Synthesis and room-temperature ferromagnetism of CeO2 nanocrystals with nonmagnetic Ca2+ doping

This work initiates an investigation on the creation of room-temperature ferromagnetism in diamagnetic solid via a nonmagnetic doping. CeO2 is taken as a prototype compound. A series of Ce1-xCaxO2-delta solid solutions with x=0-0.25 were prepared by a solution combustion method using L-glutamic acid as the fuel. Sample characterization indicates that all as-prepared samples crystallized in a pure face-centered cubic fluorite structure with crystallite sizes smaller than 10 nm. On increasing the dopant content, the crystallite size decreased from about 8 to 4 nm, while the lattice parameter increased systematically. At room temperature, all samples are ferromagnetic admixed with an apparent diamagnetic component, as composed to a mixture of paramagnetic and antiferromagnetic behaviors or the absence of the ferromagnetism reported in the literature (Bouaine et al 2007 J. Phys. Chem. C 111 2924-8; Ney et al 2008 Phys. Rev. Lett. 100 157201) for magnetically doped semiconductors such as Sn1-xCoxO2 and Zn1-xCoxO. The coercivity and saturation magnetization for x=0 are 110 Oe and 2.01 x 10(-4) emu g(-1), respectively, which slightly increased to 148 Oe and 2.26 x 10(-4) emu g(-1) on increasing the dopant content to x=0.10. Upon annealing the as-prepared samples at 800 degrees C for 2 h, these magnetic parameters were weakened. This observation was interpreted in terms of the mixed valence state of Ce3+/Ce4+ and the doping effects. Finally, the nature of the room-temperature ferromagnetism is discussed by taking into account the oxygen vacancies and defects at the surface/interfaces that act as the electron and hole traps.