EPR experiments in LiTb${\mathrm{F}}_4$, LiHo${\mathrm{F}}_4$, and LiEr${\mathrm{F}}_4$ at submillimeter frequencies

Electron-paramagnetic-resonance experiments in LiTb${\mathrm{F}}_4$, LiHo${\mathrm{F}}_4$, and LiEr${\mathrm{F}}_4$ have been performed at frequencies between 70 and 600 GHz, in magnetic fields up to 60 kG and in the temperature range $1.4<T<\mathrm{}25\mathrm{}$ K. The experiments yield precise information on the lowest levels of the rare-earth ions. We investigated the resonance line position and the linewidth as a function of frequency and temperature. The $g$ values and the couplings between the lowest levels are determined. The local field around the magnetic ion is measured directly and the Curie constant ${\Theta }_C$ can be calculated. The results give the single-ion parameters appropriate to a description of the low-temperature magnetism in these compounds.