Materials that display both magnetic order and spontaneous electric polarization. Research on these materials has enabled considerable advances to be made in understanding the interplay between magnetism and ferroelectricity. The existence of both linear and higher-order coupling terms has been confirmed, and their consequences studied. They have given rise, in particular, to a number of magnetically induced polar anomalies and have even provided an example of a ferromagnet whose magnetic moment per unit volume is totally induced by its coupling via linear terms to a spontaneous electric dipole moment.
Most known ferromagnetic materials are metals or alloys. Ferroelectric materials, on the other hand, are nonmetals by definition. It therefore comes as no surprise to find that there are no known room-temperature ferromagnetic ferroelectrics. In fact, there are no well-characterized materials which are known to be both strongly ferromagnetic and ferroelectric at any temperature.
Somewhat unaccountably, antiferromagnetic ferroelectrics are also comparative rarities in nature. Nevertheless, a few are known, and among them the barium-transition-metal fluorides are virtually unique in providing a complete series of isostructural examples. They have the chemical composition BaXF4 in which X is a divalent ion of one of the 3d transition metals, manganese, iron, cobalt, or nickel. These materials are orthorhombic and all spontaneously polar (that is, pyroelectric) at room temperature. For all except the iron and manganese materials, which have a higher electrical conductivity than the others, the polarization has been reversed by the application of an electric field, so that they are correctly classified as ferroelectric. Long-range antiferromagnetic ordering sets in at temperatures somewhat below 100 K (-280°F). Structurally the materials consist of XF6 octahedra which share corners to form puckered xy sheets which are linked in the third dimension z by the barium atoms. See Crystal
The importance of these magnetic ferroelectrics is the opportunity they provide to study and to separate the effects of a variety of magnetic and nonmagnetic excitations upon the ferroelectric properties and particularly upon the spontaneous polarization. Measurements are often made via the pyroelectric effect, which is the variation of polarization with temperature. This effect is an extremely sensitive indicator of electronic and ionic charge perturbations in polar materials. Through these perturbations the effects of propagating lattice vibrations (phonons), magnetic excitations (magnons), electronic excitations (excitons), and even subtle structural transitions can all be probed with precision. See Pyroelectricity
Of all the X ions present in the series BaXF4, the largest is Mn2+. As the temperature is reduced from room temperature, the fluorine cages contract and eventually the divalent manganous ion becomes too big for its cage, precipitating a complicated structural transition at 250 K (-10°F). One interesting effect of this phase transition is that it produces a lower-temperature phase with a crystal symmetry low enough to support the existence of the linear magnetoelectric effect, a linear coupling between magnetization and polarization. Below the antiferromagnetic transition at 26 K in BaMnF4 this linear coupling produces a canting of the antiferromagnetic sublattices through a very small angle (of order 0.2 degree of arc). The result is a spontaneous, polarization-induced magnetic moment. At low temperatures BaMnF4 is therefore technically a weak ferromagnet, although the resultant magnetic moment is extremely small, and it is more usually referred to as a canted antiferromagnet. This is the only well-categorized example of pyroelectrically driven ferromagnetism. See Antiferromagnetism, Ferroelectrics, Ferromagnetism, Magnetism