Dynamical Faraday rotation effects of sol–gel derived Al 2 O 3 –SiO 2 glass containing Eu 2+ ions T. Hayakawa * , M. Nogami Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya, Aichi 466-8555, Japan Received 16 February 2000; received in revised form 7 June 2000; accepted 29 June 2000 by H. Kamimura Abstract The optical and magnetic characterizations of Eu 2+ -doped Al 2 O 3 –SiO 2 glass, which was synthesized by a sol–gel technique, were performed on the basis of absorption–fluorescence spectra, a dynamical Faraday rotation, a static magnetic susceptibility, and an electron paramagnetic resonance (EPR). The dynamics of Faraday rotation in pulsed magnetic fields revealed the formation of anti-coupled Eu 2+ –O–Eu 2+ magnetic pairs at low temperature, as well as two contributions to the total rotation: a paramagnetic due to Eu 2+ ions and an antimagnetic presumably due to Al-associated defect centers generated by H 2 -impreg- nation. The broad feature around g  2:00 in the S-state EPR spectra implied a multi-site distribution of Eu 2+ ions in Eu,Al-rich phase produced in the sol–gel transformation. The influence of Al 2 O 3 on the aggregation of europium ions in a SiO 2 network was discussed.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Disordered system; B. Sol–gel; C. Magnetic cluster; D. Faraday rotation PACS: 75.50.Kj; 78.20.Ls 1. Introduction The magneto-optic properties of rare-earth doped glasses have stimulated considerable interest because of their advantages in optical telecommunications [1–3]. Faraday rotation glasses are promising as optical isolators, circula- tors, and switches in all opto-communication systems. Shafer and Suits [4] reported the large Faraday rotation of EuO–Al 2 O 3 –B 2 O 3 glass in the visible region. Among the 14 ions in the lanthanide series, Ce 3+ , Dy 3+ and Tb 3+ and Eu 2+ exhibit relatively large Faraday effects. It is because the lowering of the 4f–5d transition energy [5] is responsible for the large opto-magnetic properties. The magnetic rota- tion of the plane of polarization of light is usually in terms of the Verdet constant V [6–8], which is defined by the equa- tion u F  VBl 1 where u F is the rotation angle, l the length of the medium and B the density of the applied magnetic-flux. Recently, Tb 2 O 3 –B 2 O 3 –SiO 2 glasses containing highly concentrated terbium oxide have been synthesized [9]. The glasses were paramagnetic and had considerable Verdet constant at room temperature (-0.492 min/(Oe cm) at 632.8 nm 1 ), compared to other highly rare-earth doped glasses [1,4,10,11]. However, it seems impossible to incorporate by far more rare-earth ions into a glass because of their less covalent bonding to such anions in a glass network former as O 2- or F - . Even if possible, its transparency would be reduced. Hayakawa et al. [9] also observed an extraordinary temperature-dependence on the Verdet constant of the Tb 2 O 3 -containing borosilicate glasses, which did not obey the paramagnetic law of Faraday rotation [12]. Around the temperature where the derivation appeared (50 K), the lifetime of the 5 D 4 level in Tb 3+ was significantly dependent Solid State Communications 116 (2000) 77–82 0038-1098/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0038-1098(00)00282-9 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: +81-52-735-5110; fax: +81-52- 735-5294. E-mail address: hayatomo@mse.nitech.ac.jp (T. Hayakawa). 1 T. Hayakawa (unpublished data). This value was estimated in the cgs unit system. One can convert it to a value in the MKS unit system by the following equation: 1 [min/(Oe cm)]  2.909 × 10 2 [rad/(T m)], where Tesla is used as a unit for magnetic field.