NiFe2O4 Films Research Articles

Microstructural and ferromagnetic resonance properties of epitaxial nickel ferrite films grown by chemical vapor deposition

Microstructural and ferromagnetic resonance properties of epitaxial nickel ferrite (NiFe2O4) films grown by direct liquid injection chemical vapor deposition are reported. While high-quality epitaxial growth of NiFe2O4 films on (100)-oriented MgAl2O4 substrate is confirmed by high resolution transmission electron microscopy, bright field (diffraction contrast) TEM studies reveal the presence of dislocations and also dark diffused contrast areas, which originate from antiphase domains. Angle and frequency-dependent ferromagnetic resonance (FMR) experiments are conducted to determine the magnetic anisotropy and the magnetic relaxation. A low out-of-plane FMR linewidth of ∼160 Oe has been observed at a frequency of 10 GHz.

Growth of (111) oriented NiFe2O4 polycrystalline thin films on Pt (111) via sol-gel processing

Polycrystalline NiFe2O4 (NFO) thin films are grown on (111) platinized Si substrates via chemical solution processing. θ-2θ x-ray diffraction, x-ray pole figures and electron diffraction indicate that the NFO has a high degree of 〈111〉 uniaxial texture normal to the film plane. The texturing is initiated by nucleation of (111) planes at the Pt interface and is enhanced with decreasing film thickness. As the NFO magnetic easy-axis is 〈111〉, the out-of-plane magnetization exhibits improved Mr/Ms and coercivity with respect to randomly oriented films on silicon substrates. The out-of-plane Mr/Ms ratio for (111) textured NFO thin film is improved from 30% in 150 nm-thick films to above 70% in 50 nm-thick films. The improved out-of-plane magnetic anisotropy is comparable to epitaxial NFO films of comparable thickness deposited by pulsed laser deposition and sputtering.

Potential Sensor Based on Electrochemical ${\rm NiFe}_{2}{\rm O}_{4}$ Film Prepared by EB-PVD

The potential sensor is the key element and the major obstacle to realizing corrosion monitoring and control of the infrastructures in civil engineering. A novel solid-state potential sensor is prepared by the electron-beam physical vapor deposition technique. The phase composition of the oxidation film is investigated by X-ray diffraction. Also, the montmorillonite material with three-dimensional layered structure is used as the carrier material to prepare the conductive layer of the potential sensor. Furthermore, the potential stability of the sensor is verified in the simulated concrete pore solution. The results indicate that the potential sensor based on NiFe2O4 film is suitable to assembling the corrosion monitoring and control systems in civil engineering.

Electrochemical characterization of the solid-state reference electrode based on NiFe2O4 film for the corrosion monitoring of RC structures

Abstracts The electrochemical functional NiFe 2 O 4 film is applied to assemble the solid-state reference electrode, and the advanced material with three-dimensional layered structure is used as the carrier to prepare the conductive layer. The electrochemical features of the reference electrode including potential stability, influence of typical ions and temperature response are investigated in detail. Also, the polarization performance is verified. Furthermore, the electrochemical impedance spectroscopy of the reference electrode is tested. The results provide the baseline for the application of the solid-state reference electrode in the corrosion monitoring of reinforced concrete structures.

Structural features of epitaxial NiFe2O4 thin films grown on different substrates by direct liquid injection chemical vapor deposition

NiFe2O4 (NFO) thin films are grown on four different substrates, i.e., Lead Zinc Niobate–Lead Titanate (PZN–PT), Lead Magnesium Niobate–Lead Titanate (PMN–PT), MgAl2O4 (MAO) and SrTiO3 (STO), by a direct liquid injection chemical vapor deposition technique (DLI-CVD) under optimum growth conditions where relatively high growth rate (∼20nm/min), smooth surface morphology and high saturation magnetization values in the range of 260–290emu/cm3 are obtained. The NFO films with correct stoichiometry (Ni:Fe=1:2) grow epitaxially on all four substrates, as confirmed by energy dispersive X-ray spectroscopy, transmission electron microscopy and x-ray diffraction. While the films on PMN–PT and PZN–PT substrates are partially strained, essentially complete strain relaxation occurs for films grown on MAO and STO. The formations of threading dislocations along with dark diffused contrast areas related to antiphase domains having a different cation ordering are observed on all four substrates. These crystal defects are correlated with lattice mismatch between the film and substrate and result in changes in magnetic properties of the films. Atomic resolution HAADF imaging and EDX line profiles show formation of a sharp interface between the film and the substrate with no inter-diffusion of Pb or other elements across the interface. Antiphase domains are observed to originate at the film–substrate interface.

