  Due to the complex interactions between rare-earth elements and transition
metals, as well as/or themselves, rare-earth transition-metal oxides are likely
to exhibit highly intriguing and novel magnetic structures and dynamic
behaviours. Rare-earth elements in these compounds frequently demonstrate
unusual behaviours in their crystal-field (CF) excitations, which necessitate
thorough research for in-depth comprehensions. When cooling from 10 K to 1.5 K
via the magnetic ordering temperature of Er3+ at 4.1 K, we observed a
significant energy shift of the low-lying CF excitation of Er3+ in ErFeO3 from
0.35 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A
sound CF model was proposed for Er3+ in ErFeO3 by fitting to the observed CF
excitation peaks, which enables to explain all the observed experimental
results in a very consistent manner. According to the model, the ground crystal
field level of Er3+, which corresponds to the lowest Kramers doublet supposed
to be at zero energy, has been shifted by the internal magnetic fields induced
by both Er3+ and Fe3+ spin orders below and above the Er3+ ordering
temperature, respectively. Additional measurements in various magnetic fields
offer compelling evidence in favour of this hypothesis. The measured external
field dependence of the CF excitation energy led to the derivation of the
internal field of Er3+ as 0.33 meV, which is strongly corroborated by
theoretical modelling. Additionally, the effective g-factor for Er3+ in ErFeO3
showed an exceptionally significant anisotropy.
