Ferroics

Ferromagnetism is a well-known phenomenon wherein solids exhibit spontaneous magnetization even in the absence of an external magnetic field. An important characteristic of ferromagnetic materials is the hysteresis loop found in the relationship between magnetization and magnetic field. The electrical analogue of ferromagnetic materials are ferroelectric materials1 which show a hysteresis loop in the relationship between polarization and electric field, and exhibit spontaneous polarization in the absence of an external electric field. These materials therefore possess permanent dipole moments, the dipoles arising from the absence of a centre of symmetry. A wide variety of compounds are known to exhibit ferroelectricity.7 These include oxides of perovskite structure (e.g. BaTi03), hydrogen bonded solids (e.g. Rochelle salt, KH2P04), tungsten bronze type structures, pyrochlores, simple salts (e.g. (NH4)2S04, NaN02, KN03), alums, organic compounds (e.g. thiourea, glycine sulphate) and binary compounds as simple as HCl, FeS, GeTe, and V3Si.