We systematically investigated crystal-field (CF) excitations of Kramers (Nd$^{ 3+}$, Er$^{ 3+}$, Yb$^{ 3+}$) and non-Kramers (Pr$^{ 3+}$, Ho$^{ 3+}$) rare-earth ions in $REFeO_3$ using optimized CF simulations. Internal magnetic fields from the Fe$^{ 3+}$ and RE$^{ 3+}$ sublattices split the ground-state doublets of all Kramers ions (e.g. $NdFeO_3$, $ErFeO_3$, and $YbFeO_3$), generating low-energy excitations around 1 meV. In non-Kramers systems, low-energy excitations arise only when the ground state forms an accidental pseudo-doublet, as observed for Ho$^{ 3+}$ in $HoFeO_3$; such pseudo-doublets exhibit field-induced splitting analogous to Kramers ions. In contrast, true singlet ground states, exemplified by Pr$^{ 3+}$, show no zero-field splitting. Strong anisotropies are found in both internal- and external-field responses of the CF excitations in these $REFeO_3$. These results provide a unified explanation for the anomalous Zeeman splitting of CF ground states in $REFeO_3$.
Categories: cond-mat.str-el cond-mat.other