Magnetite Thin Films Research Articles

Size control of nanocrystalline magnetite thin films containing a small amount of Ge

This study investigated the preparation and particle size control of nanocrystalline magnetite (Fe 3O 4) containing a small amount of Ge. Thin films were prepared by radio-frequency sputtering with a composite target of Ge chips set on a Fe 3O 4 compound target in a mixed atmosphere of Ar and O 2. X-ray diffraction revealed that the diffraction peak of magnetite gradually broadened as the oxygen ratio increased, with the mean grain size ranging from 26 to 2 nm. Transmission electron microscopy also revealed that the magnetite structurally changed from polycrystalline single phase to isolated granular nanocrystals. Magnetization at 8 × 10 5 A/m was monotonically reduced from 0.32 to 0.04 T, and coercivity was monotonically reduced from 4.2 × 10 4 to 2.1 × 10 3 A/m with increasing the oxygen ratio from 0 to 0.4%.

Impact of ultrathin transition metal buffer layers on Fe3O4 thin films

3000 Å Fe3O4 (magnetite) thin films were simultaneously grown on (001) MgO single crystal substrates with and without 30 Å buffer layers of Fe, Cr, Mo, and Nb. For all samples, the Verwey transition temperature (TV) occurs between 119 and 125 K, indicating good oxygen stoichiometry. We observe highly oriented (001) Fe3O4 with Mo and no buffer layer, reduced (001) texture with Nb and Fe, and polycrystalline growth with Cr. Mo, Cr, and unbuffered magnetite show typical magnetic behavior, whereas Nb and Fe buffers lead to anomalous magnetic properties that may be due to interfacial reactivity.

Origin of the giant magnetic moment in epitaxialFe3O4thin films

Received 1 February 2010; revised manuscript received 25 March 2010; published 15 April 2010 We study the enhanced magnetic moment observed in epitaxial magnetite Fe3O4 ultrathin films t 15 nm grown on MgO 001 substrates by means of pulsed laser deposition. The Fe3O4 001 thin films exhibit high crystallinity, low roughness, and sharp interfaces with the substrate, and the existence of the Verwey transition at thicknesses down to 4 nm. The evolution of the Verwey transition temperature with film thickness shows a dependence with the antiphase boundaries density. Superconducting quantum interference device SQUID and vibrating sample magnetometry measurements in ultrathin films show a magnetic moment much higher than the bulk magnetite value. In order to study the origin of this anomalous magnetic moment, polarized neutron reflectivity PNR, and x-ray magnetic circular dichroism XMCD experiments have been performed, indicating a decrease in the magnetization with decreasing sample thickness. X-ray photoemission spectroscopy measurements show no metallic Fe clusters present in the magnetite thin films. Through inductively coupled plasma mass spectroscopy and SQUID magnetometry measurements performed in commercial MgO 001 substrates, the presence of Fe impurities embedded within the substrates has been observed. Once the substrate contribution has been corrected, a decrease in the magnetic moment of magnetite thin films with decreasing thickness is found, in good agreement with the PNR and XMCD measurements. Our experiments suggest that the origin of the enhanced magnetic moment is not intrinsic to magnetite but due to the presence of Fe impurities in the MgO substrates.

CVD synthesis of polycrystalline magnetite thin films: structural, magnetic and magnetotransport properties

