Oxygen vacancy mediated large magnetization in chemically synthesized Ni-doped HfO2 nanoparticle powder samples

Abstract

A cost-effective solution based chemical method of synthesizing nanostructured Hf1−xNixO2 with 0 ≤ x ≤ 0.05 in powder form, from easily available laboratory reagents is presented. Production of uniformly shaped and sized (13–16 nm) nanoparticles with excellent crystallinity is demonstrated by transmission electron microscopy, x-ray diffraction (XRD) studies and Raman spectra. The origin of ferromagnetism in the Ni-doped HfO2 nanoparticle powder samples is investigated. Magnetization studies along with x-ray photoelectron spectroscopy (XPS) studies suggest that some of the Ni-ions are substitutionally incorporated in HfO2 host matrix. The XPS studies also show the presence of a small fraction of Ni metal (most likely Ni nanoclusters), undetected in standard XRD for lightly doped samples, suggesting that the observed room temperature ferromagnetism is at least partly due to Ni nanoclusters. The observed large value (∼6 emu/g) of magnetization, may not be entirely due to the presence of Ni metal cluster, can be understood in terms of oxygen vacancies created to retain charge neutrality of HfO2 matrix as some Ni ions replace Hf4+, resulting in ferromagnetic interactions at lower concentrations.