Properties of magnetic fluid particles

Very fine particles of magnetite, nickel ferrite, and cobalt ferrite were produced by grinding coarser powders in a ball mill with a carrier fluid and a surfactant. The particles were examined by means of chemical analysis, electron microscopy, x-ray diffraction, magnetic measurements, and Mössbauer spectroscopy. Properties were determined before and after removing the surfactant coating. The most significant observation was that in some systems a large fraction of the spins was pinned in extremely high anisotropy fields as a result of bonding to the surfactant molecules. Anomalous magnetic hysteresis behavior was also noted when the surfactant coating was present.