Interface Termination Research Articles

General method for predicting ZnO–metal interface termination: Extension of the method for Al2O3–metal systems

A method for predicting interface termination between zinc oxide and various metals, in general, is discussed by extending the method previously proposed for the prediction of Al2O3–metal interface termination, which has been implemented as a free web-based software. Experimental results and first-principles calculations in references on the interface termination are carefully examined and compared with the predicted ones. It is demonstrated that the predicted results agree well with those experimental and calculated results. The method uses only basic parameters of pure elements and the formation enthalpy of oxides. Therefore, it can be applied for most of the metals in the periodic table.

Effects of Interfacial Termination, Oxidation, and Film Thickness on the Magnetic Anisotropy in Mn2.25Co0.75Ga0.5Sn0.5/MgO Heterostructures.

Perpendicular magnetic anisotropy (PMA) is a determining factor for the realization of nonvolatile information storage devices with high efficiency and thermal stability. In this work, a new spin gapless semiconductor Mn2.25Co0.75Ga0.5Sn0.5 Heusler alloy with an inter-spin zero gap was first designed theoretically. The Mn2.25Co0.75Ga0.5Sn0.5 bulk was prepared successfully in experiment. The effects of interfacial termination, oxidation, and film thickness on the magnetic anisotropy of Mn2.25Co0.75Ga0.5Sn0.5/MgO (MCGS/MgO) heterostructures are investigated systematically by first-principles calculations. The results show that all the Mn(A)Mn(C)GaSn-, Mn(A)Mn(C)CoGaSn-, Mn(B)GaSnI-, and Mn(B)GaSnII-terminated MCGS/MgO heterostructures (called as AC1, AC2, BD1, and BD2 models, respectively) present PMA, which mainly derives from the interfacial and surficial MCGS layers. Furthermore, the PMA of MCGS/MgO heterostructures can be preserved in a large range of interfacial oxidization (up to ±50%). With MCGS thickness increasing from 5 to 16 monolayers, the PMA of MCGS/MgO heterostructures with an AC-type surface decreases significantly. However, the PMA of BD-type surface models is relatively robust to the thickness of the MCGS layer, and the magnetic anisotropy always points to the out-of-plane direction. Therefore, MCGS Heusler alloy is a new promising spin gapless semiconductor candidate for spintronics applications. The robust and tunable PMA in MCGS/MgO heterostructures offers the possibility for developing nonvolatile data memory devices.

Structure of the northwestern North Anatolian Fault Zone imaged via teleseismic scattering tomography

SUMMARYInformation on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths.

Interface-engineered electron and hole tunneling.

Although the phenomenon of tunneling has been known since the advent of quantum mechanics, it continues to enrich our understanding of many fields of science. Commonly, this effect is described in terms of electrons traversing the potential barrier that exceeds their kinetic energy due to the wave nature of electrons. This picture of electron tunneling fails, however, for tunnel junctions, where the Fermi energy lies sufficiently close to the insulator valence band, in which case, hole tunneling dominates. We demonstrate the deterministic control of electron and hole tunneling in interface-engineered Pt/BaTiO3/La0.7Sr0.3MnO3 ferroelectric tunnel junctions by reversal of tunneling electroresistance. Our electrical measurements, electron microscopy and spectroscopy characterization, and theoretical modeling unambiguously point out to electron or hole tunneling regimes depending on interface termination. The interface control of the tunneling regime offers designed functionalities of electronic devices.

Robustness of the Half-Metallicity at the Interfaces in Co2MnSi -Based All-Full-Heusler-Alloy Spintronic Devices

Based on first-principles methods, we demonstrate that it would be possible to design efficient all-full-Heusler-alloy-based spintronic devices associating ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$ electrodes and nonmagnetic full-Heusler-alloy spacers selected to preserve the half-metallicity of ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$ up to the interface atomic layers. We focus on the ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}/{\mathrm{Fe}}_{2}\mathrm{Ti}\mathrm{Si}$ magnetic tunnel junction, compare the stability range of its different interface terminations, and show that the half-metallicity of the ${\mathrm{Fe}}_{2}$/$\mathrm{Mn}\mathrm{Si}$-terminated interface even withstands atomic intermixing. The robustness of the half-metallicity of this interface termination is also confirmed in ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}/{\mathrm{Fe}}_{2}\mathrm{V}\mathrm{Al}$ spin valves. With such interfaces, spintronic devices could possess very high magnetoresistive properties.

