Magnetite Films Research Articles

Wide-band enhancement of the transverse magneto-optical Kerr effect in magnetite-based plasmonic crystals

The transverse magneto-optical Kerr effect (TMOKE) in magnetite-based magneto-plasmonic crystals is studied experimentally and theoretically. We analyze angle-resolved TMOKE spectra from two types of structures where noble metallic stripes are incorporated inside a thin magnetite film or located on top of a homogeneous film. A multiple wide band enhancement of the TMOKE signal in transmission is demonstrated. The complex dielectric permittivity and gyration are experimentally determined using the ellipsometry technique as well as Faraday rotation and ellipticity measurements. The obtained parameters are used in rigorous coupled wave analysis (RCWA) calculations for studying the optical resonances. Our RCWA calculations of transmittance and TMOKE are in good agreement with the experimental data. The role of guiding and plasmonic modes in the TMOKE enhancement is revealed. We demonstrate that the TMOKE provides rich information about the studied optical resonances.

Transverse magneto-optical Kerr effect in magnetoplasmonic waveguide structures based on Fe3O4

In this study transverse magneto-optical Kerr effect in transmission for the magnetite thin film with gold grating on top was examined theoretically and experimentally. The magnetite films with thickness of 200 nm were fabricated by the laser electrodispersion technique. Arrays of gold stripes with 600 nm period and 350 nm and 500 nm stripe widths were created by lift-off e-beam lithography. It was shown experimentally that the TMOKE enhancement in structures with 350 nm width is attributed to SPP excitation on gold-magnetite surface. TMOKE value δ reaches magnitude of ±5·10−3 at the wavelengths 790 nm and 860 nm at light incidence angles of 8-10° and 12-13°, respectively. In case of 500 nm stripe width, SPP excitation is poor, however δ reaches magnitude of ±6.6·10−3 at angles of incidence more than 20°, that can be associated with the quasiguided mode resonance in such structure.

Efficiency evaluation of the transverse magneto-optical Kerr effect in magnetoplasmonic structures

In this study the new approach to the efficiency evaluation of the transverse magneto-optical Kerr effect is considered. To assess the prospects for the practical use of structures based on TMOKE, the Figure of Merit (FOM) parameter is introduced, which is the product of the reflected/transmitted light intensity and the Kerr signal of the structure. Kerr signal defined as absolute value of the difference of light intensities at opposite magnetizations. It is shown that parameter Figure of Merit (FOM) in proposed form can give more information than the most commonly used relative value of TMOKE. The advantages of the introduced parameter are demonstrated on the example of a magnetoplasmonic structure with thin magnetite films.

Study of the Electrochemical Properties of Fe3O4 Coatings on Ti6AL4V-ELI as Anticorrosive Protection

The aim of this work is produced passive films of magnetite on the Ti6Al4V-Eli alloy surface and evaluated the influence on electrochemical properties by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and polarization curves using Hank's physiological solution as a electrolyte. To obtain the magnetite films a FeCl3.6H2O (2 mmol) + FeSO4.7H2O (1 mmol) solution was used applying a potential of 1 V. Preliminary EIS results for bare metal show a lower Ztotal than Ti6Al4V-Eli+Fe3O4, however, both responses are affected by surface phenomena such as roughness or porosity characteristic of magnetite and surface preparation of Ti6Al4V. Finally, from polarization curves it was determined that the Ti6Al4V-Eli+Fe3O4 (with film) samples are more stable than the Ti6Al4V-Eli (bare), however, both samples not exhibit a pitting potential inside the polarization range applied.

Crystal texture-dependent magnetic and magnetotransport properties of half-metallic Fe3O4 films grown on oxidized Si substrates by reactive deposition

Nanocrystalline magnetite (Fe3O4) films with various preferred crystallite orientations were grown on oxidized silicon surface by reactive iron deposition in an oxygen atmosphere. Depending on the partial pressure of oxygen (PO2), the evolution of the structural, magnetic, and magnetotransport properties of the grown films was investigated. We found that the growth of films containing only the Fe3O4 phase occurs in a certain PO2 range, and the magnetite crystallites may have (311) or (110) preferred orientation. It was revealed that films with (311) and (110) textures have a column structure. An increase in the PO2 leads to a structure transformation from (311) to (110) texture with larger crystallites. A film with (110) texture showed higher values of saturation magnetization, magnetoresistance and spin polarization. The analysis of approach to magnetic saturation revealed that the local magnetic anisotropy of crystallites in textured films is much higher than the anisotropy of bulk magnetite due to the large surface contribution. The FORC diagram method in combination with the approach to magnetic saturation proved the existence of the exchange coupling between large and small grains. Our results open the reliable route for crystal texture-depending manipulation of functional properties of thin magnetite films for advanced spintronic applications.

