Interparticle magnetic correlations in dense Co nanoparticle assemblies

Resonant magnetic x-ray scattering from dense assemblies of 9-nm-diameter ε-Co and hcp-Co superparamagnetic particles is reported. For lower anisotropy ε-Co assemblies remanent scattering is significantly enhanced compared to a random orientation model, indicating that preferred intermoment orientations with antiferromagnetic character exist with spatial frequencies ranging over several nearest neighbors. This interaction-mediated collective behavior is consistent with dipolar energies and exists well into the superparamagnetic regime, revealing that such thermally activated motion is highly correlated.