Haili Bai

Large room-temperature spin-dependent tunneling magnetoresistance in polycrystalline Fe3O4 films

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin-polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance (MR) {=[ρ(H)−ρ(0)]/ρ(0)} shows linear and quadratic magnetic-field dependence in the low-field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries. At 300 K, the size of the MR reaches −7.4% under a 50-kOe magnetic field, which is a very large MR for polycrystalline Fe3O4 films.

First Principles Prediction of the Magnetic Properties of Fe-X6 (X = S, C, N, O, F) Doped Monolayer MoS2

Using first-principles calculations, we have investigated the electronic structure and magnetic properties of Fe-X6 clusters (X = S, C, N, O, and F) incorporated in 4 × 4 monolayer MoS2, where a Mo atom is substituted by Fe and its nearest S atoms are substituted by C, N, O, and F. Single Fe and Fe-F6 substituions make the system display half-metallic properties, Fe-C6 and Fe-N6 substitutions lead to a spin gapless semiconducting behavior, and Fe-O6 doped monolayer MoS2 is semiconducting. Magnetic moments of 1.93, 1.45, 3.18, 2.08, and 2.21u2005μB are obtained for X = S, C, N, O, and F, respectively. The different electronic and magnetic characters originate from hybridization between the X and Fe/Mo atoms. Our results suggest that cluster doping can be an efficient strategy for exploring two-dimensional diluted magnetic semiconductors.

Magnetism by Interfacial Hybridization and p-type Doping of MoS2 in Fe4N/MoS2 Superlattices: A First-Principles Study

Magnetic and electronic properties of Fe4N(111)/MoS2(√3 × √3) superlattices are investigated by first-principles calculations, considering two models: (I) Fe(I)Fe(II)-S and (II) N-S interfaces, each with six stacking configurations. In model I, strong interfacial hybridization between Fe(I)/Fe(II) and S results in magnetism of monolayer MoS2, with a magnetic moment of 0.33 μB for Mo located on top of Fe(I). For model II, no magnetism is induced due to weak N-S interfacial bonding, and the semiconducting nature of monolayer MoS2 is preserved. Charge transfer between MoS2 and N results in p-type MoS2 with Schottky barrier heights of 0.5-0.6 eV. Our results demonstrate that the interfacial geometry and hybridization can be used to tune the magnetism and doping in Fe4N(111)/MoS2(√3 × √3) superlattices.

Thickness dependence of magnetic and magneto-transport properties of polycrystalline Fe3O4 films prepared by reactive sputtering at room temperature

At room temperature, polycrystalline Fe3O4 films with thicknesses in the range 5–1120 nm have been prepared by reactive sputtering. Transmission electron microscopy imaging shows that uniform Fe3O4 grains are well separated by grain boundaries and their size decreases with film thickness. The change of resistivity as a function of temperature reveals a grain boundary dominated electron tunnelling mechanism. The magnetoresistance MR = (ρH − ρ0)/ρ0 measured at room temperature for the films thicker than 200 nm is ~−7.4% under a magnetic field of 46 kOe, which is among the largest values ever reported for Fe3O4 films under the same measuring conditions. As the thickness reduces from 80 to 5 nm, MR decreases from −6.5% to −1.1% due to the enhanced spin-flip scattering at film and grain surfaces.

Origin of the butterfly-shaped magnetoresistance in reactive sputtered epitaxial Fe3O4 films

Epitaxial Fe3O4 thin films were synthesized by facing-target reactive sputtering Fe targets. The epitaxy of the Fe3O4 film on MgO (100) was examined macroscopically using x-ray diffraction, including conventional θ-2θ scan, tilting 2θ scan, φ scan, and pole figure. The observed low-field butterfly-shaped magnetoresistance (MR) are explained by the primary fast rotation of the spins far away from antiphase boundaries and the high-field MR changing linearly with magnetic field can be understood by the gradual rotation of the spins near the antiphase boundaries. It is magnetocrystalline anisotropy that causes an increase in MR below Verwey transition temperature.

Spin-polarized transport of electrons from polycrystalline Fe3O4 to amorphous Si

Polycrystalline Fe3O4∕amorphous Si heterostructure was prepared by facing-target sputtering and its microstructure and electrical transport properties were studied. The polycrystalline Fe3O4 layer was grown in column structure. The electrical transport mechanism across the disordered interface between polycrystalline Fe3O4 and amorphous Si layers is tunneling above the Verwey temperature [Nature (London) 144, 327 (1939)] of 120K. Nonlinear I‐V characteristics of the Schottky diode reveal thermionic emission∕diffusion mechanism below the Verwey temperature, and Schottky barrier height is 0.27eV, calculated by a standard theory of thermionic emission∕diffusion. Based on a simplified band structure, the spin polarization of the polycrystalline Fe3O4 layer was determined to be ∼45%.

Structure and magnetotransport properties of Fe3O4–SiO2 composite films reactively sputtered at room temperature

(Fe3O4)1−x–(SiO2)x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by logu200aρ∝T−1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH−ρ0)/ρ0} of about −4.7% for Fe3O4 films and −1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x⩾0.4.

Magnetization and Resistance Switchings Induced by Electric Field in Epitaxial Mn:ZnO/BiFeO3 Multiferroic Heterostructures at Room Temperature

Electric field induced reversible switchings of the magnetization and resistance were achieved at room temperature in epitaxial Mn:ZnO(110)/BiFeO3(001) heterostructures. The observed modulation of magnetic moment is ∼500% accompanying with a coercive field varying from 43 to 300 Oe and a resistive switching ratio up to ∼10(4)% with the applied voltages of ±4 V. The switching mechanisms in magnetization and resistance are attributed to the ferroelectric polarization reversal of the BiFeO3 layer under applied electric fields, combined with the reversible change of oxygen vacancy concentration at the Mn:ZnO/BiFeO3 interface.

Effect of annealing on polycrystalline La1−xNaxMnOz ceramics

Abstract The structure, magnetic and transport properties of La 1− x Na x MnO z manganese perovskite prepared by Pechini process were investigated. Low field magnetoresistance in temperature range from 77xa0K to Curie temperature was obtained and two obvious peaks were observed in the resistivity versus temperature curves. Differential scanning calorimetry results show that the broad bumps far below Curie temperature does not originate from anomalous phase transition. The effect of annealing is carefully investigated and the results indicate that oxygen inhomogeneous distribution may be the main reason for the curious behaviors.

Enhancement of the magnetization in the Fe3O4/BiFeO3 epitaxial heterostructures fabricated by magnetron sputtering

Epitaxial Fe3O4/BiFeO3 heterostructures with different BiFeO3 thicknesses were deposited on (001) SrTiO3 substrates by magnetron sputtering. An unexpected enhancement in the magnetization measured with a high sensitivity of ∼10−7u2009emu magnetometer was observed, especially for the Fe3O4/BiFeO3 heterostructures with 22-nm-thick BiFeO3 layers. The magnetization of the heterostructures can be up to 133% of the sum of both single Fe3O4 and BiFeO3 layers deposited directly on the SrTiO3 substrate under the same conditions. The enhanced magnetization is considered to originate from the magnetic spin moments which interact and arrange ferromagnetically at the interface due to the strong interfacial coupling.