Low energy-low temperature THz excitations in RFeO₃ (R=La, Pr, Er, Lu)

We report on low temperature-low energy absorptions of antiferromagnetic and ferromagnetic magnons at  ~31.4 and ~26.7 cm^-1  in Γ₄ (G_x, A_y, F_z )-LaFeO₃ with degeneracy  lifted linearly by an applied field up to 7 T. In addition to these modes, in LuFeO₃ ligand distortions induced by  Lu 4f¹⁴ smaller radius trigger subtle changes at the perovskite B site, allowing THz Fe³⁺ Zeeman -split electronic crystal field (CF) ground state ⁶A₁(S) transitions at ~10.4 cm^-1. This local non-centrosymmetric departure is also found in ErFeO₃ (Kramers 4f¹¹ Er³⁺ ; Γ₂ (F_x, C_y, G_z)<T_SR ~93 K), but now with the Fe³⁺ Zeeman branching  strongly biased by the 3d-4f  exchange toward higher energies. At 5 K, there is no field induced  band split but a 13-fold  increase in the resonance  antiferro/ferro intensity ratio. It is also remarkable the CF applied field-dependent population, matching balance between Fe³⁺ lower and higher Zeeman -split levels. Some Er³⁺ ground state transitions coincide with phonon frequencies, suggesting strong spin-phonon interactions. Antiferro and ferro resonances turn much broader when non-Kramers Pr³⁺, containing two unpaired 4f² electrons, introduces ligand changes at the A site leading the antiferro mode and the lowest Pr³⁺ CF transition into near degeneracy. They merge into a single broad unresolved feature at 7 T.