  Magneto-optical crystals allow an efficient control of the birefringence of
light via the Cotton-Mouton and Faraday effects. These effects enable a unique
combination of adjustable linear and circular birefringence, which, in turn,
can affect the propagation of light in nonlinear Kerr media. We show
numerically that the combined birefringences can accelerate, delay, or arrest
the nonlinear collapse of (2+1)D beams, and report an experimental observation
of the acceleration of the onset of collapse in a bulk Yttrium Iron Garnet
(YIG) crystal in an external magnetic field.
