Macroscopic Measurement of Resonant Magnetization Tunneling in High-Spin Molecules

We report the observation of steps at regular intervals of magnetic field in the hysteresis loop of a macroscopic sample of oriented M${\mathrm{n}}_{12}$ ${\mathrm{O}}_{12}$(C${\mathrm{H}}_3$COO${)}_{16}$(${\mathrm{H}}_2$O${)}_4$ crystals. The magnetic relaxation rate increases substantially when the field is tuned to a step. We propose that these effects are manifestations of thermally assisted, field-tuned resonant tunneling between quantum spin states, and attribute the observation of quantum-mechanical phenomena on a macroscopic scale to tunneling in a large (Avogadro's) number of magnetically identical molecules.