Field-dependent magnetic and transport properties and anisotropic magnetoresistance in ceramic La0.67Pb0.33MnO3

A summary of detailed measurements of the held and temperature dependence of the ac susceptibility, magnetization and the longitudinal and transverse magnetoresistivities of ceramic La0.67Pb0.33MnO3 is presented. Detailed analysis of the magnetic data provides an estimate of 340.5 +/- 0.5 K for the paramagnetic to ferromagnetic transition temperature T-c, with the data closest to the critical point yielding gamma = 1.39+/-0.06 (from the temperature dependence of the susceptibility along the crossover line), beta = 0.41+/-0.02 (from the field induced variation of the temperature of the same line) and delta = 4.20+/-0.15 (from the field dependence of the critical isotherm). Nevertheless the presence of disorder-a variance in the distribution of spin-spin coupling strengths-means that Heisenberg model (asymptotic) exponents cannot be excluded; the universality class for this system cannot therefore be definitively identified. The transport data yield T-c = 339.6+/-0.4 K, and while the isotropic magnetoresistance peaks near T-c, as expected, the magnitude of the spontaneous resistive anisotropy (SRA) (the difference between the longitudinal and transverse magnetoresistance extrapolated to zero induction) increases linearly with decreasing temperature below T-c, peaks near 30 K and then falls to a smaller value (-0.2+/-0.03%) in the liquid helium range. While this latter value is in reasonable agreement with itinerant model predictions, the temperature variation in the SRA above 30 K is more convincingly reproduced by a localized model. Furthermore, the mechanisms controlling the isotropic magnetoresistance and the SRA in this system appear to be different.