# Verdet Constant of Magneto-Active Materials Developed for High-Power Faraday Devices
## 2 Characterization of the Verdet Constant as a Function of Wavelength and Temperature
### VIS-NIR Region (\(400<\lambda<1100\) nm)  
For the specified VIS-NIR region, terbium gallium garnet (TGG) crystal represents the most commonly used magneto-active material used in the FDs. One of the first investigations of this material dates back to 1970s, in which the TGG was proved to have higher Verdet constant [(77)] and higher thermal conductivity [(78)] than the often used Tb-doped magneto-optical glasses. Properties of the TGG crystal and its performance in the Faraday devices have been later investigated in many papers (e.g., in [(17; 18; 49; 56; 79; 80)]). However, the TGG crystal exhibits two major disadvantages: (a) its use in the VIS region is struggling with increasing absorption losses towards the shorter wavelengths [(59; 61)]; and (b) growth of large-aperture TGG crystals is difficult and expensive [(59; 61; 81; 82)]. Although the manufacturing process was greatly improved during the last decades, the largest aperture TGG-crystal-based FD (to the best of our knowledge) was reported in 2015 by [PERSON] et al. [(83)], having a 40-mm clear aperture. For a larger-aperture FD, the magneto-active glasses still represent a viable option for a non-repetitive high-power operation. Because of this advantage, the glass-based magneto-active materials are still being actively developed [(67; 68; 69; 84; 85; 86; 87; 88; 89)]. More and more attention is being put into the investigation of magneto-active transparent ceramics in the recent years [(34)]. A considerable amount of scientific effort has been put into the investigation of magneto-active materials, which could supersede the TGG crystal in the thermo-optical or magneto-optical characteristics or in the large-aperture scalability. One of the materials, which has been frequently co-investigated along with the TGG crystal ever since the 1960s, is the terbium aluminium garnet (TAG) crystal [(63; 56)]. The main advantage of TAG is its \(\sim\)30% higher Verdet constant [(90; 56)] and \(>\)50% higher thermal conductivity [(91)] as compared with the TGG. Nevertheless, TAG crystals are difficult to grow in dimensions suitable for practical applications because of their incongruent melting properties [(92; 90)]. The solution to this drawback has brought the development of the transparent ceramics technology, which enabled production of TAG in the form of ceramics [(93)]. A distinct enhancement of the magneto-optical properties of the TAG ceramics has been obtained by doping with: Ce [(94; 95; 96; 97)], Si [(98)], Ti [(98; 99)], Ho [(100)], Pr [(101)], Tm [(102)] or Tb\({}_{4}\)O\({}_{7}\)[(103; 104)]. A TGG transparent ceramics [(105)], with the properties similar to those of the \(<\)111\(>\)-cut of the single crystal [(45; 106)], has been also successfully manufactured and investigated in detail in the recent years [(29; 58; 107; 108; 109; 110)]. The largest piece of the TGG ceramics, a 100 mm \(\times\) 100 mm slab, was reported in [(111)], greatly overcoming the manufacturing limits of the TGG crystal. Another material, which could be used as an advantageous alternative to the TGG crystal, is the terbium scandium aluminium crystal (TSAG) [(112; 113)], which possesses \(\sim\)25% higher Verdet constant [(114; 115)], lower thermo-optical constant \(Q\) and negative optical anisotropy parameter [(41)]. All of these properties ensure that the TSAG scores much higher in the FOM parameter than TGG, which has marked this material as one of the prime candidates of several other scientific investigations [(116; 117; 118; 37)]. The sodium terbium fluoride (NTF) [(50; 119)] and the potassium terbium fluoride (KTF) crystals [(72; 120; 121)] also deserve attention: although they possess \(\sim\)20% lower Verdet constant, they exhibit a higher FOM parameter than the TGG crystal. Notable characteristics were recently reported for the rare-earth (RE) sesquioxide ceramics (RE\({}_{2}\)O\({}_{3}\); RE = Tb [(122; 123; 124; 125; 126; 127)], Dy [(128; 129; 48; 130)] or Ho [(131; 47; 132; 133)]), which possess high concentrations of the magnetically-active RE\({}^{3+}\) ions.