Low-temperature terahertz spectroscopy of LaFeO₃, PrFeO₃, ErFeO₃, and LuFeO₃: Quasimagnon resonances and ground-state multiplet transitions

We report on zone center terahertz excitations of non-Jahn Teller LaFeO3, PrFeO3, ErFeO3, and LuFeO3 distorted perovskites under external magnetic fields up 7 T. Our measurements on low-temperature/low-energy absorptions of LaFeO3 show quasiantiferromagnetic (q-AFM) and quasiferromagnetic (q-FM) magnons at omega(qAFM) similar to 31.4 cm(-1) and omega(qFM)similar to 26.7 cm(-1) in the Gamma(4) (G(x), A(y), F-z) representation with near degeneracy linearly lifted by the field. LuFeO3 is characterized by zero-field magnetic resonances at omega(qAFM) similar to 26.3 cm(-1) and omega(qFM) similar to 22.4 cm(-1) in addition to Fe3+ Zeeman-split crystal field (CF) 6A(1) ground transitions at similar to 10.4 cm(-1) triggered by subtle structural deviations induced by the Lu 4f(1)4 smaller ionic radius at the A site. This local quasinoncentrosymmetric departure is also found in ErFeO3 (Kramers 4f(11) Er3+ (I-4(15/2)); Gamma(2)(F-x, C-y, G(z)) < T-SR similar to 93 K) but with the similar to 4 cm(-1) Fe3+ Zeeman branching strongly biased toward higher energies due to 3d-4f exchange. Magnons at omega(qAFM) similar to 31.5 cm(-1) and omega(qFM) similar to 21.5 cm(-1) in ErFeO3 do not undergo field-induced band splits but a 13-fold increase in the q-AFM (omega(qAMF))/q-FM (omega(qAFM)) intensity ratio. There is a remarkable field-dependent CF matching population balance between Fe3+ higher and Er3+ lower Zeeman branches. The Er3+ (I-4(15/2)) multiplet, at the 49.5, 110.5, and 167.3 cm(-1), coincides with external lattice mode frequencies, suggesting strong lattice-driven spin-phonon interactions. Far-infrared absorption ratios under mild external fields reveal magnetic dependence only for those zone-center phonons involving moving magnetic ions. Overall, our results support the viability of magnetic state manipulation by phonons. Quasiantiferroresonances and quasiferroresonances in PrFeO3 turn much broader as non-Kramers Pr-3 introduces ligand changes at the A site, leading into near degeneracy the q-AFM mode and the lowest Pr3+ CF transition. They merge into a single broad mostly unresolved feature at 7 T. We conclude that low-energy excitations in RFeO3 (R = rare earth) strongly depend on the lanthanide ionic size, thus indivisibly tied to the mechanism associated with the origin of canted FM. In addition, minute lattice displacements also underlie considering noncentrosymmetric the most distorted RFeO3 (R = rare earth). In these perovskites, the changes triggered in the lattice by the smaller rare earth and the nonlinear intrinsic oxygen ion polarizability, known to drive lattice instabilities, provide grounds for interplay of ionic and electronic interactions yielding ferroelectric spontaneous polarization.