Magnetite Thin Films Research Articles

Spontaneous Growth of Strain-Free Magnetite Nanocrystals via Temperature-Driven Dewetting

Temperature-driven dewetting of a self-template layer has been exploited to spontaneously grow epitaxial magnetic Fe3O4 nanocrystals by pulsed laser deposition. A 10 unit cell thick Fe3O4 template layer was deposited at 400 °C on a perovskite SrTiO3(001) substrate, followed by heating at 1100 °C, which induced dewetting of the Fe3O4 template layer, forming three-dimensional islands that were uniformly distributed over the substrate surface area. These islands were used as seed crystals for the growth of spatially separated quasi-epitaxial Fe3O4 nanopyramids. Structural analysis by scanning transmission electron microscopy and X-ray diffraction revealed incoherent growth of fully relaxed (001)-oriented Fe3O4 nanocrystals on the SrTiO3 (001) substrate. Higher-order epitaxial matching of the substrate and film lattices appears to be responsible for the well-defined in-plane orientation of the Fe3O4(001) nanopyramids despite the large nominal lattice mismatch of −7.5%. The nanopyramid structure strongly affected the magnetic characteristics of magnetite, dramatically reducing the coercive field compared to conventional magnetite thin films. The coercivity of the Fe3O4 nanopyramids was also strongly size dependent. This is attributed to a transition between poly- and monodomain states. A superparamagnetic phase was detected for the smallest pyramid sizes.

Influence of the substrate on the anomalous Nernst effect of magnetite thin films

ABSTRACTWe present a comparative study of the anomalous Nernst effect (ANE), measured at room temperature for magnetite thin films deposited on different substrates in order to study the effects induced by the substrate, compressive or tensile strain and structural defects as anti-phase boundaries (APB), on the observed ANE. From our preliminary results we have observed an increase of the measured ANE in the case of compressive strain compared with the tensile one. Moreover our results also suggest that the density of APBs also play an important role in the ANE values.

Investigating the magnetic field-dependent conductivity in magnetite thin films by modelling the magnetorefractive effect

A model of the magnetorefractive effect is developed for metallic oxides which allows the MRE to be used to study the magnetic dependence of their transport and phononic properties. This model is successfully applied to Fe3O4 and compared to experiments on a series of magnetite thin films of varying thickness (10, 18, 37, 64 and 110 nm) deposited on MgO(111) substrates. Reflection spectra were modelled as a function of film thickness, calculated from the Fresnel equations using an Fe3O4 dielectric function consisting of Drude, hopping, phononic and d–s transition conductivity processes. The reflectivity spectra of the different thickness films are reasonably reproduced by the model and reveal that the Fe3O4 18.5 μm phonon peak is shifted to a shorter wavelength in the thin films, approaching the bulk value for t > 110 nm. The MRE spectra are modelled by introducing a magnetic field dependence to the hopping and phononic terms, where previous models have considered the magnetic dependence to be on the Drude term only. In addition, the position of the Fe3O4 18.5 μm phonon peak was also found to shift in energy in the applied magnetic field by 0.05 μm. These results demonstrate the potential for using the MRE technique for probing the underlying magnetoelectronic properties of thin film oxides in a quick and non-destructive way.

Raman study of the Verwey transition in magnetite thin films

We have grown epitaxial thin films of magnetite on MgO and Al2O3 substrates with sharp and distinct signatures of the Verwey transition in resistivity and magnetization. We have used Raman scattering to separate the footprint of purely structural changes from the effect of additional charge and orbital order. Raman modes related to the structural phase transition occur first at temperatures above the Verwey transition temperature. In contrast, newly emerging modes indicating additional charge and orbital order appear at the Verwey transition. These results suggest that the completion of the structural phase transition in magnetite is a necessary precursor triggering a transition into a complex charge and orbitally ordered state.

The structure of minimal magnetite cluster

Geometry and electronic structure of magnetite minimal cluster (Fe3O4)1 have been simulated by density functional theory and semiempirical PM6 method. Data on the cluster geometry obtained by different methods have been consistent; however, the ground state electronic structures as determined by different methods have not been identical. The existence of two states of similar stability, low-spin and high-spin ones, has been predicted. The correlation of the simulated geometry, vibration spectra, and magnetic properties of the cluster with experimentally determined properties of magnetite, nanomagnetite, and magnetite thin films has been discussed.

