Ignition of magnetic deflagration in Mn$_{12}$ acetate

We study the conditions for the ignition of two types of magnetic avalanches in the molecular magnet Mn$_{12}$-acetate corresponding to the major species and a fast-relaxing minor species. The minor component, which has a lower anisotropy barrier, exists in these crystals at the level of $5-7$\%. The ignition temperatures are measured using small ($30 \times 30 \mu$m$^2$) Ge thermometers. In addition, the magnetization dynamics are measured using an array of Hall sensors of comparable size. Various aspects of the ignition will be discussed, including: the reduction of the ignition threshold due to quantum tunneling, the catalytic effect of the minor species, and the shift of the ignition point as a function of external magnetic field. The work at City College was supported by NSF grant DMR-00451605. E. Z. acknowledges the support of the Israel Ministry of Science, Culture and Sports. Support for G. C. was provided by NSF grant CHE-0414555.