Tailoring interfacial properties in CaVO3 thin films and heterostructures with SrTiO3 and LaAlO3 : A DFT+DMFT study

In this paper we use density functional theory combined with dynamical mean-field theory (DFT+DMFT) to study interface effects between thin films of the correlated metal CaVO$_3$ and the two typical substrate materials SrTiO$_3$ and LaAlO$_3$. We find that the CaVO$_3$/SrTiO$_3$ interface has only a marginal influence on the CaVO$_3$ thin film, with the dominant effect being the (bulklike) epitaxial strain imposed by the large lattice mismatch, rendering the CaVO$_3$ film insulating due to the enhanced orbital polarization related to the strong level splitting between the t$_{\mathrm{2g}}$ orbitals. In contrast, at the polar CaVO$_3$/LaAlO$_3$ interface, the presence of the interface can have a huge effect on the thin film properties, depending both on the specific interface termination as well as the specific boundary conditions imposed by the multilayer geometry. We compare three different approaches to model the interface between the correlated metal CaVO$_3$ and the band insulator LaAlO$_3$, which all impose a different set of (electrostatic) boundary conditions on the electronic structure. The spectral properties obtained from our calculations reveal a strong influence of the supercell geometry, ranging from bulklike to highly doped and structurally distorted phases, indicating a potential tunability of the interfacial properties via multilayer engineering.

On the atomic structure of Pt(1 1 1)/γ-Al2O3(1 1 1) interfaces and the changes in their interfacial energy with temperature and oxygen pressure

Metal/metal-oxide interfaces are important in many applications including metal coatings, sensing, and catalysis. Here weinvestigate and predict the structure and chemical bonding of the interface between γ-Al2O3and Pt using density functional theory. Previous high-resolution transmission electron microscopy studies showed atomically flat interfaces between the (111) planes of Pt and γ-Al2O3;this interfacial orientation is studied herein. A simulatedtemperature–pressure equilibrium configurationdiagram shows howthe stability of the interfaces changes with temperature and oxygen pressure.Of the three interfacial terminations (O, Al, Al2) the oxygen-terminated γ-Al2O3(111) is the most stable at atmospheric conditions. The stability of this interface is attributed to the strong electrostatic interaction between the Pt and the oxygen atoms at the oxygen-terminated interface. This work provides essential insights into the atomic structure and the origins of the interactions at the interface between transition metals and metal-oxides, as well as interfacial configuration at a range of conditions.

Schottky barrier formation at theFe/SrTiO3(001) interface: Influence of oxygen vacancies and layer oxidation

Schottky barrier formation at metal/insulating oxide interfaces relies on complex mechanisms which are difficult to unravel. We propose a detailed numerical study of the atomic, magnetic, and electronic properties of the $\mathrm{Fe}/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ interface, in which we focused our discussion on different parameters which can affect the Schottky barrier height (SBH). The interface termination appears to be the most critical aspect to be controlled for this interface: While an ideal $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated interface would guarantee a $n$-type barrier of about 1.2--1.6 eV, the presence of a SrO termination can drastically decrease its value down to few meV. The oxidation state of the interface is also an important criterium to maintain a high barrier value. Oxygen vacancies are always cited as the source of a deterioration of the SBH. For a $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated interface, we found that in their most stable position, i.e., in the interface layer, the oxygen vacancies do not affect the value of the SBH; when moving some atomic layers away from the interface, the SBH on the contrary decreases regularly. We propose that oxidizing the interface would allow us to improve the $n$-type SBH by healing the oxygen vacancies and forming an interfacial FeO layer, which seems favorable to the formation of a higher SBH.

Stabilization of ferroelastic charged domain walls in self-assembled BiFeO3 nanoislands

Understanding the microscopic origin of exotic domain configurations and emergent properties in charged domain walls is essential for both fundamental physics and applications in next-generation nanoelectronics. In self-assembled BiFeO3 nanoislands, 71° ferroelastic charged domain walls were spontaneously formed, which were traditionally regarded as unstable architectures. Here, by combining scanning transmission electron microscopy, geometric phase analysis, and energy dispersion spectrum, we explored the microscopic mechanism of stabilizing the tail-to-tail charged domain walls with upward center-divergent quad-domain structures in BiFeO3 nanoislands. The comprehensive factors include the fully relaxed rhombohedral phase in nanoislands, negative surface screening charges, and upward built-in bias at the BiFeO3-(La,Sr)MnO3 interface, which are respectively induced by interfacial periodic dislocations, surface reconstructions, and interfacial terminations. Moreover, the unusual polarization state and domain-wall arrangement in the nanoislands were also proved to be stable even at an elevated temperature.

