Lineshapes in magnetic resonance spectra

In magnetic resonance, and in particular, in superparamagnetic resonance st udies at variable temperatures, a correlation between the apparent resonanc e magnetic field and the apparent linewidth is often observed. In order to account for this correlation, we consider the resonance Lineshapes resultin g from different phenomenological equations of damped motion of the magneti c moments in the cases of a linear paramagnet and of a perfect soft ferroma gnet. The Bloch-Bloembergen, modified Bloch, Gilbert, Landau-Lifshitz and C allen equations are analysed. In most cases we obtain analytical expression s 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 Landa u-Lifshitz lineshape provide good fits of the resonance spectra at differen t temperatures for the same magnetic and morphological parameters of the pa rticles.