JPH0545304A - Method and apparatus for observing magnetic domain using circularly polarized light of x ray - Google Patents

Abstract

PURPOSE:To make it possible to form the image of the inner magnetization of an optically opaque thick sample by alternately casting clockwise and counterclockwise circularly polarized lights of X rays on the noted place of the sample, and measuring the difference in absorbion coefficients caused by the right and left circualrly polarized lights. CONSTITUTION:Left and right circularly polarized X rays 6 and 7 emitted from an X-ray generating device (synchrotron radiation from a deflecting electromagnet) 1 are cast in the different directions. The X rays 6 and 7 are deflected into the parallel beams. with total- reflection pre-deflecting mirrors 8 and 9. Then, only the X rays having the specified wavelength are taken out with a crystal. spectroscope 10. The emitted lights from the spectroscope 10 pass through a chopper 11 and an emitted-light-intensity monitor 12. Then, the lights are condensed at one point on a sample 3 with rear-deflecting mirrors 13 and 14, which also serve the roles of optical condenser devices. The intensities IL and IR of the X rays transmitted through the sample 3 are measured with a detector 5, and the results are sent into a computer 16. When the intensities IL and IR are alternately measured, the difference corresponds to the inner magnetization of the sample. When this measurement is performed at each point of the sample 3 with the sample being scanned 15, the two-dimensional image of the magnetization can be measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for observing a magnetic domain or domain wall of a ferromagnetic material by imaging the internal magnetization of a sample in a microscope apparatus using an X-ray as a probe.

[0002]

2. Description of the Related Art Conventionally, as a method for detecting the magnetization direction of a ferromagnetic material to form an image, as described in Review of Scientific Instruments 61 (1990), pages 2501 to 2526. , Kerr effect optical microscope, Bitter method, Lorentz microscope, magnetic force scanning tunneling microscope and spin analysis scanning electron microscope are known.

[0003]

In the above prior art, the leakage magnetic field to the sample surface is detected (Bitter method, reflection type Lorentz microscope), and the internal magnetic field of a very thin sample (about 100 nm) is detected. They were observed (transmission Lorentz microscope), or only information near the sample surface was obtained (magnetic Kerr effect optical microscope, magnetic scanning tunneling microscope, and spin analysis scanning microscope). Therefore, it was not possible to image the internal magnetization of a relatively thick and optically opaque sample.

It is an object of the present invention to provide a technique for imaging the internal magnetization of a relatively thick and optically opaque sample (not transparent to visible light).

[0005]

In order to achieve the above object, the present invention utilizes the birefringence effect in left and right circularly polarized light near the absorption edge of short wavelength X-rays.

Regarding the birefringence of circularly polarized light in the X-ray region, see, for example, Physical Review Letters, Vol. 58 (1987).
(Year) pp. 737 to 740, a strong external magnetic field is applied to the sample, circularly polarized X-rays are made incident with the internal magnetization direction aligned, and the intensity of the sample transmitted light is reversed while reversing the external magnetic field. It is observed as a change in transmitted light intensity due to the parallel and antiparallel polarization directions when measured, and the external magnetic field.
Also, the above paper shows that this change appears strongly near the absorption edge of the sample constituent elements.

In the present invention, in order to detect the magnetization direction inside the sample by utilizing the birefringence in the vicinity of the absorption edge, a right-handed circularly polarized X-ray having a specific energy near the absorption edge is provided at a location of interest of the sample. Counterclockwise circularly polarized X-rays are alternately incident to measure the difference in absorption coefficient due to left and right circularly polarized light.

