Abstract
We use inelastic neutron scattering to study the crystalline electric field (CEF) excitations of in (, 0.16). For nonsuperconducting CeFeAsO, the Ce CEF levels have three magnetic doublets in the paramagnetic state, but these doublets split into six singlets when the Fe ions order antiferromagnetically. For superconducting (), where the static antiferromagnetic order is suppressed, the Ce CEF levels have three magnetic doublets at , 18.7, 58.4 meV at all temperatures. Careful measurements of the intrinsic linewidth and the peak position of the 18.7 meV mode reveal a clear anomaly at , consistent with a strong enhancement of local magnetic susceptibility below . These results suggest that CEF excitations in the rare-earth oxypnictides can be used as a probe of spin dynamics in the nearby FeAs planes.
- Received 31 July 2008
DOI:https://doi.org/10.1103/PhysRevLett.101.217002
©2008 American Physical Society