Pulsed-Radiation-Induced Magnetization Relaxation in Single-Molecule Magnets

Millimeter-wave radiation induces large dips in the magnetization of a single crystal of the Fe$_8$ single-molecule magnet (SMM) when the radiation is on resonance with transitions between energy levels. In our recent studies, we pulsed the radiation with the goal of determining T$_1$, the lifetime of the first excited state. We found that during a 0.2-ms pulse of intense radiation the spin system and the lattice are driven out of thermal equilibrium. Experiments at shorter time scales, carried out with the use of an inductive thin-film pick-up loop, revealed a surprisingly long relaxation time for magnetization on the order of $\sim$ 10 $\mu$s. A poor signal-to-noise (S/N) ratio required averaging of $\sim$ 4 $\times$ 10$^5$ individual traces to obtain acceptable data. Incorporating a superconducting interference device (SQUID) as a low-noise voltmeter into our experimental setup improves the S/N ratio, allowing us to explore the origin of the observed long relaxation time. The results of these experiments on Fe$_8$ as well as other SMMs will be presented.