[0008]

The basic principle of the present invention will be described with reference to FIG. The X-ray 2 emitted from the X-ray generator 1 that generates right-handed circularly polarized light and left-handed circularly polarized light is incident on the sample 3. The X-ray 4 transmitted through the sample is detected by the detector 5. Here, if the transmitted X-ray intensity IR when the right circularly polarized X-rays are emitted from the X-ray device and the transmitted X-ray intensity IL when the left circularly polarized X-rays are emitted are alternately measured, the difference between them is obtained. It corresponds to the internal magnetization of the sample at the location where the X-ray enters the sample. Further, a two-dimensional image of magnetization is measured by performing this measurement at each point of the sample while scanning the sample two-dimensionally.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIG. In this embodiment, synchrotron radiation from a bending magnet is used as the X-ray source. Synchrotron radiation is linearly polarized in the orbital plane, but left and right circularly polarized light (strictly speaking, elliptical polarization) is emitted in directions vertically displaced from the orbital plane. Therefore, left circularly polarized X-ray 6 and right circularly polarized X-ray 7
Are emitted in different directions. The X-rays are first deflected into parallel beams by the total reflection deflecting mirrors 8 and 9, and then only the X-rays of a specific wavelength are extracted by the crystal spectroscope 10. In a crystal spectroscope, the wavelength of emitted light is determined by the direction of X-rays that enter the spectroscope. Therefore, in order to match the wavelength of the left circularly polarized X-ray with the wavelength of the right circularly polarized X-ray, the pre-deflecting mirrors 8 and 9 which preliminarily make the respective beams parallel to each other.
is necessary. Light emitted from the crystal spectrometer is chopper 11
After passing through the outgoing light intensity monitor 12, the light is focused again on one point on the sample 3 by the post-deflection mirror / collector mirrors 13 and 14, respectively. The chopper 11 has a function of selectively switching between right and left circular deflection.

In this embodiment, the incident angles of the left-handed circularly polarized X-rays and the right-handed circularly polarized X-rays incident on the sample are slightly different, but this is because the directions of the right-handed circularly polarized light and the left-handed circularly polarized beam emitted from the light source are slightly different from each other. This is because they are out of alignment. However, in general, this deviation is less than 1 degree in terms of angle, and practically causes no problem. The intensity of the X-ray transmitted through the sample is measured by the detector 5. The sample 3 is mounted on a biaxial sample scanning mechanism 15, and this sample scanning mechanism is controlled by a computer 16. The computer also controls the chopper 11,
The intensity signals from the emitted light intensity monitor 12 and the transmitted X-ray detector 5 are fetched. Data measurement is performed in the following procedure.

(1) The position of the chopper 11 is moved to a position where leftward circular deflection can be obtained.

(2) The signal Io of the emitted light intensity monitor 12 and the signal It of the sample transmitted light detector 5 are measured and transferred to a computer.

(3) From the computer Io and It, μL × t =
ln (Io / It) is calculated and stored in the internal memory of the computer. Where t is the thickness of the sample, an unknown quantity at this point. However, as will be described later in (7), when the magnetization information M is obtained, this t is canceled, so t may remain unknown.

(4) The position of the chopper 11 is moved to a position where rightward circular deflection can be obtained.

(5) The signal Io of the emitted light intensity monitor and the signal It of the sample transmitted light detector are measured and transferred to a computer.

(6) μR × from Io and It inside the computer
Calculate t = ln (Io / It) and store it in the internal memory of the computer.

(7) Using μR and μL, M = (μR ×
t-μL × t) / (μR × t + μL × t) is calculated and stored in the internal memory.

(8) The X-ray irradiation position is changed by using the sample scanning mechanism 15.

By repeating the above scanning steps (1)-(8) and raster-scanning the sample, the two-dimensional mapping of the magnetization information M is measured, and this M is displayed on the display 17 as a luminance signal.

The present embodiment is a case of a scanning X-ray microscope. For example, if a sufficiently wide and uniform beam can be obtained by a circularly polarized X-ray source such as a circularly polarized undulator, a one-dimensional (or two-dimensional) beam can be obtained. A sample transmitted X-ray image may be detected using a detector. Also, a method of obtaining the signal proportional to the absorption coefficient by detecting the intensity of the fluorescent X-ray emitted from the sample instead of the transmitted X-ray may be used.

