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

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 {approximately}230 for La{sub 1.24}Sr{sub 1.76}Mn{sub 2}O{sub 7} to {gt}10000 for La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}, consistent with a {ital c}-axis antiferromagnetic ground state in the latter, while the {ital spin-independent} conductivity anisotropy only increases from {approximately}230 to {approximately}300. Significant {ital c}-axis ferromagnetic fluctuations are seen in La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 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. {copyright} {ital 1999} {ital The American Physical Society}