Magnetic Field-Assisted Chemical Vapor Deposition of Iron Oxide Thin Films: Influence of Field-Matter Interactions on Phase Composition and Morphology.

Magnetic field-assisted CVD offers a direct pathway to manipulate the evolution of microstructure, phase composition, and magnetic properties of the as-prepared film. We report on the role of applied magnetic fields (0.5 T) during a cold-wall CVD deposition of iron oxide from [FeIII(OtBu)3]2 leading to higher crystallinity, larger particulates, and better out-of-plane magnetic anisotropy, if compared with zero-field depositions. Whereas selective formation of homogeneous magnetite films was observed for the field-assisted process, coexistence of hematite and amorphous iron(III) oxide was confirmed under zero-field conditions. Comparison of the coercive field (11 vs 60 mT) indicated lower defect concentration for the field-assisted process with nearly superparamagnetic behavior. X-ray photoemission electron microscopy (X-PEEM) in absorption mode at the O-K and Fe-L3,2 edges confirmed the selective formation of magnetite (field-assisted) and hematite (zero-field) with coexisting amorphous phases, respectively, emphasizing the importance of field-matter interactions in the phase-selective synthesis of magnetic thin films.

Studies of electrical and magnetic properties across the Verwey transition in epitaxial magnetite thin films

The magnetic and transport properties of magnetite (Fe3O4) films grown on isostructural substrates (MgAl2O4, MgGa2O4, and CoGa2O4), with varying degrees of lattice mismatches (3.8%, −1.4%, and −0.8%, respectively), have been investigated. A significant reduction in the density of antiphase boundary defects is observed for the Fe3O4 films grown under optimal process conditions on smaller lattice mismatch substrates (MgGa2O4 and CoGa2O4) as compared to MgAl2O4. Correspondingly, films on these substrates show much improved magnetic properties and sharper transition in the resistance values at the Verwey transition. Room temperature magnetic hysteresis and ferromagnetic resonance measurement studies indicate the presence of uniaxial magnetic anisotropy induced by substrate-induced strain in the film. Temperature-dependent transport measurements confirm that film thickness has a larger effect on the sharpness of the Verwey transition temperature than the degree of lattice mismatch with substrates.

Magnetite and the Verwey transition, from γ-rays to low-energy electrons

Magnetite, a semiconducting ferrimagnetic iron spinel with a metal-insulator phase transition, the Verwey transition, has long been the subject of Mossbauer spectroscopy studies, which continue today. We review the current status of the understanding of the Mossbauer spectra of magnetite. Furthermore, magnetite is a very attractive material in current topics such as spintronics. In this particular subject, to determine the behavior of magnetic domains is paramount, and the changes ocurring on the near surface region upon undergoing the Verwey transition are relevant. In order to advance in this area, we have incorporated some new techniques, namely microscopy observations made with low-energy electrons. These observations can be performed upon changing the temperature, and can provide magnetic contrast through the use of spin-polarized electrons. By this means, we have observed the ferroelastic transformation associated with the Verwey transition, discovered an order-disorder transition of the (001) surface of magnetite and observed the changes in the magnetic domains on the same surface by changing the temperature. Low-energy electrons also are the key to the Mossbauer experiments of magnetite films and surfaces, with the promise of providing surface-sensitive spatially resolved Mossbauer spectra.

Ultrafast x-ray absorption near edge spectroscopy of Fe3O4 using a laboratory based femtosecond x-ray source.

Ultrafast time-resolved x-ray absorption near edge spectroscopy (XANES) experiment was performed on a magnetite (Fe3O4) film using a femtosecond laser plasma x-ray source delivering Bremsstrahlung radiation. Ultrafast temporal evolution of the XANES of Fe3O4 following an excitation by an infra-red (IR) laser pulse was observed in a pump-probe scheme. The Fe K x-ray absorption edge shifts towards low energy upon IR excitation as much as 12 eV, which is mainly attributed to the charge transfer between the Fe ions. The shift in the absorption edge occurred within about 150 fs, typical time of non-thermal electronic redistribution. The charge transfer also causes an ultrafast increase in the IR transmission in the similar time scale.

Superstructures on Epitaxial Fe3O4(111) Films: Biphase Formation versus the Degree of Reduction

We analyzed the formation of biphase superstructures on the (111)-oriented epitaxial magnetite films on Pt(111) as a function of controlled film stoichiometry. The stoichiometry of the films severa...

