Realization of random-field Ising ferromagnetism in a molecular magnet

The longitudinal magnetic susceptibility of single crystals of the molecular magnet Mn-12-acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase at similar to 0.9 K. With increasing magnetic field applied transverse to the easy axis, a marked change is observed in the temperature dependence of the susceptibility, and the suppression of ferromagnetism is considerably more rapid than predicted by mean-field theory for an ordered single crystal. Our results can instead be fit by a Hamiltonian for a random-field Ising ferromagnet in a transverse magnetic field, where the randomness derives from the intrinsic distribution of locally tilted magnetic easy axes known to exist in Mn-12-acetate crystals, suggesting that Mn12-acetate is a realization of the random-field Ising model in which the random field may be tuned by a field applied transverse to the easy axis.