Role of Dopant Incorporation on the Magnetic Properties of Ce_1−xNi_xO₂ Nanoparticles: An Electron Paramagnetic Resonance Study

Nickel doping has been found to produce weak room-temperature ferromagnetism (FM) in CeO₂. The saturation magnetization (M_s) of the chemically synthesized Ce_1−xNi_xO₂ samples showed a maximum for x = 0.04, above which the magnetization decreased gradually. For Ce_1−xNi_xO₂ samples with x ≥ 0.04, an activation process involving slow annealing of the sample to 500 °C increased the M_s by more than two orders of magnitude. However, no such activation effect was observed in samples with xx for x>0.04, and (ii) the dramatic increase in M_s in the activated Ce_1−xNi_xO₂ samples with x ≥ 0.04 and the absence of this behavior in samples with x1−xNi_xO₂ samples with 0.01 ⩽ x ⩽ 0.10 at 5 and 300 K indicates the presence of several paramagnetic species: (i) two magnetically inequivalent Ni²⁺ ions with the ionic spin S = 1, (ii) one Ce³⁺ ion with spin S = 1/2, and (iii) three O₂⁻ defects with S = 1/2. The spectra of the samples with x2+ EPR line ascribed to dopant ions in substitutional sites, whereas in samples with x ≥ 0.04, there is an additional EPR line attributed to Ni²⁺ ions occupying interstitial sites. In the activated sample with x = 0.08, the EPR line due to the interstitial Ni²⁺ ions is completely absent, and only the line due to substitutional Ni²⁺ ions is present, suggesting that the enhanced FM arises from migration of Ni²⁺ ions from interstitial to substitutional sites.