A distribution of internal transverse magnetic fields has been observed in single molecule magnet ${\mathrm{Mn}}_{12}-\mathrm{BrAc}$ in the pure magnetic quantum tunneling (MQT) regime. Magnetic relaxation experiments at 0.4 K are used to produce a hole in the distribution of transverse fields whose angle and depth depend on the orientation and amplitude of an applied transverse “digging field.” The presence of such transverse magnetic fields can explain the main features of resonant MQT in this material, including the tunneling rates, the form of the relaxation and the absence of tunneling selection rules. We propose a model in which the transverse fields originate from a distribution of tilts of the molecular magnetic easy axes.

• Received 14 November 2003

DOI:https://doi.org/10.1103/PhysRevB.69.020411