Xiaocha Wang

Spin Polarization Inversion at Benzene-Absorbed Fe4N Surface

We report a first-principle study on electronic structure and simulation of the spin-polarized scanning tunneling microscopy graphic of a benzene/Fe4N interface. Fe4N is a compound ferromagnet suitable for many spintronic applications. We found that, depending on the particular termination schemes and interface configurations, the spin polarization on the benzene surface shows a rich variety of properties ranging from cosine-type oscillation to polarization inversion. Spin-polarization inversion above benzene is resulting from the hybridizations between C pz and the out-of-plane d orbitals of Fe atom.

The electronic structure and spin–orbit-induced spin splitting in antimonene with vacancy defects

We study the geometric, electronic properties, and spin splitting in monovacancy (MV) and divacancy (DV) antimonene with five different models using first-principles calculations. Meanwhile the influence of spin–orbit coupling (SOC) is included. Different vacancies cause different geometric structures with or without inversion symmetry and influence the electronic structures. MV antimonene shows metallic character, however, four DV antimnoene models preserve the semiconducting character narrowing the band gap. The inversion asymmetry and SOC lead to the spin splitting in MV and two DV models. Zeeman-type spin splitting appears with out-of-plane spin polarization along M–K–Γ. Rashba and Dresselhaus effects induced spin splitting occurs at Γ and M points in MV.

Ferroelectric Metal in Tetragonal BiCoO3/BiFeO3 Bilayers and Its Electric Field Effect.

By first-principles calculations we investigate the electronic structure of tetragonal BiCoO3/BiFeO3 bilayers with different terminations. The multiferroic insulator BiCoO3 and BiFeO3 transform into metal in all of three models. Particularly, energetically favored model CoO2-BiO exhibits ferroelectric metallic properties, and external electric field enhances the ferroelectric displacements significantly. The metallic character is mainly associated to eg electrons, while t2g electrons are responsible for ferroelectric properties. Moreover, the strong hybridization between eg and O p electrons around Fermi level provides conditions to the coexistence of ferroelectric and metallic properties. These special behaviors of electrons are influenced by the interfacial electronic reconstruction with formed Bi-O electrovalent bond, which breaks OA-Fe/Co-OB coupling partially. Besides, the external electric field reverses spin polarization of Fe/Co ions efficiently, even reaching 100%.

Geometric distortion and spin-dependent electronic structure of C6H6-adsorbed Fe3O4(001): A first-principles study

The electronic structure of C6H6/Fe3O4(001) interfaces has been investigated by the density functional theory. It is found that a weaker interaction exists between C6H6 and Fe3O4(001) in the adsorption models by comparing with the previous studies. The electronic states of C6H6 show a slight spin-splitting, which has the potential applications as a molecular spin filter. Meanwhile, a valence transition from Fe3+ to Fe2+ occurs in one model, which reduces the magnetic moment. The transition can be ascribed to the geometric distortion induced by C6H6 adsorption, which depicts the re-interaction process among Fe3O4(001) atoms. Furthermore, the high spatial spin polarization of C6H6/Fe3O4(001) appears with the increased density of states at EF. The calculated results offer a new mechanism to manipulate the interfacial electronic structure through C6H6 adsorption, which has the potential applications in organic spintronic devices.

Electric-field tunable perpendicular magnetic anisotropy in tetragonal Fe4N/BiFeO3 heterostructures

Electric field control on perpendicular magnetic anisotropy (PMA) is indispensable for spintronic devices. Herewith, in tetragonal Fe4N/BiFeO3 heterostructures with the FeAFeB/Fe-O2 interface, PMA in each Fe4N layer, not merely interfacial layers, is modulated by the electric field, which is attributed to the broken spin screening of the electric field in highly spin-polarized Fe4N. Moreover, the periodical dxy+dyz+dz2 and dxy+dx2−y2 orbital-PMA oscillation enhances the interactions between adjacent FeAFeB and (FeB)2N atomic layers, which benefits the electric field modulation on PMA in the whole Fe4N atomic layers. The electric-field control on PMA in Fe4N/BiFeO3 heterostructures is favored by the electric-field-lifted potential in Fe4N.

Strain-controlled interfacial magnetization and orbital splitting in La2/3Sr1/3MnO3/tetragonal BiFeO3 heterostructures

The electronic and magnetic properties of La2/3Sr1/3MnO3(LSMO)/tetragonal BiFeO3(BFO) heterostructures with the xy-plane biaxial strain were investigated by first-principles calculations. As the xy-plane lattice constant is ranging from 3.619 to 3.921 A, a large interfacial magnetoelectric coupling (IMEC) appears in the LaO/Fe-O2 and LaO/O2-Fe models. Particularly, the interfacial Mn in the LaO/O2-Fe model shows a large spin-down t2g orbital splitting at a strain from −2% to 6%, where the maximum splitting energy is 283 meV at 2%. Additionally, the LaO/O2-Fe model is half-metallic at a strain of 2%, 4%, and 6%. The results demonstrate that IMEC, orbital occupancy, and half-metallicity of LSMO/BFO heterostructures can be effectively engineered by the interfacial coupling and in-plane strain, which paves a way for designing the novel magnetoelectric devices.

