Multiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: (a) multiferroics where ferroelectricity is driven by hybridization or purely structural effects, with BiFeO3 as the prototype material, and (b) multiferroics where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin-, charge-, or orbital-ordering, with rare-earth manganites as prototypes. As for the first class of multiferroics, first principles calculations are shown to provide an accurate qualitative and quantitative description of the physics in BiFeO3, ranging from the prediction of large ferroelectric polarization and weak ferromagnetism, over the effect of epitaxial strain, to the identification of possible scenarios for coupling between ferroelectric and magnetic order. For the second class of multiferroics, ab initio calculations have shown that, in those cases where spin-ordering breaks inversion symmetry (e.g. in antiferromagnetic E-type HoMnO3), the magnetically induced ferroelectric polarization can be as large as a few µC cm−2. The examples presented point the way to several possible avenues for future research: on the technological side, first principles simulations can contribute to a rational materials design, aimed at identifying spintronic materials that exhibit ferromagnetism and ferroelectricity at or above room temperature. On the fundamental side, ab initio approaches can be used to explore new mechanisms for ferroelectricity by exploiting electronic correlations that are at play in transition met al oxides, and by suggesting ways to maximize the strength of these effects as well as the corresponding ordering temperatures.
This has changed drastically, following a publication in Science in 2003 (Ref. @cite_58 ), which to great extent has triggered the intensive experimental and theoretical computational research on BFO during the last 5--6 years. In this study, a large spontaneous electric polarization in combination with a substantial magnetization was observed above room temperature in thin films of BFO grown epitaxially on SrTiO @math substrates. The presence of both magnetism and ferroelectricity above room temperature, together with potential coupling between the two order parameters, makes BFO the prime candidate for device applications based on multiferroic materials.
@cite_58
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Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO 3 , is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters. Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (Tc) is traditionally accomplished by chemical substitution—as in BaxSr1-xTiO3, the material widely investigated for microwave devices in which the dielectric constant (er) at GHz frequencies is tuned by applying a quasi-static electric field1,2. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees3, which is detrimental to tunability and microwave device performance. An alternative way to adjust Tc in ferroelectric films is strain4,5,6,7,8. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in Tc is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high er at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications1,2. The dependencies on strain and oxygen vacancies of the ferroelectric polarization and the weak ferromagnetic magnetization in the multiferroic material bismuth ferrite, @math , are investigated using first principles density functional theory calculations. The electric polarization is found to be rather independent of strain, in striking contrast to most conventional perovskite ferroelectrics. It is also not significantly affected by oxygen vacancies, or by the combined presence of strain and oxygen vacancies. The magnetization is also unaffected by strain, however, the incorporation of oxygen vacancies can alter the magnetization slightly, and also leads to the formation of @math . These results are discussed in light of recent experiments on epitaxial films of @math , which reported a strong thickness dependence of both magnetization and polarization. We investigate the variation of the spontaneous ferroelectric polarization with epitaxial strain for BaTiO @math , PbTiO @math , LiNbO @math , and BiFeO @math using first principles calculations. We find that while the strain dependence of the polarization is very strong in the simple perovskite systems BaTiO @math and PbTiO @math it is only weak in LiNbO @math and BiFeO @math . We show that this different behavior can be understood purely in terms of the piezoelectric and elastic constants of the unstrained bulk material, and we discuss several factors that determine the strain behavior of a certain material.