  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.
