Spin dynamics effects in submillimeter EPR spectroscopy of impurity thulium ions in synthetic forsterite

The specific features of the EPR spectra of Tm$^{3+}$ impurity ions in synthetic forsterite have been studied by continuous-wave EPR spectroscopy in the frequency range of 270–310 GHz at a temperature of 4.2 K in weak magnetic fields. Narrow resonance signals unrelated to the modulation of the resonance conditions of EPR under the modulation of the external magnetic field have been discovered in measurements at frequencies corresponding to the zero field splitting between the ground and first excited singlet electron states of Tm$^{3+}$ ions in zero magnetic field. The origin of these narrow lines is discussed.