Magnetite (Fe3O4) is predicted to be half metallic at room temperature (RT) and it shows the highest Curie temperature among oxides. The use of Fe3O4 thin films is therefore promising for spintronic devices such as magnetic tunnel junctions (MTJs) and magnetoresistive sensors. The structural, magnetic and magnetotransport properties of magnetite are reported to be strongly dependent on the growth conditions. We have developed a very simple deposition chamber for growing thin magnetite films via a chemical vapour deposition (CVD) process based on the Fe3(CO)12 carbonyl precursor. The structural, morphological, and magnetic properties of the as deposited Fe3O4 films have been investigated by means of time of flight secondary ion mass spectrometry, grazing incidence x-ray diffraction, x-ray reflectivity, atomic force microscopy, conversion electron Mössbauer spectroscopy and superconducting quantum interference device magnetometry. Magnetotransport measurements show magnetoresistance up to −2.4% at RT at the maximum applied field of 1.1 T. Resistivity measurements in the 100–300 K temperature range reveal that the magnetotransport properties of the Fe3O4 films are governed by inter-granular tunnelling of the spin-polarized electrons. The spin polarization is estimated to be around −16%. A possible route for increasing the spin-polarized performances of our magnetite films is proposed. We have also deposited Fe3O4/MgO/Co stacks by using a combined CVD and atomic layer-deposition process. The trilayer's hysteresis curve evidences the presence of two distinct switching fields making it promising for magnetite-based MTJ applications.

Interplay of bulk and interface effects in the electric-field-driven transition in magnetite

Contact effects in devices incorporating strongly correlated electronic materials are comparatively unexplored. We have investigated the electrically driven phase transition in magnetite (100) thin films by four-terminal methods. In the lateral configuration, the channel length is less than $2\text{ }\ensuremath{\mu}\text{m}$, and voltage-probe wires $\ensuremath{\sim}100\text{ }\text{nm}$ in width are directly patterned within the channel. Multilead measurements quantitatively separate the contributions of each electrode interface and the magnetite channel. We demonstrate that on the onset of the transition contact resistances at both source and drain electrodes and the resistance of magnetite channel decrease abruptly. Temperature-dependent electrical measurements below the Verwey temperature indicate thermally activated transport over the charge gap. The behavior of the magnetite system at a transition point is consistent with a theoretically predicted transition mechanism of charge gap closure by electric field.

Study of grain morphology of various magnetite samples by means of EBSD

(001)- and (111)-oriented magnetite thin films were grown on MgO substrates (film thickness 200 nm) by means of oxygen-plasma-assisted molecular beam epitaxy and by laser-ablation. Further samples are electroplated magnetite thin films on Si/copper substrates and films from biogenic magnetite. The achieved grain orientations are analysed by means of the electron backscatter diffraction (EBSD) technique. The EBSD technique enables the crystallographic orientation of individual grains to be determined with a high spatial resolution of up to 40 nm on such ceramic materials. A high image quality of the recorded Kikuchi patterns was achieved enabling multi-phase scans (e.g., Fe3O4, MgO, Fe2O3). The facets of individual grains are analysed in detail.

Magnetic properties of magnetite thin films close to the Verwey transition

Temperature dependence of the magnetic properties of magnetite thin film across the Verwey transition has been investigated. As the temperature is decreased, the magnetization of the film in a fixed field showed a sharp decrease close to the Verwey transition temperature ( T v). The M– H loops of the film have been recorded at various temperatures below and above T v. It is found that film does not saturate at any temperature and saturation becomes more difficult below T v. While cooling through T v, the extrapolated value of magnetization to infinite field ( Q), calculated from the numerical fit 4 πM= Q [1−( H*/ H) 1/2], does not show a drop, but the coefficient indicating difficulty in saturation ( H*) shows a sharp rise as does the coercivity.

Advanced microstructural analysis of ferrite materials by means of electron backscatter diffraction (EBSD)

The analysis of the achieved texture is of great importance for the performance of ferrite materials, either bulk or thin films. The recently developed electron backscatter diffraction (EBSD) technique enables a spatially resolved study of the crystallographic orientations by means of recording of Kikuchi patterns. To our knowledge, such a thorough EBSD analysis was not yet performed in any oxidic magnetic material, and only very recently on magnetite thin films by us. A good surface polishing/cleaning is essential for this analysis, as the method requires an undisturbed surface area for a high image quality (IQ). This information is recorded to each measured Kikuchi pattern, together with a parameter describing the quality of indexation. Here, the spatially highly resolved EBSD mappings provide additional information as compared to the standard analysis techniques, which can contribute to an optimization of the growth process. Furthermore, an analysis of the grain aspect ratio is possible which provides further insight to the microstructural dependence of the magnetic properties of ferrites.

