Superparamagnetic resonance of annealed iron-containing borate glass

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 670 degrees C. As the anneal temperature increases, the g(ef) = 4.3 Sharp line characteristic of isolated iron ions decreases in intensity and finally disappears. Simultaneously, a narrow Line emerges at g(ef) approximate to 2.0, 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 lognormal 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.