The Faraday effect is one of the magneto-optical phenomena and refers to the conversion of linearly polarized light passing through a magnetic material into elliptically polarized light with the main axis-containing polarization plane rotated around the propagation vector. The angle by which the polarization plane is rotated, i.e., the Faraday rotation angle, is an important parameter determining the applicability of magnetic materials in devices such as electric-current and magnetic-field sensors, optical isolators, and optical circulators. Since the Faraday effect deals with a transmitted light, the transmittance of the magnetic materials is another important factor for applications. Thus, the materials are required to show a great magneto-optical figure of merit, that is defined as Faraday rotation angle or Verdet constant divided by absorbance or optical absorption coefficient. Here, the Verdet constant is defined as the Faraday rotation angle divided by external magnetic field and light path length inside the magnetic materials. It is well known that single crystals of garnet-type ferrites such as Y₃Fe₅O₁₂ and (Gd,Bi)₃Fe₅O₁₂ exhibit a large Faraday effect and a low optical absorption in the infrared region, especially in a wavelength range from 1.3 μm to 1.5 μm, and that they are effectively utilized as an optical isolator for optical telecommunications. However, compared to the garnet-type ferrites in the infrared region, magneto-optical materials with the superior performance, are lacking in the visible to ultraviolet region. Hence, the development of such materials is still in progress.

Oxide glasses rich in rare-earth ions exhibit a great Faraday effect, especially in the visible to ultraviolet range. Although these glasses feature magnetizations smaller than those of ferro- or ferri-magnetic oxide crystals such as abovementioned Y₃Fe₅O₁₂ because the rare-earth-containing glasses are usually paramagnetic at room temperature, the transmittance of these glasses notably exceeds that of ferrite crystals in the visible to ultraviolet range. In addition, oxide glass has an advantage that it is feasible to tune continuously the composition so that optimized properties are attained and to fabricate large-sized and specific-shaped materials. In addition to the paramagnetic glasses, the Faraday effect of diamagnetic glasses is intensively investigated as well. The magnetization of diamagnetic glasses is further smaller than that of paramagnetic glasses, but the Faraday rotation angle or the Verdet constant of diamagnetic glasses is almost independent of temperature. This is an advantageous point of diamagnetic glasses, which cannot be realized in ferro-magnetic, ferri-magnetic, and para-magnetic materials. Furthermore, for wide-band gap oxide glass like SiO₂ glass, which is diamagnetic, the Faraday effect can occur even in a very short wavelength range such as the deep and vacuum ultraviolet.

This review represents recent development on oxide glasses exhibiting large Faraday rotation. The macroscopic and microscopic mechanism of the Faraday effect are explained. The microscopic mechanism is very important to select magneto-optically active elements and to design glass compositions.Also, the Faraday effect of diamagnetic glasses is described. Heavy-metal oxide glasses and sulfide glasses are intensely exploited because the magnetic susceptibility of diamagnetic materials depends on the constituent atoms (ions) and the susceptibility is proportional to the squared atomic (ionic) radius and the number of electrons contained in the atom (ion). The Verdet constants of these glasses are summarized. The applications of diamagnetic glasses are briefly mentioned.

Subsequently, the Faraday effect of paramagnetic oxide glasses containing large amounts of rare-earth ions is reviewed. The pioneering work in this field has been carried out in the mid-1960s, showing that some ions like Ce³⁺, Pr³⁺, Tb³⁺, Dy³⁺, and Eu²⁺ give rise to larger Verdet constants in the visible range. A description is given to explain why these rare-earth ions exhibit larger Faraday effects than other ones. Recent researches seem to mainly pay attention to Tb³⁺-rich oxide glasses, for which higher concentrations of Tb³⁺ ions simply enhance the Verdet constant. In particular, Tb³⁺-rich oxide glasses fabricated via containerless processing, which is an emerging method and effective to expand the glass-forming region, showing the larger Verdet constant than single-crystalline Tb₃Ga₅O₁₂ used as a commercially available optical isolator in the visible range. Furthermore, EuO-based amorphous oxides that have an unexpected ferromagnetism exhibit rather large Faraday effect.

In addition to the abovementioned diamagnetic oxide glasses and rare-earth-rich oxide glasses, a brief review concerns the Faraday effect of oxide glasses containing large amounts of 3d transition metal ions as well as glass-ceramics comprising ferro- or ferri-magnetic nano-sized crystalline particles embedded in transparent glass matrices.

oxide glass  /  Faraday effect  /  Verdet constant  /  ferromagnetic amorphous oxide  /  rare earth  /  divalent europium