Origin of hysteresis in resistive switching in magnetite is Joule heating

In many transition-metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, ${\text{Fe}}_{3}{\text{O}}_{4}$, below the Verwey temperature, an electric-field-driven transition to a state of lower resistance was recently found, with hysteretic current-voltage response. We report the results of pulsed electrical conduction measurements in epitaxial magnetite thin films. We show that while the high- to low-resistance transition is driven by electric field, the hysteresis observed in $I\text{\ensuremath{-}}V$ curves results from Joule heating in the low-resistance state. The shape of the hysteresis loop depends on pulse parameters and reduces to a hysteresis-free ``jump'' of the current provided thermal relaxation is rapid compared to the time between voltage pulses. A simple relaxation-time thermal model is proposed that captures the essentials of the hysteresis mechanism.

Probing the interface ofFe3O4/GaAsthin films by hard x-ray photoelectron spectroscopy

Magnetite (Fe3O4) thin films on GaAs have been studied with HArd X-ray PhotoElectron Spectroscopy (HAXPES) and low-energy electron diffraction. Films prepared under different growth conditions are compared with respect to stoichiometry, oxidation, and chemical nature. Employing the considerably enhanced probing depth of HAXPES as compared to conventional x-ray photoelectron spectroscopy (XPS) allows us to investigate the chemical state of the film-substrate interfaces. The degree of oxidation and intermixing at the interface are dependent on the applied growth conditions; in particular, we found that metallic Fe, As2O3, and Ga2O3 exist at the interface. These interface phases might be detrimental for spin injection from magnetite into GaAs.

Characterization of nanostructured magnetite thin films produced by sol–gel processing

Nanocrystalline films of magnetite have been prepared by a novel sol–gel route in which, a solution of iron (III) nitrate dissolved in ethylene glycol was applied on glass substrates by spin coating. Coating solution showed Newtonian behaviour and viscosity was found as 0.0215 Pa.s. Annealing temperature was selected between 291 and 350 °C by DTA analysis in order to obtain magnetite films. In-plane grazing angle XRD and TEM studies showed that magnetite phase was present upon annealing the films at 300 °C. The films had crack free surfaces and their thicknesses varied between ~10 and 200 nm. UV–Vis spectrum results showed that transmittance of the films increases with decreasing annealing temperature and increasing spinning rate. Up to 96% transmittance was observed between the wavelengths of 900–1,100 nm. Vibrating sample magnetometer measurements indicated that magnetite thin films showed ferromagnetic behavior and the saturation magnetization value was found as ~35 emu/cm3.

Surface magnetic structure of epitaxial magnetite thin films grown on MgO(001)

The crystallographic structure of antiphase domain boundaries (APBs) and the magnetic structure of large domains of magnetite (Fe3O4) thin films, which were epitaxially grown on MgO(001) substrates and annealed in an ambient condition, were investigated by scanning tunneling microscopy (STM), transmission electron microscopy (TEM), magnetic force microscopy (MFM), and scanning electron microscopy with polarization analysis (SEMPA). STM and TEM showed the morphology of the APBs in film surfaces and the bulk, whereas MFM and SEMPA showed the magnetic structure of the surfaces. As in previous paper reported by another group, STM and TEM observations revealed the antiphase domains on a scale from tens of nanometers to a few hundred nanometers. Also MFM measurement showed the magnetic domains on a scale of a few hundred nanometers and large-scale undulations of a few micrometers. Our SEMPA observations of the films displayed large-scale magnetic structures on a scale of a few hundred nanometers to a few micrometers. These large-scale magnetic structures are expected to be induced by the enlargement of magnetic domains that contain magnetic moments aligned along a magnetic easy axis. These moments have in-plane components that make the SEMPA images visible. An air-annealing process is needed to change magnetic couplings in APBs and to form large-scale magnetic ordering.