Epitaxial Growth of Fe3O4 Layers on the C-Plane of Sapphire by Pulsed Laser Deposition

We have studied the effect of substrate temperature and molecular oxygen pressure on the growth of thin (<180 nm) epitaxial Fe3O4(111) films on the C-plane of single-crystal sapphire (Al2O3(0001)) via pulsed laser evaporation. We have investigated the electrical properties, morphology, and structure of magnetite films grown under various conditions and the magnetic-field and temperature dependences of their resistivity. Fe3O4 has been shown to have a stable growth plateau (SGP) at pressures in the range (3–9) × 10–5 Torr and growth temperatures from 300 to 550°C. With increasing growth temperature, the SGP shifts to higher oxygen pressures. Analysis of the effect of film growth temperature indicates that optimal electrical characteristics and a more perfect crystal structure are ensured by elevated growth temperatures, above 400°C, whereas growth temperatures below 350°C make it possible to minimize the surface roughness of the films and improve their surface homogeneity. Annealing magnetite films under high vacuum above the growth temperature allows all of these requirements to be met.

Anticorrosion Behavior of Deposited Magnetite on Galvanized Steel in Saline Water Using RF-Magnetron Sputtering

Thin films of Magnetite have been deposited on Galvanized Steel (G-S) alloy using RF-reactive magnetron sputtering technique and protection efficiency of the corrosion of G-S. A Three-Electrodes Cell was used in saline water (3.5 % NaCl) solution at different temperatures (298, 308, 318 &amp; 328K) using potentiostatic techniques with. Electrochemical Impedance Spectroscopy (EIS) and fitting impedance data via Frequency Response Analysis (FRA) were applied to G-S alloy with Fe3O4 and tested in 3.5 % NaCl solution at 298K.Results taken from Nyquist and Bode plots were analyzed using software provided with the instrument. The results obtained show that the rate of corrosion of G.S alloy increased with increasing the temperatures from 298 to 323K; and showed that deposition of Fe3O4 caused protection efficiency to reach 79.76% for G-S in 318K. In addition the enthalpy &amp; entropy of activation were evaluated. Apparent energies of activation have been calculated for the corrosion process of uncoated and coated G.S alloy by sputtering technique in saline water (3.5 % NaCl). The morphological analysis was carried out using Scanning Electron Microscopy (SEM) technique.

Transverse Magneto-Optical Kerr Effect in Magnetite Covered by Array of Gold Nanostripes

We study theoretically and experimentally a magneto-optical Kerr effect (TMOKE) in magnetite film covered by sub-wavelength grating consisting of gold nanostripes. Our rigorous coupled wave analysis (RCWA) simulations of optical reflection and transmission coefficients of the structure under study predict a multiple enhancement of TMOKE response in transmission as compared with a plain magnetite film without gold nanostripes. We demonstrate that due to the high absorption losses in magnetite film, the quality factors of the optical resonances in our sample are low which results in wide areas on the dispersion diagram where the TMOKE can be observed.

Bio-based thin films of cellulose nanofibrils and magnetite for potential application in green electronics

The objective of this work was to prepare bio-based thin films and evaluate the additions of magnetite and glycerol on the physico-chemical (flexibility, wettability and barrier properties) and dielectric properties of cellulose/chitosan-based films. The films were prepared by solution casting and presented a suitable dispersion of the constituents observed by SEM and FTIR. The films were thermally stable up to 150 °C and had a higher flexibility, wettability and lower barrier properties upon addition of glycerol. The calculated dielectric constant (εr) for the composite films was based on measurements of capacitance, at 100 and 1000 Hz, with the additions of magnetite and glycerol more than doubling the εr increasing the charge storage capacity. The bio-based thin films have potential to be used as insulators in capacitors on the production of green electronics thus, reducing toxic and nonrenewable e-waste generation.

Influence of Step-edge induced antiphase boundary density on the magnetoresistance behavior of epitaxial Fe3O4 thin films

We have studied the influence of step-edge (SE) induced antiphase-boundaries (APB) density on the magnetoresistance (MR) behavior of epitaxial magnetite (Fe3O4) films. The APB density was varied through the step-terrace periodicity of vicinal MgO (100) surface (miscut angle between 1 and 5°). The contribution from the SEs induced APBs on the MR, ΔMR, in Fe3O4 films increases with the decrease in temperature and peaks at the Verwey transition. Magnitude of the ΔMR is found to increase with miscut angle (up to 2°) and decreases for larger miscut angles, which is related to the influence of APB related strain-fields on the degree of spin-polarization of Fe3O4 regions on terraces.

