Optical domain, ferroic coupling and birefringence studies of the magnetic phase transition in fe-i boracite

The magnetic phase transition 3ml' m with the Curie temperature TC = 30K has been studied by means of optical domain analysis and birefringence measurements using polarized light microscopy. The domain structure is consistent with the species 43ml ‘Fm with 24 partially ferromagnetic /fully ferroelectric / fully ferroelastic orientation states. The weakly ferromagnetic m phase possesses a spontaneous magnetization perpendicular to the (110)cub planes (mirror m). A coupling effect between the ferromagnetic, ferroelectric and ferroelastic domains was observed and detected. The spontaneous birefringence of magnetic and possibly coupled structural origin vanishes above Tc and exhibits a second order character of transition.