Effects of Reduced Dimensionality on Spin Dynamics in the Layered Perovskite La1.4Sr1.6Mn2O7

We report zero-field muon spin rotation data in single crystals of La1.4Sr1.6Mn2O7 for T=5–325 K. The spin-lattice relaxation rate is spatially inhomogeneous below the 3D magnetic transition temperature TC and anisotropic above TC. We find evidence against 2D spin ordering or in-plane correlations above TC. Additionally, the very slow spin fluctuations found below TC in cubic (3D) perovskites like (La,Ca)MnO3 or (La,Sr)MnO3, and attributed to relatively small magnetoelastic polarons, are absent in La1.4Sr1.6Mn2O7. This suggests that the polaron size in the layered material is significantly larger than in the 3D perovskites.