Probing metastability with viscosity isotherms

We present a general strategy for reconstructing the temperature dependence of the thermal fluctuation field and of the spectrum of metastable state free-energy excitation barriers of any magnetic material, which exhibits persistent memory from measurements of viscosity isotherms. The prescription, which is based on the hypothesis that the free energy can be decomposed into an ensemble of two-level subsystems, is applied to the analysis of an exchange-bond-disordered ferromagnetic perovskite Ca0.6Sr0.4RuO3 with a critical temperature TC∼45 K.