Anisotropic spin waves in a metallic antiferromagnetNd0.45Sr0.55MnO3

We demonstrate that ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_3$ is an example of a metallic antiferromagnetic manganite, and that it exhibits very anisotropic spin-wave dispersion relations with intra- and interplanar exchange parameters $8JS=32.6\pm 0.9\mathrm{meV}$ and ${4J}^{\prime }S=-10.1\pm 0.5\mathrm{meV},$ respectively. This anisotropy reflects underlying $d_{x^2-y^2}$-type orbital ordering, and the metallic behavior indicates that a charge ordering is decoupled from the orbital ordering in this system.