A brief introduction to the magneto-optical Kerr effect
Principle: The Kerr magneto-optical effect is divided into three types: polar direction, longitudinal direction and transverse direction, respectively corresponding to the three cases where the magnetization of the material is perpendicular to the reflective surface, parallel to the surface and the incident surface, parallel to the surface and perpendicular to the incident surface. The magneto-rotation of the pole direction and longitudinal Kerr magneto-optical effect is proportional to the magnetization. Generally, the pole direction has the strongest effect, followed by the longitudinal direction, and there is no obvious magneto-rotation in the transverse direction.
Application: The most important application of the Kerr magneto-optical effect is to observe the magnetic domains of ferromagnets (see Magnetic Media, Ferromagnetism). Different magnetic domains have different spontaneous magnetization directions, which cause different rotations of the vibration plane of the reflected light. When observing the reflected light through a polarizer, different light and dark areas corresponding to each magnetic domain will be observed. This method can also be used for dynamic observation of magnetic domain changes.
Research Experiment
Purpose
The magneto-optical Kerr effect is used to measure the magnetic signal and hysteresis loop of the magnetic film, and determine the influence of the magnetic anisotropy of the magnetic film with the thickness of the film.
To study the phenomenon of exchange bias of ferromagnetic (FM)/antiferromagnetic (AFM) bilayers.
Experimental Principle
1. Surface magneto-optic Kerr effect (SMOKE)
When linearly polarized light is incident on an opaque sample surface, if the sample is anisotropic, the reflected light will become elliptically polarized light and the polarization direction will be deflected. If the sample is ferromagnetic at this time, it will also cause the reflected light to be polarized The plane rotates an additional small angle relative to the polarization plane of the incident light. This small angle is called the Kerr rotation angle θK, which is the angle between the long axis of the ellipse and the reference axis. Unlike the absorptivity of s-polarized light, the ellipsometry of the reflected light changes even if the sample is in a non-magnetic state, while ferromagnetism causes an additional change in the ellipticity, which is called the Kerr ellipsometry εK , which is the ratio of the major and minor axes of the ellipse.
According to the different configuration states of the magnetic field relative to the incident surface, the surface magneto-optical Kerr effect can be divided into three types:
a. Polar Kerr effect, its magnetization direction is perpendicular to the sample surface and parallel to the incident surface;
b. Longitudinal Kerr effect, its magnetization direction is in the sample film plane and parallel to the incident plane;
c. Transverse Kerr effect, the magnetization direction is in the sample film plane and perpendicular to the incident plane.
2. Exchange Bias
Ferromagnetic (FM)/Antiferromagnetic (AFM) systems (such as double-layer films) are cooled from above the antiferromagnetic Neel temperature to a low temperature in an external magnetic field, the hysteresis loop of the ferromagnetic layer will deviate from the origin along the direction of the magnetic field , the deviation is called the exchange bias field, usually denoted as HE, and the coercivity increases, which is called the exchange bias.
