Effects of Reduced Dimensionality on Spin Dynamics in the Layered PerovskiteLa1.4Sr1.6Mn2O7

We report zero-field muon spin rotation data in single crystals of La{sub 1.4}Sr {sub 1.6}Mn{sub 2}O{sub 7} for T=5{endash}325 K . The spin-lattice relaxation rate is spatially inhomogeneous below the 3D magnetic transition temperature T{sub C} and anisotropic above T{sub C} . We find evidence against 2D spin ordering or in-plane correlations above T{sub C} . Additionally, the very slow spin fluctuations found below T{sub C} in cubic (3D) perovskites like (La,thinspCa)MnO{sub 3} or (La,thinspSr)MnO{sub 3} , and attributed to relatively small magnetoelastic polarons, are absent in La{sub 1.4}Sr {sub 1.6}Mn{sub 2}O{sub 7} . This suggests that the polaron size in the layered material is significantly larger than in the 3D perovskites. {copyright} {ital 1998} {ital The American Physical Society}