Exchange interaction and magnetoresistance in La₂/₃Ca₁/₃MnO₃ : experiment and models

Abstract

The magnetization M(T) and electrical resistivity ρ(T) of a La₂/₃Ca₁/₃MnO₃ film have been studied in the temperature range 5 K⩽T⩽320 K in the magnetic field intervals 10 Oe⩽H⩽400 Oe and 0⩽H⩽50 kOe, respectively. It is found that the M(T)/M(0) value is larger than that predicted by the conventional molecular field model below the Curie point T=267 K, and that the ln ρ(T) dependence is close to linear in the temperature range 80 K<T<200 K (accordingly, ∂ ln ρ/∂T is constant in this region). A model of the electrical conductivity and magnetoresistivity of the system describing qualitatively the experimental results is proposed (the Δmτ model). The model includes a thermally activated (with characteristic energy Δ) mechanism of conductivity, dependence of the concentration and the effective mass (m) of the itinerant charge carriers on the magnetization, as well as scattering (with characteristic time τ) of the charge carriers by static breakings of the translational symmetry, thermal fluctuations of the magnetic order, and phonons.