Critical behavior of magneto resistance near the metal-insulator transition of La0.7Ca0.3MnO3

The resistivity, p(T), of thin film and bulk samples of La0.7Ca0.3MnO3 is investigated in magnetic fields between 0-10T and temperatures between 10-340 K. Metallic resistivity, rho(lt)(T) = rho(t) + rho(2)T(2) + rho(4.5)T(4.5), is observed well below the metal-insulator transition (MIT) temperature T-MI, For T > T-MI the resistivity is governed by the Shklovskii-Efros variable-range hopping mechanism, giving rho(ht)(T) = rho(0) exp[(T-0/T)(1/2)], Combining the contributions of itinerant and localized carriers, that means rho(lt)(T) and rho(ht)(T), respectively, a good fit of rho(T) is obtained over the whole temperature region investigated. Below MIT, for T --> T-MI - 0, a critical behavior obeying the law rho(T) similar to (T-MI - T)(-v) is observed. For T - T-MI + 0, the critical behavior of the resistivity is determined by divergence of the correlation length (T - T-MI)(-v), due to the relation T-0 - 1/xi. In both cases, vapproximate to1. The temperature dependence of when approaching T-MI from the high-temperature side, is attributed to generation of critical clusters of the metallic phase in the semiconductor matrix. Rapid increase of the metallic component is observed when B is increased, (C) 2002 Elsevier Science B.V. All rights reserved.