In 1845, Michael Faraday first discovered the magneto-optical effect. He discovered that when an external magnetic field is applied to a glass sample, the polarization plane of the transmitted light will rotate. Then he did experiments on light reflection on a metal surface with a magnetic field applied, but because the surface he called was not flat enough, the results of the experiment were not convincing. In 1877, John Kerr discovered the magneto-optic Kerr effect when observing the reflection of polarized light from the polished electromagnet poles. In 1985, two scholars, Moog and Bader, conducted a lot of experiments on the magneto-optical Kerr effect of iron ultra-thin film epitaxial growth on the gold single crystal (100) surface, and successfully obtained the hysteresis loop of a magnetic substance with a thickness of one atomic layer. And proposed to use SMOKE (abbreviation of surface magneto-optic Kerr effect) as the surface magneto-optic Kerr effect to represent the research of applying magneto-optic Kerr effect on surface magnetism. Because this method causes the sensitivity of magnetic analysis to reach an atomic layer thickness, and the instrument is configured to work in an ultra-high vacuum system, it has become an important research method of surface magnetism. The surface magneto-optical Kerr effect experimental system is an important method in the study of surface magnetism. It is used in the study of magnetic order, magnetic anisotropy, interlayer coupling and phase transition behavior of magnetic ultra-thin films. Both have important applications. The application of this system can automatically scan the magnetic hysteresis loop of the magnetic sample to obtain information on the coercivity and magnetic anisotropy of the thin film sample.
Magneto-optical information storage is a new technology developed in recent years and an innovation to traditional information storage technology. The development of more, more superior, and practical magneto-optical media materials is an important task in the current information storage field. Measuring the Kerr rotation angle of magneto-optical media is the basic method and method for studying these materials. For non-developers, the experiment of measuring the magneto-optical Kerr angle on the one hand can improve the ability to conduct comprehensive physical experiments, and on the other hand, they will have a deeper understanding of new information storage technologies, which can inspire them to use physical principles in information The storage technology and other fields put forward new ideas and make new contributions.
Principle of effect
When a beam of monochromatic linearly polarized light irradiates the surface of the magneto-optical medium film, part of the light will be transmitted, and the polarization plane of the transmitted light is the same as that of the incident light. The ratio has a corner, and this corner is called the magneto-optical Faraday corner (). The polarization plane of the reflected light also has a rotation angle compared with the polarization plane of the incident light. This rotation angle is called the magneto-optical Kerr rotation angle (), and this effect is called the magneto-optical Kerr effect.
The magneto-optical Kerr effect includes three situations:
(1) Longitudinal Kerr effect, that is, the magnetization is parallel to the surface of the medium and parallel to the incident surface of the light. The Kerr effect; the intensity of the Kerr signal decreases with the decrease of the incident angle, and it is 0 at normal incidence. The Kerr rotation angle and Kerr ellipticity in the longitudinal Kerr signal are both an order of magnitude smaller than that of the polar Kerr signal. Therefore, the detection of the longitudinal Kerr signal is more difficult than the extreme direction. However, for thin film samples, the easy magnetic axis is generally parallel to the surface of the sample, and the magnetization of the sample is easily saturated under the longitudinal configuration, so the longitudinal Kerr effect is quite sensitive to the in-plane magnetization.
(2) Polar Kerr effect, that is, the Kerr effect that occurs when the magnetization is perpendicular to the surface of the medium; usually, the intensity of the polar Kerr effect increases with the decrease of the incident angle. It reaches its maximum at normal incidence. And the Kerr rotation angle is the largest and most obvious.
(3) Transverse Kerr effect, which is the Kerr effect that occurs when the magnetization is parallel to the surface of the medium; the polarization state of the reflected light does not change, because the photoelectric field and the magnetization vector product under this configuration There is never a component perpendicular to the direction of light propagation. Only p-polarized light (the polarization direction is parallel to the incident surface) has a small reflectivity change (generally speaking, it only causes a jump in length and does not cause a rotation of the polarization plane).
For the magneto-optical medium that has written information, it is necessary to use the magneto-optical Kerr effect to read the written information. The specific method is: focus a beam of monochromatic polarized light and irradiate it on a certain point on the surface of the medium, and distinguish the "0" or "1" of the information by detecting the magnetization direction of the magnetic domain at that point. For example, if the illuminated point is magnetized in the forward direction, the magneto-optical Kerr rotation angle of the reflected light at this point should be as shown in the figure "The polarization plane rotates when linearly polarized light is reflected by the magneto-optical medium film." The point of is the reverse magnetization, then the magneto-optical Kerr angle of the reflected light at this point should be. Therefore, if the axis of the polarization analyzer is adjusted to make an angle with the plane perpendicular to the recording medium, the reflected light at the reverse magnetization point on the medium will not pass through the polarization analyzer, and at the forward magnetization of the medium, The reflected light can pass through the polarization analyzer. This indicates that the polarization plane of the reflected light is rotated by an angle. In this way, if we place a photoelectric detection device (such as a photomultiplier tube) behind the light reflected on the surface of the magneto-optical medium and on the optical path after passing through the polarization analyzer, we can easily identify whether the reflection point is magnetized forward or reversed. To magnetize, that is, to complete the identification of "0" and "1". It can be seen that the magneto-optical Kerr rotation angle plays a very important role in the reading of magneto-optical information. If the magneto-optical medium is attached to the surface of a rotatable disc, a magneto-optical disc is formed. When the magneto-optical disc rotates, if monochromatic polarized light is focused on the surface of the magneto-optical disc at the same time, the light can be scanned point by point, that is, the information is continuously read.
The measuring system consists of the following five parts:
(1) Optical shock absorption platform.
(2) Optical path system, including input optical path and receiving optical path. The laser uses ordinary semiconductor lasers, and both the polarizing and analyzer prisms use Glan-Thompson prisms. The photoelectric detection device is composed of a hole-shaped adjustable diaphragm, an interference color filter and a silicon photocell. The mechanical adjustment structure of the Glan-Thompson prism consists of coarse angle adjustment and spiral angle measurement. The linear displacement of the micrometer head is transformed into the angular displacement of the prism rotation. The graduation value of the micrometer head is 0.01 mm, and the index value of the turntable is 1, and the measurement accuracy of the micrometer head is about 2 through the angular displacement calibration of the linear displacement of the micrometer head.
(3) Excitation power supply host and programmable electromagnet. The excitation power supply host can choose automatic and manual scanning of the magnetic field.
(4) Combination device of pre-amplifier and DC power supply. a) Amplify the Kerr signal received by the photoelectric detection device as a pre-amplifier and send it to the signal detection host. b) The magnetic field intensity signal detected by the Hall sensor is pre-amplified and sent to the detection device. c) Provide precision regulated power supply for the laser.
(5) Signal detection host. The Kerr signal and magnetic field intensity signal from the preamplifier are amplified in two stages, and then sent to the computer for processing after A/D conversion. At the same time, the size of the Kerr signal and magnetic field intensity signal is displayed by a digital voltmeter. D/A provides a quasi-triangular wave with a period of 20 s, 40 s, and 80 s as the excitation current automatic scanning signal.
The magneto-optical Kerr method is an effective method to measure material properties, especially the physical properties of thin-film materials. The surface magneto-optical Kerr effect is an important experimental method of surface magnetism , Has been widely used in the study of magnetic order, magnetic anisotropy, interlayer coupling in multilayer films, and phase transition behavior between magnetic ultra-thin films.