L. L. Tao

Ferroelectricity and tunneling electroresistance effect driven by asymmetric polar interfaces in all-oxide ferroelectric tunnel junctions

By constructing asymmetric polar interfaces, all-oxide ferroelectric tunnel junctions (FTJs) are proposed that can achieve a sizable tunneling electroresistance (TER) effect. Based on first-principles quantum transport calculations on a prototypical LaNiO3/BaTiO3/LaNiO3 junction, we predict that TER reaches 103% under a finite bias. Driven by the asymmetric polar interfaces, the resultant intrinsic electric field causes a highly asymmetric electrostatic potential in comparison to that of the FTJ with symmetric polar interfaces. As a result, the tunneling resistance changes significantly upon polarization reversal leading to sizable TER. The physical origin of the TER effect can be well understood in terms of local density of states, transport in momentum space, real-space scattering states and a free-electron tunneling model. Our results provide an insight into the understanding of ferroelectricity and the TER mechanism in FTJs and will be useful for FTJ-based devices design.

Origin of ferromagnetism in aluminum-doped TiO2 thin films: Theory and experiments

In this paper, we combine first-principles calculations and experiments to investigate the magnetic properties of aluminum-doped TiO2 films of rutile structure. Density-functional theory with generalized gradient approximation based calculations were carried out for three cases, where the TiO2 lattice contains oxygen vacancies VO only, an oxygen is substituted by a fluorine atom, or a Ti is substituted by an aluminum. Magnetic moments associated with the formation of Ti3+ ions are found in all cases but they couple differently resulting in different magnetic states. Al-doped samples prepared in our labs exhibit ferromagnetism at room temperature with a TC near 340 K. The experimental results are consistent with the first principles calculations, and the magnetism is associated with the VO defect electrons induced by the Al doping. The defect electron occupies nearby Ti sites giving rise to the Ti3+ moments and, at the same time, has spatially extended wavefunctions assuring overlapping between neighbors.

Tunneling magnetoresistance in Fe3Si/MgO/Fe3Si(001) magnetic tunnel junctions

We present a theoretical study of the tunneling magnetoresistance (TMR) and spin-polarized transport in Fe3Si/MgO/Fe3Si(001) magnetic tunnel junction (MTJ). It is found that the spin-polarized conductance and bias-dependent TMR ratios are rather sensitive to the structure of Fe3Si electrode. From the symmetry analysis of the band structures, we found that there is no spin-polarized Δ1 symmetry bands crossing the Fermi level for the cubic Fe3Si. In contrast, the tetragonal Fe3Si driven by in-plane strain reveals half-metal nature in terms of Δ1 state. The giant TMR ratios are predicted for both MTJs with cubic and tetragonal Fe3Si electrodes under zero bias. However, the giant TMR ratio resulting from interface resonant transmission for the former decreases rapidly with the bias. For the latter, the giant TMR ratio can maintain up to larger bias due to coherent transmission through the majority-spin Δ1 channel.

Ferroelectricity and tunneling electroresistance effect in asymmetric ferroelectric tunnel junctions

We report the investigation on the ferroelectricity and tunneling electroresistance (TER) effect in PbTiO3 (PTO)-based ferroelectric tunnel junctions (FTJs) using first-principles calculations. For symmetric FTJs, we have calculated the average polarizations of PTO film and effective screening lengths of different metal electrodes for a number of FTJs, which is useful for experimental research. For asymmetric FTJs, significant asymmetric ferroelectric displacements in PTO film are observed, which is attributed to the intrinsic field generated by the two dissimilar electrodes. Moreover, by performing quantum transport calculations on those asymmetric FTJs, a sizable TER effect is observed. It is found that the asymmetry of ferroelectric displacements in PTO barrier, which is determined by the difference of work functions of the electrodes, controls the observed TER effect. Our results will help unravel the TER mechanism of asymmetric FTJs in most experiments and will be useful for the designing of FTJ-base...

Tunneling magnetoresistance of FePt/NaCl/FePt(001)

We propose and theoretically investigate an interesting and potentially very attractive magnetic tunnel junction FePt/NaCl/FePt(001) for spintronics. It is attractive because the strain at the FePt/NaCl interface is relatively small and, as a result, spin injection from the FePt metal to the NaCl barrier is significant and thus a potentially large TMR ratio can be obtained. The electronic bands with the symmetry of L10 FePt cross the Fermi level for both the majority-spin and minority-spin channels, and the evanescent state with the symmetry dominates the electron transmission through the fcc NaCl barrier. Very respectable values of the tunnel magnetoresistance ratio are predicted. The microscopic physics of quantum transport in this system is systematically analyzed and understood.

