Experimental upper bound on superradiance emission from Mn 12 acetate

We used a Josephson junction as a radiation detector to look for evidence of the emission of electromagnetic radiation during magnetization avalanches in a crystal assembly of ${\mathrm{Mn}}_{12}$ acetate. The crystal assembly exhibits avalanches at several magnetic fields in the temperature range from $1.8\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}2.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with durations of the order of $1\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$. Although a recent study shows evidence of electromagnetic radiation bursts during these avalanches [J. Tejada et al., Appl. Phys. Lett. 84, 2373 (2004)], we were unable to detect any significant radiation at well-defined frequencies. A control experiment with external radiation pulses allows us to determine that the energy released as radiation during an avalanche is less than one part in ${10}^{4}$ of the total energy released. In addition, our avalanche data indicates that the magnetization reversal process does not occur uniformly throughout the sample.