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Low temperature Terahertz Spectroscopy of LaFeO, PrFeO, ErFeO, and LuFeO: Quasimagnon resonances and ground multiplet transitions
We report on zone center THz excitations of non-Jahn Teller LaFeO,
PrFeO, ErFeO, and LuFeO distorted perovskites under external
magnetic fields up 7 T. Low temperature-low energy absorptions of LaFeO
show antiferromagnetic and ferromagnetic quasimagnons at AFM ~31.4 and
FM ~26.7 cm in the 4 (Gx, Ay, Fz) representation.
LuFeO is characterized by zero field magnetic resonances at AFM
~26.3 cm and FM ~22.4 cm in addition to Fe
Zeeman-split crystal field (CF) 6A ground transitions at ~10.4 cm
triggered by structural deviations induced by smaller Lu 4f. This local
non-centrosymmetric departure is also found in ErFeO (Kramers 4f
Er (4I15/2); {\Gamma}2 (Fx, Cy, Gz) <TSR ~93 K), but with the ~4
cm Fe Zeeman branching strongly biased toward higher energies.
Quasimagnons at AFM ~31.5 cm and FM ~21.5 cm in
ErFeO do not undergo field induced band splits but a 13-fold increase in
the antiferro (AMF) /ferro(AFM) intensity ratio. There is a
remarkable field-dependent CF matching population balance between Fe
higher and Er lower Zeeman branches. Antiferro- and ferro- resonances in
PrFeO turn much broader as non-Kramers Pr introduces ligand changes at
the A site leading into near degeneracy the antiferromagnetic mode and the
lowest Pr CF transition. We conclude that low energy excitations in
RFeO (R=rare earth) strongly depend on the lanthanide ionic size. Minute
lattice displacements also underlie considering non-centrosymmetric the most
distorted RFeO (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.Comment: Full Manuscript and Supplemental Material, 73 pages, 27 figure