Self-suppression of phase transitions exemplified by the effects of (de)magnetization of magnetite at the Curie point under high pressure

Abstract

Flux linkage between coaxial current and pick-up coils wound on a magnetite core, and measurements of the secondary voltage were used as the experimental basis for the determination of permeability. Measurements were made isobarically at pressures of up to 5.6 GPa on polycrystalline samples and single crystals with (111) axis. The Curie temperature, maximum and minimum permeability values, and the temperature coefficient of the permeability were determined as functions of the pressure. The pressure coefficient of the Curie temperature was found to vary from 4.7 to 20 K GPa −1 for polycrystalline samples and from 17 to 22 k GPa −1 for single crystals. The ferri-paramagnetic transition, although sharp, occurs over a finite temperature interval. At settings within this interval of rapidly-varying magnetization, the secondary voltage, signifying the permeability, was observed to be unstable and repeatedly self-reversing, thus resulting in oscillations of the values approaching the permeability maximum and, in reverse, the permeability minimum toward the paramagnetic state. Two explanations offer themselves for self-reversals: (1) core temperature changes resulting from changes in eddy current losses; and (2) magnetocaloric oscillations associated with spontaneous (de)magnetization. A case for magnetocaloric oscillations can be made on the grounds that spontaneous magnetization is accompanied by spontaneous strains, change in the specific heat and anomalous thermal expansion. The transition thus provides an example for a situation in which the isentropic values of relative variations of temperature with relative change in volume, which define Grüneisen's gamma, are anomalous.