Investigation of structural and magnetic properties of Ni, NiFe and NiFe2O4 thin films

Soft magnetic thin films of Ni, NiFe and NiFe2O4 were prepared using reactive magnetron sputtering in various deposition conditions. Experimentally observed soft magnetic property was compared and correlated with nanocrystalline structure evolution. Ni and NiFe deposited films are textured with fcc(111) phase preferred orientation. Accordingly, grain size and lattice parameter were calculated from X-ray diffraction (111) peak line width and 2θ peak position. Addition of reactive gas oxygen in deposition process has substantial effect on crystalline structure of film. There is phase transition from the ordered NiFe (111) structure to the NiFe2O4 nanocrystalline phase. The resulting film has shown small X-ray diffraction intensity peak corresponding to (311) and (400) orientation, indicating small amount of existing NiFe2O4 phase. The mechanism has been discussed to be responsible for nanocrystallization and amorphization of NiFe2O4 films. Magnetic measurement (M-H) loop reveal soft magnetic nature of films with magnetic anisotropy. The coercivity (Hc) of films is in accordance with random anisotropy model, where Hc reduced with grain size. The structural transformation was supported by Fourier transforms infrared spectroscopy measurement. The films are highly smooth with surface roughness in the range of ∼0.53–0.93nm. NiFe2O4 films have shown lowest surface roughness with highest electrical resistivity values. The structural, surface, magnetic and infrared spectroscopy results are observed and analyzed.

Polarized neutron reflectivity study of NiFe2O4 films with very large saturation magnetization

Epitaxial (001)-oriented NiFe2O4 films have been grown by RF magnetron sputtering on (001)MgAl2O4 substrates. In agreement with previous reports, magnetization loops of the thinner films reveal a saturation magnetization up to 3 times larger than the thicker films, which display values in agreement with the bulk. Here we discuss polarized neutron reflectivity experiments which are able to disregard that the magnetization overshoot is caused at the early stages of the growth.

Magnetic Properties of Epitaxial NiFe<sub>2</sub>O<sub>4</sub> Thin Films Prepared Using Dynamic Aurora PLD in a Magnetic Field

This study examined effects of applying a magnetic field (up to 2 kG) on the magnetic properties of epitaxial NiFe2O4(NF) thin films during deposition. The NF films were deposited on Y0.15Zr0.85O1.93 (YSZ) buffered Si (001) substrates using pulsed laser deposition (PLD). Although application of magnetic field during deposition affected neither the crystal structure nor orientation of the NF thin films, it improved magnetic properties. The saturation magnetization (Ms) of NF thin films deposited at 500°C and 600°C without application of a magnetic field was as low as 40 emu/g. However, that of NF deposited under magnetic field of 2 kG got to bulk Ms (50.3 emu/g). The TEM observation results revealed that the anti-phase boundary (APB) density decreased by application of the magnetic field during deposition. Results show that magnetic properties of NF thin films are controllable using the magnetic field during deposition.

A robust approach for the growth of epitaxial spinel ferrite films

Heteroepitaxial spinel ferrites NiFe2O4 and CoFe2O4 films have been prepared by pulsed laser deposition at various temperatures (175–690 °C) under ozone/oxygen pressure of 10 mTorr. Due to enhanced kinetic energy of ablated species at low pressure and enhanced oxidation power of ozone, epitaxy has been achieved at significantly lower temperatures than previously reported. Films grown at temperature below 550 °C show a novel growth mode, which we term “vertical step-flow” growth mode. Epitaxial spinel ferrite films with atomically flat surface over large areas and enhanced magnetic moment can be routinely obtained. Interestingly, the growth mode is independent of the nature of substrate (spinel MgAl2O4, perovskite SrTiO3, and rock-salt MgO) and film thickness. The underlying growth mechanism is discussed.

Formation of antiphase domains in NiFe2O4 thin films deposited on different substrates

Thin films of NiFe2O4 have been deposited on various substrates using pulsed laser deposition and the defect structures investigated by transmission electron microscopy. Owing to the simultaneous nucleation of cation-disordered sites during the nonequilibrium growth, the NiFe2O4 films exhibit antiphase domains of ∼20 nm, irrespective of the substrate symmetry. For growth on isostructural spinel substrates, the density of antiphase appears to decrease with decreasing lattice mismatch. Aberration corrected high resolution transmission electron microscopy reveals that the interchange of equivalent tetrahedral cation positions in the host oxygen sublattice as one of the possible mechanisms leading to the formation of antiphase domains.

