Oxygen-vacancy-induced diffusive scattering in Fe/MgO/Fe magnetic tunnel junctions

Terahertz Spin-Conductance Spectroscopy: Probing Coherent and Incoherent Ultrafast Spin Tunneling

Thin-film stacks F |H consisting of a ferromagnetic-metal layer F and a heavy-metal layer H are spintronic model systems. Here, we present a method to measure the ultrabroadband spin conductance across a layer X between F and H at terahertz frequencies, which are the natural frequencies of spin-transport dynamics. We apply our approach to MgO tunneling barriers with thickness d = 0-6 Å. In the time domain, the spin conductance Gs has two components. An instantaneous feature arises from processes like coherent spin tunneling. Remarkably, a longer-lived component is a hallmark of incoherent resonant spin tunneling mediated by MgO defect states, because its relaxation time grows monotonically with d to as much as 270 fs at d = 6.0 Å. Our results are in full agreement with an analytical model. They indicate that terahertz spin-conductance spectroscopy will yield new and relevant insights into ultrafast spin transport in a wide range of spintronic nanostructures.

Elucidate interfacial disorder effects on the perpendicular magnetic anisotropy at Fe/MgO heterostructure from first-principles calculations

The interfacial perpendicular magnetic anisotropy (PMA) plays a key role in spintronic applications such as memory recording and computational devices. Despite robust PMA being reported at the Fe/MgO interface, there are still inconsistencies in the disorder effects on the interfacial magnetic anisotropy. Here we reported a comprehensive study of the influence of the interfacial disorder, including the underoxidization, overoxidization, and oxygen migration, on the PMA of the Fe/MgO interface using first-principles calculations. Compared to the pristine Fe/MgO interface, the underoxidation at the Fe/MgO interface keeps the interfacial PMA but reduces the interfacial anisotropy constant (K_i). The overoxidization and oxygen migration at the interface both reduce theK_iand even switch the easy magnetization axis from the out-of-plane to in-plane direction at high oxygen percentage. In all the cases, theK_iwas found strongly correlated to the difference of the orbital magnetic moment along the in-plane and out-of-plane direction. Calculated layer-resolved and orbital-resolvedK_irevealed that the orbital coupling between thed_xyanddx2-y2states of the interfacial Fe layer plays a key role in determining the interfacial magnetic anisotropy. This work provides deep insights into the oxidation effects on the interfacial magnetic anisotropy of Fe/MgO system and a possible avenue to tune theK_ivia interfacial engineering.

High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Subnanometer Thick Al2O3 Tunnel Barriers Fabricated Using Atomic Layer Deposition

Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) are a key to obtaining high-tunneling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including atomic-scale control over the TB thickness, conformal coating, and a low defect density. Motivated by this, this work explores the fabrication and characterization of spin-valve Fe/ALD-Al₂O₃/Fe MTJs with an ALD-Al₂O₃ TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ∼77 and ∼90% have been obtained, respectively, at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlO_x TBs with optimized device structures. In situ scanning tunneling spectroscopy characterization of the ALD-Al₂O₃ TBs has revealed a higher TB height (E_b) of 1.33 ± 0.06 eV, in contrast to E_b ∼ 0.3-0.6 eV for their AlO_x TB counterparts, indicative of significantly lower defect concentrations in the former. This first success of the MTJs with subnanometer thick ALD-Al₂O₃ TBs demonstrates the feasibility of in situ ALD for the fabrication of pinhole-free and low-defect ultrathin TBs for practical applications, and the performance could be further improved through device optimization.

Pathways for charge transport through material interfaces

Modeling charge transport across material interfaces is important for understanding the limitations of electronic devices such as transistors, electrochemical cells, sensors, and batteries. However, modeling the entire structure and full dimensionality of an interface can be computationally demanding. In this study, we investigate the validity of an efficient reduced one-dimensional Hamiltonian for calculating charge transport along interfaces by comparing to a two-dimensional model that accounts for additional charge transport pathways. We find that the one-dimensional model successfully predicts the qualitative trend of charge transmission probability among Pt/Fe₂O₃ and Ag/Fe₂O₃ interfaces. However, the two-dimensional model provides additional information on possible pathways that are not perpendicular to the interface direction. These charge transport pathways are directed along the lowest potential energy profile of the interface that correlates with the crystal structure of the constituting materials. However, the two-dimensional paths are longer and take more scattering time. Therefore, the one-dimensional model may hold sufficient information for qualitative estimation of charge transport through some material interfaces.

