Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA.
Nature. 2010 Aug 19;466(7309):954-8. doi: 10.1038/nature09331.
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 ferromagnetism. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena 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 multiferroics. 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 today. Recently, however, a new route to ferroelectric ferromagnets was proposed 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, EuTiO(3), was predicted to exhibit strong ferromagnetism (spontaneous magnetization, approximately 7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, approximately 10 microC cm(-2)) simultaneously under large biaxial compressive strain. 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 mechanism. Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics.
铁电铁磁体极为罕见，它们是具有基础科学意义的多铁性材料，有望在新的技术中得到应用，将场效应电子学的低功耗和高速与电压控制铁磁性的永久性和可扩展性结合起来。此外，同时表现出这些现象的少数几种化合物的性能与有用的铁电体或铁磁体相比微不足道：它们的自发极化或磁化强度小了 1000 倍或更多。具有磁电多铁性的化合物也是如此。由于单相多铁性材料的性能较弱，因此，当今最接近应用的是涉及应变耦合压电和磁致伸缩组件的复合和多层方法。然而，最近提出了一种制备铁电铁磁体的新途径，通过使用单一控制参数应变将既不是铁电体也不是铁磁体的磁有序绝缘体转变为铁电铁磁体。目标系统 EuTiO(3) 预计在大双轴压缩应变下同时表现出强铁磁性（自发磁化，约每个 Eu 为 7 玻尔磁子）和强铁电性（自发极化，约 10 微 C cm(-2))。这些值比任何已知的铁电铁磁体高几个数量级，与仅为铁电体或铁磁体的最佳材料相当。由于缺乏提供所需压缩的合适衬底，我们转而采用拉伸应变。在这里，我们通过实验和理论都证明了在双轴拉伸下出现多铁态，出乎意料的是，所需的应变更小，从而允许更厚的高质量晶体薄膜。这种强铁磁性铁电体的实现为这种自旋晶格耦合机制的高温表现指明了方向。我们的工作表明，单个实验参数应变同时控制多个序参量，是一种可行的替代调谐参数，可用于通过组成来创造多铁性材料。
