Magnetic properties and structure of pulsed laser deposited nickel ferrite films
Series of heteroepitaxy spinel ferrite thin films (NixFe 3-xO4) were prepared by the alternating target laser ablation deposition (ATLAD) technique. Different from conventional pulsed laser ablation deposition (PLD), the AT-LAD technique allows different laser pulses incident upon multiple targets alternatively to grow the film in an atomic layer-by-layer fashion. In this research, targets of NiO and Fe2O3 were used to grow Nix Fe3-xO4 films on single crystal MgO (111) substrates. The substrate temperature and oxygen pressure were fixed at 900°C and 5 mTorr, respectively. Values of x were varied by changing the number of laser pulses on NiO and Fe2O3 targets within each cycle.
NixFe3-xO 4 films, with spinel structure oriented along <111> direction, were characterized by CuKα x-ray diffraction (XRD). The maximum lattice parameter (8.353 Å) was obtained at a laser shot ratio Fe:Ni∼2 which is also larger than the bulk value (8.339 Å) . We studied the surface and cross-section morphology using scanning electron microscope (SEM). The SEM images revealed triangle shaped grains oriented along the substrate orientation, the <111> direction. Energy dispersive x-ray analysis (EDAX) was utilized to identify the atomic ratios of Ni and Fe elements. It is revealed Fe:Ni is close to 2 when the laser pulses on NiO and Fe2O3 target were 5 and10 per ATLAD cycle, respectively. The saturation magnetization, was 4πMs, of the NiFe 2O4 thin films, measured by vibrating sample magnetometer (VSM), ∼4,000G, which is larger than the bulk value (∼3,400G).
Microwave properties were measured using a ferromagnetic resonance (FMR) spectrometer. Using the Smit & Beljar FMR equations, the minimum anisotropy constants, K⊥ = -238700 (erg/cm 3), obtained corresponding to an atomic ratio Fe:Ni∼2. The maximal FMR linewidth, ΔH(Oe)=1061(Oe), was also obtained from this film. The corresponding effective gyromagnetic factor (g eff) is 2.00, which is obtained using the plot of f as a function of Hi.
We calculated the cation distribution based on EDAX determined atomic ratio (Fe:Ni) and the measured 4πMS. This information can also be confirmed by synchrotron based x-ray absorption spectra analysis. The results indicated more Ni ions on A sites in ATLAD films compared with bulk. This explains the expanded lattice parameter and the increase of 4π MS.