wavelength modulated by the photonic crystal, and changes in magnetization orientation due to a physical event in the vicinity of the multi-cavity magnetophotonic crystal by the light of the specific wavelength detected by the detection unit. It is characterized by three-dimensional display.

Further, in the magneto-optical imaging according to the present invention, a leakage magnetic field generated from the physical event by the application of the excitation unit is calculated in advance by a finite element method, and a plurality of the multi-cavity magnetic photonic crystals are selected according to the leakage magnetic field. The distance between the magnetic layers is determined. Here, when the physical event is a magnetic field generated from an electric current, it can function as an ammeter, and when it is a defect present in a prefectural bond object, it can function as a flaw detector. Can do.

Furthermore, the magneto-optical recording medium according to the present invention is a magneto-optical recording medium using any one of the first to fourth multi-cavity magnetic photonic crystals according to the present invention, wherein the plurality of magnetic layers are formed. It is used as a magnetic recording layer, and is characterized by being multi-valued by modulated light reproduced in multiple gradations according to the magnetization direction recorded in a plurality of layers.

Finally, an arithmetic element according to the present invention is an arithmetic element using any one of the first to fourth multi-cavity magnetic photonic crystals according to the present invention, and is two-dimensionally arranged on the plurality of magnetic layers. The magnetic pixels are arranged in a plurality of layers, and multi-gradation operations are processed by the light transmitted through the magnetic pixels of each magnetic layer.

A multi-cavity magnetic photonic crystal having a defect layer (or magnetic layer) composed of a plurality of polycrystalline magnetic garnet thin films having the above-described characteristics has been developed. The defect layers are installed at different lift-off positions from the defects to be inspected, and are optically designed so that the localization of light can be selected according to the wavelength of the reproduction light. Each of the defect layers can modulate the magnetization direction according to a three-dimensional magnetic field distribution.

Furthermore, by changing the wavelength of the reproduction light, the magnetization direction of each defect layer can be obtained as a high-contrast MO image. Therefore, since the three-dimensional magnetic field distribution can be derived from the obtained MO image image and the spread of the leakage magnetic field varies depending on the defect depth, the defect depth, etc. can be determined using the multi-cavity MPC according to the present invention. Internal information can be detected.

Explanatory drawing of the simulation model which concerns on Example 1 of this invention FIG. 3 is a distribution diagram of a vertical component of a leakage magnetic field according to Embodiment 1 of the present invention, (a) a vertical component of a magnetic field generated from a defect having a depth of 10 μm, and (b) a vertical component of a magnetic field generated from a defect having a depth of 20 μm Structure diagram of multi-cavity magnetic photonic crystal according to Example 1 of the present invention The graph which showed the reflectance and Faraday rotation angle of the multicavity magnetic photonic crystal which concern on Example 1 of this invention Spatial distribution diagram of leakage magnetic field by multi-cavity magnetic photonic crystal according to Example 1 of the present invention, (a) leakage magnetic field when lift-off is 3.7 μm, defect depth is 10 μm, (b) lift-off is 3.7 μm, defect Leakage magnetic field at a depth of 20 μm, (c) Leakage magnetic field at a lift-off of 9.5 μm, defect depth of 10 μm, (d) Leakage magnetic field at a lift-off of 9.5 μm, defect depth of 20 μm Explanatory drawing of two-dimensional light modulation by multilayered magnetic photonic crystal according to Example 2 of the present invention Principle diagram of magneto-optical recording according to Embodiment 3 of the present invention Structure diagram of multi-cavity magnetic photonic crystal for multilevel reproduction of reproduction signal according to Embodiment 3 of the present invention FIG. 6 is a structural diagram of a multi-cavity magnetic photonic crystal for increasing storage capacity by wavelength selectivity according to a third embodiment of the present invention.

First, a magnetic photonic crystal for three-dimensional MO imaging on the order of 10 μm will be described. The magneto-photonic crystal is composed of a plurality of magneto-optic layers, and the magneto-optic layer has a cavity structure that can individually localize light having a design wavelength. By designing the thickness of the magneto-optical layer and the multilayer dielectric film, the Faraday rotation angle and reflectance of the readout light can be controlled.

