Magnetic Properties of Microcrystals Studied by Mössbauer Spectroscopy

Abstract

The magnetic properties of microcrystals are discussed on the basis of results obtained by use of Mössbauer spectroscopy. Spectra of microcrystals with dimensions smaller than about 100 A are influenced by magnetic fluctuation effects such as superparamagnetic relaxation and collective magnetic excitations, and these effects allow a determination of the product of the particle size and the magnetic anisotropy energy constant. Measurements on ferromagnetic and ferrimagnetic particles as a function of applied magnetic fields allow a determination of the particle volume. The anisotropy energy constants of microcrystals of Fe, Co, Ni and Fe3O4 are found to change upon chemisorption. Spectra of small particles at low temperatures also give information on the magnetic properties of the atoms in the surface layer. These studies as well as thin film studies show that no "magnetically dead layers" are present at the surface of the samples. In the case of metallic iron an enhanced magnetic hyperfine field is found, in agreement with theoretical results.