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 the interaction. Non-adiabatic spin flips involving multispin processes which compensate the large Zeeman splitting energy are considered within the frame of a one dimensional X-Y model. An extended scheme of non-resonant phononless flip-flop transitions is applied to spin relaxation inn Crystal consisting of magnetic molecules Mn₁₂O₁₂ with acetate ions. It is shown that unusual features in the magnetic field dependence of the relaxation rate can be accounted for by a crude generalization of the X-Y model to three dimensions.