The impact of finite size effects on spin waves in CoO*

We studied the spin waves in nanoscaled CoO using inelastic neutron scattering. The zero-field measurements were carried out on Co/CoO nanoparticles, CoO nanoparticles, and the bulk powder of CoO in the temperature range of 15 -- 300 K. The temperature-dependent inelastic intensity at 2.5 meV, found in all samples, was ascribed to CoO spin waves. We observed an increase at least of factor of 100 in the inelastic intensity for Co/CoO as compared to the CoO bulk, and shift of intensity towards larger scattering vectors. We suggest that new boundary conditions imposed by the nanoparticle surface and the breaking of the symmetry are mainly responsible for this effect. Similar enhancement of the spin wave spectrum was also predicted in thin films [1,2]. [1] S. Reshetnyak et al PMC Physics B 2008 [2] Y.Gorobets et al, Tech. Phys. 1998