Ion valence state and magnetic origin of PbPd 1−x Ni x O 2 nanograin films with a high-temperature ferromagnetism

The PbPd1 − xNi x O2 nanograin films with different Ni-doping levels (x = 0.037–0.150) were synthesized by the sol–gel spin-coating method and an oxidation treatment. The films with a thickness of about 210 nm were found to be single phase with a body-centered orthorhombic structure and to possess a large number of Pb vacancies. High-temperature ferromagnetism discovered in these films can be maintained above 380 K. Paramagnetism was also found in the films with high Ni-doping levels. The analysis on the XANES spectra and their first derivatives offered the evidences for the facts that the film’s ferromagnetism is intrinsic and the increase in the Pb valence from 2+ towards 4+, caused by the appearance of large amount of Pb vacancies and low electronegativity of Pb2+ ion, provides magnetic moments to the film’s magnetism. A carrier-mediated mechanism bridged to the bound magnetic polaron model based on the Pb vacancies, the doped Ni ions and the Pb ions with a valence higher than 2+ was adopted to interpret the origin of the magnetism within these PbPd1 − xNi x O2 nanograin films. The variation of the specific area of the grain boundary was believed to have a great influence on the proportions of the film’s ferromagnetism and paramagnetism.