EBSD analysis of electroplated magnetite thin films

By means of electron backscatter diffraction (EBSD), we analyse the crystallographic orientation of electroplated magnetite thin films on Si/copper substrates. Varying the voltage during the electroplating procedure, the resulting surface properties are differing considerably. While a high voltage produces larger but individual grains on the surface, the surfaces become smoother on decreasing voltage. Good quality Kikuchi patterns could be obtained from all samples; even on individual grains, where the surface and the edges could be measured. The spatial resolution of the EBSD measurement could be increased to about 10 nm; thus enabling a detailed analysis of single magnetite grains. The thin film samples are polycrystalline and do not exhibit a preferred orientation. EBSD reveals that the grain size changes depending on the processing conditions, while the detected misorientation angles stay similar.

Thermal and irradiation induced interdiffusion in magnetite thin films grown on magnesium oxide (0 0 1) substrates

Epitaxial Fe 3O 4(0 0 1) thin films (with a thickness in the range of 10–20 nm) grown on MgO substrates were characterized using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS) and investigated using Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). The Mg out-diffusion from the MgO substrate into the film was observed for the directly-deposited Fe 3O 4/MgO(0 0 1) films. For the Fe 3O 4/Fe/MgO(0 0 1) films, the Mg diffusion was prevented by the Fe layer and the surface layer is always a pure Fe 3O 4 layer. Annealing and ion beam mixing induced a very large interface zone having a spinel and/or wustite formula in the Fe 3O 4-on-Fe film system.

Structure-Disorder induced Magneto-Resistance intensification on spin-dependent-conduction in magnetite (Fe3O4) thin film produced by RF magnetron sputtering method

The intensification of magneto-resistance (MR) caused by structure-disorder has been observed in magnetite (Fe3O4) thin film (MTF) produced by the rf magnetron sputtering method. The MTFs with the thickness of 20nm, 50nm and 100nm were deposited on the substrates of SiO2-glass, MgAl2O4 (100) and MgO (100) single crystals. We have observed that the MR of the MTF on SiO2-glass substrate changes 3–5 times larger than that of the MTFs on MgAl2O4 and MgO substrates. From the AFM, SEM and XRD measurements, we have found that the MTFs are composed of magnetite nano-particles (MNPs) and the crystal-axis directions of the MNPs in the MTF on SiO2-glass are mutually random and those of the MTFs on MgAl2O4 and MgO are almost aligned along (100) direction. In addition, from the electrical and magnetization measurements, we have found that the MNPs in the MTF on SiO2-glass show the amorphous-like behavior and those of the MTFs on MgAl2O4 and MgO indicate the crystal-like behavior.

X-ray Absorption Spectroscopic Studies of Pulsed-Laser-Deposited Thin Films of Fe3O4 on Si(111) Substrate

We report on the growth and the characterization of magnetite thin films grown on Si(111) substrates by using pulsed laser deposition at different substrate temperatures (TS = 350, 450 and 550 ◦C) by performing X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) experiments. The O K− and Fe L3,2−edge XAS spectra reveal that the couplings of the O 2p with Fe 3d orbitals highly rely on the growth processes. The XMCD data of well-characterized thin films exhibit characteristic contributions from Fe A ions in tetrahedral sites and Fe 2+ B and Fe B ions in octahedral sites and have spectral features similar to those of the Fe3O4 single crystal. The temperature dependence of the XMCD line shape features remain unchanged below and above the Verwey transition. These investigations demonstrate the experimental conditions for controlled growth of Fe3O4 thin films on Si(111) substrates and suggest that the Fe3O4/Si(111) might a promising spintronic materials for future technology.

