Dispersionless Spin Waves and Underlying Field-Induced Magnetic Order in Gadolinium Gallium Garnet

We report the results of neutron diffraction and inelastic neutron scattering on a powder sample of Gd3Ga5O12 at high magnetic fields. Analysis of the diffraction data shows that in high fields (B greater than or similar to 1.8 T) the spins are not fully aligned, but are canted slightly as a result of the dipolar interaction. The magnetic phase for fields less than or similar to 1.8 T is characterized by antiferromagnetic peaks at (210) and an incommensurate wave vector. The dominant contribution to inelastic scattering at large momentum transfers is from a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction, and that the spectrum at high fields B greater than or similar to 1.8 T is well described by a spin wave theory.