Low temperature Terahertz Spectroscopy of LaFeO3, PrFeO3, ErFeO3, and LuFeO3: Quasimagnon resonances and ground multiplet transitions
Abstract
We report on zone center THz excitations of non-Jahn Teller LaFeO3, PrFeO3, ErFeO3, and LuFeO3 distorted perovskites under external magnetic fields up 7 T. Low temperature-low energy absorptions of LaFeO3 show antiferromagnetic and ferromagnetic quasimagnons at ωAFM ~31.4 and ωFM ~26.7 cm-1 in the 4 (Gx, Ay, Fz) representation. LuFeO3 is characterized by zero field magnetic resonances at ωAFM ~26.3 cm-1 and ωFM ~22.4 cm-1 in addition to Fe3+ Zeeman-split crystal field (CF) 6A1 ground transitions at ~10.4 cm-1 triggered by structural deviations induced by smaller Lu 4f14. This local non-centrosymmetric departure is also found in ErFeO3 (Kramers 4f11 Er3+ (4I15/2); 2 (Fx, Cy, Gz) <TSR ~93 K), but with the ~4 cm-1 Fe3+ Zeeman branching strongly biased toward higher energies. Quasimagnons at ωAFM ~31.5 cm-1 and ωFM ~21.5 cm-1 in ErFeO3 do not undergo field induced band splits but a 13-fold increase in the antiferro (ωAMF) /ferro(ωAFM) intensity ratio. There is a remarkable field-dependent CF matching population balance between Fe3+ higher and Er3+ lower Zeeman branches. Antiferro- and ferro- resonances in PrFeO3 turn much broader as non-Kramers Pr3 introduces ligand changes at the A site leading into near degeneracy the antiferromagnetic mode and the lowest Pr3+ CF transition. We conclude that low energy excitations in RFeO3 (R=rare earth) strongly depend on the lanthanide ionic size. Minute lattice displacements also underlie considering non-centrosymmetric the most distorted RFeO3 (R=rare earth). Changes triggered by the smaller rare earth and the nonlinear intrinsic oxygen ion polarizability provide grounds for interplay of ionic and electronic interactions yielding ferroelectric spontaneous polarization.