Critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.7Pb0.05Na0.25MnO3

Critical behavior in La_0.7Pb_0.05Na_0.25MnO₃ has been investigated by dc magnetization measurements. Magnetic data indicate that the compound exhibits a continuous (second-order) paramagnetic (PM) to ferromagnetic (FM) phase transition. Estimates of critical exponents yield δ=4.80±0.01, γ=1.296±0.002 and β=0.344±0.007 (consistent with both the predictions for the three-dimensional-Heisenberg model and with those reported for materials when the FM transition is ascribed to the double exchange (DE) mechanism as a major origin) with T_C=334.54±0.08. The critical exponent γ is slightly inferior than predicted from the 3D Heisenberg model. Such a difference may be due, within the context of the quenched disorder, to the presence of some alterations of short-range magnetic order of FM clusters in the PM phase. The temperature variation in the effective exponent (γ_eff) is similar to those for disordered ferromagnets.