Structural phase transition, electronic structure, and magnetic properties of sol-gel-prepared inverse-spinel nickel-ferrites thin films

Abstract

X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM) were used to investigate the influence of Ni ions on the structural, electronic, and magnetic properties of nickelferrites (NixFe3-xO4). Spinel NixFe3-xO4 (x ≤ 0.96) samples were prepared as polycrystalline thin films on Al2O3 (0001) substrates, using a sol-gel method. XRD patterns of the nickel-ferrites indicate that as the Ni composition increases (x > 0.3), a structural phase transition takes place from cubic to tetragonal lattice. The XPS results imply that the Ni ions in NixFe3-xO4 substitute for the octahedral sites of the spinel lattice, mostly with the ionic valence of +2. The minority-spin d-electrons of the Ni2+ ions are mainly distributed below the Fermi level (EF), at around 3 eV; while those of the Fe2+ ions are distributed closer to EF (~1 eV below EF). The magnetic hysteresis curves of the NixFe3-xO4 films measured by VSM show that as x increases, the saturation magnetization (Ms) linearly decreases. The decreasing trend is primarily attributable to the decrease in net spin magnetic moment, by the Ni2+ (2 μB) substitution for octahedral Fe2+ (4 μB) site. © The Korean Magnetics Society. All rights reserved.