Abrupt Transition between Thermally-Activated Relaxation and Quantum   Tunneling in a Molecular Magnet
  We report Hall sensor measurements of the magnetic relaxation of Mn$_{12}$ acetate as a function of magnetic field applied along the easy axis of magnetization. Data taken at a series of closely-spaced temperatures between 0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order'' transition between thermally-assisted relaxation and magnetic decay via quantum tunneling. 
K. M. Mertes, Y. Zhong, M. P. Sarachik, Y. Paltiel, H. Shtrikman, E.   Zeldov, E. Rumberger and D. N. Hendrickson