Large magnetoresistance of paracyclophane-based molecular tunnel junctions: A first-principles study

We report a theoretical study of magnetoresistance and spin-polarized transport of a series of paracyclophane-based molecular tunnel junctions. We predict that the molecular tunnel junction using [2.2]-paracyclophane barrier has the desired low resistance area product in combination with high magnetoresistance ratio. In addition, we find the spin-polarized conductance decreases exponentially with increasing the molecular length, indicating a nonresonant tunneling mechanism. In particular, the characteristic decay constant can be theoretically evaluated from the complex band structure of periodic paracyclophane molecule. The spin-polarized transport mechanism is systematically analyzed.

Strain-tunable ferroelectricity and its control of Rashba effect in KTaO3

The effects of epitaxial strain on the ferroelectric, structural properties of KTaO3 are studied by means of first-principles calculations. We show that the ferroelectric polarization magnitude as well as the orientation can be tuned by an in-plane strain: the c-phase is energetically more stable than the aa-phase at a large compressive strain while a phase transition from c- to aa-phase is observed at a large tensile strain, owing to the significant polarization-strain coupling. More importantly, based on relativistic first-principles calculations, we demonstrate a large Rashba spin splitting in the strained KTaO3. Interestingly, the spin textures in momentum space can be controlled and switched via polarization switching. Our tight-binding analysis indicates that the combination of spin-orbit coupling and ferroelectric distortion plays a key role for the observed Rashba spin splitting. Our results present some fundamental understanding of the interplay between Rashba effect and ferroelectricity in oxide...

Effect of a semiconductor electrode on the tunneling electroresistance in ferroelectric tunneling junction

We report the tunneling electroresistance effect (TER) in a Pt/BaTiO3(BTO)/Nb:SrTiO3 (n-STO) ferroelectric tunnel junction (FTJ). Using transmission electron microscopy, X-ray photoelectron spectroscopy, and piezoresponse force microscopy, we find that the thick BaTiO3 (5 nm) film is epitaxial and of high quality. A large ON/OFF resistance ratio of more than 104% at room temperature is observed. Our experimental results as well as theoretical modeling reveal that the depletion region near the BTO/n-STO interface can be electrically modulated via ferroelectric polarization, which plays a key role for the TER effect. Moreover, both long retention and high switching reproducibility are observed in the Pt/BTO/n-STO FTJ. Our results provide some fundamental understandings of the TER mechanism in the FTJs using a semiconductor electrode and will be useful for FTJ-based nonvolatile devices design.

External electric field effects on electronic and magnetic properties at molecule-metal interfaces: Cu-phthalocyanine adsorbed on Fe(001) surface

We report the first-principles studies of adsorption of Cu-phthalocyanine (CuPc) molecules on Fe(001) surfaces using density functional theory with generalized gradient approximation. The van der Walls interaction between CuPc molecules and Fe surfaces was included with Grimme approximation. The detailed structural and electronic and magnetic properties at the interface between the CuPc and Fe(001) were obtained. More importantly, based on the understanding of adsorption configurations, we further studied the interfacial properties with considering the effect of external electric field. The results demonstrate that both the transferred charge and localized magnetic moments of adsorbed molecules can be tuned by external applied electric field.

All-electrical generation of spin-polarized currents in quantum spin Hall insulators

The control and generation of spin-polarized currents (SPCs) without magnetic materials and external magnetic field is a big challenge in spintronics and normally requires spin-flip mechanism. In this work, we propose a novel method to control and generate SPCs in stanene nanoribbons in the quantum spin Hall (QSH) insulator regime by all electrical means without spin-flip mechanism. This is achieved with intrinsic spin-orbit coupling in stanene nanoribbons by tuning the relative phase of spin up and down electrons using a gate voltage, which creates a time delay between them thereby producing alternative SPCs driven by ac voltage. The control and generation of SPCs are demonstrated numerically for ac transport in both transient and ac regime. Our results are robust against edge imperfections and generally valid for other QSH insulators such as silicene and germanene, etc. These findings establish a novel route for generating SPCs by purely electrical means and open the door for new applications of semiconductor spintronics.