A strong ferroelectric ferromagnet created by means of spin–lattice coupling

Abstract

Ferroelectric ferromagnets, or multiferroics, are of significant technological interest because they combine the low power and high speed of field-effect electronics with the permanence and routability of voltage-controlled ferromagnetism. Unfortunately, they are rare, and those that do exist have ferroelectric and ferromagnetic properties that are typically weak compared with conventional useful ferroelectrics and ferromagnets. A new route to fabricating multiferroics was recently predicted: in theory, magnetically ordered insulators that are neither ferroelectric nor ferromagnetic — of which there are many — can be turned into ferroelectric multiferroics by strain from the underlying substrate. June Hyuk Lee et al. now realize this route experimentally for EuTiO3. Their demonstration that a single experimental parameter, strain, can simultaneously control multiple order parameters opens up exciting possibilities for creating useful multiferroic materials. Ferroelectric ferromagnets — materials that are both ferroelectric and ferromagnetic — are of significant technological interest. But they are rare, and those that do exist have weak ferroelectric and ferromagnetic properties. Recently a new way of fabricating such materials was proposed, involving strain from the underlying substrate. This route has now been realized experimentally for EuTiO3. The work shows that a single experimental parameter, strain, can simultaneously control multiple order parameters. Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism1,2. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena1,2,3,4,5 are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics6,7,8. Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today1,2. Recently, however, a new route to ferroelectric ferromagnets was proposed9 by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO3, was predicted to exhibit strong ferromagnetism (spontaneous magnetization, ∼7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, ∼10 µC cm−2) simultaneously under large biaxial compressive strain9. These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin–lattice coupling mechanism10. Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition11 for creating multiferroics.