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The coexistence of magnetic and ferroelectric orderings have been generally regarded as "mutually excusive" inoxide perovskites. Their coexistence in some magnetoelectric multiferroics has evoked tremendous interest in recentyears because of the interesting underlying physics and also the possibility of designing new generation sensor,actuator and memory devices.
The observation of an anomaly in the dielectric constant at the magnetic transition temperature is often taken asan evidence of magnetoelectric coupling of multiferroic origin. It has, however, been pointed out that such a dielectricanomaly may also result from magnetoresistive contributions due to space charge polarizations, especially in granularsystems. It does not therefore necessarily provide evidence for intrinsic magnetoelectric coupling of multiferroic origin.In the (Bi1-xAx) (Fe1-xTix)O3 (A = Ba2+, Pb2+) and (1-x)BiFeO3 – x Pb(Fe0.5 Nb0.5)O3 systems, we have recentlyshown1,2 that the temperature variation of the high frequency dielectric constant, free from space charge contributions,still exhibits an anomaly at the magnetic transition temperature. Further, it is accompanied with a pronouncedchange in the lattice parameters at the magnetic transition temperature revealing strong magnetoelastic coupling.More interestingly, there is a significant shift in the atomic positions below the magnetic transition temperature. Wehave shown that these shifts are due to a rare type of isostructural phase transition driven by one of the irreps of theferroelectric space group. The ionic polarization calculated form the positional coordinates of the atoms in the unitcell is not only found to increase in the magnetic phase but also scale linearly with magnetization. Our observationsprovide the first direct and atomic level evidence for intrinsic magnetoelectric coupling of multiferroic origin in BiFeO3based systems.
1) Anar Singh et al, Phy. Rev. Letters 101, 247602 (2008)
2) Shuvrajyoti Bhattacharjee et al, Appl. Phys. Letters 94 (2009) 012906
3) Jayprakash Patel et al , to be published
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