Structural, magnetic and spectroscopic study of a diluted magnetic oxide: Co doped CeO2−δ
Résumé
A structural, spectroscopic and magnetic study of Co doped CeO2−δ diluted magnetic oxide (DMO) thin films grown by pulsed laser deposition is presented. No secondary phase or metallic clusters could be detected. The samples are ferromagnetic at room temperature and epitaxial films display a large magnetic anisotropy with an out-of-plane easy axis. The evolution of the magnetization with temperature did not reveal any superparamagnetic signal related to nanosized clusters, reinforcing the conclusion that the ferromagnetism is intrinsic in this system. The magnetization at saturation has the same magnitude (~1.4–1.5 μB/Co) in epilayers and textured films and shows no clear dependence on the structural quality, contrary to other DMOs. It is also shown that ferromagnetism is not restricted to a particular region of the films, such as the interface. The ferromagnetic signal depends sensitively on the oxygen pressure during growth and post-growth annealing. The valence of Ce and Co was deduced from x-ray spectroscopies, revealing a predominant Co2+ state. The origin of ferromagnetism in Co doped CeO2−δ is discussed in connection with possible charge-compensating defects and existing models describing indirect exchange in DMOs.