  A novel method to study the fundamental problem of quantum double well
potential systems that display magnetic hysteresis is proposed. The method,
coined quantum-first-order reversal curve (QFORC) analysis, is inspired by the
conventional first-order reversal curve (FORC) protocol, based on the Preisach
model for hysteretic phenomena. We successfully tested the QFORC method in the
peculiar hysteresis of the Mn12Ac molecular magnet, which is governed by
macroscopic quantum tunneling of magnetization. The QFORC approach allows one
to quickly reproduce well the experimental magnetization behavior, and more
importantly to acquire information that is difficult to infer from the usual
methods based on matrix diagonalization. It is possible to separate the thermal
activation and tunneling contributions from the magnetization variation, as
well as understand each experimentally observed jump of the magnetization curve
and associate them with specific quantum transitions.
