Magneto-optic Kerr effect

In physics the magneto-optic Kerr effect (MOKE) or the surface magneto-optic Kerr effect (SMOKE) is one of the magneto-optic effects. It describes the changes to light reflected from a magnetized surface. It is used in materials science research in devices such as the Kerr microscope, to investigate the magnetization structure of materials.

Light that is reflected from a magnetized surface can change in both polarization and reflected intensity. The effect is similar to the Faraday effect: the Faraday effect describes changes to light transmitted through a magnetic material, while the Kerr effect describes changes to light reflected from a magnetic surface. Both effects result from the off-diagonal components of the dielectric tensor ${\displaystyle \varepsilon }$. These off-diagonal components give the magneto-optic material an anisotropic permittivity, meaning that its permittivity is different in different directions. The permittivity affects the speed of light in a material:

${\displaystyle v_{p}={\frac {1}{\sqrt {\varepsilon \mu }}}}$

...
Wikipedia