Critical behavior and transport properties of single crystal Pr1-xCaxMnO3 (x=0.27, and 0.29)

A summary of analyses of critical behavior in both the ac susceptibility and dc magnetization in single crystal Pr1-xCaxMnO3 (x=0.27, and 0.29) is presented supplemented by magnetotransport data. The magnetic data indicate that both single crystals exhibit a continuous/second-order paramagnetic to ferromagnetic phase transition but with marginally different critical exponents. Estimates of the latter for the x=0.27 sample yield delta=4.81 +/- 0.02, gamma=1.36 +/- 0.02, and beta=0.36 +/- 0.02 (consistent with both the predictions for the nearest-neighbor three-dimensional-Heisenberg model and with those reported for its double exchange-dominated metallic counterparts) with T-C=127 +/- 0.5 K; whereas at x=0.29, delta=4.62 +/- 0.05, gamma=1.38 +/- 0.01, and beta=0.37 +/- 0.02 with T-C=114 +/- 0.5 K. The absence of metallicity-and by inference, the suppression of charge fluctuations-does not, therefore, appear to influence the universality class of the transition. No evidence for a Griffiths-type phase was found in either specimen. These single crystals also display comparable values of the acoustic spin-wave stiffness D similar to 70 meV A(2) well below that found in the charge-ordered field-induced metallic regime of the same system. These samples remain insulators below ferromagnetic ordering temperature T-C in fields up to 90 kOe; they display nonabiabatic small polaron mediated transport behavior in the paramagnetic regime in zero field characterized by activation energies comparable to those reported previously for a range of doped perovskites.