Spin polarization and magnetic characteristics at C6H6/Co2MnSi(001) spinterface

Organic materials with mechanical flexibility, low cost, chemical engineering, and long spin lifetime attract considerable attention for building spintronic devices. Here, a C6H6/Co2MnSi(001) spinterface is investigated by first-principles calculations and spin-polarized scanning tunneling microscopy simulations. Several high symmetry adsorption sites are discussed, together with two possible surface terminations of Co2MnSi(001). An inversion of the spin polarization is induced near EF even in the case of an external electric field, indicating that C6H6 can act as a spin filter to exploit the spin injection efficiency in organic spintronic devices. Unlike previous studies on molecule/ferromagnet interfaces, this inversion is closely related to the electronic structure of the atoms in the subsurface layer of Co2MnSi according to the orbital symmetry analysis. Furthermore, the magnetic moment and magnetic anisotropic energy (MAE) in the outermost Co2MnSi layer are studied. Particularly, in the most stable configuration, the sign of MAE is inversed due to hybridization between C p and Co dz2 orbitals, which suggests that a greater modification on MAE can be achieved by the use of a highly chemically reactive organic molecule. These findings improve the study on the engineering of magnetic properties at molecule/ferromagnetic interfaces through a single π-conjugated organic molecule.

Perpendicular Magnetic Anisotropy Preserved by Orbital Oscillation in Strained Tetragonal Fe4N/BiFeO3 Bilayers

Orbital performances are important for inducing and manipulating the perpendicular magnetic anisotropy (PMA) in spintronic devices. Herewith, the orbital-mediated PMA in highly spin-polarized Fe4N are investigated in strained tetragonal Fe4N/BiFeO3(001) heterostructures with the FeAFeB/Fe-O2 termination using the first-principles calculations. Different from the d2 = dxz + dyz + dz2 favored PMA in previously reported Fe film, for all the Fe4N atomic layers at the biaxial strain of S, all d orbitals (i.e., d1 = dxy + dx2-y2 and d2) make contributions to the PMA at S = 0% and in-plane magnetic anisotropy (IMA) at S = -2 and 2%. Specifically, the d1-d2 orbital oscillation preserves (or favors) the PMA in 0% strained Fe4N, where the stronger MAE contribution alternates between d1 and d2 in adjacent Fe4N layers. However, at S = -2 and 2%, the whole Fe4N shows IMA with stable d1 and d2 contributions. Moreover, the PMA in the unstrained Fe4N can be transformed into the IMA by a strain of -2% with a high spin polarization, where Fe4N/BiFeO3 interfacial effects are crucial. The PMA preserved by the controllably orbital oscillation in highly spin-polarized Fe4N paves a way for developing novel spintronic devices.

Strain and electric-field tunable valley states in 2D van der Waals MoTe2/WTe2 heterostructures.

The strain and electric-field effects on the electronic structure of MoTe2/WTe2 van der Waals heterostructures are investigated by first-principles calculations. The MoTe2/WTe2 heterostructures are indirect band gap semiconductors under different strains except for 2%. At a strain from  -6% to 6% under a zero electric field, the band gap is 0.56, 0.62, 0.69, 0.62, 0.46, 0.37 and 0.29 eV, respectively. Meanwhile, spin splitting at the conduction band minimum (CBM) decreases monotonically from 76-1 meV, and that at the valance band maximum (VBM) is 232, 266, 292, 307, 319, 302 and 283 meV. At an electric field from  -0.3 to 0.3 V Å-1 under a 2% strain, VBM splitting decreases from 499-77 meV, but CBM splitting almost remains at 33 meV. A semiconductor-metal transition appears at an electric field of  -0.3 V Å-1. At different electric fields under a  -4% strain, CBM splitting monotonically increases from 37-154 meV, but VBM splitting is 437, 438, 378, 273, 150, 78 and 134 meV, respectively. Our results can provide a more significant basis for spintronic and valleytronic devices.

Spin-dependent electronic transport characteristics in Fe4N/BiFeO3/Fe4N perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (MTJs) have attracted increasing attention owing to the low energy consumption and wide application prospects. Herewith, against Jullieres formula, an inverse tunnel magnetoresistance (TMR) appears in tetragonal Fe4N/BiFeO3/Fe4N perpendicular MTJs, which is attributed to the binding between the interface resonant tunneling state and central (bordered) hot spots. Especially, antiferromagnetic BiFeO3 shows an extra spin-polarized resonant state in the barrier, which provides a magnetic-barrier factor to affect the tunneling transport in MTJs. Meanwhile, due to the spin-polarized transport in Fe4N/BiFeO3/Fe4N MTJs, the sign of TMR can be tuned by the applied bias. The tunable TMR and resonant magnetic barrier effect pave the way for clarifying the tunneling transport in other junctions and spintronic devices.