Template-assisted fabrication of macroporous NiFe2O4 films with tunable microstructural, magnetic and interfacial properties

Nickel ferrite (NiFe2O4) films with macroporous frameworks are successfully fabricated on silicon substrates via a novel template-assisted route. The fabrication approach mainly involves the formation of two-phase composite (desired nickel ferrite and nickel oxide phases) films by thermal conversion of Ni–Fe layered double hydroxide precursor films followed by a selective leaching of self-generated NiO template from the two-phase composite films. The microstructural, magnetic and interfacial properties of the resulting films were characterized by means of X-ray diffraction (XRD), chemical analyses, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM). The results revealed that the microstructures (e.g. particle sizes and spacing among particles), magnetism and surface hydrophobic property of as-fabricated NiFe2O4 films could be finely tuned by varying the composition of precursor films. The increasing Ni/Fe ratio in precursor films leads to reduced particle sizes and enlarged spacing among ferrite particles in NiFe2O4 films. Furthermore, the as-fabricated NiFe2O4 films show tunable saturation magnetization varying from 36 to 55 emu/g and excellent surface hydrophobicity ranging from 139 to 152° after surface modification of films. We believe that such unique macroporous NiFe2O4 films can open up new opportunities for a wide range of advanced applications.

Effect of growth temperature on the magnetic, microwave, and cation inversion properties on NiFe2O4 thin films deposited by pulsed laser ablation deposition

First principles band structure calculations suggest that the preferential occupation of Ni2+ ions on the tetrahedral sites in NiFe2O4 would lead to an enhancement of the exchange integral and subsequently the Néel temperature and magnetization. To this end, we have deposited NiFe2O4 films on MgO substrates by pulsed laser deposition. The substrate temperature was varied from 700to900°C at 5mTorr of O2 pressure. The films were annealed at 1000°C for different times prior to their characterization. X-ray diffraction spectra showed either (100) or (111) orientation with the spinel structure dependent on the substrate orientation. Magnetic studies showed a magnetization value of 2.7kG at 300K. The magnetic moment was increased to the bulk value as a result of postdeposition annealing at 1000°C. The as produced films show that the ferromagnetic resonance linewidth at 9.61GHz was 1.5kOe, and it was reduced to 0.34kOe after postannealing at 1000°C. This suggests that the annealing led to the redistribution of Ni2+ ions to their equilibrium octahedral sites. Further, it is shown that the magnetically preferred direction of Ha can be aligned perpendicular to the film plane when films are grown with a fixed oxygen pressure of 5mTorr for films deposited at 700 and 900°C.

Hybrid perovskite-spinel magnetic tunnel junctions based on conductive ferrimagnetic NiFe2O4

Ultrathin epitaxial films of NiFe2O4, when grown under specific conditions, show a large enhancement of their magnetic moment and electrical conductivity. This allowed us to use such layers as electrodes in magnetic tunnel junctions. In this paper, we report on the observation of a large tunnel magnetoresistance at low temperature in La2∕3Sr1∕3MnO3∕SrTiO3∕NiFe2O4 junctions. From these results we infer a spin polarization of up to 45% for NiFe2O4, which stays constant up to ∼300K. NiFe2O4 can therefore be considered as an alternative to Fe3O4 as a high-temperature spin-polarized Fe oxide. Furthermore, our findings provide a way to design materials for spintronics based on the stabilization of nonthermodynamically stable magnetic phases of spinel oxides.

High-temperature cluster glass state and photomagnetism in Zn- and Ti-substituted NiFe2O4 films

We report on the cluster glass nature and photoinduced magnetization (PIM) of (Ni,Zn,Fe,Ti)3O4 thin films prepared using a pulsed-laser deposition (PLD) technique. The films exhibit cluster glass behaviors with a spin-freezing temperature Tf of approximately 230 K. It was found that the magnetization increases following light irradiation below Tf. The experimental results suggest that the electronic transitions induced by photons are intrinsic to the observed PIM rather than the thermal heating effects resulting from light irradiation. The change in the magnetization from the original dark value at 10 K was approximately 40% for the Ni0.7Zn0.3Fe1.7Ti0.3O4 film, and it was approximately 5% for the Ni0.4Zn0.6Fe2O4 film when the excitation intensity is 1.48mW∕mm2. The analysis of the excitation energy dependence of PIM suggests that an intervalence charge transfer Ti4++Fe2+→Ti3++Fe3+ contributes to the observed enhancement in the PIM of the Ti-substituted films.

