NONADIABATIC SPIN TRANSITIONS IN LARGE MAGNETIC-FIELDS

The longitudinal relaxation of interacting spins is studied under the influence of a strong external magnetic field exceeding the scale of t he interaction. Nonadiabatic spin flips involving multispin processes which compensate the large Zeeman splitting energy are considered with in the frame of a one dimensional X-Y model. An extended scheme of non -resonant phononless flip-flop transitions is applied to spin relaxati on in a crystal consisting of magnetic molecules Mn12O12 with acetate ions. It is shown that unusual features in the magnetic field dependen ce of the relaxation rate can be accounted for by a crude generalizati on of the X-Y model to three dimensions.