Magnetic hyperfine splitting in Moessbauer spectra of microcrystals

Below the superparamagnetic blocking temperature of a microcrystal the magnetization direction is in general not fixed, but fluctuates in directions close to an easy direction of magnetization. Such fluctuations (collective magnetic excitations) result in a reduction in the magnetic splitting of the Moessbauer spectrum. A low-temperature approximation for this reduction is derived for microcrystals with arbitrary magnetic energy. Moreover, explicit expressions are presented for particles with special types of magnetic anisotropy and for particles exposed to external magnetic fields. The reduction in the magnetic hyperfine field has its maximum value just below the blocking temperature but does not exceed 5-15% in isolated particles with uniaxial anisotropy in zero applied magnetic field. However, ferro- and ferrimagnetic particles, exposed to external magnetic fields, and particles for which exchange anisotropy is predominant, may exhibit any magnetic hyperfine splitting between zero and the saturation value. It is shown that in special cases an assembly of microcrystals in close contact with each other may behave like a spin-glass. We discuss how studies of the magnetic hyperfine splitting of Moessbauer spectra of microcrystals give information on the particle size and the prevailing magnetic anisotropies. (orig.)