Defect-implantation for the all-electrical detection of non-collinear spin-textures

The viability of past, current and future devices for information technology hinges on their sensitivity to the presence of impurities. The latter can reshape extrinsic Hall effects or the efficiency of magnetoresistance effects, essential for spintronics, and lead to resistivity anomalies, the so-called Kondo effect. Here, we demonstrate that atomic defects enable highly efficient all-electrical detection of spin-swirling textures, in particular magnetic skyrmions, which are promising bit candidates in future spintronics devices. The concomitant impurity-driven alteration of the electronic structure and magnetic non-collinearity gives rise to a new spin-mixing magnetoresistance (XMR_defect). Taking advantage of the impurities-induced amplification of the bare transport signal, which depends on their chemical nature, a defect-enhanced XMR (DXMR) is proposed. Both XMR modes are systematised for 3d and 4d transition metal defects implanted at the vicinity of skyrmions generated in PdFe bilayer deposited on Ir(111). The ineluctability of impurities in devices promotes the implementation of defect-enabled XMR modes in reading architectures with immediate implications in magnetic storage technologies.

Precise detection of magnetic skyrmions is a key prerequisite to exploit them in future magnetic storage technologies. The authors, using first principles studies, propose two novel spin-mixing magnetoresistance effects enabled by defects, which allow for a highly efficient all-electrical detection of spin-swirling textures.

Realizing giant tunneling electroresistance in two-dimensional graphene/BiP ferroelectric tunnel junction

Ferroelectric tunnel junctions (FTJs) composed by sandwiching a thin ferroelectric layer between two leads have attracted great interest for their potential applications in nonvolatile memories due to the tunnel electroresistance (TER) effect. So far, almost all FTJs studied focus on adopting three dimensional (3D) ferroelectric materials as the tunnel barrier. Recently, many two-dimensional (2D) ferroelectric materials with in-plane or out-of-plane spontaneous polarization have been theoretically proposed or even fabricated, providing a new type of candidate as the tunnel barrier in FTJs. However, very little has been known about whether such 2D ferroelectric materials may lead to an excellent TER effect. In this work, using first-principles calculations, we demonstrate that a giant TER effect of around 623%, which is comparable to 3D FTJs, can be realized through the ferroelectric tunnel junction constructed with the 2D ferroelectric materials BiP and B/N-doped graphene. The analysis of the effective potential and electronic structure indicates that the large TER ratio arises from the unsymmetrical screening effects of the B/N-doped vertical van der Waals graphene/BiP leads. Our findings demonstrate the great potential of novel application of 2D ferroelectric BiP in FTJs.

In situ soft x-ray absorption spectroscopic study of polycrystalline Fe/MgO interfaces

Interfacial interactions between a layer of iron and MgO hold the key to various phenomena like tunnel magnetoresistance, perpendicular magnetic anisotropy, interlayer exchange coupling, observed in the system. Interface structure has been studied in situ during deposition of iron on MgO surface, using soft x-ray absorption spectroscopy (SXAS). Sub-monolayer sensitivity of SXAS, combined with in situ measurements as a function of iron layer thickness, allowed one to study the evolution of interface with film thickness. Two different substrates namely MgO (0 0 1) single crystal, and a polycrystalline MgO film on Si substrate have been used in order to elucidate the role of the state of MgO surface in controlling the interface structure. It is found that at the interface of iron and MgO film, about two monolayers of Fe₃O₄ are formed. Fe₃O₄ being the oxide of iron with the highest heat of formation, the reaction appears to be controlled thermodynamically. On the other hand, on the interface with MgO (0 0 1) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behavior of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.

Quantum Transport in Gated Dangling-Bond Atomic Wires

A single line of dangling bonds (DBs) on Si(100)-2 × 1:H surface forms a perfect metallic atomic-wire. In this work, we investigate quantum transport properties of such dangling bond wires (DBWs) by a state-of-the-art first-principles technique. It is found that the conductance of the DBW can be gated by electrostatic potential and orbital overlap due to only a single DB center (DBC) within a distance of ∼16 Å from the DBW. The gating effect is more pronounced for two DBCs and especially, when these two DB "gates" are within ∼3.9 Å from each other. These effective length scales are in excellent agreement with those measured in scanning tunnelling microscope experiments. By analyzing transmission spectrum and density of states of DBC-DBW systems, with or without subsurface doping, for different length of the DBW, distance between DBCs and the DBW, and distance between DB gates, we conclude that charge transport in a DBW can be regulated to have both an on-state and an off-state using only one or two DBs.