<Simulation method and simulation model>
The simulation was performed using a finite element analysis program (COMSOL Ver. 4.3). The subject was assumed to be pure iron, and its permeability was set to 4000. In order to magnetize the subject, the same pure iron magnetizer was assumed and a magnetic field was applied. The magnetizer was assumed to have a magnetic flux density of 132 T and a magnetic field generated in the air was 450 Oe. This value corresponds to a magnetic field generated when a maximum current of 4 A is passed through the magnetizer used for acquiring the MO image by actual measurement. For the defect, a triangular crack with an opening width of 1 μm was placed in the center of the subject, and the depth was used as a variable. The upper part of the object boundary and the inside of the defect were given a permeability of 1 as air. As a model used for the simulation, FIG. 1 shows the case where the depth is 10 μm and 20 μm.

<Study on simulation results and depth evaluation method>
FIG. 2A shows a vertical component of a leakage magnetic field generated from a defect having a depth of 10 μm, and FIG. 2B shows a vertical component of a leakage magnetic field generated from a defect having a depth of 20 μm. The Faraday effect increases in proportion to a magnetic field parallel to the traveling direction of light. The reason why the vertical component of the leakage magnetic field is shown is that the vertical component of the leakage magnetic field is considered to be caused by the Faraday rotation of the magneto-optical material.

  From the simulation results, it can be seen that a leakage magnetic field is generated around the defect, and the magnetic field spreads and is spatially distributed as the distance from the object surface increases. On the other hand, comparing FIG. 2 (a) and FIG. 2 (b), it has been shown that the spread of the leakage magnetic field varies depending on the defect depth. Therefore, by acquiring a two-dimensional leakage magnetic field distribution at a plurality of positions at different distances from the object, the defect depth can be predicted from the difference in the distribution. Therefore, it was thought that the defect depth could be evaluated from the image by changing the lift-off, which is the distance between the object and the magneto-optical material, and acquiring MO images at a plurality of positions. However, it can be seen from FIG. 2 that for that purpose, it is necessary to control the lift-off on the order of several μm to several tens of μm. If a piezo stage or the like is used, it is possible to control the order of μm. However, in order to apply to nondestructive inspection, faster and easier control is desired.

<Design of multi-cavity magnetic photonic crystal>
As a result of calculating the leakage magnetic field distribution around the defect by using the finite element method, it was shown that the magnetic field distribution is widened as the distance from the object surface increases, and that the magnetic film is preferably thin. Further, since the leakage magnetic field distribution changes according to the defect depth, it is considered that the defect depth can be evaluated by obtaining a plurality of spatial distributions of the leakage magnetic field. Therefore, in order to image magnetic fields with different heights of about several μm, MPC is applied to design an MPC in which five magnetic layers having different optical film thicknesses are arranged at a period of several μm in the height direction. Hereinafter, an MPC having a plurality of magneto-optical layers is referred to as a multi-cavity MPC.

  In addition, the optical properties of the multi-cavity MPC to be designed are calculated using a matrix approach. Magnetic garnet has a relatively large absorption coefficient in the visible light region with a light wavelength of about 500 nm. Therefore, when the multi-cavity MPC is designed in the visible light region, light absorption is increased by the plurality of magnetic garnet layers, and the light utilization efficiency is lowered. Therefore, a multicavity MPC was designed on the longer wavelength side than the wavelength of 1000 nm where the absorption coefficient of the magnetic garnet is small. The optical film thickness d of the magnetic garnet layer of the multi-cavity MPC is given by d = λ / (2n). In this case, λ is a light wavelength to be designed, and n is a refractive index. Note that the optical film thickness of the magnetic garnet layer increases from the above equation in proportion to the designed light wavelength.

FIG. 3 shows the structure of the designed multi-cavity MPC and the lift-off value when the multi-cavity MPC is arranged on the target surface. The substrate was SGGG, the substrate-side dielectric multilayer film was (SiO ₂ / Ta ₂ O ₅ ) 4 pairs, and the reflection-side dielectric multilayer film was (SiO ₂ / Ta ₂ O ₅ ) 8 pairs. The magnetic garnet was BiDyAl: YIG (hereinafter referred to as Bi: YIG), and five layers were inserted while shifting the optical film thickness by 50 nm from the design wavelength of 1350 nm to 1550 nm. The dielectric multilayer film between Bi: YIG was (SiO ₂ / Ta ₂ O ₅ ) 2.5 pairs. Table 1 shows the design wavelength and physical film thickness of the designed multicavity MPC.

