  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 due to dipolar interactions. With increasing magnetic field
applied transverse to the easy axis, the transition temperature decreases
considerably more rapidly than predicted by mean field theory to a T=0 quantum
critical point. Our results are consistent with an effective Hamiltonian for a
random-field Ising ferromagnet in a transverse field, where the randomness is
induced by an external field applied to Mn$_{12}$-acetate crystals that are
known to have an intrinsic distribution of locally tilted magnetic easy axes.
