High-temperature magnetic-field-induced activation of room-temperature ferromagnetism in Ce1-xNixO2

We report room-temperature ferromagnetism in nickel doped ceria (Ce1-xNixO2) 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 (8.6x10(-4) mu(B)/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 degrees 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 CeO2 with saturation magnetization M-s 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. (c) 2007 American Institute of Physics.