Recent experimental measurements of large flexoelectric coefficients in ferroelectric ceramics suggest that strain gradients can affect the polarization and permittivity behaviour of inhomogeneously strained ferroelectrics. Here we present a phenomenological model of the effect of flexoelectricity on the dielectric constant, polarization, Curie temperature (T C ), temperature of maximum dielectric constant (T m ) and temperature of the onset of reversible polarization (T ferro ) for ferroelectric thin films subject to substrate-induced epitaxial strains that are allowed to relax with thickness, and the qualitative and quantitative predictions of the model are compared with experimental results for (Ba 0.5 Sr 0.5 )TiO 3 thin films on SrRuO 3 electrodes. It is shown that flexoelectricity can play an important role in decreasing the maximum dielectric constant of ferroelectric thin films under inhomogeneous in-plane strain, regardless of the sign of the strain gradient.

X-ray analysis of ferroelectric thin layers of Ba 1/2 Sr 1/2 TiO3 with different thickness reveals the presence of internal strain gradients across the film thickness and allows us to propose a functional form for the internal strain profile. We use this to calculate the direct influence of strain gradient, through flexoelectric coupling, on the degradation of the ferroelectric properties of thin films with decreasing thickness, in excellent agreement with the observed behaviour. This work highlights the link between strain relaxation and strain gradients in epitaxial films, and shows the pressing need to avoid strain gradients in order to obtain thin ferroelectrics with bulk-like properties.

Single-phase magnetoelectric multiferroics are ferroelectric materials that display some form of magnetism. In addition, magnetic and ferroelectric order parameters are not independent of one another. Thus, the application of either an electric or magnetic field simultaneously alters both the electrical dipole configuration and the magnetic state of the material. The technological possibilities that could arise from magnetoelectric multiferroics are considerable and a range of functional devices has already been envisioned. Realising these devices, however, requires coupling effects to be significant and to occur at room temperature. Although such characteristics can be created in piezoelectric-magnetostrictive composites, to date they have only been weakly evident in single-phase multiferroics. Here in a newly discovered room temperature multiferroic, we demonstrate significant room temperature coupling by monitoring changes in ferroelectric domain patterns induced by magnetic fields. An order of magnitude estimate of the effective coupling coefficient suggests a value of ~1 × 10−7 sm−1.

In an attempt to reproduce the functional properties associated with relaxor electroceramics, pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced at stacking periodicities of a few unit cells, consistent with relaxor behavior. However, enhancement in dielectric constant was generally associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency shows that fine-scale superlattices conform to Maxwell–Wagner behavior. This suggests that the observed enhancement of the real part of the dielectric constant is an artifact produced by carrier migration to interfaces within the dielectric. A comparison of this data with that already published on dielectric superlattices suggests that previous claims of an enhancement in dielectric constant may also be attributed to the Maxwell–Wagner effect.

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We present a study of the crystallography and transport properties of NdNiO 3 thin films, grown by pulsedlaser deposition, on a variety of substrates and with a range of thicknesses. Results highlight the importance of epitaxy, and show that NdNiO 3 , with a sharp metal-insulator phase transition, can be fabricated without the need for high-pressure processing. The conductivity of the nickelate films was found to be well described by a linear sum of activated transport and Mott's variable range hopping in the entire measured temperature range of the semiconducting state, and this description was also found to provide an accurate fit for previously published transport properties of bulk ceramics. The transition was subsequently modeled using a percolative approach. It was found that the temperature of the metal-insulator phase transition, in both our films and in bulk, corresponded to a critical percolation threshold where the volume fraction of the semiconducting phase (V s ) was 2 3 , as expected for a three-dimensional cubic lattice. For the thinnest films grown on NdGaO 3 , a possible crossover to two-dimensional percolation was indicated by V s ϭ 1 2 .

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields,and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of mesoscopic dipole closure patterns in free-standing, single-crystal lamellae of BaTiO3. Formation of these patterns is a dynamical process resulting from system relaxation after the BaTiO3 has been poled with a uniform electric field. The flux-closure states are composed of shape conserving 90° stripe domains which minimize disclination stresses.

Ferroelectric and ferroelastic domain walls are two-dimensional (2D) topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional properties, such as polarity in non-polar systems or electrical conductivity in otherwise insulating materials, it becomes clear that domain walls represent a new and exciting state of matter. In this review, we discuss the exotic polarisation profiles that can arise at domain walls with multiple order parameters and the different mechanisms that lead to domain wall polarity in non-polar ferroelastic materials. The emergence of energetically degenerate variants of the domain walls themselves suggests the existence of interesting quasi-1D topological defects within such walls. We also provide an overview of the general notions which have been postulated as fundamental mechanisms responsible for domain wall conduction in ferroelectrics. We then discuss the prospect of combining domain walls with transition regions observed at phase boundaries, homo-and heterointerfaces, and other quasi-2D objects, enabling emergent properties beyond those available in today's topological systems. Key points• In ferroelectrics, the emergence of a second polarisation component leads to analogues of Bloch and Néel walls. The stabilization of these walls opens the possibility for quasi-1D topological defects separating wall regions of opposite polarities.• Polar domain walls in ferroelastics rely on two mechanisms: a polarity imposed by the natural symmetry of strain-compatible domain walls, which can often be described by flexoelectric gradient coupling, and the emergence of a potentially switchable polarity when their natural symmetry is broken.• Several mechanisms are responsible for domain wall conduction in ferroelectrics: extrinsic intra-bandgap defect states, intrinsic suppression of the conduction band and intrinsic shift of the band structure induced by local electric fields.• Transition regions occurring at phase boundaries, homo-and heterointerfaces, and other quasi-2D objects probably exist at a smaller length scale, in the vicinity of domain walls, and could lead to exceptional properties and coupling phenomena.