  We show an example of a purely magnetic spin resonance in EuTiO3 and the
resulting new record high Faraday rotation of 590 deg/mm at 1.6 T for 1 cm
wavelengths probed by a novel technique of magneto-optical GHz time-domain
ellipsometry. From our transmission measurements of linear polarized light we
map out the complex index of refraction in the GHz to THz range. We observe a
strong resonant absorption by magnetic dipole transitions involving the Zeeman
split S=7/2 magnetic energy levels of the Eu 2+ ions, which causes a very large
dichroism for circular polarized radiation.
