  The oscillatory interlayer exchange interaction between two magnetic layers
separated by a metallic spacer is one of the few coherent quantum phenomena
that persist at room temperature. Here we show that this interaction can be
controlled dynamically by illuminating the sample (e.g. a spin valve) with
radiation in the 10-100 THz range. We predict that the exchange interaction can
be changed from ferromagnetic to anti-ferromagnetic (and vice versa) by tuning
the amplitude and/or the frequency of the radiation. Our chief theoretical
result is an expression that relates the dynamical exchange interaction to the
static one that has already been extensively measured.
