Field-Induced Crossover in La_0,7Pb_0.3MnO₃

A field-induced crossover is observed both resistively and magnetically in a single crystal of La_0.7Pb_0.3MnO₃, a compound that exhibits colossal magnetoresistance (CMR) and has a second-order ferromagnetic transition. The resistivity (p) data observed in high magnetic fields show a universal decrease with the magnetization (M) and little dependence on temperature. In contrast, p is independent of M for lower fields. An analysis of the magnetization data finds that the critical exponents have different values depending on whether the high field data or the low field data are analyzed. The results are consistent with the interpretation that at low fields, the compound behaves as a collection of magnetic clusters with mean-field-like inter-cluster interactions, and at high fields the compound behaves as a system of individual spins with short-range interactions.