High-temperature magnetic-field-induced activation of room-temperature ferromagnetism in Ce1-x Nix O2

We report room-temperature ferromagnetism in nickel doped ceria (Ce1-x Nix O2) powders prepared using the sol-gel process. Magnetometry studies on as-prepared samples reveal a weak ferromagnetic (FM) behavior in the range 0<x<0.04 (8.6× 10-4 μB Ni ion for 4%). For x>0.04, the FM magnetization steadily decreases for increased dopant concentration. The weak FM behavior of the samples improved dramatically after undergoing high-temperature (≤500 °C) activation in the presence of an external magnetic field (H≤1 T). The magnitude of the improvement, which was very reproducible, was strongest for 10% Ni doped Ce O2 with saturation magnetization Ms increasing by 600 times. Curie temperatures of the activated samples varied in the 597-646 K range depending on the Ni concentration and preparation conditions. X-ray diffraction studies did not reveal any noticeable structural changes as a result of the activation process.