Spin Waves Across 3-Dimensional, Close-Packed Nanoparticles*

While there is much practical and theoretical interest in characterizing magnons within 3-dimensional self-assembled nanoscale systems, few experimental techniques are appropriate. Here inelastic neutron scattering, although intensity limited, is utilized to measure inter-nanoparticle spin waves, or magnons, which arise from coupling between 8.4 nm manganese ferrite nanoparticles that are self-assembled into a close-packed lattice, yet physically separated by oleic acid surfactant. The observed magnons are dispersive, respond to an applied magnetic field, and display the expected temperature-dependent Bose population factor. Moreover, the dispersion yields a non-negative energy gap only when the effective Q is reduced by the inter-particle spacing, confirming that it is a collective excitation between the nanoparticles, rather than originating within individual nanoparticles. The experimental results are well explained by a limited parameter model which treats the 3-dimensional ordered, magnetic nanoparticles as dipolar-coupled superspins.