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Magnetic susceptibility, electron paramagnetic resonance ͑EPR͒, and optical properties have been studied in a glass system ͕20La 2 O 3 − 22Al 2 O 3 − 23B 2 O 3 −35͑SiO 2 +GeO 2 ͖͒ with a part of La 2 O 3 substituted by Gd 2 O 3 in different concentrations. Positive Weiss constants have been found in the more heavily doped glasses and ascribed to clustering of Gd 3+ ions. Two magnetic phase transitions at 55 and 12 K were detected and ascribed, respectively, to ferromagnetic and antiferromagnetic clusters containing Gd ions. The overall shape of the EPR spectra shows the presence of clustering at the higher Gd contents. At low temperatures the cluster-related resonance signal is altered in shape, indicating an onset of magnetic anisotropy field. This signal is convincingly fitted to superparamagnetic resonance arising from ferromagnetic nanoparticles. The clustering, depending on the Gd concentration, correlates with a significant shift to lower energies of the strong optical absorption band edge, ascribed to a charge transfer transition between Gd ions. A nonmonotonous change of refractive index with the increase of the Gd content indicates changes in the glass matrix and in Gd cluster structure.

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