Typical spontaneous symmetry breakdown in solid state as revealed by Fe3O4

Abstract

Several of our experimental results, as seen from the side of the symmetry breakdown of this most characteristic material, will be shown. X‐ray study reveals that the lattice constant decreases approximately by 7×10⁻⁶ per degree from room temperature to the Verwey temperature T_v. At T_v, the cell volume increases by about 6×10⁻⁴, suggesting the presence of covalent nature in the crystal. At T_v, the crystal transforms to monoclinic. Although there is a report of another transition at about 12°K, very careful work by X‐ray and NMR showed no discontinuous change in this temperature region in the accuracy up to 10⁻⁴. Detailed measurements of magnetoelectric effect were performed at 4.2°K and up to 34°K statically and dynamically. The induced electric potentials between the two planar electrodes formed parallel to a‐, b‐, or c‐plane were observed as a function of the direction of the magnetic field in a‐plane. Magnetic field cooling along [112], squeezing along [111] and electrically polarizing along a‐, b‐, or c‐axis were simultaneously performed. We found that the electrically polarizing at and below T_v is only effective along a‐axis. The crystal symmetry revealed is nearly monoclinic and may be triclinic. Again no appreciable discontinuous signal is observed near 12°K.