Nature of Magnetoelectric coupling in corundum antiferromagnet Co4Ta2O9

Nature of Magnetoelectric coupling in corundum antiferromagnet Co4Ta2O9 S. Chaudhary, P. Srivastava and S. Patnaik School of Physical Sciences, Jawaharlal Nehru University, New Delhi-110067, India

Abstract We study the magnetocapacitance (MC) effect and magnetoelectric (ME) coupling in spin-flop driven antiferromagnet Co4Ta2O9. The magnetocapacitance data at high magnetic fields are analyzed by phenomenological Ginzburg-landau theory of ferroelectromagnets and it is found that change in dielectric constant is proportional to the square of magnetization. The saturation polarization and magnetoelectric coupling are estimated to be 52µC/m2 and γ = 1.4 x10-3
(emu/g)-2 respectively at 6 Tesla. Electric polarization is achieved below Neel temperature only when the sample is cooled in the presence of magnetic field and it is established that the ground state is non-ferroelectric implying that magnetic lattice does not lead to spontaneous symmetry breaking in Co4Ta2O9.

Keywords: Magnetocapacitance, Multiferroic materials, Electric polarization, Magnetization.

*The Authors S. Chaudhary and P. Srivastava contributed equally to this work.

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I. Introduction The magnetic control of an electrically ordered state or electric control of a magnetically ordered state promises to usher in a plethora of advanced technologies 1-2 along with the possibility of finding new functionalities hitherto unknown. At the center of such juxtaposed phenomena is the magneto-electric (ME) effect that was discovered over a century ago by Curie.3 However, the strength of ME coupling had remained extremely weak until its prediction in antiferromagnetic Cr2O3 by Dzyaloshinskii.4 Over the years, several insulating magnetic oxides have been identified with strong ME coupling and a system of recent interest includes corundum of the general formula Z4A2O9 (with Z= Co, Fe, Mn, and A = Nb, Ta).5 In particular, two members of this family, Co4Nb2O9 and Co4Ta2O9 have attracted sustained scientific scrutiny. The identification of microscopic origin of such codependence that relates to spatial inversion and time reversal symmetry breaking is of current interest.
Experimentally, the ME coupling is estimated from the magnetic field dependence of the dielectric constant (so called Magnetocapacitance) and based on this, mutiferroic compounds are grouped into two sets. One set belongs to the case where the magnetic structure drives the onset of ferroelectric phase (e.g. TbMnO3, GdFeO3 etc) that does not require poling in the presence of external field.6-8 The second group shows linear ME effect (e.g. Cr2O3, MnTiO3, NdCrTiO5, Co3O4 etc) where presumably the spontaneous electric polarization is absent at the ground state.5,6,9 Only when an external magnetic field is applied during cooling, the electric polarization is developed, the magnitude of which increases linearly with field .10,11,12 Further, such co- dependence are widely studied under the general framework of Ginzburg–Landau theory for second-order phase transition of ferroelectromagnets.13-17 Recently, Fang et al. have reported that electric polarization can be induced in the Co4Ta2O9 only under the condition that the sample is poled in the presence of magnetic field.18 But the cause of this ME effect in Co4Ta2O9 and its ground state property has remained unknown. Moreover, magnetic spin-flop transition in Co4Ta2O9 has been indicated but the magnitude of ME effect is not estimated. In this paper, we report a detailed investigation of magnetoelectric effect in polycrystalline Co4Ta2O9 that includes (i) determination of Magneto- electric (ME) coefficient, (ii) correlation between magnetism and magneto-capacitance in Co4Ta2O9 (iii) the effect of spin flop transition on large ME coupling and (iv) establishment of non-ferroelectric ground state in Co4Ta2O9.

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II. Experiments

The polycrystalline Co4Ta2O9 samples were prepared by solid state reaction method. Stoichiometric amount of Co3O4 and Ta2O5 were used as starting material. The mixture was ground together and calcined at 1000°C for 10 hrs in air. The powder was then reground and pressed into pellets of diameter 10mm and 1mm thickness and sintered at 1100°C for another 10 hrs. Both heating and cooling rates were kept at 5 K/min. Phase purity was confirmed at room temperature by powder X-ray diffraction (Rigaku Miniflex X-ray diffractometer with Cu-Kα radiation). Lattice parameters were obtained from Reitveld refinement of XRD data using FullProf software. The DC magnetization measurements were done in vibrating sample magnetometer (VSM) mode of a Cryogenic 14 Tesla Physical Property Measurement System (PPMS). The dielectric measurements were done using an Agilent E4980A LCR meter. For pyroelectric and dielectric measurements, electrodes were prepared on the sample by painting silver paste on the planar surfaces. The pyroelectric measurement was perfo

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