Magnetic, dielectric and calorimetric studies on 0.9BiFeO 3 -0.1BaTiO 3 indicate strong magnetoelectric coupling. XRD studies reveal a very remarkable change in the rhombohedral distortion angle and a significant shift in the atomic positions at the magnetic Tc due to an isostructural phase transition. The calculated polarization using Rietveld refined atomic positions scales linearly with magnetization. Our results provide the first unambiguous evidence for magnetoelectric coupling of intrinsic multiferroic origin in a BiFeO 3 based system.

Magnetization, frequency dependent dielectric, and structural studies on 0.73BiFeO 3 -0.27PbTiO 3 in the temperature range from 300 to 600 K reveal anomalies in the unit cell parameters and the intrinsic value of the dielectric constant, free from space charge contributions, at the antiferromagnetic transition temperature ͑T N ͒. Our results provide unambiguous confirmation of magnetoelectric coupling of multiferroic origin at T N and evidence for monoclinic distortion of the ferroelectric phase. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3068000͔ Bismuth based transition metal oxides ͑BiBO 3 ͒ and their solid solutions with other perovskites are known for their important multiferroic properties ͑see the recent reviews͒ [1][2][3] where the ferroelectricity arises due to the 6s lone pair chemistry of Bi +3 , while the magnetic order results from the Fe/ Mn/Cr ions occupying the B site. Of these compounds, BiFeO 3 is of special interest as its ferroelectric and magnetic transition temperatures are located well above the room temperature, raising possibilities of room temperature multiferroic devices. 1-3 Bulk BiFeO 3 shows a rhombohedrally distorted perovskite structure in the R3c space group with a ferroelectric transition temperature T C of ϳ1103 K ͑Ref. 4͒ and G-type antiferromagnetic ͑AFM͒ spin configuration with an incommensurate cycloidal spin structure 5 with a magnetic T N ϳ 643 K. 6 The long period modulated magnetic structure leads to the cancellation of net macroscopic magnetization and hence the absence of linear magnetoelectric coupling. 1 The linear magnetoelectric effect has been reported in bulk BiFeO 3 after breaking the modulated magnetic structure in the presence of high external magnetic field. 7,8 It has been suggested that chemical substitutions may also suppress the cycloidal spin structure. [9][10][11] The observation of an anomaly in the temperature dependence of dielectric constant around the magnetic transition temperature is often taken as an evidence for magnetoelectric coupling. 12,13 However, a dielectric anomaly at the magnetic transition temperature can also result from the magnetoresistance effects in granular systems 14 because of the large difference in the resistivities of the grains and grain boundaries. For an unambiguous confirmation of multiferroic coupling, it is therefore essential to demonstrate that the observed dielectric anomaly at the magnetic transition temperature is not due to the magnetoresistance at various interfaces such as grain boundaries. It should also be supported by other auxiliary evidence such as magnetoelastic coupling in the bulk phase through structural studies. 11 Here, we present the results of magnetization, dielectric, and structural studies as a function of temperature on 0.73BiFeO 3 -0.27PbTiO 3 ͑BF-0.27PT͒ ceramics, which reveal monoclinic distortion of the ferroelectric phase and intrinsic coupling between the AFM and ferroelectric order parameters. The particular composition ͑x = 0.27͒ that we have investigated is of special in...

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