  A magnetization-induced rotation in the third-order nonlinear optical
response is observed in out-of-plane-magnetized epitaxial EuO films. We discuss
the relation of this nonlinear magneto-optical rotation to the linear Faraday
rotation. It is allowed in all materials but, in contrast to the linear Faraday
rotation, not affected by the reduction of the thickness of the material. Thus,
the third-order magneto-optical rotation is particularly suitable for probing
the magnetization of functional magnetic materials such as ultra-thin films and
multilayers.
