The magneto-optic Kerr effect is a change in the polarization of an electromagnetic wave upon reflection from a magnetized material. The magnetic response can change both polarization orientation and ellipticity. It differs from the Faraday effect, which is observed in transmission.
A polarizer, reflective sample, analyzer and photodetector turn a small polarization change into an intensity signal. Sweeping an applied magnetic field while recording the signal can trace changes of magnetization and switching. The probed component depends on optical and magnetic geometry, so interpreting the trace requires calibration rather than identifying arbitrary brightness with total magnetization. [ref]
https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=620499. NIST-hosted magnetic-imaging chapter: reflected Kerr versus transmitted Faraday response and geometry dependence
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The Magneto-Optic Kerr Effect (MOKE) is a phenomenon that describes the interaction between light and magnetized materials. It occurs when polarized light reflects off or transmits through a magnetized surface, leading to a change in the polarization state of the light. This effect is typically observed in ferromagnetic materials and is named after the physicist Johannes Peter Gregor Mendel (Kerr).