Targeted chemical pressure yields tuneable millimetre-wave dielectric.

Epitaxial strain can unlock enhanced properties in oxide materials, but restricts substrate choice and maximum film thickness, above which lattice relaxation and property degradation occur. Here we employ a chemical alternative to epitaxial strain by providing targeted chemical pressure, distinct from random doping, to induce a ferroelectric instability with the strategic introduction of barium into today's best millimetre-wave tuneable dielectric, the epitaxially strained 50-nm-thick n = 6 (SrTiO3)nSrO Ruddlesden-Popper dielectric grown on (110) DyScO3. The defect mitigating nature of (SrTiO3)nSrO results in unprecedented low loss at frequencies up to 125 GHz. No barium-containing Ruddlesden-Popper titanates are known, but the resulting atomically engineered superlattice material, (SrTiO3)n-m(BaTiO3)mSrO, enables low-loss, tuneable dielectric properties to be achieved with lower epitaxial strain and a 200% improvement in the figure of merit at commercially relevant millimetre-wave frequencies. As tuneable dielectrics are key constituents of emerging millimetre-wave high-frequency devices in telecommunications, our findings could lead to higher performance adaptive and reconfigurable electronics at these frequencies.

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics.

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.

A strong ferroelectric ferromagnet created by means of spin-lattice coupling.

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.

Strain effect on magnetic-exchange-induced phonon splitting in NiO films.

NiO thin films with various strains were grown on SrTiO3 (STO) and MgO substrates using a pulsed laser deposition technique. The films were characterized using an x-ray diffraction, atomic force microscopy, and infrared reflectance spectroscopy. The films grown on STO (001) substrate show a compressive in-plane strain which increases as the film thickness is reduced resulting in an increase of the NiO phonon frequency. On the other hand, a tensile strain was detected in the NiO film grown on MgO (001) substrate which induces a softening of the phonon frequency. Overall, the variation of in-plane strain from -0.36% (compressive) to 0.48% (tensile) yields the decrease of the phonon frequency from 409.6 cm-1 to 377.5 cm-1 which occurs due to the ∼1% change of interatomic distances. The magnetic exchange-driven phonon splitting Δω in three different samples, with relaxed (i.e. zero) strain, 0.36% compressive strain and 0.48% tensile strain, was measured as a function of temperature. The Δω increases on cooling in NiO relaxed film as in the previously published work on a bulk crystal. The splitting increases on cooling also in 0.48% tensile strained film, but Δω is systematically 3-4 cm-1 smaller than in relaxed film. Since the phonon splitting is proportional to the non-dominant magnetic exchange interaction J 1, the reduction of phonon splitting in tensile-strained film was explained by a diminishing of J 1 with lattice expansion. Increase of Δω on cooling can be also explained by rising of J 1 with reduced temperature.

Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films.

MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition technique, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (0 0 1)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, in paramagnetic phase, the frequency of doubly-degenerate phonon increases with the strain, and splits below Néel temperature T N due to the magnetic-exchange interaction. The phonon splitting in the MnO (0 0 1) films is 20% larger than that of the bulk MnO. Films grown on (1 1 0)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below T N, the lower-frequency phonon splits in the IR spectra and the higher-frequency phonon strongly hardens in AFM phase; these features are evidences for a spin-order-induced structural phase transition from tetragonal to a lower symmetry phase. Total phonon splitting is 55 cm-1 in (1 1 0)-oriented MnO film, which is more than twice the value in bulk MnO, but since the splitting is present already in paramagnetic phase, we cannot clearly correlate it with the value of exchange coupling constant. Nevertheless, at least observation of enhanced phonon splitting in strained MnO (0 0 1) films show that the exchange coupling could be enhanced by the compressive strain which supports recent theoretical predictions published by Wan et al (2016 Sci. Rep. 6 22743) and Fischer et al (2009 Phys. Rev. B 80 014408).

Growth and lattice dynamics of ultrathin BaO films on Pt(0 0 1).

We report on the surface phonons of long-range ordered BaO thin films grown on Pt(0 0 1). In the thickness range between 4 and 28 ML, we find unstrained BaO(0 0 1)-([Formula: see text]) bulk-like terminated films which coincide with a Pt(0 0 1)-c([Formula: see text]) lattice periodicity. The dipole-active lattice vibrations were determined using high-resolution electron energy loss spectroscopy. For all BaO film thicknesses, a single Fuchs-Kliewer phonon-polariton is observed. Its intensity increases and its frequency softens with increasing film thickness. These thickness-dependent properties and the spectral shape are quantitatively discussed on the basis of dielectric theory, where a proper modeling requires three components: the dielectric response of the BaO film itself, the plasmonic response of the metallic substrate, and a weak damping due to a defect-induced doping within the oxide film. For a full description, also the quantization of the phonon wavevector due to the confinement within the film of finite thickness has to be taken into account.