Large Energy Shifts of Crystal-field Excitations in Erbium Orthoferrite Driven by Internal Magnetic Fields

Due to the complex interactions between rare-earth elements and transition metals, as well as between 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 studies for in-depth comprehensions. When cooling from 10 K to 1.5 K through the magnetic ordering temperature of Er^3+ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of Er^3+ in erbium orthoferrite (ErFeO_3) from 0.32 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for Er^3+ in ErFeO_3 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 Er^3+, which corresponds to the lowest Kramers doublet supposed to be at zero energy transfer, has been shifted by the internal magnetic fields induced by both Er^3+ and Fe^3+ spin orders below and above the Er^3+ 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 a derivation of the internal field of Er^3+ as 0.54 T, which is strongly corroborated by theoretical modelling. Additionally, the g-factor for the Er^3+ ground state in ErFeO_3 shows an exceptionally significant anisotropy.