Thickness dependence of the resistivity tensor in epitaxial magnetite thin films

We report a systematic study of the thickness dependence of the resistivity tensor of epitaxial thin films of magnetite (Fe3O4). We find that decreasing film thickness decreases the relative magnitude of the terms related to crystal symmetry but increases their field dependence. We attribute this behavior to the presence of antiphase boundaries in thin films of magnetite and the dependence of their density on the film thickness.

Stoichiometric magnetite grown by infrared nanosecond pulsed laser deposition

Pulsed laser deposition (PLD) is a versatile technique for the fabrication of nanostructures due to the possibilities it offers to control size and shape of nanostructured deposits by varying the laser parameters. Magnetite nanostructures are currently promising materials to be used in computing, electronic devices and spintronic applications. For all these uses the fabrication of uniform nanostructured pure magnetite thin films is highly advantageous. In PLD of magnetite, the laser irradiation wavelength and substrate temperature crucially affect the composition, crystallinity, surface structure and the magnetic properties of the grown samples. This work shows that the use of nanosecond IR laser at 1064nm enhances the quality of the resulting magnetite thin films, compared to the extensively used UV wavelengths. Deposition at 1064nm, upon heating the substrate at 750K, produces thin films constituted by stoichiometric magnetite nanoparticles with sharp edges and sizes ranging from 80 to 150nm, with a Verwey transition at 119K and a coercivity of 232Oe at room temperature, close to those of pure bulk magnetite. Thus, IR-PLD of self-prepared hematite sintered targets constitutes a low-cost procedure of fabrication of pure magnetite nanostructured thin films.

The structure, magnetic and transport properties of Fe3O4 thin films on different substrates by pulsed laser deposition

Magnetite thin films (10–300 nm) are deposited on different substrates including single crystals ((0001)-Al2O3, (001)-Si, (110)-SiO2), textured polycrystalline oxide underlayers (ZnO), amorphous glass, at temperature from room temperature to 550 °C by the pulsed laser deposition technique. Strong (111)-texture can be obtained on single crystal Si, SiO2, amorphous glass, and (0001)-textured ZnO layer, no matter whether they have either a huge lattice mismatch or no matching at all with Fe3O4. The film on single crystal Al2O3 (sapphire) shows a (111)-epitaxial structure with a strong in plane compressive strain. The saturated magnetization for magnetite thin films grown on SiO2 and Si substrates can reach 440 emu/cc while the epitaxial films grown on Al2O3 have a magnetization around 310 emu/cc when the film thickness is 40 nm. The substrate-induced strain in the films grown on Al2O3 substrates significantly affects the magnetic anisotropy and the magnetic coupling at the anti-phase boundaries in the films.

On the transition point of thermally activated conduction of spinel-type MFe2O4 ferrite thin films (M = Zn, Co, Ni)

Thin films of zinc ferrite (ZnFe2O4), cobalt ferrite, and nickel ferrite have been grown on SrTiO3(100) substrates. By changing the growth temperature TG, the electrical conductivity σ of the thin films can be switched from insulating to conducting. An increased lattice constant of thin films grown at low temperatures (e.g., TG(ZnFe2O4)<450 °C) indicates the formation of defects, which seems to promote the hopping conductivity. Temperature-dependent measurements of σ reveal two thermally activated conduction processes. Interestingly, the transition point of these two processes at Tx=107 K−140 K is close to the Verwey transition known from structurally similar magnetite thin films.

Observation of the spin Seebeck effect in epitaxial Fe3O4 thin films

We report the experimental observation of the spin Seebeck effect in magnetite thin films. The signal observed at temperatures above the Verwey transition is a contribution from both the anomalous Nernst (ANE) and spin Seebeck (SSE) effects. The contribution from the ANE of the Fe3O4 layer to the SSE is found to be negligible due to the resistivity difference between Fe3O4 and Pt layers. Below the Verwey transition, the SSE is free from the ANE of the ferromagnetic layer and it is also found to dominate over the ANE due to magnetic proximity effect on the Pt layer.

