Half-metallic Magnetite Research Articles

Reactively sputtered half-metallic Fe3O4 thin films at room temperature on polymethyl methacrylate: A perspective for flexible spintronics

For plastic spintronics, it has thus far been a challenge to develop a (magnetic nanostructures)/polymer heterostructure at low temperature due to the thermal sensitivity of polymers. In this context, half-metallic magnetite (Fe3O4) films of different thicknesses were coated on flexible polymethyl methacrylate (PMMA) substrates by reactive sputtering at 300 K. Grain size and thickness increased from ~10 nm to ~34 nm and 50 nm to 400 nm with an increase in the deposition time from 165 to 1335 s, respectively. Moreover, a sharp rise in grain size was observed when the thickness of the film reached 200 nm, and finally, saturation took place at 400 nm. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS) studies confirmed the absence of the γ-Fe2O3 phase and thus the successful deposition of Fe3O4 thin films on the PMMA substrate. Parameters as such as Verwey transition (Tv) (~125 K), saturation magnetization (Ms) (~354 emu/cm3) and magnetoresistance (MR) (−8.6% under 60 KOe at 300 K) under H ∥ Film plane were obtained for Fe3O4/PMMA heterostructures with a film thickness of 200 nm. The resistivity (ρ), Tv, Ms and MR were hardly affected on several bending tests, proving that Fe3O4/PMMA heterostructures are exciting materials for future flexible spintronics.

An approach to reduce the antiferromagnetic coupling of antiphase boundaries in half-metallic magnetite films

Highly conductive (∼105 μΩ cm) Mn doped epitaxial Fe3O4 films were fabricated by reactive sputtering. The larger size of magnetic domains compared to grain size with the increasing Mn content indicates that the partial antiferromagnetic coupling across the antiphase boundaries has been weakened, which was further demonstrated by the smaller exchange bias, faster saturated magnetization, and decreasing exchange interaction JAF. The decrease of antiferromagnetic strength originates from the larger Mn-O bond length than that of Fe-O bond. The first-principle calculation shows that the half-metallic feature (100% spin polarization) of Fe3O4 was unchanged with the incorporation of Mn atoms.

Assembly of Iron Oxide Nanocrystal Superstructures

Self-organization of colloidal magnetic nanocrystals (NCs) into ordered superstructures is of significant interest as an opportunity for the controlled design and fabrication of mesoscopic magnetic materials that could find use in magnetic data storage, sensors and biomedicine. For example, an assembly of monodisperse magnetic NCs with controlled interparticle spacing can allow a fine tuning of spindependent transport which is particularly important when single electron tunneling (SET) and spin-dependent transport (SDT) are targeted in nano-sized devices [1, 2]. Using ad-hoc self-assembly procedures, highly ordered supercrystal structures have been prepared in recent years, with interesting collective magnetic properties that are different from those exhibited by their isolated NC building blocks, corresponding disordered ensembles or their bulk materials counterpart. In large superstructures of colloidal iron oxide NCs, the interplay of magnetic dipole-dipole interactions with the NC magnetocrystalline anisotropy can lead to enhanced saturation magnetization and coercivity, while dilute assemblies exhibit superparamagnetic behaviour [3, 4]. Collective magnetic properties are indeed extremely sensitive to mutual interactions among NCs, depending on the superlattice structure and interparticle spacing. This calls for efficient design of NC arrays with different dimensions for integration into functional device structures. Here, we report on the magnetic fieldassisted assembly of iron oxide NCs with inverse spinel cubic structure of mixed-phase halfmetallic magnetite (Fe3O4) /insulating maghemite (γ-Fe2O3) [5]. Self-assembly was carried out by subjecting toluene solutions of purified NCs to slow solvent evaporation in a quasi toluene-vapour-saturated ambient. A perpendicular to plane magnetic field (0.5 T) was applied to facilitate growth of superlattices. Two common procedures were employed for NC self-assembly, namely dip coating and drop casting. In the former approach, a substrate was dipped into a NC solution with a permanent magnet positioned below and kept there over a period of 3 days maintaining a saturated toluene vapour ambience. Then the substrate was extracted, washed and dried. In the drop-casting method, few drops of NC solution were placed over the substrate and the solvent was allowed to slowly evaporate. NC concentrations were varied in the range of 0.3-7 μM. Self-assembly was investigated on two kinds of surfaces: (100 nm thick) Au and (3nm/100nm) SiO2/Au, prepared on SiO2/Si substrates using thermal evaporation and electron beam techniques. The template surfaces were also modified with organic ligands to evaluate the effect of NC-substrate interactions on the selfassembly. The superlattices could be tailored in a versatile way, from thin films up to macroscopic supercrystals in the millimeter range, by acting on NC solution concentration and the mode of casting. Although NC self-assembly into superlattices is largely entropically driven, beyond crystal size distribution and NC density, ordering is also governed by NC-NC and NCsubstrate interactions, and the presence of external forces. Interparticle interactions may include van der Waals attraction, steric repulsion, dipole-dipole interactions and Coulomb interactions between charged ligands. Changing these interactions can significantly modify the self-assembly process. In particular, the organic ligands passivating the NC surface or the specific substrate chemistry were found to change the extent of the mutual interactions between neighbouring particles as well as the strength of NC adhesion on the substrate, thereby modifying the surface mobility which plays a critical role [5].

