Magnetotransport properties of CrxFe3−xO4 (x≤0.95) thin films prepared using a sol-gel method were measured and analyzed in comparison with their magnetic properties. The samples were polycrystalline and exhibited a slight decrease in lattice constant as the Cr composition (x) increases. The observed decreasing trend in the saturation magnetization (Ms) of CrxFe3−xO4 with increasing x can be explained in terms of the decrease in net spin magnetic moment due to the substitution of Cr3+(3 μB) for octahedral Fe3+(5 μB). The magnetoresistance (MR) of the CrxFe3−xO4 films was found to increase in a linear manner with increasing external field (H≤5 kOe) while the observed magnetization of the samples started to saturate near H=2 kOe. For a given H, MR decreases with increasing x. The observed increase in MR with increasing H in CrxFe3−xO4 is mainly attributable to the reduction in tunneling resistance of spin-polarized carriers through grain boundaries (GBs). Transition layers of magnetization formed around magnetic defects in the GBs are likely to be connected to the spin-dependent tunneling. The decrease in MR with increasing x is attributable to the disorder-induced scattering.
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 (Ni x Fe 3-x O 4 ). Spinel Ni x Fe 3-x O 4 (x ≤ 0.96) samples were prepared as polycrystalline thin films on Al 2 O 3 (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 Ni x Fe 3-x O 4 substitute for the octahedral sites of the spinel lattice, mostly with the ionic valence of +2. The minority-spin d-electrons of the Ni 2+ ions are mainly distributed below the Fermi level (E F ), at around 3 eV; while those of the Fe 2+ ions are distributed closer to E F (~1 eV below E F ). The magnetic hysteresis curves of the Ni x Fe 3-x O 4 films measured by VSM show that as x increases, the saturation magnetization (M s ) linearly decreases. The decreasing trend is primarily attributable to the decrease in net spin magnetic moment, by the Ni 2+ (2 µ B ) substitution for octahedral Fe 2+ (4 µ B ) site.
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