  Crystal-field excitations, for example in transition-metal oxides where a
rare-earth element is used as a spacer between the transition-metal-oxide
tetrahedra and octahedra, are assumed to be extremely robust with respect to
external perturbations such as temperature. Using inelastic neutron scattering
experiments, a giant shift of the energy of the lowest crystal-field excitation
of Er3+ (4I15/2) in ErFeO3 from 0.30(2) meV to 0.75(2) meV was measured below
the magnetic-ordering temperature of erbium at 4.1 K. Quantum-mechanical
point-charge calculations of the crystal-field levels indicate that the shift
is caused by the internal magnetic field created by the erbium spins
themselves, which causes a Zeeman splitting of the erbium 4f electronic levels,
and therefore a change in the energies of crystal-field transitions. To verify
this explanation, the effect of an external magnetic field on the crystal-field
excitations was measured by inelastic neutron scattering and compared to the
field-dependent point-charge calculations. The existence of an internal
magnetic exchange interaction will have implications for a deeper understanding
of a broader group of phenomena such as multiferroic properties or spin
frustration, which are a consequence of various competing electronic and
magnetic exchange interactions.
