Joseph A. Lahut

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 Mossbauer 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.

Spin pinning at ferrite‐organic interfaces

We have previously reported a drastic moment decrease in NiFe2O4 fine particles (∠100A dia.) coated with an organic surfactant such as oleic acid. Removal of the organic coating restored the moment of the particles. Continued investigation has demonstrated that the apparent moment decrease is due to strong pinning of the spins of those ferrite cations that are bonded to the organic molecules. The evidence for this model includes: (1) UNcoated NiFe2O4 particles, prepared in an otherwise identical fashion and with the same size distribution showed no decrease in moment. This observation eliminates the possibility that defects or abnormal surface morphology are responsible for the decreased moment. (2) Low temperature Mossbauer measurements in zero field of coated and uncoated particles showed identical spectra associated with ordered fine particle NiFe2O4. (3) Mossbauer data taken on coated particles in a field of 68.6 kOe applied along the direction of γ‐ray emission showed only a small decrease of the Pm=...

Superparamagnetic Ferrite Particles Produced by Milling

Superparamagnetic particles of Ni‐, Co‐, Fe‐, and NiZn ferrites were prepared by the milling techniques described by Kaiser and Miskolczy. The particles were extracted from the carrier liquid and structural and magnetic properties were measured before and after removing the organic surfactant layer. The lattice spacings and Curie temperatures of the particles were not significantly changed by the milling. However, the presence of the surfactant layer reduced σs by >50%, indicating that the chemical bonding of the organic molecules to the ferrite decreased the magnetization of the surface layers of the particles. Removal of the surfactant restored σs to nearly bulk values. In addition, the presence of an anomalously high anisotropy was indicated by very large coercive forces at 10 °K. The possibility that a surface anisotropy is responsible for this behavior is discussed.