Switchable Fermi surface sheets in greigite

Greigite (Fe${}_{3}$S${}_{4}$) and magnetite (Fe${}_{3}$O${}_{4}$) are isostructural and isoelectronic ferrimagnets with quite distinct properties. Electronic structure calculations reveal greigite is a normal metal in contrast to half-metallic magnetite. Greigite shows a complex Fermi surface with a unique influence of relativistic effects: The existence of sheets of the Fermi surface depends on the direction of the magnetization. This enables spinorbitronics, spintronics on the level of a single compound rather than a device. Due to its relativistic origin, spin contamination is irrelevant in spinorbitronics and the entire periodic table is available for optimizations.

Half-metallicity, magnetic moments, and gap states in oxygen-deficient magnetite for spintronic applications

The electronic structure near oxygen vacancies in half-metallic magnetite has been calculated using first principles methods. Oxygen vacancies are responsible for the existence of gap states occupied by majority and minority spin electrons. We discuss whether these defects modify the spin magnetic moments, the magnetization, the magnetic coupling between Fe ions, and the half-metallic behaviour of magnetite. These results, which contribute to remove stumbling blocks to magnetite-based spintronic devices, could be useful to analyze the conductivity, the magnetotransport and magnetic properties, the electron and optical spectra of actual magnetite electrodes.

Growth of mixed phased films of γ-Fe2O3 and half metallic Fe3O4 using (100) MgO/Fe under layer for spin electronic devices

Half metallic magnetite films using (100) MgO/Fe under layer deposited at room temperature were prepared using facing targets sputtering method. X-ray diffractometry and TEM analyses confirmed epitaxial growth of (100) oriented Fe3O4 layer on (100) oriented MgO under layer prepared on 5nm-thick Fe buffer layer on glass substrates. Resistivity analyses clarified that higher oxygen partial pressure caused higher oxidation of Fe ions in the films which tend to result in γ-Fe2O3. Substrate heating is effective to improve crystallinity and magnetic properties of Fe3O4 film. Oxygen partial pressure of 7.5%, substrate temperature of 150 °C and sputtering discharge current of 0.45A was suitable to prepare magnetite Fe3O4 film with (100) preferential orientation and good magnetic properties.

Magneto-resistance control by use of interparticle-coupling-form variation in half-metallic magnetite nano-particle sinter

We have studied the magneto-resistance (MR) control by use of a variation of the interparticle-coupling-form (IPCF) in half-metallic magnetite (Fe3O4) nano-particle sinter (MNPS). The average nano-particle size of the MNPS is 30nm and the measurements were carried out between 4K and 300K. The variation of the IPCF in the MNPS was performed by the heat treatment (HT) between 400°C and 800°C. The behavior of the MR and electrical resistance (ER) considerably changes at the HT temperature of around 600°C. Below around 600°C the ER indicates the variable-range-hopping conduction behavior in a wide temperature region, whereas above around 600°C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal. We have considered that the electrical conduction switching in the MNPS occurs at the HT temperature of around 600°C. Namely, below around 600°C the ER is dominated by the grain-boundary conduction and above around 600°C the ER is determined by the inter-grain conduction. We have found that at room temperature the MR for the MNPS made at the HT temperature of 400°C shows approximately 20 times that of a bulk single crystal. The application of the magnetic field to grain-boundary conduction is considered to cause the large enhancement of the MR.

Transport and Magneto-Transport Characteristics of Fe$_{3}$O$_{4}$/GaAs Hybrid Structure

We have investigated the transport and magneto-transport characteristics of magnetite (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) thin films deposited on top of GaAs (100). The magnetoresistance has been measured in both the longitudinal and the transverse directions, and the results show that magnetoresistance of the thin Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> films is due to anisotropic magnetoresistance (AMR) and there is no evidence of anti-phase boundaries from the magnetoresistance measurements. The current-voltage (I-V) characteristics across the interface have been measured to explore the nature of the magnetite/GaAs contact, and the results show typical characteristics of Schottky barrier contact with a moderate barrier height of 0.34 eV, which might favour high efficient spin injection from the half metallic magnetite into GaAs for spintronic applications

Transport characteristics of magnetite thin films grown onto GaAs substrates

Magnetite thin films with a preferred (111) orientation have been deposited by reactive dc magnetron sputtering from a pure Fe target onto (100) GaAs substrates at 400 °C. The films show a clear Verwey transition in both the magnetization and sheet resistance as functions of temperature. For films deposited onto semiconducting n-type GaAs substrates, we have obtained asymmetric current–voltage (I–V) characteristics with a Schottky diodelike behavior in forward bias. Activation energy plots of the I–V data as a function of temperature indicate a barrier height of 0.3–0.4 eV. This does not take into account the contribution from tunneling across the narrow depletion layer in these junctions, so should be considered a lower bound to the actual Schottky barrier height. Our work points to the potential integration of half-metallic magnetite with GaAs-based heterostructures for spin-electronic devices.

Large magnetoresistance in tunnel junctions with an iron oxide electrode

We report on the fabrication and properties of (cobalt/alumina/iron oxide) tunnel junctions. We observe magnetoresistance (MR) effects reaching 43% at 4.2 K and 13% at room temperature. This large MR is ascribed to the presence of a Fe3−xO4 (close to half-metallic magnetite) phase identified by electron diffraction. At low temperature, the MR drops sharply when the bias voltage is smaller than 10 mV, which suggests that the magnetoresistance originates from the activation of tunneling channels through spin polarized states below and above the Fermi level in the iron oxide.