Ground-state tunneling in Mn 12 -acetate

We report Hall sensor measurements of the magnetic relaxation of ${\mathrm{Mn}}_{12}$-acetate as a function of magnetic field applied along the easy axis of magnetization for a series of closely spaced temperatures between 0.24 K and 1.9 K. The occasional absence (or suppression) of ground-state tunneling under conditions where one would expect it to be readily observable is attributed to the presence of a broad distribution of tunnel splittings.