Structural and magnetic phase transitions in Fe - I boracite,

The structural and magnetic phase transitions of Fe - I boracite have been studied by means of optical examinations of the domain structure and by measurements of the spontaneous birefringence using polarized-light microscopy. These phase transitions have also been characterized by dielectric and spontaneous polarization studies, realized on ferroelastic/ferroelectric single-domain states, which were obtained by electric field poling along and directions, with simultaneous visual control of the domain state. In the trigonal phase, an electric field of one polarity produced a monodomain with the spontaneous polarization and the optical axis perpendicular to the plane, while that of the reversed polarity gave rise to domains with the three other preferential directions of , non-collinear with E , and to a polarization of , indicating the non- -reversibility of along . The magnetic phase transition resulted in anomalies both in the dielectric permittivity (up to 2 MHz) and in the spontaneous polarization at the Curie temperature . This suggests an interaction between the magnetic moment and the electric polarization, consistent with the ferroelectric/ferroelastic/ferromagnetic coupling effect previously found for the m phase.