Application of high-frequency electron paramagnetic resonance/electron spin echo for the identification of the impurity composition and electronic structure of ceramics based garnets

Electron paramagnetic resonance (EPR) spectra of Ce$^{3+}$, Yb$^{3+}$, Cr$^{3+}$, and Gd$^{3+}$ impurity ions in yttrium aluminum garnet Y$_{3}$Al$_{5}$O$_{12}$ (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic $g$-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd$^{3+}$ ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.