Anisotropic spin waves and exchange interactions in theA-type antiferromagnetic state ofPr0.5Sr0.5MnO3

Abstract

Inelastic neutron scattering is used to investigate the spin dynamics in the layered $A$-type antiferromagnetic state of ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The spin wave dispersion, observed between 2 and $12\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, for neutron momentum transfer, $\mathbf{Q}$, parallel to the ferromagnetic planes is found to be much steeper than the dispersion when $\mathbf{Q}$ is perpendicular to the ferromagnetic planes, indicating strong anisotropy. Here we show that the three-dimensional Heisenberg model with nearest neighbor exchange interactions, an interlayer antiferromagnetic coupling, an intralayer ferromagnetic coupling, and a single ion anisotropy can account for these dispersions. A comparison of the ratio of exchange couplings in ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ and $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ shows that the anisotropy of exchange interactions strongly depends on the type of orbital ordering.