Low temperature oxidation mechanisms of nanocrystalline magnetite thin film

A detailed investigation of the mechanisms related to the low temperature oxidation of nanocrystalline magnetite thin films into maghemite is presented. Despite strong differences in the functional properties of these two phases, structural similarities make it difficult to distinguish between them, and to quantify the oxidation process, particularly in the case of nanostructured polycrystalline layers. Contrary to the case of bulk materials or monocrystalline films and particles, the oxidation processes in nanocrystalline thin film have only scarcely been studied. In this work, structural and optical techniques, including X-ray diffraction (XRD), EXAFS/X-ray absorption near edge structure, FTIR, and Raman scattering, have been used to estimate the oxidation rate of magnetite. The overall oxidation reaction rates are discussed in the framework of two limiting cases corresponding to intra grain diffusion and to grain boundary diffusion. SIMS profiling and electrical measurements were also carried out to better assess the oxidation quantification in order to conclude on the predominant oxidation mechanisms in this heterogeneous material. We propose a qualitative model for the structure, in terms of insulating zone distribution, for partially oxidized films.

Selective growth of pure magnetite thin films and/or nanowires grown in situ at a low temperature by pulsed laser deposition

Pure Fe3O4 thin films and/or nanowires were selectively grown onto c-Al2O3 (0001) substrates in situ at different substrate temperatures using pulsed laser deposition (PLD). The epitaxial Fe3O4 thin films were grown at deposition temperatures that ranged from 300 to 600 °C, while nanowires with a single-crystalline nature were grown at temperatures of more than 600 °C without catalysts. The in-plane compressive stress of the films increased as deposition temperature increased. Protrusions to relieve the compressive stress typically formed at the low-angle grain boundaries and were then developed to make nanowires. Both the films and the nanowires showed a ferromagnetic nature and a magnetic anisotropy at room temperature.

Functionalized magnetite silica thin films fabricated by MAPLE with antibiofilm properties

We report on the fabrication of magnetite/salicylic acid/silica shell/antibiotics (Fe3O4/SA/SiO2/ATB) thin films by matrix-assisted pulsed laser evaporation (MAPLE) to inert substrates. Fe3O4-based powder have been synthesized and investigated by XRD and TEM. All thin films were studied by FTIR, SEM and in vitro biological assays using Staphylococcus aureus and Pseudomonas aeruginosa reference strains, as well as eukaryotic HEp-2 cells. The influence of the obtained nanosystems on the microbial biofilm development as well as their biocompatibility has been assessed. For optimum deposition conditions, we obtained uniform adherent films with the composition identical with the raw materials. Fe3O4/SA/SiO2/ATB thin films had an inhibitory activity on the ability of microbial strains to initiate and develop mature biofilms, in a strain- and antibiotic-dependent manner. These magnetite silica thin films are promising candidates for the development of novel materials designed for the inhibition of medical biofilms formed by different pathogenic agents on common substrates, frequently implicated in the etiology of chronic and hard to treat infections.

Intensified magneto-resistance by rapid thermal annealing in magnetite (Fe3O4) thin film on SiO2 glass substrate

We have observed large magneto-resistance (MR) intensified by rapid thermal annealing (RTA) in magnetite (Fe3O4) thin film (MTF) on SiO2 glass (a-SiO2) substrate. The MTF was produced by the RF magnetron sputtering method by using a magnetite target. The electrical resistivity (ER) of as-grown MTF (AG-MTF) showed the Mott's variable range hopping behavior, which implies that the AG-MTF is amorphous-like. Although the magneto-resistance (MR) ratio of bulk single crystal is very small except around the Verwey transition temperature (VTT), that of the AG-MTF showed moderately large below room temperature. Due to RTA of the AG-MTF by use of an IR image furnace, the MR ratio of MTFs was intensified, and especially by the annealing around the Curie temperature (585°C) of magnetite. Furthermore the ER of the rapid thermally annealed MTF (RTA-MTF) showed a slight kink at around the VTT, which indicates that the crystallinity of the RTA-MTF is higher than that of the AG-MTF The MTF produced by the RF magnetron sputtering method are composed of magnetite fine particles (MFPs). We consider that the directions of magnetic moments of MFPs in the MTF were spatially randomized by the RTA and the strong spin scattering of itinerant electrons transferring between adjacent MFPs caused the intensification of the MR ratio.

Fe3O4(1 1 1) thin films with bulk-like properties: growth and atomic characterization

Growth defects, domain structures and off-stoichiometric compositional deviation are the cause of significant deviation of magnetic and transport properties of magnetite (Fe3O4) thin films. We show that these anomalous properties can be fully controlled by postannealing in CO/CO2 atmosphere. Aberration corrected electron microscopy has revealed a full structural transformation from Fe/FeOx to Fe3O4 and a drastic reduction of growth defects in the postannealed films. The magnetic and magnetotransport measurements show that postannealed film properties are bulk-like: the magnetization was fully saturated with ∼480 emu cm−3, as in bulk single crystal, and the negative magnetoresistance was suppressed by an order of magnitude.

