SUPERPARAMAGNETIC RESONANCE OF FERRIC IONS IN DEVITRIFIED BORATE GLASS

Drastic changes are observed in magnetic resonance spectra of berate g lasses doped with low concentrations of iron oxide, after heat treatme nt above the glass transition temperature. A sharp g(ef) = 4.3 line, c haracteristic of diluted Fe3+ ions in the vitreous matrix, progressive ly 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 superi mposed features: a narrow one and a broad one. The narrow feature is d ominant at lower treatment temperatures, while the broader one grows r apidly 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 vitr eous matrix and characterised by a magnetocrystalline anisotropy const ant, a demagnetisation field and a line shape function depending on th e particle volume. The two-line pattern observed is shown to result fr om a unimodal log-normal distribution of particle diameters, with the most probable diameter value increasing with treatment temperature. Su ch a distribution results from a very inhomogeneous iron distribution in the glass matrix.