Spin-polarized tunneling in the half-metallic ferromagnets ${\mathrm{La}}_{0.7-x}{\mathrm{Ho}}_x{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ $(x=0\mathrm{}$ and 0.15): Experiment and theory

The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin-polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin-polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean-field approximation. We apply this model to our experimental data on two half-metallic ferromagnets, ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.55}{\mathrm{Ho}}_{0.15}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ in the temperature range 5–300 K. We find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization. We discuss the significance of our results in the light of the recent finding by A. Biswas et al. [Phys. Rev. B 59, 5368 (1999)] regarding the evolution of the total density of states at the Fermi level as a function of temperature in colossal magnetoresistive materials.