Wide-band EPR spectroscopy of a Lu$_3$Al$_5$O$_{12}$:Tb$^{3+}$ crystal

In a single crystal of lutetium aluminum garnet (Lu$_3$Al$_5$O$_{12}$, LuAG) in the frequency range of 37–210 GHz, at a temperature of 4.2 K, the EPR spectra of non-Kramers Tb$^{3+}$ impurity ions were recorded. The measurement results indicate that Tb$^{3+}$ ions replace Y$^{3+}$ ions in the dodecahedral position with local D$_2$ symmetry. The value of the $g$ factor, the hyperfine structure constant, and the energy interval between the ground and the first excited nondegenerate sublevels of the ground multiplet are determined. Weak satellite signals were also registered, the origin of which is associated with the formation of antisite defects in the environment of paramagnetic Tb$^{3+}$ centers.