In perovskite-structure epitaxial films, it has been theoretically predicted that the polarization and the coherence of polar order can increase with increasing crystallographic strain. Experimental evidence of strain-induced long-range ferroelectric order has not been obtained thus far, posing the fundamental question of whether or not strain can induce the long-range polar order. Here we demonstrate the existence of strain-induced ferroelectric order in quantum paraelectric ${\mathrm{KTaO}}_3$ by combining experimental investigations of epitaxial ${\mathrm{KTaO}}_3$ films and density-functional-theory calculations. The long-range ferroelectric order does exist under a large enough epitaxial strain. We suggest that a region of short-range polar order might appear between paraelectric and ferroelectric states in the strain-temperature phase diagrams.

In scientifically intriguing and technologically important multifunctional ABO₃ perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.

Dielectric anomalies observed in heterostructures of the epitaxial thin films of $({\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3{)}_{0.68}{-(\mathrm{P}\mathrm{b}\mathrm{T}\mathrm{i}\mathrm{O}}_3{)}_{0.32},$ fabricated by in situ pulsed laser deposition on ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_3/\mathrm{M}\mathrm{g}\mathrm{O}$ (100), and with metal top electrodes, were analyzed. The contribution of the film-electrode interfaces to the properties of the heterostructures was evaluated and the true properties of the films were reconstructed. Deviation from Curie-Weiss behavior, temperature evolution of the local order parameter, the Vögel-Fulcher relationship, and temperature evolution of the relaxation time spectrum were found in the films. The relaxor ferroelectric properties of the films were essentially similar to those of the single crystal. Also, it was shown that an apparent “relaxorlike” behavior in ferroelectric thin film heterostructures, evidenced only by a broad maximum and frequency dispersion of the dielectric permittivity, can be determined by the film-electrode interface rather than by the relaxor properties of the films.

Polarization state in epitaxial thin films of relaxor ferroelectric PbMg1∕3Nb2∕3O3 (PMN) was experimentally studied at the nanoscale using a piezoresponse force microscopy (PFM). In the absence of a dc bias applied to the PFM tip, no piezoelectric activity could be found on most of the surface of the film. Under a moderate voltage (>1.5–2V), a polar state with a nonzero piezoresponse could be induced. Longer poling resulted in a significant reduction of the initial piezoresponse. After removing the bias field, a long-term relaxation of the piezoelectric signal obeying a Kohlrausch-Williams-Watt dependence was observed.

The dielectric behavior of pulsed laser deposited ferroelectric $({\mathrm{Pb}}_{\text{1−x}}{\mathrm{La}}_x){\mathrm{(Zr}}_{\mathrm{0.65}}{\mathrm{Ti}}_{\mathrm{0.35}}{\mathrm{)O}}_{\mathrm{3}}$ films (PLZT $\text{x/65/35,}$ $\text{x=0–9.75\hspace{0.17em}}\mathrm{at.\hspace{0.17em}\%}$ ) has been studied experimentally. Epitaxial stoichiometric PLZT films were formed on a pulsed laser deposited layer of ${\mathrm{La}}_{\mathrm{0.5}}{\mathrm{Sr}}_{\mathrm{0.5}}{\mathrm{CoO}}_{\mathrm{3}}$ (LSCO) on MgO (100) single-crystal substrates. The dielectric permittivity and loss tangent of the resulting heterostructures were measured in the temperature range of 20–350 °C at a frequency of 100 Hz–1 MHz. A peak around 130–350 °C was observed in the dielectric permittivity versus temperature curves. The peak exhibited a relaxor type behavior. Its position was a nonmonotonic function of the La content and depended on the microstructure of the film. The broadening of the peak of the dielectric permittivity was larger than that in the ceramic PLZT and it also depended on the La content and microstructure of the film. The broadening depended on the temperature and frequency ranges: master curves of the normalized dielectric permittivity versus normalized temperature were obtained for PLZT films. The results are discussed in terms of the random field theory for relaxor ferroelectrics and the models for finite-size ferroelectrics.

We use computational and experimental methods to study $d^0$ ferromagnetism at a charge-imbalanced interface between two perovskites. In ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ superlattice calculations, the charge imbalance introduces holes in the ${\mathrm{SrTiO}}_3$ layer, inducing a $d^0$ ferromagnetic half-metallic 2D hole gas at the interface oxygen $2p$ orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type $d^0$ perovskite interfaces. Atomically sharp epitaxial $d^0$ ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$, ${\mathrm{SrTiO}}_3/{\mathrm{KNbO}}_3$, and ${\mathrm{SrTiO}}_3/{\mathrm{NaNbO}}_3$ interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen $t_{1g}$ band in comparison with the more studied $d$ band $t_{2g}$ symmetry electron gas.