Magnetic nanoparticles in borate glasses: Identification and sizing

Heat treatment of borate glasses co-doped with low contents of iron and larger radius elements: Dy, Tb, Gd, Ho, Er, Y and Bi results in formation of magnetic nanoparticles, radically changing their physical properties. Transmission electron microscopy and synchrotron radiation-based techniques: XRD, EXAFS, XANES and SAXS, show a broad distribution of nanoparticle sizes with characteristic depending on the treatment regime; a crystalline structure of these nanoparticles is detected in heat treated samples. Magnetic circular dichroism (MCD) studies of samples subjected to heat treatment as well as of maghemite, magnetite and iron garnet allow to unambiguously assigning the nanoparticle structure to maghemite. Different features observed in the MCD spectra are related to different electron transitions in Fe³⁺ ions gathered in the nanoparticles. Variable-temperature electron magnetic resonance (EMR) studies confirm the formation of magnetic nanoparticles and the identification of their nature. Computer simulations of the EMR spectra corroborate the broad distribution of nanoparticle sizes found by “direct” techniques.