Temperature changes of the ${\mathbf{Fe}}_8$ molecular magnet during its spin reversal process

Tunneling of the spins in the ${\mathrm{Fe}}_8$ 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 ${\mathrm{Fe}}_8$ crystals.

Figure 1

The experimental setup which is mounted on a cold finger and connected to the ${}^3\mathrm{He}$ refrigerator. This setup is in the center of a magnet with the field pointing along the crystal $z$ direction.

Figure 2

Temperature versus magnetic field for sample 1, using a four-point probe measurement to determine the resistivity of the thermistor. Each panel shows results for a different magnetic field sweep rate ${\alpha }_H$.

Figure 3

Temperature versus magnetic field for sample 2, using a 2-point probe measurement to determine the resistivity of the thermistor. Each panel shows results for a different magnetic field sweep rate ${\alpha }_H$.

Figure 4

The magnetic field sweep rate plotted as a function of the first magnetization jump $n=1$ of ${\mathrm{Fe}}_8$. The solid line is a fit to Eq. (7) derived in the text. The inset is raw hysteresis loop data taken from Ref. [6] from which the magnetization jumps are derived.