Critical behavior of magnetoresistance near the metal–insulator transition of La0.7Ca0.3MnO3

The resistivity, ρ(T), of thin film and bulk samples of La0.7Ca0.3MnO3 is investigated in magnetic fields between 0–10 T and temperatures between 10–340 K. Metallic resistivity, ρlt(T)=ρr+ρ2T2+ρ4.5T4.5, is observed well below the metal–insulator transition (MIT) temperature TMI. For T>TMI, the resistivity is governed by the Shklovskii-Efros variable-range hopping mechanism, giving ρht(T)=ρ0exp[(T0/T)1/2]. Combining the contributions of itinerant and localized carriers, that means ρlt(T) and ρht(T), respectively, a good fit of ρ(T) is obtained over the whole temperature region investigated. Below MIT, for T→TMI−0, a critical behavior obeying the law ρ(T)∼(TMI−T)−ν is observed. For T→TMI+0, the critical behavior of the resistivity is determined by divergence of the correlation length ξ∼(T−TMI)−ν, due to the relation T0∼1/ξ. In both cases, ν≈1. The temperature dependence of ξ, when approaching TMI 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.