Ruqian Wu

First principles determination of the effects of phosphorus and boron on iron grain boundary cohesion.

Toward an electronic level understanding of intergranular embrittlement and its control in steels, the effects of phosphorus and boron impurities on the energy and electronic properties of both an iron grain boundary and its corresponding intergranular fracture surface are investigated by the local density full potential augmented plane wave method. When structural relaxations are taken into account, the calculated energy difference of phosphorus in the two environments is consistent with its measured embrittlement potency. In contrast to the nonhybridized interaction of iron and phosphorus, iron-boron hybridization permits covalent bonding normal to the boundary contributing to cohesion enhancement. Insights into bonding behavior offer the potential for new directions in alloy composition for improvement of grain boundary-sensitive properties.

Intrinsic spin Seebeck effect in Au/YIG.

The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin polarized density functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.

Spin–orbit induced magnetic phenomena in bulk metals and their surfaces and interfaces

Abstract First-principles electronic structure studies based on local spin density functional theory and performed on extremely complex simulations of ever increasingly realistic systems, play a very important role in explaining and predicting surface and interface magnetism. This review deals with what is a major issue for first-principles theory, namely the theoretical/computational treatment of the weak spin–orbit coupling in magnetic transition metals and their alloys and its important physical consequences: magneto-crystalline anisotropy, magnetostriction, magneto-optical Kerr effects and X-ray magnetic circular dichroism. As is demonstrated, extensive first-principles calculations and model analyses now provide simple physical insights and guidelines to search for new magnetic recording and sensor materials.

Conductometric chemical sensor based on individual CuO nanowires

CuO nanowires with high crystalline quality are synthesized via a simple thermal oxidation method. Charge conduction on individual nanowires under a transverse electric field exhibits an intrinsic p-type semiconducting behavior. Variations in signal transducer in different chemical gas environments are measured on individual CuO nanowire field effect transistors. They demonstrate good performance to both NO(2) and ethanol gasses. In particular, the nanowire chemical sensor reveals a reverse response to ethanol vapor under temperature variation. Experimental results and first-principles calculations indicate that ethanol is oxidized in air at high temperature, resulting in the production of CO(2) and H(2)O. The strong H(2)O adsorption leads to the reversal behavior, due to the electron transfer from H(2)O molecules to the CuO surface.

Iron Pyrite Thin Films Synthesized from an Fe(acac)3 Ink

Iron pyrite (cubic FeS2) is a promising candidate absorber material for earth-abundant thin-film solar cells. Here, we report on phase-pure, large-grain, and uniform polycrystalline pyrite films that are fabricated by solution-phase deposition of an iron(III) acetylacetonate molecular ink followed by sequential annealing in air, H2S, and sulfur gas at temperatures up to 550 °C. Phase and elemental compositions of the films are characterized by conventional and synchrotron X-ray diffraction, Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy (XPS). These solution-deposited films have more oxygen and alkalis, less carbon and hydrogen, and smaller optical band gaps (E(g) = 0.87 ± 0.05 eV) than similar films made by chemical vapor deposition. XPS is used to assess the chemical composition of the film surface before and after exposure to air and immersion in water to remove surface contaminants. Optical measurements of films rich in marcasite (orthorhombic FeS2) show that marcasite has a band gap at least as large as pyrite and that the two polymorphs share similar absorptivity spectra, in excellent agreement with density functional theory models. Regardless of the marcasite and elemental impurity contents, all films show p-type, weakly activated transport with curved Arrhenius plots, a room-temperature resistivity of ~1 Ω cm, and a hole mobility that is too small to measure by Hall effect. This universal electrical behavior strongly suggests that a common defect or a hole-rich surface layer governs the electrical properties of most FeS2 thin films.

