Unusual Magnetic Phase Transitions in Gd Clusters in Multicomponent Oxide Glasses

Magnetic susceptibility, EPR and optical properties have been studied in a glass system {20La2O3-22Al2O3-23B2O3-35(SiO2+GeO2)} with a part of La2O3 substituted by Gd2O3 in different concentrations. Positive Weiss constants have been found in more heavily doped glasses; two magnetic transitions at 55 and 12 K have been detected and ascribed, respectively, to ferromagnetic and antiferromagnetic clusters of Gd ions. The EPR spectra confirm the clustering at higher Gd contents. At low temperatures the cluster-related resonance indicates an onset of magnetic anisotropy. The clustering, depending on the Gd concentration, correlates with a shift to lower energies of the strong optical absorption band edge.