  The symmetry of magnetic quantum tunneling (MQT) in the single molecule
magnet Mn12-acetate has been determined by sensitive low-temperature magnetic
measurements in the pure quantum tunneling regime and high frequency EPR
spectroscopy in the presence of large transverse magnetic fields. The combined
data set definitely establishes the transverse anisotropy terms responsible for
the low temperature quantum dynamics. MQT is due to a disorder induced locally
varying quadratic transverse anisotropy associated with rhombic distortions in
the molecular environment (2nd order in the spin-operators). This is
superimposed on a 4th order transverse magnetic anisotropy consistent with the
global (average) S4 molecule site symmetry. The hard axes associated with these
forms of the transverse anisotropy are not collinear, leading to a complex
interplay between local and global symmetries, the consequences of which are
analyzed in detail.
