Magnetic Dynamics of Cobalt Nanoparticle Dispersions

We have used inelastic neutron scattering studies to study the dynamics and spatial correlations of monodisperse suspensions of Co nanoparticles. At high temperatures, the scattering is broad and quasielastic, reflecting the reversal of nanoparticle moments between states separated by an anisotropy barrier. Surprisingly, the scattering is strong even at wave vectors q as large as 2-3 A$^{-1}$, suggesting that the reversal of the overall nanoparticle moment involves localized and rapid fluctuations of smaller groups of Co moments. Spectral weight is transferred to increasingly small energies as the blocking temperature T$_{B}$ is approached, with the slowest fluctuations associated with the smallest wave vectors. Below T$_{B}$ the quasielastic scattering in the dynamically frozen state becomes inelastic, growing in intensity and hardening with decreasing temperature. This excitation is analogous to a bulk spin wave, albeit at a much lower energy.