Spin-independent and spin-dependent conductance anisotropy in layered colossal-magnetoresistive manganite single crystals

Abstract

Results are reported for the conductance anisotropies in single crystals of the layered colossal magnetoresistive manganites. Well below the magnetic ordering temperatures, the zero-field anisotropy increases from \ensuremath{\sim}230 for ${\mathrm{La}}_{1.24}{\mathrm{Sr}}_{1.76}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ to g10 000 for ${\mathrm{La}}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7},$ consistent with a c-axis antiferromagnetic ground state in the latter, while the spin-independent conductivity anisotropy only increases from \ensuremath{\sim}230 to \ensuremath{\sim}300. Significant c-axis ferromagnetic fluctuations are seen in ${\mathrm{La}}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7},$ but no evidence is found for the recently reported two-dimensional magnetic ordering transition at higher temperatures. Direct evidence suggests that spin-orbit coupling is weaker than any other possible sources of orbital ordering.