Magnetotransport in polycrystalline La_2/3Sr_1/3MnO₃ thin films of controlled granularity

Abstract

Polycrystalline La_2/3Sr_1/3MnO₃ thin films were synthesized by pulsed laser ablation on single crystal (100) yttria-stabilized zirconia substrates to investigate the mechanism of magnetotransport in a granular manganite. Different degrees of granularity are achieved by using the deposition temperatures (T_D) of 700 and 800 °C. Although no significant change in magnetic order temperature (T_C) and saturation magnetization is seen for these two types of films, the temperature and magnetic field dependence of their resistivity (ρ(T,H)) is strikingly dissimilar. While the ρ(T,H) of the 800 °C film is comparable to that of epitaxial samples, the lower growth temperature leads to a material which undergoes insulator-to-metal transition at a temperature (T_P≈170 K) much lower than T_C. At T«T_P, the resistivity is characterized by a minimum followed by ln T divergence at still lower temperatures. The high negative magnetoresistance (≈20%) and ln T dependence below the minimum are explained on the basis of kondo-type scattering from blocked Mn spins in the intergranular material. Further, a striking feature of the T_D=700∅ °C film is its two orders of magnitude larger anisotropic magnetoresistance (AMR) as compared to the AMR of epitaxial films. We attribute it to unquenching of the orbital angular momentum of 3d electrons of Mn ions in the intergranular region where crystal field is poorly defined.