Title:
Evidence of electronic phase arrest and glassy ferromagnetic behaviour in (Nd(0.4)Gd(0.3))Sr(0.3)MnO3 manganite: comparative study between bulk and nanometric samples.

Abstract:
The effect of the doping of rare earth Gd(3+) ions replacing Nd(3+) in Nd(0.7)Sr(0.3)MnO(3) is investigated in detail. Measurements of resistivity, magnetoresistance, magnetization, and linear and nonlinear ac magnetic susceptibility on chemically synthesized (Nd(0.7-x)Gd(x))Sr(0.3)MnO(3) show various interesting features with doping level x=0.3. A comparative study has been carried out between a bulk and a nanometric sample (grain size ∼60 nm) synthesized from the same as a prepared powder to maintain an identical stoichiometry. The resistivity of the samples shows strong dependence on the magnetic field-temperature history. The magnetoresistance of the samples also shows strong irreversibility with respect to sweeping of the field between the highest positive and negative values. Moreover, the 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 effects and the arrest of phases in the samples. Furthermore, the bulk sample displays a spin glass like behaviour as evident from the frequency dependence of linear ac magnetic susceptibility and critical divergence of the nonlinear ac magnetic susceptibility. The experimentally obtained characteristic time τ(0) after dynamical scaling analysis of the frequency dependence of the ac susceptibility is found to be 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 the 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.