Anisotropic spin waves and exchange interactions in the a -type antiferromagnetic state of Pr0.5 Sr0.5 Mn O3

Inelastic neutron scattering is used to investigate the spin dynamics in the layered A -type antiferromagnetic state of Pr0.5 Sr0.5 Mn O3 at 20 K. The spin wave dispersion, observed between 2 and 12 meV, for neutron momentum transfer, Q, parallel to the ferromagnetic planes is found to be much steeper than the dispersion when 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 Pr0.5 Sr0.5 Mn O3 and LaMn O3 shows that the anisotropy of exchange interactions strongly depends on the type of orbital ordering.