Modeling the Interface between Lithium Metal and Its Native Oxide.

Owing to their high theoretical capacities, batteries that employ lithium (Li) metal as the negative electrode are attractive technologies for next-generation energy storage. However, the successful implementation of lithium metal batteries is limited by several factors, many of which can be traced to an incomplete understanding of surface phenomena involving the Li anode. Here, first-principles calculations are used to characterize the native oxide layer on Li, including several properties associated with the Li/lithium oxide (Li2O) interface. Multiple interface models are examined; the models account for differing interface (chemical) terminations and degrees of atomic ordering (i.e., crystalline vs amorphous). The interfacial energy, formation energy, and strain energies are predicted for these models. The amorphous interface yields the lowest interfacial formation energy, suggesting that it is the most probable model under equilibrium conditions. The work of adhesion is evaluated for the crystalline interfaces, and it is found that the O-terminated interface exhibits a work of adhesion more than 30 times larger than that of the Li-terminated model, implying that Li will strongly wet an oxygen-rich Li2O surface. The electronic structure of the interfaces is characterized using Voronoi charge analysis and shifts in the Li 1s binding energies. The width of the Li/Li2O interface, as determined by deviations from bulklike charges and binding energies, extends beyond the region exhibiting interfacial structural distortions. Finally, the transport of Li ions through the amorphous oxide is quantified using ab initio molecular dynamics. Facile transport of Li+ through the native oxide is observed. Thus, increasing the percentage of amorphous Li2O in the solid electrolyte interphase may be beneficial for battery performance. In total, the phenomena quantified here will aid in the optimization of batteries that employ high-capacity Li metal anodes.

Experimental observation of large tunneling anisotropic magnetoresistance in a magnetic tunnel junction without heavy metals

Tunneling anisotropic magnetoresistance (TAMR) in the magnetic tunnel junctions (MTJs) driven by spin–orbit coupling (SOC) has attracted much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, it has been believed that heavy metals are indispensable for the large TAMR. Here we experimentally show a TAMR of up to 46% in a La0.7Sr0.3MnO3 (LSMO) based MTJ without heavy metals. This value represents the largest TAMR for MTJs with half-metallic electrodes. We demonstrate that the TAMR ratio is enhanced by over one order of magnitude by tuning interface termination layer, achieving quasilocalized states at the interface Fermi level on the basis of magnetization orientation. These results are also supported by our first-principles density functional calculations. This finding illustrates a crucial role of interface tuning in the TAMR effect, opening a route for engineering the large anisotropic magnetoresistance by interface termination control and manipulating high spin polarization without using heavy metals.

Electric field modulation of magnetism in ferrimagnetic Heusler heterostructures

To date the realization of magnetoresistive random access memory (RAM) and magnetoelectric RAM (MeRAM) devices relies primarily on ultrathin ferromagnetic-based (FeCoB/MgO) magnetic tunnel junctions. On the other hand, the Heusler family of intermetallics is considered very promising for spintronic applications. Nevertheless, the voltage controlled magnetic anisotropy (VCMA) in ultrathin Heusler-based magnetic-tunnel junction stacks remains unexplored. Here, using the ferrimagnetic Heusler ${\mathrm{Mn}}_{3}\mathrm{Ga}$ as a prototype system, we report ab initio calculations of the electric field modulation of magnetism in the $\mathrm{Ir}/{\mathrm{Mn}}_{3}\mathrm{Ga}$/MgO heterostructure. The trilayer structures with one and three monolayer Ir caps and Mn-Mn termination exhibit large perpendicular magnetic anisotropy in contrast to those with Mn-Ga termination which yield in-plane magnetization orientation. We predict giant VCMA coefficients the magnitude and sign of which depend on both the interface termination and the Ir cap thickness. The underlying atomistic mechanism lies on the electric-field-induced shifts of the spin-orbit coupling energies of the spin-polarized $\mathrm{Ir}/d$ orbitals with different orbital angular momentum symmetries. Our paper paves the way for exploiting the unique magnetic properties of ferrimagnetic Heusler compounds for next generation MeRAM devices.

Polarization Screening Mechanisms at La0.7Sr0.3MnO3-PbTiO3 Interfaces.

The structural, electronic, and magnetic properties of interfaces between epitaxial La0.7Sr0.3MnO3 and PbTiO3 have been explored via atomic resolution transmission electron microscopy of a functional multiferroic tunnel junction. Measurements of the polar displacements and octahedral tilting show the competition between the two distortions at the interface and demonstrate strong dependence on the polarization orientation. The density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.

