Room-temperature multiferroic magnetoelectrics

A short review is given of the recent work on single-phase crystals that are ferroelectric and ferromagnetic at or near room temperature. BiFeO3 is mentioned only briefly, because it has been reviewed in detail elsewhere very recently; emphasis instead is on copper oxide, perovskite oxides with mixed B-site occupancy (such as Pb(Fe1/2Ta1/2)x(ZryTi1−y)1−xO3 and related Nb and W compounds), Fe-, Co-, and Mn-based Aurivilius-phase oxides and hexaferrites. Seven years ago, Wilma Eerenstein, Neil Mathur and Jim Scott published a Venn diagram (above) showing the overlap of piezoelectricity, ferroelectricity (green circle), ferromagnetism (black circle), and magnetoelectricity (blue hatched center circle); and soon thereafter Manuel Bibes put into each sector the crystals which were thought to belong. The overlap region between green and black are multiferroic ferromagnetic-ferroelectrics. Not all were correct; BiMnO3 is NOT ferroelectric. Of these materials, only Cr2O3 and BiFeO3 function at room temperature (or are magnetoelectric, and the former is neither a ferromagnet nor ferroelectric). Today’s review is an update on the new multiferroic and/or magnetoelectric materials that function at or near ambient temperatures and pressures: Cupric oxide (not actually magnetoelectric), iron magnesium hexaferrites, and perovskite oxides based upon PbTiO3. Multiferroics combine two properties not typically found together: ferroelectricity and ferromagnetism. With complex structures and properties, such materials are undeniably intriguing. James F. Scott reviews single-phase multiferroics that exhibit the magnetoelectric effect — whereby magnetic polarization is induced by applying an external electric field or, conversely, an electric polarization is induced through a magnetic field — at or near room temperature. In particular, four classes of materials — copper oxides, perovskite oxides, ‘Aurivillius-phase’ layered oxides and manganese hexaferrites — are discussed. Multiferroic magnetoelectrics have potential data storage applications and should ideally combine properties desirable for both magnetic and ferroelectric random-access memory. Various limitations such as low operational temperatures, small polarizations, and possibly slow switching speeds (still unmeasured for most new materials) had hindered their use, but these are likely to be overcome in future. The materials reviewed here seem very promising, advances in processing may improve the properties of multiferroic magnetoelectrics, and these compounds could also prove suitable for other memory devices.