Lineshapes in magnetic resonance spectra

In magnetic resonance, and in particular, in superparamagnetic resonance studies at variable temperatures, a correlation between the apparent resonance magnetic field and the apparent linewidth is often observed. In order to account for this correlation, we consider the resonance Lineshapes resulting from different phenomenological equations of damped motion of the magnetic moments in the cases of a linear paramagnet and of a perfect soft ferromagnet. The Bloch-Bloembergen, modified Bloch, Gilbert, Landau-Lifshitz and Callen equations are analysed. In most cases we obtain analytical expressions for the apparent resonance-field shift. Finally we report an experimental variable-temperature study of the superparamagnetic resonance of ultrafine Fb(2)O(3) particles in sol-gel glass. Computer simulations using the Landau-Lifshitz lineshape provide good fits of the resonance spectra at different temperatures for the same magnetic and morphological parameters of the particles.