The thermodynamic effect of nonhydrostatic stress on the Verwey transition

Abstract

Experimental results on the change in the Verwey transition temperature in magnetite under compression exhibit puzzling variability, with slopes ranging from −6 to +16K/GPa. Our thermodynamic analysis of the Verwey transition in magnetite explains much of this variability in terms of the transformation strain as magnetite changes in crystal structure from cubic to monoclinic. Because this strain involves a much larger change in shape than in volume, the change in temperature of the phase boundary (Tv) can be much more sensitive to nonhydrostatic stress than to hydrostatic pressure. Uniaxial compression and tension both are predicted to increase Tv, an effect that is opposite in sign and, for favorable orientations, up to six times greater per gigapascal than for pressure. Moreover, because the monoclinic twin orientation with highest Tv is thermodynamically favored, our treatment also shows how any desired twin may be selected uniquely by applying stress of appropriate orientation, thereby removing obstacles to understanding many kinds of low-temperature phenomena in magnetite caused by the presence of multiple twins. A similar analysis for the thermodynamic effects of strong magnetic fields is outlined, predicting that Tv will generally be lowered. Again, the twin for which Tv is highest (i.e., is lowered the least) will be the most stable. Twin selectivity per gigapascal of stress is up to two to three times stronger than that per Tesla of magnetic field.