Electrically enhanced magnetization in highly strained BiFeO3 films

The control of magnetism via an electric field has attracted substantial attention because of potential applications in magnetoelectronics, spintronics and high-frequency devices. In this study, we demonstrate a new approach to enhance and control the magnetization of multiferroic thin film by an electric stimulus. First, to reduce the strength of the antiferromagnetic superexchange interaction in BiFeO3, we applied strain engineering to stabilize a highly strained phase. Second, the direction of the ferroelectric polarization was controlled by an electric field to enhance the Dzyaloshinskii–Moriya interaction in the highly strained BiFeO3 phase. Because of the magnetoelectric coupling in BiFeO3, a strong correlation between the modulated ferroelectricity and enhanced magnetization was observed. The tunability of this strong correlation by an electric field provides an intriguing route to control ferromagnetism in a single-phase multiferroic

The ELBA Force Field for Coarse-Grain Modeling of Lipid Membranes

A new coarse-grain model for molecular dynamics simulation of lipid membranes is presented. Following a simple and conventional approach, lipid molecules are modeled by spherical sites, each representing a group of several atoms. In contrast to common coarse-grain methods, two original (interdependent) features are here adopted. First, the main electrostatics are modeled explicitly by charges and dipoles, which interact realistically through a relative dielectric constant of unity (). Second, water molecules are represented individually through a new parametrization of the simple Stockmayer potential for polar fluids; each water molecule is therefore described by a single spherical site embedded with a point dipole. The force field is shown to accurately reproduce the main physical properties of single-species phospholipid bilayers comprising dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE) in the liquid crystal phase, as well as distearoylphosphatidylcholine (DSPC) in the liquid crystal and gel phases. Insights are presented into fundamental properties and phenomena that can be difficult or impossible to study with alternative computational or experimental methods. For example, we investigate the internal pressure distribution, dipole potential, lipid diffusion, and spontaneous self-assembly. Simulations lasting up to 1.5 microseconds were conducted for systems of different sizes (128, 512 and 1058 lipids); this also allowed us to identify size-dependent artifacts that are expected to affect membrane simulations in general. Future extensions and applications are discussed, particularly in relation to the methodology's inherent multiscale capabilities

Effect of 90° domain walls on the low-field permittivity of PbZr(0.2)Ti(0.8)O3 thin films.

We report on the contribution of 90° ferroelastic domain walls in strain-engineered PbZr(0.2)Ti(0.8)O(3) thin films to the room-temperature permittivity. Using a combination of phenomenological Ginzburg-Landau-Devonshire polydomain thin-film models and epitaxial thin-film growth and characterization, the extrinsic or domain wall contribution to the low-field, reversible dielectric response is evaluated as a function of increasing domain wall density. Using epitaxial thin-film strain we have engineered a set of samples that possess a known quantity of 90° domain walls that act as a model system with which to probe the contribution from these ferroelastic domain walls. We observe a strong enhancement of the permittivity with increasing domain wall density that matches the predictions of the phenomenological models. Additionally, we report experimentally measured bounds to domain wall stiffness in such PbZr(0.2)Ti(0.8)O(3) thin films as a function of domain wall density and frequency

Stabilization of mixed-phase structures in highly strained BiFeO 3 thin films via chemical-alloying

Chemical-alloying is demonstrated to stabilize the mixed-phase structure of highly strained epitaxial BiFeO 3/LaAlO 3 (001) heterostructures. Such mixed-phase structures are essential for the large electromechanical responses (4%-5% strains under applied electric field); however, films with thickness exceeding 250 nm undergo an epitaxial breakdown to a non-epitaxial bulk-like rhombohedral-phase. Such an irreversible transformation of the mixed-phase structure limits the magnitude of the net surface displacement associated with these field-induced phase transformations. Using high-resolution x-ray diffraction reciprocal space mapping and scanning-probe-based studies, we show that chemical-alloying of BiFeO 3 thin films can stabilize these mixed-phase structures and delay the onset of epitaxial breakdown. © 2012 American Institute of Physics

Effect of 90° domain walls and thermal expansion mismatch on the pyroelectric properties of epitaxial PbZr0.2Ti0.8O3 thin films.

We have investigated the contribution of 90° domain walls and thermal expansion mismatch to pyroelectricity in PbZr(0.2)Ti(0.8)O(3) thin films. The first phenomenological models to include extrinsic and secondary contributions to pyroelectricity in polydomain films predict significant extrinsic contributions (arising from the temperature-dependent motion of domain walls) and large secondary contributions (arising from thermal expansion mismatch between the film and the substrate). Phase-sensitive pyroelectric current measurements are applied to model thin films for the first time and reveal a dramatic increase in the pyroelectric coefficient with increasing fraction of in-plane oriented domains and thermal expansion mismatch

Effect of 90° domain walls and thermal expansion mismatch on the pyroelectric properties of epitaxial PbZr0.2Ti0.8O3 thin films.

We have investigated the contribution of 90° domain walls and thermal expansion mismatch to pyroelectricity in PbZr(0.2)Ti(0.8)O(3) thin films. The first phenomenological models to include extrinsic and secondary contributions to pyroelectricity in polydomain films predict significant extrinsic contributions (arising from the temperature-dependent motion of domain walls) and large secondary contributions (arising from thermal expansion mismatch between the film and the substrate). Phase-sensitive pyroelectric current measurements are applied to model thin films for the first time and reveal a dramatic increase in the pyroelectric coefficient with increasing fraction of in-plane oriented domains and thermal expansion mismatch

Improved pyroelectric figures of merit in compositionally graded PbZr1-xTixO3 thin films.

Pyroelectric materials have been widely used for a range of thermal-related applications including thermal imaging/sensing, waste heat energy conversion, and electron emission. In general, the figures of merit for applications of pyroelectric materials are proportional to the pyroelectric coefficient and inversely proportional to the dielectric permittivity. In this context, we explore single-layer and compositionally graded PbZr1-xTixO3 thin-film heterostructures as a way to independently engineer the pyroelectric coefficient and dielectric permittivity of materials and increase overall performance. Compositional gradients in thin films are found to produce large strain gradients which generate large built-in potentials in the films that can reduce the permittivity while maintaining large pyroelectric response. Routes to enhance the figures of merit of pyroelectric materials by 3-12 times are reported, and comparisons to standard materials are made