Role of Dopant Incorporation on the Magnetic Properties of Ce\u3csub\u3e1−x\u3c/sub\u3eNi\u3csub\u3ex\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e Nanoparticles: An Electron Paramagnetic Resonance Study

Nickel doping has been found to produce weak room-temperature ferromagnetism (FM) in CeO2. The saturation magnetization (Ms) of the chemically synthesized Ce1−xNixO2 samples showed a maximum for x = 0.04, above which the magnetization decreased gradually. For Ce1−xNixO2 samples with x ≥ 0.04, an activation process involving slow annealing of the sample to 500 °C increased the Ms by more than two orders of magnitude. However, no such activation effect was observed in samples with xx for x\u3e0.04, and (ii) the dramatic increase in Ms in the activated Ce1−xNixO2 samples with x ≥ 0.04 and the absence of this behavior in samples with x1−xNixO2 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