Some pecularities of the 94 GHz EPR spectra of Tb$^{3+}$ in yttrium-aluminum garnet

In terbium-containing yttrium-aluminum garnet crystals, along with Tb$^{3+}$ ions located in the yttrium position in a regular environment, EPR of terbium centers was observed with the initial splitting of the non-Kramers quasidoublet near the photon energy of 94 GHz was observed. They belong to terbium ions near which there are YAl antisite defects (yttrium ions in octahedral aluminum positions). Using the technique of magnetic field modulation, operating frequency modulation and electron spin echo, the features of the EPR spectra of such centers in low magnetic fields were studied. EPR spectra of new centers with a hyperfine structure characteristic of Tb$^{3+}$ and low intensity (approximately 0.05% compared to the main centers) were also discovered. The number of lines and the symmetry of these centers indicate that they may belong to Tb$^{3+}$ ions replacing Al$^{3+}$ ions in the octahedral sites of the YAG crystal lattice.