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 Ce O2. The saturation magnetization (Ms) of the chemically synthesized Ce1-x Nix O2 samples showed a maximum for x=0.04, above which the magnetization decreased gradually. For Ce1-x Nix O2 samples with x0.04, 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 x<0.04. Electron paramagnetic resonance (EPR) technique has been exploited to understand (i) the gradual decrease in the FM and subsequent disappearance of FM with increase in x for x>0.04, and (ii) the dramatic increase in Ms in the activated Ce1-x Nix O2 samples with x0.04 and the absence of this behavior in samples with x<0.04. Detailed analysis by simulation of the EPR data on several as-prepared Ce1-x Nix O2 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=12, and (iii) three O2- defects with S=12. The spectra of the samples with x<0.04 are dominated by a single Ni2+ EPR line ascribed to dopant ions in substitutional sites, whereas in samples with x0.04, 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.