Transport properties of Fe₃O₄ magnetite at high pressure up to 24 GPa: a search for crossovers

Abstract

The thermoelectric power (Seebeck effect) and the electrical resistivity of the cubic-spinel Fe₃O₄ magnetite were investigated under high pressures up to 24 GPa at room temperature. Comparative experiments were performed on several micro-samples cut from the same single-crystalline ingot of stoichiometric magnetite. The pressure dependencies of the thermopower, the resistivity, and a temperature difference along a sample (it is related to the thermal conductivity) demonstrated that a kink (bend) at 5–7 GPa is the only crossover that is reproduced in all the samples for all pressure cycles. In one of the samples studied, the beginning of some electronic transition was distinctly registered at 19.5–20 GPa. We discuss some hidden and probably optional electronic transition in Fe₃O₄ which occurs nearby the onset of the transformation to the high-pressure orthorhombic phase. The recently predicted two magnetic transitions in Fe₃O₄, namely, the valence transition from the inverse to the normal electron configuration and the spin transition of the Fe²⁺ ions, are analysed.

Keywords:

• magnetite
• high pressure
• thermopower
• polaron hopping conductivity
• octahedral network
• valence transition
• inverse and normal spinel

Acknowledgements

The authors are grateful to Dr A.E. Karkin (IMP) for assistance in the characterization of the samples. The work was partly supported by the RFBR, #07-08-00338. S.V.O. also thanks the JSPS and the ISSP for the financial support.