Magnetic properties of epitaxial Fe3_3O4_4 films with various crystal orientations and TMR effect in room temperature

Fe$_3$O$_4$ is a ferrimagnetic spinel ferrite that exhibits electric conductivity at room temperature (RT). Although the material has been predicted to be a half metal according to ab-initio calculations, magnetic tunnel junctions (MTJs) with Fe$_3$O$_4$ electrodes have demonstrated a small tunnel magnetoresistance effect. Not even the sign of the TMR ratio has been experimentally established. Here, we report on the magnetic properties of epitaxial Fe$_3$O$_4$ films with various crystal orientations. The films exhibited apparent crystal orientation dependence on hysteresis curves. In particular, Fe$_3$O$_4$(110) films exhibited in-plane uniaxial magnetic anisotropy. With respect to the squareness of hysteresis, Fe$_3$O$_4$ (111) demonstrated the largest squareness. Furthermore, we fabricated MTJs with Fe$_3$O$_4$(110) electrodes, and obtained an TMR effect of -12\% at RT. The negative TMR ratio corresponded to the negative spin polarization of Fe$_3$O$_4$ predicted from band calculations

Mesoscovic magnetic/semiconductor heterostructures

We report the experimental results of Fe and Fe3O4 nanostructures on GaAs(100) surfaces and hybrid Ferromagnetic/Semiconductor/Ferromagnetic (FM/SC/FM) spintronic devices. Element specific x-ray magnetic circular dichroism (XMCD) measurements have shown directly that Fe atoms on the GaAs(100)-4 x 6 surface are ferromagnetic. Within coverages of 2.5 to 4.8 ML superparamagnetic nanoclusters are formed and exhibiting strong uniaxial anisotropy, of the order of 6.0 x 10(5) erg/cm(3). The coercivities of epitaxial Fe dot arrays films grown on GaAs(100) were observed to be dependent on the separation and size of the dots indicating that interdot dipolar coupling affects the magnetization processes in these dots. In addition Fe3O4 films grown on deformed GaAs(100) substrates have been observed to form nanostripes following the topography of the substrate and magneto-optical Kerr effect (MOKE) measurements showed that these nanostripes have uniaxial magnetic anisotropy with easy axis perpendicular to the length of the nanostripes. Meanwhile the FM/SC/FM vertical device has exhibited a biasing current dependent on MR characteristics, with a maximum change of 12% in the MR observed, indicating for the first time a large room temperature spin injection and detection

Magnetoelectric properties of magnetite thin films

Resistivity, DC Hall effect and transverse magnetoresistance measurements were made on polycrystalline thin films of magnetite (Fe3O4) from 104K to room temperature. The Verwey transition is observed at TV=123K, about 4K higher than reported for bulk magnetite. The ordinary and extraordinary Hall coefficients are negative over the entire temperature range, consistent with negatively charged carriers. The extraordinary Hall coefficient exhibits a rho 1/3 dependence on the resistivity above TV and a rho 2/3 dependence below TV. The magnetoresistance is negative at all temperatures and for all magnetic field strengths. The planar Hall effect signal was below the sensitivity of the present experiment

Investigation of Electrodeposited Magnetite Films: Formation and Characterization

Magnetite (Fe3O4) is of both scientific and technological interest because of its fascinating magnetic properties. It has a high Curie temperature of 860 K and a theoretical 100% spin polarization at the Fermi level. There are a variety of deposition techniques to form thin films of magnetite, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), iron oxidation, sputtering and so on. In comparison with other deposition methods mentioned above, electrodeposition has a key advantage of relatively low processing temperature. The intention of this work was to investigate magnetite (Fe3O4) thin films grown via an electrochemical route by using various kinds of characterization techniques, especially on morphology, chemical composition, structure and magnetic properties. Fe3O4 thin films were obtained by using a galvanostatic or potentiostatic deposition from simple aqueous solutions of ferrous salts. Iron oxide thin films have been grown at different current densities and temperatures onto polycrystalline copper substrates. XRD results indicate that Fe3O4 is formed at 90 oC at an applied current density of 0.05 mA·cm-2. Lower growth temperatures can cause the formation of another phase, α-FeOOH at a certain concentration of Fe2+ and pH buffer. Time-dependent growth of the iron oxides exhibits nucleation and coalescence. In order to obtain uniform Fe3O4 film surface, longer deposition times are needed. The influence of applied potential on the characteristics of the deposited iron oxide was examined. The formation of Fe3O4 in a low potential regime (< 100 mV) vs. gold reference electrode while iron oxyhydroxides such as goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) are favoured for E > 100 mV. The magnetic properties of the films were found to be strongly dependent on the deposition potential. The multi-layer structure of Fe3O4/α-FeOOH/Fe3O4 onto NiO/Ni substrates has been demonstrated via successive deposition. A TEM cross-section image shows α-FeOOH is coherently formed between two ferromagnetic layers. ADF-STEM micrographs show that Fe3O4 has a columnar structure and has less composition variation compared to that grown onto a polycrystalline copper substrate. Synchrotron techniques, i.e. x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD), were performed to examine the iron oxide film. Fe K-edge x-ray absorption spectra demonstrate that the films grown at low potential regime (< 100 mV) have a comparable valency state with the standard Fe3O4 sample. The identification of the iron oxide was further confirmed by using XMCD technique. The calculation of the asymmetry ratio suggests that the total magnetic moment increased with decreasing applied potential. In addition, vibrating sample magnetometer (VSM) data show that the magnetic response is somewhat slower for the iron oxide grown at higher potential regime. A change of pH in the electrolyte does not change the lattice constant and film morphology or texture but does affect particle sizes in Fe3O4 thin films. This decrease with the pH is due to the reaction of FeOH+ ions with molecular oxygen in electrolyte

Novel Multifunctional Materials Based on Oxide Thin Films and Artificial Heteroepitaxial Multilayers

Transition metal oxides show fascinating physical properties such as high temperature superconductivity, ferro- and antiferromagnetism, ferroelectricity or even multiferroicity. The enormous progress in oxide thin film technology allows us to integrate these materials with semiconducting, normal conducting, dielectric or non-linear optical oxides in complex oxide heterostructures, providing the basis for novel multi-functional materials and various device applications. Here, we report on the combination of ferromagnetic, semiconducting, metallic, and dielectric materials properties in thin films and artificial heterostructures using laser molecular beam epitaxy. We discuss the fabrication and characterization of oxide-based ferromagnetic tunnel junctions, transition metal-doped semiconductors, intrinsic multiferroics, and artificial ferroelectric/ferromagetic heterostructures - the latter allow for the detailed study of strain effects, forming the basis of spin-mechanics. For characterization we use X-ray diffraction, SQUID magnetometry, magnetotransport measurements, and advanced methods of transmission electron microscopy with the goal to correlate macroscopic physical properties with the microstructure of the thin films and heterostructures.Comment: 21 pages, 21 figures (2 figures added, typos corrected