Influence of Miscut Angle on the Magnetoresistance Behavior of Epitaxial Fe3O4 Thin Films Grown on Vicinal Substrates

This present work reported here the influence of miscut angle of vicinal MgO (100) substrates (with miscut angle between 1-5� along &lt;011&gt; direction) on the magnetoresistance (MR) behavior of epitaxial magnetite (Fe3O4) films. By varying the miscut angle, we are able to control the step-terrace periodicity of the substrate surface, which allows us to tailor the density of the step-edges induced antiphase boundaries, APBs (nanoscale liner defects formed during growth due to coalescence of neighbouring domains), in epitaxial Fe3O4 films. We found that the contribution from the step-edges induced APBs to the magnetoresistance (MRSE) in Fe3O4 films increases with increase in miscut (up to 2�) and decreases for larger miscut angles. From temperature dependent studies, we observed that the magnitude of MRSE increased with decrease in temperature and peaks at the Verwey transition. The decrease in MRSE for larger miscut samples is attributed to the influence of APB strain-fields on the degree of spin-polarization of Fe3O4 regions on terraces.

The Lateral Photovoltaic Effect in the Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;/SiO&lt;sub&gt;2&lt;/sub&gt;/p-Si Structure

We report on the results of the study of the lateral photovoltaic effect in the Fe3O4/SiO2/p-Si structure. It is found that maximum of the lateral photovoltage is localized near the measuring contacts and rapidly attenuates when the light spot moves away from them. Correspondence of the photovoltage sign to the conductivity type of the silicon substrate is achieved only taking into account the interface states at the SiO2/p-Si interface. The extreme dependence of the lateral photovoltage on the thickness of the Fe3O4film is observed, which is due to the fact that the barrier height is laterally inhomogeneous at small thicknesses of magnetite film, whereas at higher thicknesses of the film the lateral photovoltaic effect is short-circuited by this film.

Failure Analysis of Boiler Water Wall Tube for Power Generation in District Heating

A corrosion failure of materials, often exposed to the corrosive environment in the district heating facility, has a critical influence on structural integrity. Thus, it is very important to investigate the cause of the failure for the establishment of effective prevention methodology. In this study, the failure analysis was performed in the case that a water wall tube for a boiler in district heating was fractured along the axial direction of the tube that is SA-210 Grade A1, which is frequently used for the boiler. The feed water temperature was about 220°C and the operating pressure was about 10 MPa. For the visual inspection of the failed part, the wall thinning and failure were observed in the location where the heat was directly reached. Sodium and iron oxides were only found in the inner surface of the tube, as confirmed by x-ray diffraction, and it indicates that hydrogen is generated by the mechanism of the caustic corrosion, resulting in hydrogen embrittlement near the inner surface. The microstructural changes along the direction of radial and hoop were intensively examined. A number of microvoids were formed along the grain boundary, leading to some microcracks in the location inside the tube. As a result, it is concluded that an unstable magnetite film at elevated temperature under tensile residual stresses developed along the hoop direction in the interior of the tube undergoes the caustic corrosion, and microvoids or microcracks along the grain boundary formed due to hydrogen embrittlement cause a final fracture of the tube.

Impact of Strain and Morphology on Magnetic Properties of Fe₃O₄/NiO Bilayers Grown on Nb:SrTiO₃(001) and MgO(001).

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe3O4/NiO bilayers grown on both MgO(001) and SrTiO3(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO3. In the case of Fe3O4, the misfit strain on MgO(001) and SrTiO3(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe3O4 films grown on NiO/SrTiO3 exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO3 the NiO films showed strong strain relaxation. Fe3O4 films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO3. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO3 due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.

Magnetic properties of Fe3O4 antidot arrays synthesized by AFIR: atomic layer deposition, focused ion beam and thermal reduction.

Magnetic films of magnetite (Fe3O4) with controlled defects, so-called antidot arrays, were synthesized by a new technique called AFIR. AFIR consists of the deposition of a thin film by atomic layer deposition, the generation of square and hexagonal arrays of holes using focused ion beam milling, and the subsequent thermal reduction of the antidot arrays. Magnetic characterizations were carried out by magneto-optic Kerr effect measurements, showing the enhancement of the coercivity for the antidot arrays. AFIR opens a new route to manufacture ordered antidot arrays of magnetic oxides with variable lattice parameters.