Peculiar magnetism of transition metal cluster compounds

We discuss a class of solids which have in their structure fairly separated tetrahederal clusters of transition metals: vanadium and molybednum. With decreasing temperature these compounds undergo a structural transition from cubic to rhombohederal phase at a temperature T-s, followed by a transition to a ferromagnetic phase at a lower temperarure T-c. Magnetic measurements indicate the presence of moments of spin half per cluster. Between T-c and T-s susceptibility shows large departures from Curie-Weiss behavior and specific heat has an unusually large non-phononic contribution. We present a model in which each cluster acts as a unit associated with a magnetic moment of spin half and a structural degree of freedom related to distortions of the tetrahedron. Our model accounts for all the observed features including effects due to external pressure and magnetic field. Copyright (C) EPLA, 2013