Magnons and crystal field transitions in hexagonal REMnO3 (RE = Er, Tm,   Yb, Lu) single crystals
  Far-infrared optical transmission spectra of the antiferromagnetic resonances, or magnons, and crystal field transitions have been studied in hexagonal REMnO3 (RE = 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 K and 100 K. The magnon frequency increases systematically with a decrease of the RE ion radius. The magnetic ordering of RE ions (RE=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 main magnetic interactions in the system including exchange of the Mn3+ spins with RE3+ paramagnetic moments. 
E. C. Standard, T. Stanislavchuk, A. A. Sirenko, N. Lee, and S.-W.   Cheong
cond-mat.mtrl-sci cond-mat.str-el