Magnons and crystal-field transitions in hexagonal RMnO ₃ (R = Er, Tm, Yb, Lu) single crystals

Far-infrared optical transmission spectra of the antiferromagnetic resonances, or magnons, and crystal-field (CF) transitions have been studied in hexagonal RMnO ₃ (R = Er, Tm, Yb, Lu) single crystals. The magnon and CF frequencies, their oscillator strengths, and effective g factors have been measured using external magnetic fields up to 9 T in the temperature range between 1.5 and 100 K. The magnon frequency increases systematically with a decrease of the R ion radius. The magnetic ordering of R ions (R = Er, Tm, Yb) was observed at low temperatures T<3.5 K and in strong external magnetic fields. The observed effects are analyzed taking into account the main magnetic interactions in the system including exchange of the Mn3 ⁺ spins with R3 ⁺ paramagnetic moments.