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Evidence of electronic phase arrest and glassy ferromagnetic behaviour in (Nd0.4Gd0.3)Sr0.3MnO3 manganite : Comparative study between bulk and nanometric samples

The effect of doping of rare earth Gd 3+ ion replacing Nd 3+ in Nd0.7Sr0.3MnO3 is investigated in details. Measurements of resistivity, magnetoresistance, magnetization, linear and non linear ac magnetic susceptibility on chemically synthesized (Nd0.7-xGdx)Sr0.3MnO3 shows various interesting features with doping level x=0.3. Comparative study has been carried out between a bulk and a nanometric sample (grain size ~ 60 nm) synthesized from the same as prepared powder to maintain identical stoichiometry. Resistivity of the samples shows strong dependence on the magnetic field - temperature history. The magnetoresistance of the samples also show strong irreversibility with respect to sweeping of the field between highest positive and negative values. Moreover, resistivity is found to increase with time after field cooling and then switching off the field. All these phenomena have been attributed to phase separation effect and arrest of phases in the samples. Furthermore, the bulk sample displays a spin glass like behaviour as evident from frequency dependence of linear ac magnetic susceptibility and critical divergence of the nonlinear ac magnetic susceptibility. The experimentally obtained characteristic time t after dynamical scaling analysis of the frequency dependence of the ac susceptibility is found to be t=10-17 s which implies that the system is different from a canonical spin glass. An unusual frequency dependence of the second harmonic of ac susceptibility around the magnetic transition temperature led us to designate the magnetic state of the sample to be glassy ferromagnetic. On reduction of grain size low field magnetoresistance and phase arrest phenomena are found to enhance but the glassy state is observed to be destabilized in the nanometric sample.