Low temperature magneto-dielectric measurements on BiFeO3 lightly substituted by cobalt

Dielectric and magnetodielectric measurements are done on BiFe1-xCoxO3: x=0, 0.01, and 0.02 in the temperature range 70-300K and up to magnetic field 1.3?T. The dielectric data are well described by HaverliakNegami expression plus an additional term for the Maxwell Wagner (MW) type relaxations, whose contribution is dominant near room temperature. The parameters obtained from the fitting of data using the above mentioned expression, suggest slowing down of relaxation and approach towards ideal Debye type relaxations, as the temperature is lowered. The dielectric relaxations obey polaronic variable range hopping model with distinct activation energies (Ea) in the extrinsic (6.67T(3/4)meV) and intrinsic (2.88T(3/4)meV) regions for the parent sample (x=0), and thus a distinct transition from extrinsic to intrinsic behavior is seen at 215?K while lowering the temperature. This distinct transition is missing for Co substituted samples probably due to the extrinsic region values of E-a (3.42T(3/4)meV and 2.42T(3/4)meV for x=0.01 and 0.02, respectively) comparable to that of the intrinsic region (see x=0). The magnetodielectric measurement shows positive magnetodielectricity (MD) in the intrinsic region (T<215K for x=0) and negative MD in the extrinsic region (T>215K for x=0). The extrinsic region is found to be dominated by MW and magnetoresistance effects, whereas MD in intrinsic regions is due to the spin reorientation transitions. The Co substitution is found to increase the extrinsic and non-Debye contributions to dielectricity, which becomes so large that no spin reorientation transitions are seen in x=0.02 sample. The pyroelectric active region in x=0 is found to be dominated by the diffusive behavior having contribution of the form omega(-0.5). (C) 2015 AIP Publishing LLC.