Lawrence Berkeley National Laboratory

Resonant magnetic x-ray scattering from dense self-assemblies of 9-nm diameter epsilon-Co and hcp-Co particles is reported. For lower anisotropy epsilon-Co we find remanent magnetic scattering that is significantly enhanced, indicating preferred inter-particle moment orientations of both antiferromagnetic and ferromagnetic character. This interaction-mediated collective behavior is consistent with dipolar fields and exists well above the isolated particles blocking temperature where thermal activation is operative, suggesting that magnetostatic super-spin waves exist in such systems.