Atomic insight into the interfacial bonding and role of carbon atoms on β-SiC(1 1 1)/Al2MgC2(0 0 0 1): A first-principles study

Al2MgC2 may form on the interface of SiC/Mg composite, and as substrate of heterogeneous nucleation, Al2MgC2 could refine α-Mg grain. So, interface system SiC/Al2MgC2/Mg is interesting to be further clarified, while the study about SiC/Al2MgC2 is still insufficient. In present work, interfacial bonding on SiC(1 1 1)/Al2MgC2(0 0 0 1) and role of carbon atoms are investigated by using density functional theory (DFT) method. Considering different interfacial termination and stacking sites, totally 18 models are examined. Si/C-terminated and top-site stacked model (denoted as Si/C_top) is identified as the most stable one. Interfacial fracture toughness is predicted as critical stress intensity factor KIcint = 2.34∼2.99 MPa∙m1/2. Interfacial bonding mostly contributes from carbon atoms, which behave as charge acceptors, and charge transfer between interfacial C-Si atoms is confirmed. Peaks in PDOS of interfacial C atoms shift towards negative side. Especially for C atom in SiC(1 1 1), shift from −1.33 eV to less than −2.9 eV, which generates stronger interfacial bonding. For C atom in Al2MgC2(0 0 0 1), new peak forms around −8.08 eV, and mainly from C-2p2 orbital. Meanwhile, its s orbital peak negatively shifts from −9.71 eV to −11.61 eV. The interfacial Si-C covalent bonds are predominantly composed as hybridizations of C-2p2 and Si-3s2 around −8.1 eV, C-2p2 and Si-3s2 around −11.5 eV.

First-principles investigations of effects of solute elements on stability and electronic structure of laves phase/matrix interface in Ni-based superalloys

It is acknowledged that solute elements like Ru exert great influence on precipitation of deleterious topologically close-packed (TCP) phases. In this work, by means of first principle calculation based on density functional theory (DFT), the interface structure between C14-laves phase and matrix γ phase has been established for the first time, and stable interfacial termination type was achieved. Effects of solute atoms like Ru on interface stability, electronic structure, and bond characteristics were further calculated. It has been proved that, unlike other elements, Ru can notably elevate the interfacial energy of C14-laves/γ system, lower interface stability, and increase the difficulty of nucleation of C14-laves phase; on the contrary, W will decrease the interfacial energy of C14-laves/γ and make it easier for C14-laves phase to nucleate; Re, Mo, and Cr will also increase the interfacial energy, but nowhere near the effect of Ru. Besides, in comparison with other solute elements, C14-laves/γ substituted by Ru has the smallest degree of interfacial charge accumulation, lowest interfacial atomic bonding strength, and poorest interfacial stability. All the reasons discussed above lead to the blocking effect of Ru on the nucleation of C14-laves phase.

First-principles theoretical and experimental studies of effects of ruthenium on precipitation behavior of μ phase and μ/matrix interface stability in Ni-based single crystal superalloys

Precipitation of deleterious topologically close-packed (TCP) phases worsens the mechanical behavior of Ni-based single crystal superalloys. It is acknowledged that Ru will inhibit precipitation of TCP phase. In this paper, precipitation behaviors of TCP in alloy with and without Ru have been investigated through experimental characterization means, combined with the first principles calculations. For the first time, the interface structure between precipitated μ phase and matrix γ phase has been established, and stable interfacial termination types in different orientation relationships were obtained. Effects of solute atoms like Ru on interface stability and site substitution behaviors of different solute elements in two phases were further calculated, and compared with precipitation behavior and composition information of TCP phases in two alloys. It is verified that Ru is different with other elements, which can remarkably increase the interfacial energy of μ/γ interface, decrease interface stability, and rise the difficulty of nucleation of μ phase; conversely, W will lower the interfacial energy of μ/γ, making it easier for μ phase to nucleate; the effects of Co and Cr are not that obvious; the effects of Re and Mo on interfacial energy and nucleation of μ phase vary with orientation relationships. Also, Re, Cr, and Ru have larger trends to diffuse from matrix γ to μ phase during growing process of μ phase. The above reasons give rise to blocking effect of Ru on the nucleation of TCP phase, but Ru has trifling influence on inhibition of precipitation growth.

