Magnetic structure and CEF levels in CeNiAsO

Some HF materials like CeCu6-xAux and YbRh2Si2 have been interpreted as exhibiting unconventional local criticality, rather than a conventional magnetic instability. However, the situation is far from settled so there is great interest in exploring new HF systems in the proximity of magnetic instabilities. Recent reports on CeNiAsO indicate this is a good candidate for such studies. CeNiAsO is isotructural to the 1111 Fe-based superconductors but exhibits a different type of itinerant magnetism associate with the rare earth site. CeNiAsO has two successive phase transitions at 9.3K and 7.3K that were tentatively associated with antiferromagnetism. To understand these transitions, we carried out magnetic neutron diffraction experiments at the Spallation Neutron Source. We have identified the long range magnetic order and associate the lower transition with a reorientation of the magnetic moment. We also report inelastic neutron powder experiments that determine the crystal field level scheme, which differs from that reported for CeFeAsO and corroborate our analysis of the magnetic structure.