Tuning the growth orientation of NiFe2O4 films by appropriate underlayer selection

We have grown NiFe2O4 thin films on (001)-oriented SrTiO3 and (001)-oriented Pt underlayers. Although these two templates present a similar lattice mismatch with NiFe2O4 (about -6%), the ferrite grows cube-on-cube with a (001) orientation on SrTiO3 and (111)-textured on Pt, with four different in-plane variants. This evidences that the interface energy between NiFe2O4 and Pt or SrTiO3 is the key parameter ruling the film texture, while the elastic energy appears as a second-order factor. We compare the structural and magnetic properties of these two films and discuss possible applications of our findings.

Electrochemical Preparation of Ni and Fe Hydroxide/Oxide Films Using Polyethylenimine

Composite films consisting of Fe hydroxide, Ni hydroxide and polyethylenimine (PEI) were obtained by the combined electrolytic-electrophoretic deposition method. The proposed method for PEI charging and deposition is based on the use of cationic polymer-metal ion complexes. Cathodic deposits were obtained on Pt and Ag substrates. By varying both the concentration of PEI in solutions and the deposition time, the amount of material deposited and its composition can be varied. The method was used to fabricate NiFe2O4 films. The deposits were studied by thermogravimetric analysis, X-ray diffraction analysis and SEM. The mechanism of electrochemical intercalation of PEI into hydroxide deposits is discussed.

Magneto-optical Polar and Longitudinal Kerr Spectra of NiFe2O4 Single-crystals.

Measurements and analysis of spectroscopic reflection ellipsometry and magneto-optical polar and longitudinal complex Kerr effect are reported on a natural (111) fece of a high quality NiFe2O4 single-crystal. The experimental reflection data is expressed in terms of the permittivity tensor. This information is employed in modeling experimental polar Kerr spectra of a NiFe2O4 film and a satisfactory agreement has been achieved. These results demonstrate the importance of the spectroscope magneto-optic reflection ellipsometry in the diagnostics of magnetic oxide films with thicknesses in the range 101 to 102 nm.

Formation of Nickel Ferrite Films by Solid-State Reaction

Nickel ferrite (NiFe2O4) thin films are of potential interest for magnetic applications. In the present study, the production of NiFe2O4 by solid-state reaction between thin films of hematite (α-Fe2O3) and nickel oxide (NiO) on (0001) sapphire (α-Fe2O3) substrates has been examined. The NiFe2O4 thin films were prepared by two different methods. In the first case the NiFe2O4film was grown in situ in the deposition system, while in the second case the NiFe2O4 film was formed ex situ by reacting at elevated temperatures in air. These two methods of reaction lead to interesting morphological differences in the ferrite layers.Epilayers of α-Fe2O3 followed by NiO were deposited onto (0001) α-Al2O3 by pulsed-laser deposition (PLD) in 6 mTorr O2. NiFe2O4 films were obtained by reacting the starting films in two different ways: in situ (during film growth) and ex situ.. In both cases, the α-Fe2O3 films were grown under the same conditions while those for the deposition of the NiO layers were different.

Microstructure and magnetic properties of spinel and hexagonal ferrimagnetic films prepared by pulsed laser deposition

Thin films of NiFe2O4, CoFe2O4, Ni0.5Zn0.5Fe2O4, and BaFe12O19, deposited at temperatures higher than 500°C are isotropic and polycrystalline, the thickness (25 nm up to 1.5 μm), the grain size (50 nm to 1 μm) and surface roughness (1 to 100 nm) depending upon deposition parameters. Saturation magnetizations are close to the bulk values, but coercivities are rather high: up to 500 Oe for Ni-ferrites and more than 3 kOe for the hexaferrite.

Epitaxial growth and magnetic behavior of NiFe2O4 thin films

Thin films of NiFe2O4 were deposited on SrTiO3 (001) and Y0.15Zr0.85O2 (yttria-stabilized zirconia) (001) and (011) substrates by 90°-off-axis sputtering. Ion channeling, x-ray diffraction, and transmission electron microscopy studies reveal that films grown at 600 °C consist of ∼300 Å diameter grains separated by thin regions of highly defective or amorphous material. The development of this microstructure is attributed to the presence of rotated or displaced crystallographic domains and is comparable to that observed in other materials grown on mismatched substrates (e.g., GaAs/Si or Ba2YCu3O7/MgO). Postdeposition annealing at 1000 °C yields films that are essentially single crystal. The magnetic properties of the films are strongly affected by the structural changes; unannealed films are not magnetically saturated even in an applied field of 55 kOe, while the annealed films have properties comparable to those of bulk, single crystal NiFe2O4. Homoepitaxial films grown at 400 °C also are essentially single crystal.