Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass

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: 0.63B(2)O(3)-0.37Li(2)O-0.75 x 10(-3) Fe2O3 in mole %) were annealed at increasing temperatures T-a, starting at the glass transition temperature. A new composite resonance at g(ef) approximate to 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 T-a while the broad component increases in intensity as T-a 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 T-a increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with T-a. At lower anneal temperatures T-a, a striking increase occurs in the particle diameters, while at higher T-a 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. (C) 2000 American Institute of Physics. [S0021-8979(00)04710-1].