Magneto-transport study of magnetite (Fe3O4) nanoparticles between Au nanogap electrodes on surface-oxidized Si substrate

We have studied the magneto-transport of magnetite (Fe3O4) nanoparticles (MNPs) between Au nanogap electrodes (ANGEs) on surface-oxidized Si substrate. The MNP sizes are approximately 40nm and the 100nm thick magnetite thin film (which is formed of the MNP aggregation) was prepared between and around the ANGEs by use of the RF reactive magnetron sputtering method. The distance between the ANGEs and the bridge width of the ANGEs are approximately 50nm and 1.5μm, respectively. The ANGEs were produced by the tilted-angle-deposition method. The optical microscope, scanning electron microscope and atomic force microscope were used to observe the sample surface structure and morphology. To investigate the crystal structure and crystallinity of the MNPs, the X-ray diffraction measurement was performed. The electrical resistance and magneto-resistance ratio of the MNPs between the ANGEs were measured as a function of temperature. The magneto-transport mechanism is discussed on the basis of the spin dependent transport.

Investigation of high quality magnetite thin films grown on SrTiO3(001) substrates by pulsed laser deposition

A systematic study of Fe3O4 epitaxial films grown on single-crystalline SrTiO3(100) substrates using pulsed laser deposition has been conducted in order to elaborate magnetite films with properties suitable for their integration into spintronics heterostructures. Structural, morphologic, magnetic and transport properties of the films as a function of the deposition conditions were extensively explored in situ and ex situ. The conditions for the growth of high quality magnetite thin films were determined. The critical temperatures and magnetization close to the bulk values, low roughness and high conductivity make these samples perfectly suitable for future use in the spintronics heterostructures.

Pyroelectric detection of spontaneous polarization in magnetite thin films

We have investigated the spontaneous polarization in Fe${}_{3}$O${}_{4}$ thin films by using dynamic and static pyroelectric measurements. The magnetic and dielectric behavior of Fe${}_{3}$O${}_{4}$ thin films grown on Nb:SrTiO${}_{3}$(001) substrates was consistent with bulk crystals. The well-known metal-insulator (Verwey) transition was observed at 120 K. The appearance of a pyroelectric response in the Fe${}_{3}$O${}_{4}$ thin films just below the Verwey temperature shows that spontaneous polarization appeared in Fe${}_{3}$O${}_{4}$ at the charge-ordering transition temperature. The polar state characteristics are consistent with bond- and site-centered charge ordering of Fe${}^{2+}$ and Fe${}^{3+}$ ions sharing the octahedral $B$ sites. The pyroelectric response in Fe${}_{3}$O${}_{4}$ thin films was dependent on the dielectric constant. Quasistatic pyroelectric measurement of Pd/Fe${}_{3}$O${}_{4}$/Nb:SrTiO${}_{3}$ junctions showed that magnetite has a very large pyroelectric coefficient of 735 nC cm${}^{\ensuremath{-}2}$ K${}^{\ensuremath{-}1}$ at 60 K.

X-ray diffraction study on size effects in epitaxial magnetite thin films on MgO(0 0 1)

Epitaxial ultrathin iron oxide films of different thicknesses were grown by reactive molecular beam epitaxy in 10−6 mbar oxygen atmosphere on MgO(0 0 1) single crystal substrates at room temperature. Afterwards, the films were studied by x-ray diffraction, x-ray reflectivity and x-ray photoelectron spectroscopy to provide information regarding film structure as well as chemical composition of the films. Except for a very thin interface layer of subnanometre thickness, the iron oxide films have magnetite stoichiometry and structure and Mg does not diffuse from the substrate into the iron oxide film. The interface layer has a wuestite structure as determined by kinematic diffraction analysis. The magnetite films exhibit very homogeneous thickness while the vertical lattice constant decreases gradually towards its bulk value.

Thin magnetite films on an oxidized silicon surface: Raman spectroscopy study

Polycrystalline films of magnetite (Fe3O4) formed by the reactive sputtering of iron in oxygen on Si(001) substrates covered by thin (1.4 nm) or thick (1200 nm) SiO2 layers have been studied by Raman spectroscopy. It is established that (i) the α-Fe2O3 phase is formed due to the laser-induced heating in magnetite films synthesized on thick SiO2 layers and (ii) the formation of α-Fe2O3 phase depends on the thickness of the buffer SiO2 layer.