<Characteristics of multi-cavity magnetic photonic crystal>
FIG. 4 shows the reflectivity and Faraday rotation angle of the multicavity MPC calculated using the matrix approach method. In FIG. 4, the horizontal axis represents wavelength, the left vertical axis represents reflectance, and the right vertical axis represents Faraday rotation angle. From the figure, it can be seen that in the vicinity of the design wavelength of 1350 nm to 1550 nm, an increase in the optical localization peak of the MPC and the Faraday rotation angle corresponding to the design wavelength of each Bi: YIG layer is obtained. This means that by shifting the wavelength of light incident on the MPC, images of magnetic field distributions having different heights can be obtained on the order of several μm.

For example, the distribution of the vertical component of the leakage magnetic field with a wavelength of 1350 nm and a lift-off of 3.7 μm as viewed from above is shown in FIGS. (C) and (d). Thus, if the defect depth is different, the distribution of the leakage magnetic field is also different accordingly, so that the defect depth can be evaluated.

Therefore, the multi-cavity MPC can acquire the MO image corresponding to the leakage magnetic field at the position where the lift-off from the subject is different by shifting the wavelength, and the defect depth and the like can be obtained from the difference in the spatial distribution of the leakage magnetic field. Can be evaluated.

On the other hand, since the leakage magnetic field can be detected using the multicavity MPC, it can be said that the magnetic field generated around the current can be detected by the multicavity MPC. In general, the magnitude and position of the current can be detected by the magnitude and distribution of the magnetic field. According to the present invention, by making the MPC multi-layered, it is possible to accurately grasp the position where the current flows, and to increase the accuracy of the current value. In addition, since the magneto-optical effect of the polycrystalline magnetic garnet film has a strong dependence on temperature, the physical properties are changed by changing the composition of each defect layer, and the physical properties of each defect layer, that is, the magnetic material, are changed between temperature and The relationship between optical effects can be shown. A highly accurate ammeter can be realized by detecting a magneto-optical effect corresponding to a temperature change and a magnetic field from each defective layer and performing temperature correction.

<Logical operations using light patterns>
Here, application to parallel optical computation using a multi-cavity MPC will be described. By performing threshold processing or the like, digital calculation can be performed with light.

Optical information processing using a two-dimensional image can be performed by switching light ON / OFF using a two-dimensionally arranged pixel group and performing digital processing.

<Application of optical computing for multi-layered MPC>
As a spatial light modulator for optical computing, an optical addressed multi-layer MPC is expected. FIG. 6 shows a structure diagram of the multilayer MPC. Thermomagnetic writing is performed using light on the transparent magnetic material to be the defect layer, and the pixels are two-dimensionally arranged. When light is incident on the formed magnetic pixel group, only the pixel driving unit rotates the plane of polarization due to the magneto-optic effect. Two-dimensional information can be obtained by detecting light transmitted through the MPC through a polarizer.

In addition, when the magnetic material is displayed in a plurality of layers and the same detection is performed, multi-tone information processing can be performed. As a merit of using the multilayer MPC structure,
-Drive is very fast due to magnetization reversal-Writing and signal detection can be performed only by incident light-Multiple spatial light modulators do not need to be arranged, so the optical system can be reduced and simplified For example, there is no mechanical structure, and no diffracted light that can be a noise when detecting light is generated.

<Magnetic recording disk>
The magneto-optical recording disk is a method for performing high-density magnetic recording by thermomagnetic writing using condensed light.
The magneto-optical recording material uses a magnetic thin film having perpendicular magnetization in order to form magnetic recording bits at high density. First, the magnetization direction is aligned in one direction (FIG. 7A). The magnetic thin film is heated to the Curie temperature using a pulse laser, and the magnetization is locally lost (FIG. 7B). Magnetization is reversed by applying a reversal magnetic field and an external magnetic field of the magnetic film (FIG. 7C). Light irradiation ends and the temperature is sufficiently reduced to complete recording (FIG. 7D). As a magneto-optical disk in practical use, there is MD (Mini Disc).

For reproduction, the recording film is irradiated with laser light having a lower output than that at the time of writing. The laser beam is linearly polarized light, and the plane of polarization rotates according to the recorded magnetization orientation by the magneto-optical effect when reflected or transmitted by the recording film. This light is detected as intensity information through an analyzer.

