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Author(s): Julia A. Mundy [1]; Charles M. Brooks [2]; Megan E. Holtz [1]; Jarrett A. Moyer [3]; Hena Das [1]; Alejandro F. Rbola [1]; John T. Heron [2, 4]; James [...]

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 (1). Appropriate systems such as [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability (1-3). 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--[Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases (4,5)--in which [(SrO).sub.2] crystallographic shear (6,7) planes provide an alternative to the formation of point defects for accommodating non-stoichiometry (8,9). Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability (10) in bi-axially strained [Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases with n [greater than or equal to] 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics--doping (1-3,11,12) or strain (13-16)--in this unique system an increase in the separation between the [(SrO).sub.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 (3)., Ferroelectric thin films possessing a nonlinear dielectric response to a quasi-static electric field have been widely pursued for tunable dielectric devices (17-20) that work at gigahertz frequencies. [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] is the [...]

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 (1,2). Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena (1-5) 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 multiferroics (6-8). 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 today (1,2). Recently, however, a new route to ferroelectric ferromagnets was proposed (9) 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, EuTi[O.sub.3], was predicted to exhibit strong ferromagnetism (spontaneous magnetization, ~ (7) Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, ~ 10 µC [cm.sup.-2]) simultaneously under large biaxial compressive strain (9). 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 (10). 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., Using epitaxy and the mis-fit strain imposed by an underlying substrate, it is possible to strain dielectric thin films to per cent levels--far beyond where they would crack in bulk. [...]

Electric fields offer an innovative means of controlling condensed-matter systems. The approach has been applied to nanoscale oxide interfaces, for studying the physics of two-dimensional Superconductors. ********** In recent years, [...]

This Letter determined that EuTi[O.sub.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 [...]