Symmetry of Magnetic Quantum Tunneling in Single Molecule Magnet Mn12-acetate

The symmetry of magnetic quantum tunneling has been studied in the prototype single molecule magnet Mn$_{12}$-acetate using a micro-Hall effect magnetometer and superconducting high field vector magnet system. An average crystal fourfold symmetry is shown to be due to local molecular environments of twofold symmetry that are rotated by 90$^o$ with respect to one another, confirming that disorder which lowers the molecule symmetry is at important to magnetic quantum tunneling. We have studied a subset of these lower (twofold) site symmetry molecules and present evidence for a Berry phase effect consistent with a local twofold symmetry.