Phase separation and the effect of quenched disorder in Pr(0.5)Sr(0.5)MnO(3).

The nature of phase separation in Pr(0.5)Sr(0.5)MnO(3) has been probed by linear, as well as nonlinear, magnetic susceptibilities and resistivity measurements across the second order paramagnetic to ferromagnetic transition (T(C)) and first order ferromagnetic to antiferromagnetic transition (T(N)). We found that the ferromagnetic (metallic) clusters, which form at T(C), continuously decrease their size with a decrease in temperature and coexist with non-ferromagnetic (insulating) clusters. These non-ferromagnetic clusters are identified to be antiferromagnetic. It is shown that they do not arise because of the superheating effect of the lower temperature first order transition. This reveals phase coexistence in manganite, around half-doping, encompassing two long-range order transitions. Substitution of quenched disorder (Ga) at Mn-sites promotes antiferromagnetism at the cost of ferromagnetism without adding any magnetic interaction or introducing any significant lattice distortion. Moreover, an increase in disorder decreases the ferromagnetic cluster size and with 7.5% Ga substitution cluster size reduces to the single-domain limit. Resistivity measurements also reveal the phase coexistence identified from the magnetic measurements. It is significant that, an increase in disorder up to 7.5% increases the resistivity of the low temperature antiferromagnetic phase by about four orders.