We report Hall sensor measurements of the magnetic relaxation of Mn12 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.
@article{arxiv.cond-mat/0012247,
title = {Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet},
author = {K. M. Mertes and Y. Zhong and M. P. Sarachik and Y. Paltiel and H. Shtrikman and E. Zeldov and E. Rumberger and D. N. Hendrickson},
journal= {arXiv preprint arXiv:cond-mat/0012247},
year = {2009}
}