Magnetic ordering and spin fluctuations in nearly half-doped manganites

We discuss unusual behavior discovered in spin dynamics in the nearly half-doped manganite systems. Nd0.45Sr0.55MnO3 is a metallic antiferromagnet, and exhibits very anisotropic spin wave dispersion relations. The ferromagnetic (FM) state of Pr1/2Sr1/2MnO3 and Nd1/2Sr1/2MnO3 also exhibit clear anisotropic dispersion relations. Such anisotropy reflects the underlying d(x2−y2)-type orbital ordering. We also demonstrate that the high temperature paramagnetic state without charge ordering in doped manganites is characterized by the FM spin fluctuations with an anomalously small energy scale for a wide range of doping concentrations.