Giant planar Hall effect in epitaxialFe3O4thin films and its temperature dependence

We have investigated the giant planar Hall effect $({\ensuremath{\rho}}_{xy})$ in epitaxial thin films of ${\text{Fe}}_{3}{\text{O}}_{4}$ grown on MgO (001) substrates with thickness ranging from 5 to 150 nm. A record value at room temperature of $|{\ensuremath{\rho}}_{xy}|\ensuremath{\approx}60\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{cm}$ was obtained for the 5-nm-thick film. Below room temperature, $|{\ensuremath{\rho}}_{xy}|$ shows a huge enhancement when crossing the Verwey transition, reaching the value of $16\text{ }\text{m}\ensuremath{\Omega}\text{ }\text{cm}$ at $T=73\text{ }\text{K}$ for the 20-nm-thick film. This work demonstrates the potentiality of the planar Hall effect in magnetite thin films for studies of magnetization processes as well as for sensitive low-field detection. The influence of the superparamagnetic tendency for the thinnest films is highlighted. The results are also discussed in the context of fundamental aspects of ${\text{Fe}}_{3}{\text{O}}_{4}$ below the Verwey transition.

Effect of growth temperature on the structural and transport properties of magnetite thin films prepared by pulse laser deposition on single crystal Si substrate

Magnetite (Fe 3O 4) thin films are prepared by pulsed laser deposition using an α-Fe 2O 3 target on silicon (111) substrate in the substrate temperature range of 350 °C to 550 °C. X-ray diffraction (XRD) measurement shows that the film deposited at 450 °C is a single phase Fe 3O 4 film oriented along [111] direction. However, the film grown at 350 °C reveals mixed oxide phases (FeO and Fe 3O 4), while the film deposited at 550 °C is a polycrystalline Fe 3O 4. X-ray photoelectron spectroscopy study confirms the XRD findings. Raman measurements reveal identical spectra for all the films deposited at different substrate temperatures. We observe abrupt increase in the resistivity behavior of all the films around Verwey transition temperature ( T V) (125 K–120 K) though the transition is broader in the film deposited at 350 °C. We observe that the optimized temperature for the growth of Fe 3O 4 film on Si is 450 °C. The electrical transport behavior follows Shklovskii and Efros variable range hopping type conduction mechanism below T V for the film deposited at 450 °C possibly due to the granular growth of the film.

Magnetic Properties of Ultrathin Magnetite Films Grown by Molecular Beam Epitaxy

Magnetic properties as a function of film thickness (2-55 nm) for magnetite (Fe3O4) thin films grown on MgO (100) substrates show that the small thickness films (les 5 nm) are ferromagnetic and possess magnetization values which are much greater than that of bulk Fe3O4 . The inverse thickness dependence of the magnetization points to an interface related mechanism. Whereas, our detailed spin polarized density functional theoretical calculations suggests that there is only a marginal enhancement in the local spin moments of Fe-atom in the vicinity of the Fe3O4-MgO interface and it turns out that the noncompensation of spin moments between the two magnetic sublattices of spinel Fe3O4 is a major factor contributing to the enhancement of magnetization.

Direct deposition of magnetite thin films on organic semiconductors

Technological procedures able to produce high quality electrodes from magnetic oxides in vertical organic-inorganic hybrid devices is a challenging task in the field of organic spintronics. Thin films of magnetite (Fe3O4) have been successfully grown directly on top of organic semiconductor layers, tris(8-hydroxyquinoline)aluminium(III) (Alq3), by pulsed-electron ablation technique. The films show ferromagnetic behavior and good structural quality, properties detected by magneto-optical Kerr effect, superconductor quantum interference device, micro-Raman spectroscopy, and Atomic Force Microscopy. The ferromagnetic behavior persists even for 10nm thick films. Charge injection at magnetite-organic interface has been finally demonstrated by detecting electroluminescence from Alq3.