Paramagnetic Tb$^{3+}$ centers in yttrium aluminum garnet

Yttrium-aluminum garnet crystals with terbium impurity were studied by high-frequency EPR in a wide frequency range (70–200 GHz). Along with the Tb$^{3+}$ ions located in the yttrium position in a regular environment, a number of terbium centers with a lower concentration and changed values of the initial splitting of the non-Kramers quasi-doublets were observed. The change in the initial splitting is associated with the presence of antisite defects in the terbium environment. The terbium centers with a smaller initial splitting as compared to the main Tb$^{3+}$ were found and attributed to terbium ions, near which there are AlY antisite defects (aluminum ions in dodecahedral positions of yttrium).