# Verdet Constant of Magneto-Active Materials Developed for High-Power Faraday Devices
## 4 Conclusions
Most of these new successors of the TGG crystal are also Tb-based, having a higher concentration of the Tb\({}^{3+}\) ions. The other commonly investigated materials contain Ce, Pr, Ho or Dy ions, in a combination with the Tb ions. From these, we may mention TAG, TSAG, RE:TGG, or RE:TAG, all of them having approximately a few tenths of percent higher Verdet constant than the TGG. The record holds the Tb\({}_{2}\)O\({}_{3}\) ceramics with more than three times higher Verdet constant than that of the TGG crystal. Reports of Tb-less compounds are less common, but, among those, materials such as Dy\({}_{2}\)O\({}_{3}\), Ho\({}_{2}\)O\({}_{3}\), CeF\({}_{3}\), CeAlO\({}_{3}\) or NaCe(MoO\({}_{4}\))\({}_{2}\) show promising magneto-optical characteristics. The extended FOM parameters in Equations (2)-(4), which offer a much more realistic comparison of these materials under high-power operation, are, unfortunately, for the vast majority of these materials unknown. It is, therefore, one of the main tasks for future research to evaluate the performance of these promising materials under the thermal loading by a high-power laser. A comparison of some of these materials was already reported by the authors of [16; 38] based on a maximum achievable power in a Faraday isolator ensuring a 30 dB isolation ratio. The last spectral region reviewed is the NIR-MIR region (\(\lambda>1100\) nm), for which the ferrimagnetic YIG crystal represents the most commonly used material for low-power laser applications. Most recently, magneto-optical characteristics of a few paramagnetic materials were reported, i.e., Dy-based sesquioxide ceramics or EuF\({}_{2}\) crystal, which could be used as an alternative to the YIG. However, due to the low values of the Verdet constant, their utilization in a FD would demand relatively strong magnetic fields >2 T for achieving standard rotation angles. Therefore, it is crucial to continue the investigation of new materials with higher magneto-optical activity. Conceptualization, D.V., O.S. and A.L. ; methodology, D.V. and O.S. ; software, D.V. and O.S. ; validation, D.V. and O.S. ; formal analysis, D.V. and O.S. ; investigation, D.V. and O.S. ; resources, A.L. and T.M. ; data curation, D.V. and O.S. ; writing--original draft preparation, D.V. ; writing--review and editing, O.S. and A.L. ; visualization, D.V. and O.S. ; supervision, O.S., A.L. and T.M. ; project administration, D.V., O.S., A.L. and T.M. ; and funding acquisition, A.L. and T.M. This article was co-financed by the European Regional Development Fund and the state budget of the Czech Republic (project HiLASE CoE: Grant No. CZ.02.1.01/0.0/0.0/15_006/0000674) and by the European Union's Horizon 2020 research and innovation programme under grant agreement No. 739573. This work was also supportedby the Ministry of Education, Youth and Sports of the Czech Republic (Programmes NPU I Project No. LO1602, and Large Research Infrastructure Project No. LM2015086). The authors would like to express their gratitude to [PERSON] from the National Institute for Fusion Science, Japan, for the fruitful collaboration on the characterization experiments of new magneto-active materials for the high-power Faraday devices. The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.