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 ${\mathrm{LaFeO}}_{3}, {\mathrm{PrFeO}}_{3}, {\mathrm{ErFeO}}_{3}$, and ${\mathrm{LuFeO}}_{3}$ distorted perovskites under external magnetic fields up 7 T. Our measurements on low-temperature/low-energy absorptions of ${\mathrm{LaFeO}}_{3}$ show quasiantiferromagnetic (q-AFM) and quasiferromagnetic (q-FM) magnons at ${\ensuremath{\omega}}_{q\mathrm{AFM}}\ensuremath{\sim}31.4\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{q\mathrm{FM}}\ensuremath{\sim}26.7\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in the ${\mathrm{\ensuremath{\Gamma}}}_{4}\phantom{\rule{0.28em}{0ex}}({G}_{x},\phantom{\rule{0.28em}{0ex}}{A}_{y},\phantom{\rule{0.28em}{0ex}}{F}_{z})$ representation with near degeneracy linearly lifted by the field. ${\mathrm{LuFeO}}_{3}$ is characterized by zero-field magnetic resonances at ${\ensuremath{\omega}}_{q\mathrm{AFM}}\ensuremath{\sim}26.3\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{q\mathrm{FM}}\ensuremath{\sim}22.4\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in addition to ${\mathrm{Fe}}^{3+}$ Zeeman-split crystal field (CF) $6{\mathrm{A}}_{1}$ ground transitions at $\ensuremath{\sim}10.4\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ triggered by subtle structural deviations induced by the $\mathrm{Lu}\phantom{\rule{0.28em}{0ex}}4{f}^{1}4$ smaller ionic radius at the A site. This local quasinoncentrosymmetric departure is also found in ${\mathrm{ErFeO}}_{3}$ (Kramers $4{f}^{11} {\mathrm{Er}}^{3+}\phantom{\rule{0.28em}{0ex}}(^{4}\mathrm{I}_{15/2})$; ${\mathrm{\ensuremath{\Gamma}}}_{2}({F}_{x},\phantom{\rule{0.28em}{0ex}}{C}_{y},\phantom{\rule{0.28em}{0ex}}{G}_{z})<{T}_{\mathrm{SR}}\ensuremath{\sim}93\phantom{\rule{0.28em}{0ex}}\mathrm{K})$ but with the $\ensuremath{\sim}4\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1} {\mathrm{Fe}}^{3+}$ Zeeman branching strongly biased toward higher energies due to $3d\text{\ensuremath{-}}4f$ exchange. Magnons at ${\ensuremath{\omega}}_{q\mathrm{AFM}}\ensuremath{\sim}31.5\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{q\mathrm{FM}}\ensuremath{\sim}21.5\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ in ${\mathrm{ErFeO}}_{3}$ do not undergo field-induced band splits but a 13-fold increase in the q-AFM (${\ensuremath{\omega}}_{q\mathrm{AMF}}$)/q-FM (${\ensuremath{\omega}}_{q\mathrm{AFM}}$) intensity ratio. There is a remarkable field-dependent CF matching population balance between ${\mathrm{Fe}}^{3+}$ higher and ${\mathrm{Er}}^{3+}$ lower Zeeman branches. The ${\mathrm{Er}}^{3+}\phantom{\rule{0.28em}{0ex}}(^{4}\mathrm{I}_{15/2})$ multiplet, at the 49.5, 110.5, and $167.3\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, 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 ${\mathrm{PrFeO}}_{3}$ turn much broader as non-Kramers ${\mathrm{Pr}}^{3}$ introduces ligand changes at the A site, leading into near degeneracy the q-AFM mode and the lowest ${\mathrm{Pr}}^{3+}$ CF transition. They merge into a single broad mostly unresolved feature at 7 T. We conclude that low-energy excitations in $R{\mathrm{FeO}}_{3}$ ($R=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth}$) 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 $R{\mathrm{FeO}}_{3}$ ($R=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth}$). 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.