Wideband EPR spectroscopy of Y$_3$Al$_5$O$_{12}$ : Ho$^{3+}$ crystals

EPR spectra of Ho$^{3+}$ impurity ions were recorded in single crystals of yttrium aluminum garnet (Y$_3$Al$_5$O$_{12}$) in the frequency range of 114–170 GHz, at a temperature of 4.2 K. The measurement results indicate the substitution of Ho$^{3+}$ ions for Y$^{3+}$ ions in the dodecahedral position with D$_2$ local symmetry. The magnitude of the $g$-factor, the hyperfine structure constant, and the energy interval between the main and the first excited nondegenerate sublevels of the main multiplet $^5$I$_8$ are determined. Weak satellite signals have also been registered, the origin of which is associated with the formation of antisite defects in the environment of Ho$^{3+}$ paramagnetic centers.