Growth, Structure and Properties of BiFeO3-BiCrO3 Films obtained by Dual Cross Beam PLD

The properties of epitaxial Bi2FeCrO6 thin films, recently synthesized by pulsed laser deposition, have partially confirmed the theoretical predictions (i.e. a magnetic moment of 2 muB per formula unit and a polarization of ~80 microC/cm2 at 0K). The existence of magnetic ordering at room temperature for this material is an unexpected but very promising result that needs to be further investigated. Since magnetism is assumed to arise from the exchange interaction between the Fe and Cr cations, the magnetic behaviour is strongly dependent on both their ordering and the distance between them. We present here the successful synthesis of epitaxial Bi2FexCryO6 (BFCO x/y) films grown on SrTiO3 substrates using dual crossed beam pulsed laser deposition. The crystal structure of the films has different types of (111)-oriented superstructures depending on the deposition conditions. The multiferroic character of BFCO (x/y) films is proven by the presence of both ferroelectric and magnetic hysteresis at room temperature. The oxidation state of Fe and Cr ions in the films is shown to be 3+ only and the difference in macroscopic magnetization with Fe/Cr ratio composition could only be due to ordering of the Cr3+ and Fe3+ cations therefore to the modification of the exchange interaction between them.Comment: Manuscript accepted for publication in IEEE-UFF

Infrared and magnetic characterization of the multiferroic Bi2FeCrO6 thin films in a broad temperature range

Infrared reflectance spectra of an epitaxial Bi2FeCrO6 thin film prepared by pulsed laser deposition on LaAlO3 substrate were recorded between 10 and 900 K. No evidence for a phase transition to the paraelectric phase was observed, but some phonon anomalies were revealed near 600 K. Most of the polar modes exhibit only a gradual softening, which results in a continuous increase of the static permittivity on heating. It indicates that the ferroelectric phase transition should occur somewhere above 900 K. Magnetic measurements performed up to 1000 K, revealed a possible magnetic phase transition between 600 and 800 K, but the exact critical temperature cannot be determined due to a strong diamagnetic signal from the substrate. Nevertheless, our experimental data show that the B-site ordered Bi2FeCrO6 is one of the rare high-temperature multiferroics.Comment: subm. to PR

Epitaxial Bi2FeCrO6 Multiferroic Thin Films

We present here experimental results obtained on Bi2FeCrO6 (BFCO) epitaxial films deposited by laser ablation directly on SrTiO3 substrates. It has been theoretically predicted, by Baettig and Spaldin, using first-principles density functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2 Bohr magneton per formula unit) and ferroelectric (with a polarization of ~80 microC/cm2 at 0K). The crystal structure has been investigated using X-ray diffraction which shows that the films are epitaxial with a high crystallinity and have a degree of orientation depending of the deposition conditions and that is determined by the substrate crystal structure. Chemical analysis carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy (XPS) indicates the correct cationic stoichiometry in the BFCO layer, namely (Bi:Fe:Cr = 2:1:1). XPS depth profiling revealed that the oxidation state of Fe and Cr ions in the film remains 3+ throughout the film thickness and that both Fe and Cr ions are homogeneously distributed throughout the depth. Cross-section high-resolution transmission electron microscopy images together with selected area electron diffraction confirm the crystalline quality of the epitaxial BFCO films with no identifiable foreign phase or inclusion. The multiferroic character of BFCO is proven by ferroelectric and magnetic measurements showing that the films exhibit ferroelectric and magnetic hysteresis at room temperature. In addition, local piezoelectric measurements carried out using piezoresponse force microscopy (PFM) show the presence of ferroelectric domains and their switching at the sub-micron scale.Comment: Accepted for publication in Philosophical Magazine Letter

Ferroelectricity induced by interatomic magnetic exchange interaction

Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science[1-9]. However, the coexistence of magnetism and conventional ferroelectricity is physically unfavoured[10]. Recently several remedies have been proposed, e.g., improper ferroelectricity induced by specific magnetic[6] or charge orders[2]. Guiding by these theories, currently most research is focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature, consequently far from the practical application. Simple collinear magnets, which can have high magnetic transition temperature, have never been considered seriously as the candidates for multiferroics. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic (AFM) insulator MnO, can display a magnetically induced ferroelectricity under a biaxial strain

Ordered arrays of multiferroic epitaxial nanostructures

Epitaxial heterostructures combining ferroelectric (FE) and ferromagnetic (FiM) oxides are a possible route to explore coupling mechanisms between the two independent order parameters, polarization and magnetization of the component phases. We report on the fabrication and properties of arrays of hybrid epitaxial nanostructures of FiM NiFe2O4 (NFO) and FE PbZr0.52Ti0.48O3 or PbZr0.2Ti0.8O3, with large range order and lateral dimensions from 200 nm to 1 micron