Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table

Nature of the ferroelectric phase transition in multiferroic BiFeO3 from first principles

International audienceA first-principles-based scheme is used to investigate the nature of the ferroelectric phase transition in the multiferroic BiFeO3. This transition possesses several order parameters (that are components of the polarization and of the tilting of oxygen octahedra) with none of them being primary and is solely driven by the collaborative coupling between the polarization and the other order parameters. This phase transition is therefore neither proper nor improper but rather can be regarded as a special trigger-type transition. Our atomistic simulations also reveal the precise origins and mechanisms of this trigger-type phase transition. © 2009 The American Physical Society

The nature of ferroelectricity under pressure

International audienceAdvances in first-principles computational approaches have, over the past fifteen years, made possible the investigation of physical properties of ferroelectric systems. In particular, such approaches have led to a microscopic understanding of the occurrence of ferroelectricity in perovskite oxides at ambient pressure. In this article, we report ab initio studies on the effect of hydrostatic pressure on the ferroelectricity in perovskites and related materials. We found that, unlike common belief, these materials exhibit ferroelectricity at high enough pressure. We also explain, in detail the (unusual) nature of this ferroelectricity

A simple law governing coupled magnetic orders in perovskites

An energetic expression containing four different macroscopic terms is proposed to explain and understand coupled magnetic orders (and the directions of the simultaneously occurring ferromagnetic and/or antiferromagnetic vectors) in terms of anti-phase and/or in-phase tilting of oxygen octahedra in magnetic and multiferroic perovskites. This expression is derived from a suggested simple microscopic formula, and has its roots in the Dzyaloshinsky-Moriya interaction. Comparison with data available in the literature and with first-principles calculations we conduct here confirms the validity of such a simple and general law for any tested structural paraelectric and even ferroelectric phase, and for any chosen direction of any selected primary magnetic vector. © 2012 IOP Publishing Ltd

Magnetoelectricity in BiFeO3 films First-principles-based computations and phenomenology

A first-principles-based effective Hamiltonian is used to compute linear and quadratic magnetoelectric (ME) coefficients in epitaxial (001) BiFeO 3 thin films. Its predictions are analyzed within a phenomenological model that provides analytical expressions of the ME coefficients in terms of polarization, as well as dielectric and magnetic susceptibilities. Its main discoveries are (i) the quadratic ME coefficient is dramatically enhanced by increasing the magnitude of the compressive strain within the Cc phase, similar to the previously reported enhancement of the linear ME coefficient in these films; (ii) the enhancements of the linear and quadratic ME coefficients have the same macroscopic origin, namely an increase in the dielectric permittivity; and (iii) the relative contribution of two different free-energy terms on the total linear ME coefficient is extracted from the simulations. The analytical expressions also help in understanding other ME effects. © 2011 American Physical Society

Wang-Landau Monte Carlo formalism applied to ferroelectrics

International audienceThe Wang-Landau Monte Carlo algorithm is implemented within an effective Hamiltonian approach and applied to BaTiO 3 bulk. The density of states obtained by this approach allows a highly accurate and straightforward calculation of various thermodynamic properties, including phase transition temperatures, as well as polarization, dielectric susceptibility, specific heat, and electrocaloric coefficient at any temperature. This approach yields rather smooth data even near phase transitions and provides direct access to entropy and free energy, which allow us to compute properties that are typically unaccessible by atomistic simulations. Examples of such latter properties are the nature (i.e., first order versus second order) of the phase transitions for different supercell sizes and the thermodynamic limit of the Curie temperature and latent heat

Properties of multiferroic BiFeO3 under high magnetic fields from first principles

Properties of BiFeO3 under high magnetic fields applied in the plane perpendicular to the polarization are investigated via an original first-principles-based effective Hamiltonian. Different phenomena are found, depending if the magnetic fields lie (a) along the initial direction of the antiferromagnetic vector, (b) perpendicular to it, or (c) in between these two latter directions. For instance, a spin-flop transition occurs for case (a), while a continuous transition occurs, for which both the antiferromagnetic vector and the field-induced magnetization rotate, for case (c). Such latter rotation leads to a controllable large enhancement of the magnetoelectric coefficient. © 2009 The American Physical Society

Erratum Properties of multiferroic BiFe O3 under high magnetic fields from first principles (Phys. Rev. B (2009) 79 (012101))

[No abstract available

Finite-temperature properties of multiferroic BiFeO3

International audienceAn effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i)the symmetry of the ground state, (ii)the Néel and Curie temperatures, and (iii)the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a)an overlooked phase above TC 1100K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b)improperlike dielectric features above TC, and (c)that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases. © 2007 The American Physical Society

Phase stability and structural temperature dependence in powdered multiferroic BiFeO3

International audienceWe report a temperature-dependent investigation of the multiferroic perovskite bismuth ferrite BiFeO3 (BFO) by using x-ray powder diffraction together with differential scanning calorimetry measurements. Our results provide evidence that the paraelectric phase above Tc =820°C is not cubic but distorted and can be well refined in a monoclinic P 21 /m space group. An equivalent structure can be reconstructed based on the C2/m monoclinic space group and by assuming two types of bismuth sites. The marked change of the cell volume at Tc provides evidence for the first-order nature of the R3c -to- P 21 /m transition. The high-temperature P 21 /m phase is centrosymmetric and characterized by (i) strong oxygen octahedra tilting along the b axis; (ii) the occurrence of antiferroelectric displacements of the Fe cations; and (iii) an interesting lamellar structure characterized by two different types of BiO12 cages. The temperature-induced lamellar structure suggests a significant electronic rearrangement in terms of chemical bonding, which in turn might condition anisotropic electronic properties. The occurrence of a lamellar structure provides also an understanding of why BFO decomposes suddenly at higher temperatures. Finally, an anomaly in the evolution of the cell parameters at TN underlines the spin-lattice coupling in proximity of the magnetic transition. © 2008 The American Physical Society