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 $E{r}^{3+}$ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of $E{r}^{3+}$ in erbium orthoferrite ($ErFe{O}_3$) from 0.32 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for $E{r}^{3+}$ in $ErFe{O}_3$ 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 $E{r}^{3+}$, 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 $E{r}^{3+}$ and $F{e}^{3+}$ spin orders below and above the $E{r}^{3+}$ 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 $E{r}^{3+}$ as 0.54 T, which is strongly corroborated by theoretical modelling. Additionally, the g-factor for the $E{r}^{3+}$ ground state in $ErFe{O}_3$ shows an exceptionally significant anisotropy.