We present an analytical theory on the heights of the quantum steps observed in the hysteresis loops of molecular magnets. By considering the dipolar interaction between molecular spins, our theory successfully yields the step heights measured in experiments, and reveals a scaling law for the dependence of the heights on the sweeping rates hidden in the experimental data. With this theory, we show how to accurately determine the tunnel splitting of a single molecular spin from the step heights and the sample geometry.

• Received 11 February 2002

DOI:https://doi.org/10.1103/PhysRevB.65.224401