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=...