In this embodiment, the synchrotron radiation is used as the X-ray source, and the left and right circularly polarized light emitted in the directions vertically displaced from the orbital plane is used. However, the circular polarization undulator having a variable polarization direction is used. A circularly polarized X-ray source may be used.

Further, in this embodiment, the difference in absorption coefficient when the polarization of the X-rays irradiated on the sample is changed is measured.
A transmission X-ray detector that has a detection efficiency that depends on the polarization direction by irradiating a sample with left and right circularly polarized light at the same time (for example, a ferromagnetic substance with magnetic circular dichroism to which a saturation magnetic field is applied from the outside). Can be realized by using a detector having a photocathode as a photocathode), and a method of measuring the absorption of left circularly polarized light and the absorption of right circularly polarized light can be used.

[0023]

According to the present invention, there is an effect that an internal magnetization of a thick opaque sample which does not transmit an electron beam or visible light can be detected to form an image.

In addition, in the present invention, (1) since X-rays hardly interact with the magnetic field, it is possible to obtain information only on the magnetization direction of the sample. (2) Unlike electron beams, X-rays penetrate deep inside the sample (usually about 10 μm). Therefore, even thick samples that do not allow electron beams to pass can be measured by the transmission method. Not only the information including the internal magnetization can be obtained. (3) In the present invention, the magnetization information M is the absorption coefficient μR for right-handed circularly polarized light and the absorption coefficient μL for left-handed circularly polarized light.
Can also be expressed in a format such as M = (μR−μL) / (μR + μL), so that the contrast due to uneven thickness of the sample can be erased and expressed as an image of only the magnetization direction and strength. ..

[Brief description of drawings]

FIG. 1 is a diagram showing a basic principle of the present invention.

FIG. 2 is a schematic diagram of an embodiment of the present invention.

[Explanation of symbols]

1 ... X-ray generator, 2 ... X-ray, 3 ... sample, 4 ... X-ray transmitted through sample, 5 ... X-ray detector transmitted through sample, 6 ... Counterclockwise circularly polarized X-ray, 7 ... Right Circularly polarized X-ray, 8 ... Left-handed circularly polarized X-ray total reflection pre-deflection mirror, 9 ... Right-handed circularly polarized X
Total reflection pre-deflecting mirror for line, 10 ... Crystal spectroscope, 11 ... Chopper, 12 ... Spectrometer output light intensity monitor, 13 ... Left-handed circularly polarized post-deflection mirror / condenser, 14 ... Right-handed circularly polarized light X-ray post-deflection mirror and focusing mirror, 15 ... Biaxial sample scanning mechanism, 16 ...
Calculator, 17 ... Display.

Claims (4)

[Claims] 1. In an X-ray microscopic method of irradiating a sample with X-rays and measuring an image by its absorption contrast, clockwise and counterclockwise circularly polarized X-rays are made incident on substantially the same place of the sample, Further, a magnetic domain observation method characterized in that magnetic circular dichroism, which is a difference in absorption between the right-handed circularly polarized light and the left-handed circularly polarized light, is obtained, and the internal magnetization of the sample is detected by this. 2. The magnetic domain observing method according to claim 1, wherein an image is formed by utilizing magnetic circular dichroism obtained over substantially the entire surface of a sample on which circularly polarized X-rays are incident. 3. An X-ray source, which deflects right-handed circularly polarized light and left-handed circularly polarized light emitted above and below the orbital plane of the X-ray source, and
A reflecting mirror that guides the sample to substantially the same place on the sample, a chopper for alternately irradiating the right-handed circularly polarized light and the left-handed circularly polarized light, and a magnetic circle of the sample using the right-handed circularly polarized light and the left-handed circularly polarized light. A device for measuring chromaticity, comprising a device for imaging the internal magnetization of a sample from the magnetic circular dichroism of the sample obtained over almost the entire surface of the sample on which the right-handed circularly polarized light and the left-handed circularly polarized light are incident. A circularly polarized X-ray microscope. 4. The circularly polarized X-ray microscope according to claim 3, further comprising a device for scanning the circularly polarized X-rays over substantially the entire surface of the sample.