Giant tunnel magneto-resistance in graphene based molecular tunneling junction

We propose and theoretically investigate a class of stable zigzag graphene nanoribbon (ZGNR) based molecular magnetic tunneling junctions (MTJs). For those junctions having pentagon-connecting formations, huge tunnel magneto-resistance (TMR) is found. Different from most of the other proposed molecular junctions, the huge TMR in our structures is generic, and is not significantly affected by external parameters such as bias voltage, gate voltage, length of the molecule and width of the ZGNRs. The double pentagon-connecting formation between the molecule and ZGNRs is critical for the remarkable TMR ratio, which is as large as ∼2 × 10(5). These molecular MTJs behave as almost perfect spin filters and spin valve devices. Other connecting formations of the ZGNR based MTJs lead to much smaller TMR. By first principles analysis, we reveal the microscopic physics responsible for this phenomenon.

Transient dynamics of magnetic Co-graphene systems

We report the investigation of response time of spin resolved electron traversing through a magnetic Co-graphene nano-device. For this purpose, we calculate the transient current under a step-like upward pulse for this system from first principles using non-equilibrium Green's function (NEGF) formalism within the framework of density functional theory (DFT). In the absence of dephasing mechanisms, transient current shows a damped oscillatory behavior. The turn-on time of the magnetic Co-graphene nano-device was found to be around 5-20 femtoseconds, while the relaxation time can reach several picoseconds due to the damped oscillation of transient current for both majority spin and minority spin. The response time was determined by the resonant states below the Fermi level, but does not depend on the chirality of graphene and the amplitude of pulse bias. Each resonant state contributes to the damped oscillation of transient current with the same frequency and different decay rates. The frequency of the oscillation is half the pulse bias and the decay rate equals the lifetime of the corresponding resonant state. When inelastic phase-relaxing scattering is considered, the long duration oscillatory behavior of the transient current is suppressed and the relaxation time is reduced to hundreds of femtoseconds.

Predictive DFT-based approaches to charge and spin transport in single-molecule junctions and two-dimensional materials: successes and challenges

CONSPECTUS: The emerging field of flexible electronics based on organics and two-dimensional (2D) materials relies on a fundamental understanding of charge and spin transport at the molecular and nanoscale. It is desirable to make predictions and shine light on unexplained experimental phenomena independently of experimentally derived parameters. Indeed, density functional theory (DFT), the workhorse of first-principles approaches, has been used extensively to model charge/spin transport at the nanoscale. However, DFT is essentially a ground state theory that simply guarantees correct total energies given the correct charge density, while charge/spin transport is a nonequilibrium phenomenon involving the scattering of quasiparticles. In this Account, we critically assess the validity and applicability of DFT to predict charge/spin transport at the nanoscale. We also describe a DFT-based approach, DFT+Σ, which incorporates corrections to Kohn-Sham energy levels based on many-electron calculations. We focus on single-molecule junctions and then discuss how the important considerations for DFT descriptions of transport can differ in 2D materials. We conclude that when used appropriately, DFT and DFT-based approaches can play an important role in making predictions and gaining insight into transport in these materials. Specifically, we shall focus on the low-bias quasi-equilibrium regime, which is also experimentally most relevant for single-molecule junctions. The next question is how well can the scattering of DFT Kohn-Sham particles approximate the scattering of true quasiparticles in the junction? Quasiparticles are electrons (holes) that are surrounded by a constantly changing cloud of holes (electrons), but Kohn-Sham particles have no physical significance. However, Kohn-Sham particles can often be used as a qualitative approximation to quasiparticles. The errors in standard DFT descriptions of transport arise primarily from errors in the Kohn-Sham energy levels (self-energy errors). These errors are small in the strong-coupling regime where the molecular levels are significantly broadened at the Fermi level but are large in the coherent off-resonant tunneling regime where DFT overestimates conductance by orders of magnitude. The DFT+Σ approach uses a physically motivated, parameter free estimate of the self-energy corrections to correct the energy levels in DFT, giving conductance in quantitative agreement with experiment for a large but nonexhaustive class of single-molecule junctions. In 2D materials, the self-energy error is relatively small, and critical issues stem instead from the large length scales in experiments, which make it necessary to consider band-bending within the 2D material, as well as scattering due to electron-phonon interactions, spin-flip interactions, defects, etc.

Thermal spin transfer in Fe-MgO-Fe tunnel junctions

We compute thermal spin transfer (TST) torques in Fe-MgO-Fe tunnel junctions using a first principles wave-function-matching method. At room temperature, the TST in a junction with 3 MgO monolayers amounts to 10(-7) J/m(2)/K, which is estimated to cause magnetization reversal for temperature differences over the barrier of the order of 10 K. The large TST can be explained by multiple scattering between interface states through ultrathin barriers. The angular dependence of the TST can be very skewed, possibly leading to thermally induced high-frequency generation.

Resonant tunneling through electronic trapping states in thin MgO magnetic junctions

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15  V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.