We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

Dynamics of domain interfaces in a broad range of functional thin-film materials is an area of great current interest. In ferroelectric thin films, a significantly enhanced piezoelectric response should be observed if non-180 degrees domain walls were to switch under electric field excitation. However, in continuous thin films they are clamped by the substrate, and therefore their contribution to the piezoelectric response is limited. In this paper we show that when the ferroelectric layer is patterned into discrete islands using a focused ion beam, the clamping effect is significantly reduced, thereby facilitating the movement of ferroelastic walls. Piezo-response scanning force microscopy images of such islands in PbZr0.2Ti0.8O3 thin films clearly point out that the 90 degrees domain walls can move. Capacitors 1 microm2 show a doubling of the remanent polarization at voltages higher than approximately 15 V, associated with 90 degrees domain switching, coupled with a d33 piezoelectric coefficient of approximately 250 pm V-1 at remanence, which is approximately three times the predicted value of 87 pm V-1 for a single domain single crystal.

Articles you may be interested inStrain relaxation of epitaxial ( Ba 0.6 Sr 0.4 ) ( Zr 0.3 Ti 0.7 ) O 3 thin films grown on SrTiO 3 substrates by pulsed laser deposition

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A map showing regions of stability of possible domain structures and relative fractions of domains in a polytwin structure is developed for epitaxial heterostructures with active layers which undergo a cubic–tetragonal or a tetragonal–orthorhombic transformation. This map, which is also applicable to epitaxial film–substrate systems, shows the dependence of the polytwin structure on misfit strain, lattice parameters of the product phase, and mechanical stress. A uniaxial stress field applied during cooling down from the growth temperature strongly affects the domain structure selection and perfect single-domain structures may be obtained with such fields. For polytwin layers with thicknesses close to the critical thickness for domain formation, microstresses must be taken into consideration and the domain stability map is modified accordingly. Misfit dislocation generation at the deposition temperature is taken into account through a temperature dependent effective substrate lattice parameter. As examples, PbTiO3(001) films grown on MgO(001) and SrTiO3(001) substrates are analyzed. Theoretical predictions are in good agreement with experimental results from the literature. The maps for epitaxial BaTiO3(001) films grown on various substrates are also included. Experimental observations for YBa2Cu3O7−x films strongly support the ideas developed in this paper.

Magnetoelectric (ME) materials that exhibit an induction of magnetization by an electric field or an induction of electric polarization by a magnetic field offer increased functionality and entirely new applications for electronic devices.[1] Examples of potential applications include multistate memories and logic, tunable filters, sensors, and new types of electromechanical devices. The ME response exhibited by all known singlephase materials is either too weak or occurs at temperatures too low for practical exploitation. [1,2] In contrast, laminated ceramic composites, fabricated from piezoelectric and magnetostrictive components, have been found to exhibit strain-mediated ME responses an order-of-magnitude larger than those observed in single-phase materials. [3,4] Several types of devices incorporating such composites have been demonstrated. [5][6][7] Unfortunately, existing bulk ME composites are not amenable to on-chip integration, which is a prerequisite for incorporation into microelectronic devices. The principal problem in implementing strain-mediated layered ME thin-film composites is related to the strong clamping effects of the substrate, which effectively inhibit the ME response. [8][9][10] The clamping can be significantly reduced by generating transversely modulated composite films with interphase boundaries perpendicular to the substrate. Recently, transverse nanostructures have been synthesized using epitaxial self-assembly of CoFe 2 O 4 (ferrimagnetic) and BaTiO 3 (ferroelectric) phases on singlecrystal (001) SrTiO 3 substrates. [8] These films, which consisted of CoFe 2 O 4 nanorods in a BaTiO 3 matrix, were shown to exhibit substantial ME coupling (in contrast to the layered heterostructures), which was attributed to 1) a reduced clamping effect by the substrate, and 2) efficient strain coupling resulting from the nanometer scale of the component phases and coherency of the interfaces. Subsequently, similar self-assembled nanostructures were obtained in composite CoFe 2 -O 4 -PbTiO 3 [11] and CoFe 2 O 4 -BiFeO 3 films; [12] in the latter system, a polarization reversal by the external magnetic field was demonstrated. All multiferroic nanostructures reported to date have been limited to CoFe 2 O 4 pillars embedded in a ferroelectric perovskite-type matrix.In the present report, we demonstrate that the morphologies of self-assembled multiferroic nanostructures can be varied over a wide range by modifying the epitaxial stress state in the film. In particular, morphologies ranging from rodlike (either magnetic or ferroelectric) to lamellar-like were obtained by varying the substrate orientation and phase fractions in epitaxial composite films containing ferrimagnetic CoFe 2 O 4 and ferroelectric PbTiO 3 . The approach relies on the epitaxial stress-controlled self-assembling of the component phases.

A thermodynamic model is presented that describes the polarization and the dielectric response of ferroelectric-paraelectric bilayers and multilayers. It is shown that a strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical paraelectric layer thickness. The bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie–Weiss temperature in homogeneous ferroelectrics at this critical thickness. A numerical analysis is carried out for a pseudomorphic (001) BaTiO3∕SrTiO3 heteroepitaxial bilayer on (001) SrTiO3 and a stress-free BaTiO3∕SrTiO3 bilayer. Complete polarization suppression and a dielectric peak are predicted to occur at approximately 66% and 14% of SrTiO3 in these two systems, respectively.

The thermodynamic principles of the formation of a new class of materials, polydomain heterostructures, are formulated. The polydomain heterostructures can be formed as a result of phase transformations in constrained layers, composed of epitaxial couples or multilayers. Due to the elastic interaction between the layers of a heterostructure, these layers transform into sets of periodically alternating lamellae, or elastic domains. A polydomain layer can consist of either differently oriented domains of the same phase ͑twins͒ or domains of different phases. The goal of this article is to determine the parameters of the polydomain heterostructures and the conditions for their formation, i.e., their dependence on the characteristics of phase transformations, lattice misfits, and the film thickness, as well as external fields. Heterostructures containing ferroelectric and superconductor oxide layers will be considered as examples. In part I the necessary conditions for the formation of polydomain heterostructures have been established on the basis of the analysis of thermodynamic effects of internal and external macrostresses on phase transformations. The thermodynamic criteria are formulated in terms of a few energy parameters: the misfit energies of the phases that form the heterostructure and the energies of interaction between domains. Together with the free energies of the unconstrained phases, these parameters give a quantitative thermodynamic description of polydomain heterostructures provided that the interface effects are negligible. It is shown that polytwin structures are always more stable than single domain structures and therefore, their formation is always possible at some external stresses and temperatures. The stability of heterophase polydomain structures depends on the parameter of incompatibility between the domains, i.e., the ratio of the energy of direct interactions between the domains through their interface to the energy of indirect interaction between domains through the elastic surrounding matrix. If the interdomain interfaces are mobile, their movement may result in superelastic deformation and essential softening of the effective elastic modulus. ͓S0021-8979͑98͒05901-5͔