E. Y. Jiang

Bond-order–bond-length–bond-strength (bond-OLS) correlation mechanism for the shape-and-size dependence of a nanosolid

A bond-order–bond-length–bond-strength (bond-OLS) correlation mechanism is presented for consistent insight into the origin of the shape-and-size dependence of a nanosolid, aiming to provide guidelines for designing nanomaterials with desired functions. It is proposed that the coordination number imperfection of an atom at a surface causes the remaining bonds of the lower-coordinated surface atom to relax spontaneously; as such, the bond energy rises (in absolute value). The bond energy rise contributes not only to the cohesive energy (ECoh) of the surface atom but also to the energy density in the relaxed region. ECoh relates to thermodynamic properties such as self-assembly, phase transition and thermal stability of a nanosolid. The binding energy density rise is responsible for the changes of the system Hamiltonian and related properties, such as the bandgap, core-level shift, phonon frequency and the dielectrics of a nanosolid of which the surface curvature and the portion of surface atoms vary with particle size. The bond-OLS premise, involving no assumptions or freely adjustable parameters, has led to consistency between predictions and experimental observations of a number of outstanding properties of nanosolids.

Bond contraction and lone pair interaction at nitride surfaces

It is shown that bond contraction and nonbonding lone-pair interaction dominate at nitride surfaces. The maximum elastic recovery of a nitride surface was found to be 100% under a relatively lower nanoindentation load (<1.0 mN) and the hardness of the surface was found to be 100% higher than the bulk value. It is interpreted that the spontaneous bond contraction, estimated at 12%–14%, strengthens the binding energy and hence the hardness and Young’s modulus at the surface. The lone-pair weak interaction claims the responsibility for (i) the high elastic recovery, (ii) the lower Raman frequencies of vibration, and (iii) the existence of critical loads for slide friction or lone-pair broken.

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%.

Facing targets sputtered Fe‐N gradient films

Fe‐N gradient films were prepared with a facing targets sputtering system. During deposition, the nitrogen pressure increased linearly up to a value, which is called the ‘‘ultimate pressure.’’ Composition profiles, microstructure, magnetic properties, and corrosion resistance of the films were investigated by various methods. The experimental results indicate that the Fe‐N films possess some composition and structural gradients. The Fe concentration decreases from the substrate to the film surface from 100 to 66 at. %. The phases α‘‐Fe16N2, γ’‐Fe4N, e‐FexN(2<x≤3) and ζ‐Fe2N are present in the gradient films at different depths. Ms under the ultimate nitrogen pressure of 0.05 Pa has a value of 1803 emu/cc which is higher than that of bulk iron, this is attributed to the presence of Fe16N2. Increasing further the ultimate nitrogen pressure, Ms decreases monotonically. The corrosion resistance of the gradient film with higher nitrogen concentration near the surface is good enough for magnetic recording heads.

The structures and magnetic properties of FeN films prepared by the facing targets sputtering method

The Fe nitride films were easily produced by facing targets sputtering (FTS). The structures and magnetic properties of the films depend on the reactive pressure (PN) and dc substrate biasing voltage (Vb). α‐Fe, α‘‐Fe16N2, γ’‐Fe4N, e‐Fe3N, and ζ‐Fe2N as well as the amorphous FeN phase have been produced at the PN ranges of 0–8×10−3 Torr. The magnetic moment per iron atom increases with PN until PN=1.0 mTorr at which the α’‐Fe16N2 phase appears with the coercive force Hc=103 A/m, susceptibility χm=161, and μS=2.85 μB. The relation between conductivity and temperature σ‐T indicates that Fe nitride films resemble a semiconductor. The Curie temperature of these films is reduced with the increase of PN. Magnetic relaxation phenomena were observed in the vicinity of 540 °C.

Effects of surface passivation and interfacial reaction on the size-dependent 2p-level shift of supported copper nanosolids

Abstract Effects of surface passivation and particle-substrate interfacial reaction on the size dependent 2p 3/2 -level shift of nanosolid Cu have been numerically analyzed, leading to information about the reactivity of Cu nanosolid with different substrates, such as graphite, polymer and alumina. It has been found that Ar + bombardment promotes the Cu-polymer reaction and N + passivation strengthens the surface bond due to nitride formation. Cu atom interacts with alumina slightly stronger at room temperature than at 80 K. Matching predictions to the measured size-dependent Cu-2p level shift reveals that the intra-atomic trapping energy of a core electron at the Cu 2p 3/2 -level is −931.0 eV and the bulk crystal bonding intensity to the 2p 3/2 electron is −1.70 eV, which is beyond the scope of conventional approaches.

Enhanced Hall effect in FexGe1−x nanocomposite films

Enhanced Hall effect has been observed in the FexGe1−x nanocomposite films composed of nanoscale Fe grains embedded in amorphous Ge matrix. The Fe grain size, the saturation magnetization, and the interparticle interaction increase with increasing Fe atomic fraction x. The transport mechanism of the films changes from semiconducting to metallic character as x increases, and the percolation phenomenon ocurrs at x∼0.5. The Hall resistivity (ρxy) reaches its maximum of ∼126μΩcm at x=0.5, which is ∼140 times larger than that of Fe films. The ordinary and extraordinary Hall coefficients are enhanced by two orders in magnitude compared to pure Fe films and four orders compared to the bulk Fe. Upon annealing at 400°C, the enhanced Hall effect disappears with the disappearance of the Fe grains. This enhanced Hall effect can be attributed to the local quantum interference effect due to the presence of ∼1–2nm Fe grains.

Current-perpendicular-to-plane transport properties of polycrystalline Fe3O4/α-Fe2O3 heterostructures

Current-perpendicular-to-plane transport properties of sputtered polycrystalline Fe3O4/α-Fe2O3 heterostructures were investigated. A rectifying behavior was observed. The voltage shift increases linearly with temperature and turns from negative to positive at 230 K. The current-dependent magnetoresistance (MR) changes from negative to positive in the temperature range of 230–260 K. The largest negative MR is −32% at 230 K, and the positive MR at 305 K reaches 80% at 1.0 mA. The characteristic MR is thought to be caused by the rectifying effect and band structure at the Fe3O4/α-Fe2O3 interface.

Structure, magnetic, and transport properties of sputtered Fe/Ge multilayers

The structure, magnetic, and transport properties, especially the Hall effect, of Fe∕Ge multilayers fabricated by magnetron sputtering were investigated. Structure characterization indicates a periodic modulated structure with alternately deposited polycrystalline Fe and amorphous Ge layers. The room-temperature magnetic measurements reveal that the uniaxial magnetic anisotropy Ku of the Fe∕Ge multilayers with a period of 5.2nm is 2.27×103J∕m3. The temperature coefficient of resistivity of all the films is positive at room temperature but turns to be negative at low temperatures due to the weak localization effect. The Fe∕Ge multilayers show anomalous Hall effect and the Hall sensitivity KH is independent of the temperature, showing that Fe∕Ge multilayers have the potential applications in the field of magnetic sensors. When the period Λ is 5.2nm, the anomalous Hall coefficient Rs reaches its largest value of 1.8×10−7Ωm∕T, which is three orders of magnitude larger than that of the bulk Fe material.