  We demonstrate that yttrium iron garnet microspheres support optical
whispering gallery modes similar to those in non-magnetic dielectric materials.
The direction of the ferromagnetic moment tunes both the resonant frequency via
the Voigt effect as well as the degree of polarization rotation via the Faraday
effect. An understanding of the magneto-optical coupling in whispering gallery
modes, where the propagation direction rotates with respect to the
magnetization, is fundamental to the emerging field of cavity optomagnonics.
