Crystalline, Shape, and Surface Anisotropy in Two Crystal Morphologies of Superparamagnetic Cobalt Nanoparticles by Ferromagnetic Resonance

Ferromagnetic resonance (FMR) techniques are used to investigate superparamagnetic cobalt nanoparticles (NP's) with different crystalline structures and sizes ranging from 4 to 9 nm in diameter. Magnetic contributions from NC shape, crystallographic structure, defects, and surface structure are discussed. An independent-superparamagnetic-grain model is employed to simulate the FMR measurements. The results from both single crystalline and polycrystalline cobalt NP's reveal that a particle's effective anisotropy, and thus its magnetic properties, are extremely sensitive to internal structure as well as overall particle shape. Finally, surface chemical properties were found to yield unique FMR signatures for NP's at low temperatures.