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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We report the near-edge x-ray absorption spectroscopy (NEXAFS) at the Co/Mn L(3,2) edge and oxygen K edge of the well-characterized Bi-substituted Co(2)MnO(4) multiferroic samples. The evolution of peak features in NEXAFS spectra of the Co/Mn L(3,2) edge and O K edge show the Bi-induced redistribution of magnetic cations (Co/Mn). The variation in valence states of Co and Mn in all the substituted compositions is consistent with the observed ferrimagnetic behaviour of the samples. Magnetization data show the decrease in molecular field complementing the ferrimagnetism. The role of Bi in the enhancement of magnetic interactions as well as the appearance of ferroelectricity in Bi(x)Co(2-x)MnO(4) (0≤x≤0.3) is discussed.

Micro crystalline materials of BiCoO 3 and Ni 0.5 Bi 0.5 CoO 3 have been prepared by solid state reaction technique. XRD studies of these polycrystalline materials confirmed the cubic structure with 197 I 23 space group. The substitution of nickel in place of bismuth resulted in lattice contraction. The thermoelectric properties were investigated in the temperature ranging from 300˚C to 700˚C. The samples showed positive Seebeck coefficient. Nickel substitution with Bismuth is found to decrease the Seebeck coefficient and thermal conductivity but increase the electrical conductivity. The figure of merit (ZT) of the material was enhanced on nickel substitution. The ZT values increased with the increase of temperature which enables its utility in high temperature thermoelectric applications.

We report the structural and transport properties of the Bi-substituted Co 2 MnO 4 multiferroic materials. Samples synthesized using the solid state reaction route with the composition Bi x Co 2−x MnO 4 ͑Ͻ0x Ͻ 0.3͒ exhibit a single phase behavior with a cubic spinel structure ͑space group Fd3m͒. The lattice parameter was found to increase with the Bi substitution. The dc-conductivity studies reveal that all the samples possess a semiconducting behavior. The resistivity was found to decrease with the increase in the Bi substitution. The dc-as well as ac-conductivity data were analyzed in the light of various conduction models. The dc-conductivity data are explained using the variable range hopping model. The ac conductivity calculated from the dielectric data as a function of temperature and frequency demonstrates the cross over from small polaron tunneling to correlated barrier hopping type conduction in these materials.