Increasing the band gap of iron pyrite by alloying with oxygen

Systematic density functional theory studies and model analyses have been used to show that the band gap of iron pyrite (FeS(2)) can be increased from ~1.0 to 1.2-1.3 eV by replacing ~10% of the sulfur atoms with oxygen atoms (i.e., ~10% O(S) impurities). O(S) formation is exothermic, and the oxygen atoms tend to avoid O-O dimerization, which favors the structural stability of homogeneous FeS(2-x)O(x) alloys and frustrates phase separation into FeS(2) and iron oxides. With an ideal band gap, absence of O(S)-induced gap states, high optical absorptivity, and low electron effective mass, FeS(2-x)O(x) alloys are promising for the development of pyrite-based heterojunction solar cells that feature large photovoltages and high device efficiencies.

Control of the magnetism and magnetic anisotropy of a single-molecule magnet with an electric field.

Through systematic density functional calculations, the mechanism of the substrate induced spin reorientation transition in FePc/O-Cu(110) is explained in terms of charge transfer and rearrangement of Fe-3d orbitals. Moreover, we find giant magnetoelectric effects in this system, manifested by the sensitive dependence of its magnetic moment and magnetic anisotropy energy on an external electric field. In particular, the direction of magnetization of FePc/O-Cu(110) is switchable between in-plane and perpendicular axes, simply by applying an external electric field of 0.5 eV/Å along the surface normal.

MAGNETOCRYSTALLINE ANISOTROPY OF INTERFACES - FIRST-PRINCIPLES THEORY FOR CO-CU INTERFACE AND INTERPRETATION BY AN EFFECTIVE LIGAND INTERACTION-MODEL

The state tracking method proposed recently is employed for the first-principles local density determination of interface magnetocrystalline anisotropy (MCA) energy by the full potential linearized augmented plane wave method. The interface MCA mechanism involving Co is studied with the Co-Cu interface as an example. The free standing Co monolayer is found to exhibit a strong negative MCA (easy axis in the layer plane), -1.35 meV, due to the spin-orbit coupling between spin-down bonding z(2) and anti-bonding xz or yz states along in the Brillouin zone, and between anti-bonding z(2) and bonding xz and yz states near (M) over bar. At the Co-Cu interface, the out-of-plane Co bonding z(2), xz and yz states interact strongly with the Cu states, giving rise to the main change: a decrease in the magnitude of this negative contribution. Together with the effect of changes in band filling and the contribution from the spin-orbit coupling between opposite spins, the interface MCA energy of a Co layer is -0.38 meV for a Co overlayer on a Cu(001) substrate, and near zero (- 0.01 meV) for a Co layer sandwiched between a Cu(001) matrix. These results are in very good agreement with recent in situ experimental measurements. An effective ligand interaction model is developed which successfully interprets the first principles results and further shows how the interface MCA depends on the energy of the d orbitals of the interface atoms and the strength of the interface bonds.

Giant magnetic anisotropy of transition-metal dimers on defected graphene.

Continuous miniaturization of magnetic units in spintronics and quantum computing devices inspires efforts to search for magnetic nanostructures with giant magnetic anisotropy energy (MAE) and high structural stability. Through density functional theory calculations, we found that either Pt-Ir or Os-Ru dimer forms a stable vertical structure on the defected graphene and possess an MAE larger than 60 meV, sufficient for room-temperature applications. Interestingly, their MAEs can be conveniently manipulated by using an external electric field, which makes them excellent magnetic units in spintronics and quantum computing devices.

First-principles determinations of magneto-crystalline anisotropy and magnetostriction in bulk and thin-film transition metals

Abstract Recent developments in the first-principles determination of magneto-crystalline anisotropy (MCA) and magnetostrictive coefficients in transition metal systems are reviewed. With the aid of our newly developed state tracking and torque approaches, high numerical stability can be achieved for the MCA energy, the essential ingredient of the magnetostriction. Very smooth monotonic behavior of the MCA energy with respect to the lattice strain was found for most of the systems studied. The calculated magnetostrictive coefficients are positive for BCC Fe and FCC Co but negative for FCC Ni — a result which agrees well with experiments. This can be explained simply through the strain induced d-band broadening and shifting. For Co/Cu(0 0 1), Co/Pd(0 0 1) and Co/Pd(1 1 1) thin films, the magnetostrictive coefficients are found to be very sensitive to the change of substrates and even orientation.