Magnetically Driven Ferroelectricity
Magnetically driven multiferroics are insulating materials, mostly oxides, in which macroscopic electric polarization is induced by magnetic long-range order. A necessary but not sufficient condition for the appearance of spontaneous electric polarization is the absence of inversion symmetry. We can distinguish between proper and improper ferroelectric (FE). The difference lie in the driving force (the primary order parameter) that lead to ferroelectricity: In the case of proper FE, the primary order parameter is the ferroelectric distortion. One example of proper FE is BaTiO3 where a covalent bonding between the transition metal and the oxygen happens to allow a polar state.
In the case of improper FE, the primary order parameter is not the ferroelectric distortion but another type of phase change, like magnetic ordering or a structural change. The FE distortion is a secondary order parameter in the sense that it is driven by the presence of other order parameters. One example of improper FE is when the inversion symmetry of a crystal is broken by magnetic structure like spiral magnetic ordering. This is the spin-driven ferroelectric. The microscopic mechanism of magnetoelectric (ME) coupling in spiral multiferroics involves spin-orbit coupling. The polarization is smaller than the one of proper FE. ME coupling is very strong because ferroelectricity is driven by magnetic order and do not exist without the latter. That means that any change in the magnetic order will have an impact on the ferroelectricity.
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