Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn(12)-acetate.

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn(12)-acetate. A clear fourfold variation in the resonance positions may be attributed to an intrinsic fourth-order transverse anisotropy (O(4)/(4)). Meanwhile, a fourfold variation of the EPR line shapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy [O (2)/(2) identical with E(S (2)/(x)-S(2)/(y))]. These findings explain most aspects of the magnetic quantum tunneling observed in Mn(12)-acetate.