Spin-phonon avalanches in Mn-12 acetate

We report experimental studies and suggest a quantitative model of spin rel axation in Mn-12 acetate in a pulsed magnetic held in the temperature range 1.9-5.0 K. When the field applied along the anisotropy axis is swept at 14 0 T/s through a nonmagnetized Mn-12 acetate sample, the sample's magnetizat ion switches, within a few milliseconds, from zero to saturation at a well- defined field whose value depends on temperature but is quantized in units of 0.46 T. A quantitative explanation of the effect is given in terms of a spin-phonon avalanche combined with thermally assisted resonant spin tunnel ing. [S0163-1829(99)11529-7].