We report remarkable multiferroic effects in polycrystalline Bi 2 Fe 4 O 9 . High-resolution X-ray diffraction shows that this compound has orthorhombic structure. Magnetic measurements confirm an antiferromagnetic transition around 260 K. A pronounced inverse S-shape anomaly in the loss tangent of dielectric measurement is observed near the Néel temperature. This feature shifts with the application of magnetic field. These anomalies are indicative of substantial coupling between the electric and magnetic orders in this compound.
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Hexagonal YMnO 3 with space group P6 3 cm is one of the rare examples of multiferroic materials that exhibit co-dependence of a robust ferroelectric phase along with antiferromagnetic ordering. This is primarily ascribed to tilting of the MnO 5 polyhedra driven by magnetoelectric coupling. We report on the effect of magnetic field at the atomic level in YMnO 3 by utilizing magnetic field and temperature dependent neutron diffraction measurements. We show that near the Néel temperature the lattice parameters, effective magnetic moment per site, Mn-O bond lengths and O-Mn-O bond angles show large variations and these are further modified in the presence of magnetic field. We observe distinct changes with the application of magnetic field in the paramagnetic state both in atomic positions and in the bulk dielectric constant. Our results provide unambiguous confirmation of the role played by exchange-striction over and above the much studied magnetoelectric coupling in this frustrated antiferromagnet.
We present the structural, dielectric and magnetization study of single phase polycrystalline Bi x Co 2−x MnO 4 ͑0 ഛ x ഛ 0.3͒, synthesized by a conventional solid state route. All the samples have the cubic spinel structure with Fd3m space group. Bi-substitution in Co 2 MnO 4 stabilizes the ferroelectric transition at a temperature of ϳ350 K and enhances the dielectric constant with a relaxor behavior. The capacitance-voltage ͑C-V͒ measurements confirm the ferroelectric nature at room temperature. Ferrimagnetic nature of the Co 2 MnO 4 is preserved in the Bi-substituted samples. Magnetocapacitive coupling proves candidature of these materials from an application point of view. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2894518͔In multiferroic materials, magnetism and ferroelectricity ͑FE͒ coexist. These materials have attracted considerable attention in recent years. [1][2][3][4][5][6][7] The simultaneous occurrence of ferromagnetism/ferrimagnetism ͑FM͒ and FE and the coupling between these two order parameters could lead to the emergence of new storage media, which enable electrically reading/writing of the magnetic memories and vice versa, yielding more degrees of freedom from device application point of view. Multiferroics with coupled electrical and magnetic properties are termed as magnetoelectric multiferroics. However, there are only very few multiferroic materials with a sufficient amount of magnetoelectric coupling because of the contrasting origins of these properties. Among recently established magnetoelectric multiferroic materials, 8 frustrated magnets and geometrical frustration of lattice degrees of freedom have been found to be the leading mechanisms for perovskite manganites and cubic spinel systems, respectively. In this context, for FE and FM to coexist in single phase, the atom which moves off the center to induce the electric dipole moment should be different from those that carry the magnetic moment ͑atoms with partially filled d orbitals, responsible for FM͒. Recent ab initio calculations for existing ferroelectrics suggest that atoms with d 0 configuration create more off center distortion. 2,4 In principle, coexistence of FE and FM can be achieved through either an alternative mechanism like a non-d electron for magnetism or through an alternative mechanism for FE. In practice, alternative mechanisms for FE are pursued. 7 One such alternative followed is the induction of nonmagnetic ions having stereochemically active lone pair of electrons that may introduce off centering in the structure containing transition metal ions. 9 Also, spin-phonon coupling that may lead to dielectric anomalies has been envisaged for geometrically frustrated ZnCr 2 O 4 spinel. 10 Multiferroicity in conventional spinel oxides has been predicted 11 and studied. However, the strength of magnetoelectric coupling was found to be weak. 12 One can think of a way to engineer a new class of materials, combining the mechanisms discussed in previous paragraph to achieve multiferroicity in spinel materials. Inc...
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