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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 Fe2O3 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.

In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in the particle diameters, while at higher Ta these diameters reach a limit value. When the measurement temperature is increased, the resonance spectra show a reversible narrowing and an increase in intensity. The temperature dependence of the individual linewidths is attributed to thermal fluctuations of the orientations of the magnetic moments with respect to the magnetic anisotropy axes.

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A lithium borate glass containing a small amount of iron oxide is studied by electron magnetic resonance at room temperature after repeated annealing steps between 460 and . As the anneal temperature increases, the sharp line characteristic of isolated iron ions decreases in intensity and finally disappears. Simultaneously, a narrow line emerges at , superposed with a broader one, the narrow and the broader components predominating respectively after annealing at lower and at higher temperatures. Computer simulations of spectra have been carried out, based on a model of resonance of ferromagnetic single-domain nanoparticles randomly dispersed in the devitrified glass (superparamagnetic resonance). As the anneal temperature increases, the most probable particle diameter obtained assuming a log-normal distribution of diameters increases from 2.9 to 4.7 nm showing a saturation at higher anneal temperatures, whereas the relative number of larger particles grows continuously.

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