Formation of two-dimensional electron and hole gases at the interface of half-Heusler semiconductors

Heuslers are a prominent family of multi-functional materials that includes semiconductors, half metals, topological semimetals, and magnetic superconductors. Owing to their same crystalline structure, yet quite different electronic properties and flexibility in chemical composition, Heusler-based heterostructures can be designed to show intriguing properties at the interface. Using electronic structure calculations, we show that two dimensional electron or hole gases (2DEG or 2DHG) form at the interface of half-Heusler (HH) semiconductors without any chemical doping. We use CoTiSb/NiTiSn as an example, and show that the 2DEG at the TiSb/Ni(001) termination and the 2DHG at the Co/TiSn(001) termination are intrinsic to the interface, and hold rather high charge densities of 3x10^14 carriers/cm^2. These excess charge carriers are tightly bound to the interface plane and are fully accommodated in transition-metal d sub-bands. The formation of 2DEG and 2DHG are not specific to the CoTiSb/NiTiSn system; a list of combinations of HH semiconductors that are predicted to form 2DEG or 2DHG is provided based on band alignment, interface termination, and lattice mismatch.

Atomistic study of atomic structures and dislocation nucleation at Al/Al2Cu interfaces

The plate-like phase of Al2Cu necessitates a detailed understanding of the interface structure, which significantly governs the structural applications of AlCu alloys. Different from a metal/metal interface where only one stable interface exists, the metal eutectic composites possess multiple possible interfaces. Here, atomistic simulations were implemented to reveal a strong dependence of dislocation structure on the termination plane at the Al/Al2Cu hetero-interface. Different termination of Al2Cu phase at interfaces exhibited distinct atomic arrangements, leading to variations of γ -surface and ultimately dictate the dislocation structure. Among the four possible termination at Al2Cu (110) plane, two of them are stable in Al/Al2Cu hetero-interface, named as AlB-terminated and AlA2-terminated interfaces. The networks of misfit dislocations with different Burgers vectors of these two interfaces are also identified. Disregistry investigations of the interface showed that the misfit dislocation lines in AlB-terminated interface are curved while that in AlA2-terminated interface are straight. This is related to the relative interface energy between the stable coherent structures. These dislocation networks in turn have a bearing on the mechanical response of interfaces, as manifested by the different nucleation mechanisms of lattice dislocations. Although this work is performed on a particular Al/Al2Cu, the commonalities also exist in other eutectic composites. The detailed correlation between the interface structure and termination at metal-intermetallic interface has been recognized for the first time, which could offer novel avenues for fine-tuning eutectic composites with enhanced properties.

Low-field switching of noncollinear spin texture at La0.7Sr0.3MnO3−SrRuO3 interfaces

Interfaces of ferroic oxides can show complex magnetic textures which have strong impact on spintronics devices. This has been demonstrated recently for interfaces with insulating antiferromagnets such as $\mathrm{BiFe}{\mathrm{O}}_{3}$. Here, noncollinear spin textures which can be switched in very low magnetic field are reported for conducting ferromagnetic bilayers of $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{SrRu}{\mathrm{O}}_{3}$ (LSMO-SRO). The magnetic order and switching are fundamentally different for bilayers coherently grown in reversed stacking sequence. The SRO top layer forms a persistent exchange spring which is antiferromagnetically coupled to LSMO and drives switching in low fields of a few milliteslas. Density functional theory reveals the crucial impact of the interface termination on the strength of Mn-Ru exchange coupling across the interface. The observation of an exchange spring agrees with ultrastrong coupling for the $\mathrm{Mn}{\mathrm{O}}_{2}/\mathrm{SrO}$ termination. Our results demonstrate low-field switching of noncollinear spin textures at an interface between conducting oxides, opening a pathway for manipulating and utilizing electron transport phenomena in controlled spin textures at oxide interfaces.

Role of the Interfacial Rh-layer on Robust Ferromagnetism and Large Perpendicular Magnetic Anisotropy of FeRh Films on MgO(0 0 1)

Abstract Employing first-principles calculations we reveal the strong interface termination dependences of the magnetic order and perpendicular magnetic anisotropy (PMA) in FeRh films on MgO(0 0 1). While the magnetic ground state of the Fe-terminated FeRh(0 0 1) is an antiferromagnetic, the Rh-termination favors ferromagnetism within the underlying FeRh layers. This ferromagnetism can persist even at the interface with MgO, which in turn leads to a few times enhancement of PMA compared with the Fe-interface in FeRh/MgO(0 0 1). The underlying mechanism for this large PMA is an interplay between the spin-polarized Rh d xy and d x 2 - y 2 orbital states with large spin-orbit coupling at the interface. These results unveil a crucial role of the interface termination on magnetism of FeRh in antiferromagnet spintronics.