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.

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss--Srn+1TinO3n+1 phases--in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics-doping or strain-in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.

Nature 466, 954-958 (2010) This Letter determined that EuTiO 3 , when appropriately strained, becomes a strong ferroelectric ferromagnet, in agreement with prediction. Strong ferroelectrics are proper ferroelectrics, having polarization as their order parameter, with high paraelectric-to-ferroelectric transition temperatures (T c ). Such ferroelectrics are manifested by a high T c and a high peak at T c in the dielectric constant versus temperature behaviour, signifying that ferroelectricity is driven by the soft mode, which is indicative of proper ferroelectricity. Our measurements of strained EuTiO 3 demonstrate both of these characteristics (shown in Fig. 3 of our Letter), and led us to conclude that strained EuTiO 3 is a strong ferroelectric. In contrast, all well-established prior single-phase ferroelectric ferromagnets are improper or pseudoproper ferroelectrics (that is, with weak ferroelectricity resulting in minuscule P s ). We did not present P s values in our Letter. Second harmonic generation measurements do not provide quantitative values of P s and attempts to determine P s via pyroelectric measurements (Yan, L., Li, J. F. & Viehland, D., personal communication)1 resulted in unphysically high values, presumably owing to electrical leakage. Nonetheless, the magnitude of the P s of our strained EuTiO 3 films can be estimated as follows. In their classic work, Abrahams, Kurtz, and Jamieson 1 established a correlation between P s and T c for displacive ferroelectrics. By studying numerous displacive ferroelectrics they found 5) where T c is the paraelectric-to-ferroelectric transition temperature in K, Dz is the atomic displacement of the 'homopolar' metal atom in Å , and P s is the spontaneous polarization of the ferroelectric in mC cm 22 . Combining these equations to eliminate Dz allows P s to be estimated from T c in displacive ferroelectrics. The huge anomaly of the soft optical phonon near T c that we observe ( Supplementary Fig. 1 of our Letter) shows that strained EuTiO 3 is a displacive ferroelectric, making the aforementioned correlation applicable. Plugging in our measured value of T c (Fig. 3 in our Letter) yields P s 5 29 6 2 mC cm 22 for our strained EuTiO 3 films from this established correlation. This rough estimate is consistent with our first-principles theoretical predictions-P s 5 21 mC cm 22 for EuTiO 3 under 11.1% biaxial tension, corresponding to the strain of our commensurate EuTiO 3 films grown on (110) DyScO 3 . Thus, the data in our Letter shows that appropriately strained EuTiO 3 is a strong ferroelectric ferromagnet.

Optical transmission spectroscopy and spectroscopic ellipsometry were used to extract the optical properties of an epitaxially grown quasi-tetragonal BiFeO 3 thin film in the near infrared to near ultraviolet range. The absorption spectrum is overall blue shifted compared with that of rhombohedral BiFeO 3 , with an absorption onset near 2.25 eV, a direct 3.1 eV band gap, and charge transfer excitations that are ϳ0.4 eV higher than those of the rhombohedral counterpart. We interpret these results in terms of structural strain and local symmetry breaking.

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In order to investigate band gap tunability in polar oxides, we measured the optical properties of a series of Bi͑Fe 1−x Mn x ͒O 3 thin films. The absorption response of the mixed metal solid solutions is approximately a linear combination of the characteristics of the two end members, a result that demonstrates straightforward band gap tunability in this system.

Epitaxial strain can induce the formation of morphotropic phase boundary in lead free ferroelectrics like bismuth ferrite, thereby enabling the coexistence of tetragonal and rhombohedral phases in the same film. The relative ratio of these phases is governed by the film thickness and theoretical studies suggest that there exists a monoclinic distortion of both the tetragonal as well as the rhombohedral unit cells due to imposed epitaxial strain. In this work we show that optical second harmonic generation can distinguish the tetragonal-like phase from the rhombohedral-like phase and enable detection of monoclinic distortion in only a pure tetragonal-like phase.

The in-plane dielectric and ferroelectric properties of coherent anisotropically strained SrTiO 3 thin films grown on orthorhombic ͑101͒ DyScO 3 substrates were examined as a function of the angle between the applied electric field and the principal directions of the substrate. The dielectric permittivity revealed two distinct maxima as a function of temperature along the ͓100͔ p and ͓010͔ p SrTiO 3 pseudocubic directions. These data, in conjunction with optical second-harmonic generation, show that the switchable ferroelectric polarization develops first predominantly along the in-plane axis with the larger tensile strain before developing a polarization component along the perpendicular direction with smaller strain as well, leading to domain twinning at the lower temperature. Finally, weak signatures in the dielectric and second-harmonic generation response were detected at the SrTiO 3 tilt transition close to 165 K. These studies indicate that anisotropic biaxial strain can lead to new ferroelectric domain reorientation transitions that are not observed in isotropically strained films.