Magneto-Optical Imaging in an Undergraduate Laboratory Course

Abstract

Magneto-optical imaging has become a powerful technique for the measurement of local magnetic fields. The technique consists in measuring the rotation in the light polarization plane when light travels through a sensitive garnet (Yttrium Iron Garnet, YIG). The plane rotation is a function of the local magnetic field at which the garnet is exposed. We describe an experiment for an undergraduate laboratory course in optics and electromagnetism that consists of two applications of the technique: (i) The characterization of the properties of the YIG as a function of the magnetic field generated by a Helmholtz pair of coils, and (ii) The measurement of the magnetization of previously recorded audio tapes. In the first case, the magnetic field is calculated by modeling the Helmholtz coils as two parallel circular coils of radius R separated at a distance 2R. The measured change in the light polarization plane is described as the corresponding to magneto-optical layers with in plane anisotropy. In the second application, computer generated functions (e.g. square and sawtooth) of different frequencies were directly recorded with a tape recorder. The magnetized tape patterns are observed using the same characterized YIG and a commercial polarized light microscope.