Abstracts Mathematics

Field-dependent magnetic and transport properties and anisotropic magnetoresistance in ceramic La|0|.|6|7Pb|0|.|3|3MnO|3

by Amra Peles

Detailed measurements of the field and temperature dependence of the ac susceptibility, magnetization and the transverse and longitudinal magnetoresistivities of ceramic La0.67Pb0.33MnO 3 has been made, and are presented. Detailed analysis of the magnetic data provide an estimate of 340.5 +- 0.5 K for the paramagnetic to ferromagnetic temperature T'c', with data closest to the critical point yielding the set of critical exponent values i.e. _ = 1.39 +- 0.06, B = 0.41 +- 0.2 and [delta] = 4.20 +- 0.15. The presence of disorder, that is, a variance in the distribution of spin - spin coupling strength, means that Heisenberg model asymptotic exponents cannot be excluded; the universality class for this system cannot therefore be definitely 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) 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 temperature region. The latter value is in reasonable agreement with itinerant model predictions, while the temperature variation in the SRA above about 30 K is more convincingly reproduced by a localized model. Furthermore, the mechanism controlling the isotropic magnetoresistance and the SRA in this system appear to be different.