Temperature changes of the Fe8 molecular magnet during its spin reversal process

Tunneling of the spins in the Fe8 molecular magnet from a metastable ground state to an excited state is accompanied by a decay of these spins to the global ground state and an increase of the crystal temperature. We measured this temperature using two thermometers, one strongly coupled and the other weakly coupled to the thermal bath. We found that the temperature increases to no greater than 2.2 K. This upper limit agrees with the flame temperature derived from deflagration theory and previous measurements. In light of this temperature increase we re-examine the Landau, Zener, and Stuckelberg (LZS) theory of spin tunneling in large Fe8 crystals.