Some Superparamagnetic Properties of Fine Particle δFeOOH

Various samples of δFeOOH have been prepared all with substantially the same particle shape and size but with differing low-temperature saturation magnetizations. The particles were found to be hexagonal platelets with a mean diameter of 200 A and a thickness of 30 A. The magnetization of the samples was varied by varying the preparation conditions and also by partially decomposing the material to αFeOOH and αFe2O3. It has been shown that at a little above room temperature the powders were all superparamagnetic and that the variations of magnetization with preparation conditions qualitatively support a model proposed by Francombe and Rooksby. The variation of the Blocking temperature of a particular preparation as its magnetization was reduced by successive decomposition is also discussed. The results are tentatively interpreted in terms of a constant term corresponding to the normal anisotropy energy and a variable one due to the Lorentz internal field.