Orbital-ordering-induced anomalous softening of the ferromagnetic spin waves in perovskite manganites

Spin wave excitations were measured in the ferromagnetic phase of Nd${}_{1\mathrm{/}2}$Sr${}_{1\mathrm{/}2}$MnO${}_3$ by neutron scattering. This compound is located in proximity to the A-type antiferromagnetic state, and it shows a clear anisotropy and anomalous softening of the spin wave excitations. The softening in the ferromagnetic phase is induced by the orbital ordering.