Effects of a transverse magnetic field on spin tunneling in Mn12 acetate: Evidence of internal transverse anisotropy

We present magnetization measurements for a single crystal of Mn12 acetate. The effects of a transverse magnetic field H _x on the magnetization relaxation were investigated by measuring the angle dependence of the magnetization relaxation. The effect of H _x on thermally activated relaxation of the magnetization is well explained by a classical reduction of the energy barrier. By introducing an effective transverse anisotropy E, we could also explain quantitatively the dependence on H _x of the relaxation associated with resonant spin tunneling.