Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite

Abstract

We analyze the coupling between the ferroelectric and magnetic order parameters in the magnetoelectric multiferroic BiFeO 3 using density functional theory within the local spin density approximation ͑LSDA͒ and the LSDA+ U method. We show that weak ferromagnetism of the Dzyaloshinskii-Moriya type occurs in this material, and we analyze the coupling between the resulting magnetization and the structural distortions. We explore the possibility of electric-field-induced magnetization reversal and show that, although it is unlikely to be realized in BiFeO 3 , it is not in general impossible. Finally, we outline the conditions that must be fulfilled to achieve switching of the magnetization using an electric field. There has been increasing recent interest in magnetoelectric multiferroics, 1-5 which are materials that show spontaneous magnetic and electric ordering in the same phase. In addition to the fascinating physics resulting from the independent existence of two or more ferroic order parameters in one material, 6 the coupling between magnetic and electric degrees of freedom gives rise to additional phenomena. The linear and quadratic magnetoelectric ͑ME͒ effects, in which a magnetization linear or quadratic in the applied field strength is induced by an electric field ͑or an electric polarization is induced by a magnetic field͒, are already well established. 5 Recently, more complex coupling scenarios have been investigated. Examples are the coupling of the antiferromagnetic and ferroelectric domains in hexagonal YMnO 3 , 1 or the large magnetocapacitance near the ferromagnetic Curie temperature in ferroelectric BiMnO 3 . 3 Especially interesting are scenarios where the direction of the magnetization or electric polarization can be modified by an electric or magnetic field, respectively. Such a coupling would open up entirely new possibilities in data storage technologies, such as ferroelectric memory elements that could be read out nondestructively via the accompanying magnetization. Some progress has been made in this direction. Recently, the small ͑0.08 C/cm 2 ͒ electric polarization in perovskite TbMnO 3 was rotated by 90°using a magnetic field at low temperatures ͑ϳ10-20 K͒. 4 Conversely, early work on nickel-iodine boracite 7 showed that, below ϳ60 K, reversal of the spontaneous electric polarization rotates the magnetization by 90°, indicating that the axis of the magnetization, but not its sense, can be controlled by an electric field. In fact, it was believed 8,9 that electric-field-induced 180°switching of the magnetization should be impossible, because a reversal of the magnetization corresponds to the operation of time inversion, whereas the electric field is invariant under this operation. In this work we show that such behavior is not generally impossible by using multiferroic bismuth ferrite, BiFeO 3 , as a test case to analyze the coupling between magnetism and ferroelectricity.BiFeO 3 has long been known to be, in its bulk form, an antiferromagnetic, ferroelectric multiferroic, 10,11 with anti...

Structural and magnetic properties of isovalently substituted multiferroic BiFeO3: Insights from Raman spectroscopy

“…10 In the absence of cycloidal spin order, as is achieved both in BiFeO 3 thin films 1 and isovalently substituted BiFe 1−x Mn x O 3 , 7 linear magnetoelectric effects in the noncentrosymmetric phases of BiFeO 3 related ceramics become symmetry allowed and activate by a coupling between the A-site ferroelectric translation and the B-site magnetization via the antiferrodistortive a − a − a − tilt of the oxygen octahedra. 11 Although the linear magnetoelectric effect is symmetry forbidden on a macroscopic scale, the DzyaloshinskiiMoriya mechanism can still be used locally in bulk BiFeO 3 to achieve an electric switching of the spin cycloid. 12 Upon isovalent substitution on the A site, disorder is introduced which eventually leads to a decoherence of the Bi lone pair and a concurrent change in space group from the polar R3c to a nonpolar space group.…”

Strain Control of Oxygen Vacancies in Epitaxial Strontium Cobaltite Films

“…[15][16][17][18][19] Recent computational studies have investigated the effects of epitaxial strain on oxygen vacancies in conventional perovskites. While they predict that the activation energies of these vacancies can be tuned by tenths of an eV under modest strains of a few 3 percent, the initial energies of >2 eV render these changes relatively insignificant at reduced temperatures.…”

Role of strain and nanoscale defects in modifying the multiferroicity in nanostructured BiFeO3films

“…[20,30] Also, it has been theoretically proved that the epitaxial strain modifies the spiral spin structure resulting into the canting of spins and weak ferromagnetism in the multiferroic BFO compounds. [31] In the presently studied films, it is observed that SHI irradiation results in the reduction in structural strain thereby leading to decrease in magnetisation. Also, SHI-induced increase in the amorphised nanoscale defect regions in the film results in the increase in inhomogeneous spin structure and oxygen non-stoichiometry leading to the suppression in magnetisation.…”