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.
The large electric polarization, which appeared to be at odds with bulk single crystal measurements from 1970 @cite_97 , was originally assumed to be due to epitaxial strain, which results from the lattice constant mismatch between BFO and the substrate material SrTiO @math . It is known that epitaxial strain can have drastic effects on the properties of thin film ferroelectrics. For example, it can lead to a substantial enhancement of electric polarization and can even induce ferroelectricity at room temperature in otherwise non-ferroelectric SrTiO @math @cite_30 @cite_50 .
@cite_30 @cite_97 @cite_50
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Biaxial compressive strain has been used to markedly enhance the ferroelectric properties of BaTiO 3 thin films. This strain, imposed by coherent epitaxy, can result in a ferroelectric transition temperature nearly 500°C higher and a remanent polarization at least 250 higher than bulk BaTiO 3 single crystals. This work demonstrates a route to a lead-free ferroelectric for nonvolatile memories and electro-optic devices. Single crystals of BiFeO3 of perovskite structure were grown with dimensions of greater than 1 mm from a Bi2O3 flux. Dielectric hysteresis loops were measured on these crystals. While the loops were not fully saturated, they confirm that BiFeO3 is ferroelectric. 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.