Low-energy magnetic excitations in Co/CoO core/shell nanoparticles

We have used inelastic neutron scattering measurements to study the magnetic excitations of Co core/CoO shell nanoparticles for energies from 0 to 50 meV. Above the blocking temperature T_B, broad quasielastic scattering is observed, corresponding to the reorientation of the Co core moments and to paramagnetic CoO scattering. Below T_B, two nearly dispersionless inelastic peaks are found, whose energies increase with decreasing temperature as order parameters, controlled by the nanoparticle Néel temperature T_N=235 K, and saturating as T→0 at 2.7 and 6.7 meV, respectively. Similar excitations were observed in a powdered single crystal of CoO, indicating that both are intrinsic excitations of CoO, resulting from the exchange splitting of single-ion states for T≤T _N. Pronounced finite-size effects are observed for the scattering from the CoO nanoparticle shells, whose thicknesses range from 1.7 to 4.5 nm. These include an enhanced excitation linewidth, as well as a response that is not only spread over a much wider range of wave vectors, but is also significantly more intense in the nanoparticles than in bulk CoO.