Correlation of conductivity and magnetization in epitaxial La2/3Ca1/3MnO3 thin films.

We fabricated ∼200 nm thin (001)-oriented films of the ferromagnetic metallic perovskite La_2/3Ca_1/3MnO₃ on single crystal (001)-SrTiO₃ substrates by dc-sputtering at high oxygen pressure. The samples feature a Curie temperature T_C∼260 K and a magnetic moment μ(T→0 K)∼3 μ_B per Mn atom. The magnetization loops are nearly square-shaped with a coercive field H_c(5 K)=0.03 T that decreases linearly in temperature down to T_C. At low temperature, the magnetization shows a decrease ΔM∼T² and the resistivity an increase Δρ∼T² as is expected for itinerant electron ferromagnets where single particle excitations dominate. As a further indication of the high quality of the samples, the resistivity without external magnetic field peaks right at T_C with a maximum value ρ(H=0,T=T_C) of only ∼2 mΩ cm. The magnetoresistance ratio Δρ/ρ₀=[ρ(H,T)-ρ(H=0,T)]/ρ(H=0,T) also reaches its maximum value at T_C, with Δρ/ρ₀(H=6 T,T=T_C)∼-50%. The complete absence of this colossal magnetoresistance effect in the low-temperature limit is a further indication of the high sample quality which may be due to a high degree of oxygenation as a result of our preparation condictions. The relation ρ(H,T)=ρ_mexp[-M(H,T)/M₀] observed below T_C as well as the temperature characteristic of the resistivity above T_C can both be explained by a simple magnetic polaron hopping tunneling model where the tunneling barrier height depends in a simple way on the relative orientation of the local magnetization at both ends of the hopping path. However, here it must be considered that the polarons completely change their character at T_C from large and delocalized to small and highly localized entities. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]