Superparamagnetic resonance of ferric ions in devitrified borate glass

Drastic changes are observed in magnetic resonance spectra of borate glasses doped with low concentrations of iron oxide, after heat treatment above the glass transition temperature. A sharp g(ef) = 4.3 line, characteristic of diluted Fe3+ ions in the vitreous matrix, progressively disappears in the course of treatment. A new two-line pattern grows in the vicinity of g(ef) approximate to 2.0, consisting of two superimposed features: a narrow one and a broad one. The narrow feature is dominant at lower treatment temperatures, while the broader one grows rapidly at higher treatment temperatures. Computer simulations succeed in fitting the spectra as being due to superparamagnetic resonance of static monodomain magnetic particles, randomly distributed in the vitreous matrix and characterised by a magnetocrystalline anisotropy constant, a demagnetisation field and a line shape function depending on the particle volume. The two-line pattern observed is shown to result from a unimodal log-normal distribution of particle diameters, with the most probable diameter value increasing with treatment temperature. Such a distribution results from a very inhomogeneous iron distribution in the glass matrix.