<Magnetic optical disk using multi-layer MPC>
By using a magnetic material in the MPC structure as a recording layer, it can be used as a magneto-optical disk. With the multilayer structure, it is possible to increase the recording capacity by utilizing multi-valued recording / reproducing signals and utilizing wavelength selectivity.

<Multi-level playback signal>
FIG. 8 shows a schematic diagram of multi-level reproduction of a reproduction signal using a multi-layer MPC. A signal is written by thermomagnetic recording for each layer in the MPC. For reproduction, linearly polarized light is incident in the same manner as a normal magnetic disk. At that time, light outside the wavelength of the photonic band gap is used. Polarization plane rotation due to the magneto-optic effect is determined by the direction of magnetization. Here, assuming that the magnetization in the left direction is −1 and the magnetization in the right direction is +1, the three-layer structure shown in FIG. 8 can reproduce a rotation angle of 4 gradations.

<Increase in storage capacity>
The magnetic materials in the multilayer structure are designed to be localized at different light wavelengths as shown in FIG. The increase in magneto-optical effect due to the MPC structure is because light is localized in the magnetic material. In the structure shown in FIG. 9, the structure is localized at three wavelengths of red, green, and blue. By selecting the light to be used for reproduction, only the information recorded on the specific recording layer and the synthesized information are reproduced.

Claims (9)

A plurality of magnetic layers stacked with appropriate intervals in the direction of the optical axis of the irradiated light, and a plurality of dielectric layers stacked with the magnetic layers sandwiched therebetween, the adjacent magnetic layers The multi-cavity magnetic photonic crystal characterized in that the appropriate distance is maintained by a dielectric layer laminated between the body layers. The film thicknesses of the magnetic material layer and the dielectric material layer are calculated by a matrix approach method, and each magnetic material layer has arbitrarily different localized wavelengths depending on the calculated film thickness. The described multi-cavity magnetic photonic crystal. The multi-cavity magnetic photonic crystal according to claim 1, wherein the magnetic layer is a polycrystalline magnetic garnet thin film. 4. The dielectric multilayer film in which at least the outermost layer of the dielectric layers is a dielectric multilayer film in which two or more silicon oxide films and tantalum pentoxide films are laminated. Multi-cavity magnetic photonic crystal. 5. A magneto-optical imaging apparatus using the multi-cavity magneto-photonic crystal according to claim 1, wherein the multi-cavity magneto-photonic crystal is parallel to a stacking direction of the multi-cavity magneto-photonic crystal. A light source that emits light having an optical axis, an excitation unit that applies a magnetic field perpendicular to the optical axis direction of the light from the light source to the inspection object, and light of a specific wavelength modulated by a multicavity magnetic photonic crystal is detected. And a three-dimensional display of a change in magnetization orientation due to a physical event in the vicinity of the multi-cavity magnetophotonic crystal by light of a specific wavelength detected by the detection unit. Imaging device. A leakage magnetic field generated from the physical event by application of the excitation unit is calculated in advance by a finite element method, and intervals between the plurality of magnetic layers of the multi-cavity magnetic photonic crystal are determined according to the leakage magnetic field. Item 6. The magneto-optical imaging apparatus according to Item 5. The flaw detection apparatus using the magneto-optical imaging apparatus according to claim 5 or 6, wherein the multi-cavity magnetic photonic crystal is disposed with an outermost dielectric layer facing a surface of an inspection object, The light source is irradiated with light on the stacked inspection object, and the physical event is a defect existing in the inspection object, and the inspection object is detected by light of a specific wavelength detected by the detection unit. A flaw detection apparatus characterized by three-dimensionally displaying the shape and depth of a defect existing in a wafer. 5. A magneto-optical recording medium using the multi-cavity magneto-photonic crystal according to claim 1, wherein the plurality of magnetic layers are used as magnetic recording layers and are recorded in a plurality of layers. A magneto-optical recording medium which is multi-valued by modulated light reproduced in multiple gradations depending on the magnetization direction. 5. An arithmetic element using the multi-cavity magnetophotonic crystal according to claim 1, wherein magnetic pixels are two-dimensionally arranged in the plurality of magnetic layers, and each of the magnetic layers has a magnetic pixel. An arithmetic element characterized in that multi-gradation arithmetic is processed by light that passes through.