Current Plane Research Articles

Band-Edge Mixture Engineered Giant and Switchable Shift Current Generation.

Two-dimensional materials have enormous development prospects in the bulk photovoltaic effect (BPVE). The enhancement and manipulation of the BPVE are some of the key roles of its various applications. Through a simplified Hamiltonian model, this work shows that a substantial band mixture between occupied and unoccupied states could produce a large optical absorption rate with trivial topological features, resulting in a significantly enhanced shift current generation. Furthermore, this mechanism is illustrated in a realistic C3B/C3N bilayer material based on density functional theory calculation and tight-binding model. As each layer of C3B/C3N is centrosymmetric, the in-plane shift current arises from the interfacial interaction. The electron transfer between the layers gives a controllable band mixture, which offers a giant shift current reaching over ∼1500 μA/V2. In addition, we propose that interlayer sliding could reverse the in-plane shift current. Our work suggests a feasible approach for giant and switchable nonlinear optical processes.

Isothermal magnetization and magnetoresistive variations in trilayer (La2/3Sr1/3MnO3/γ-Fe2O3/La2/3Sr1/3MnO3) hetero-structure

We report the magnetic field dependent magnetization and magnetoresistance (MR) properties of La2/3Sr1/3MnO3 (LSMO)/ γ -Fe2O3/LSMO trilayer heterostructure and single layer LSMO film grown on SiO2/Si (100) substrates utilizing Pulsed Laser Deposition technique. The metal- insulator-metal configuration is a magnetic tunnel junction topology, which is a parallel network of two metallic layers (LSMO) and one insulating layer ( γ -Fe2O3) in current-in-plane (CIP) geometry. The intrinsically inhomogeneous polycrystalline trilayer film shows much lower (almost half ) coercivity, compared to single layer LSMO film. The MR-H [MR = (ρ (H)-ρ (0))/ρ (0)] behavior of the films is studied under two regimes namely, Low Field Magnetoresistance (LFMR) and High Field Magnetoresistance (HFMR) at 5 K and 300 K in the field range of 0–7 T. Several equations were developed to simulate the experimental MR-H data of the studied samples. For both the films, in the LFMR region (0 T < H ≤ 1 T @ 5 K), the variation in MR exhibits a linear increase with H, followed by a logarithmic behavior in the HFMR region (1 T < H ≤ 7 T @ 5 K). For the trilayer film in CIP geometry, a negative MR of 16% is achieved at 5 K and 1 T field conditions, while LSMO single layer shows a negative MR of 13% at same temperature and field conditions. The MR obtained at high field (7 T) for both the films is quantitatively equal with a value of 38% @ 5 K and 15% @ 300 K. We obtained significantly better MR-H results for the trilayer in current-perpendicular-to-plane (CPP) configuration, where the two metallic layers and an insulating layer are in series. In the CPP configuration, MR is 21% @ 5 K and 1 T field, while it reached to 44% at 5 K and 7 T field. At room temperature, the trilayer heterostructure shows MR of 17% at 7 T field condition. At a higher field of 9 T, trilayer film in CPP mode exhibits MR value of 48% @ 5 K and 23% @ 300 K. Further, the anti-parallel and parallel magnetization states of the MIM device are well manifested in CIP and CPP geometries.

Exploring surface charge dynamics: implications for AFM height measurements in 2D materials.

An often observed artifact in atomic force microscopy investigations of individual monolayer flakes of 2D materials is the inaccurate height derived from topography images, often attributed to capillary or electrostatic forces. Here, we show the existence of a Joule dissipative mechanism related to charge dynamics and supplementing the dissipation due to capillary forces. This particular mechanism arises from the surface conductivity and assumes significance specially in the context of 2D materials on insulating supports. In such scenarios, the oscillating tip induces in-plane charge currents that in many circumstances constitute the main dissipative contribution to amplitude reduction and, consequently, affect the measured height. To investigate this phenomenon, we conduct measurements on monolayer flakes of co-deposited graphene oxide and reduced graphene oxide. Subsequently, we introduce a general model that elucidates our observations. This approach offers valuable insights into the dynamics of surface charges and their intricate interaction with the tip.

Current-perpendicular-to-plane transport properties of 2D ferromagnetic material CrTe2

Abstract In recent years, heterostructures of van der Waals (vdW) ferromagnetic materials have become a focal point in the study of low-dimensional spintronic devices. The current direction in spin valves is commonly perpendicular to the plane (CPP). However, the transport properties of the CPP mode remain largely unexplored. In this work, current-in-plane (CIP) mode and CPP mode for CrTe2 thin films have been carefully studied. The temperature-dependent longitudinal resistance transitions from metallic (CIP) to semiconductor behavior (CPP), with the electrical resistivity of CPP increased by five orders of magnitude. More importantly, the transport properties of the CPP can be categorized into a single-gap Tunneling Through model with the activation energy (E a) of ~1.34 meV/gap at 300-150 K, Variable Range Hopping (VRH) model with a linear Negative Magnetic Resistance (NMR) at 150 -20 K, and weak localization (WL) region with a nonlinear MR at below 20 K. This study explores the vertical transport in CrTe2 materials for the first time, contributing to understand its unique properties and pave the way for its potential in spin valve devices.

Large out-of-plane spin–orbit torque in topological Weyl semimetal TaIrTe4

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin–orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin–torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ ⁄ 2e) (Ωm)−1, which is an order of magnitude higher than the reported values in other materials.

Mechanically manipulated in-plane electric currents and thermal control in piezoelectric semiconductor films

Mechanically manipulated in-plane electric currents and thermal control in piezoelectric semiconductor films

Dissipationless Counterflow Currents above T_{c} in Bilayer Superconductors.

We report the existence of dissipationless currents in bilayer superconductors above the critical temperature T_{c}, assuming that the superconducting phase transition is dominated by phase fluctuations. Using a semiclassical U(1) lattice gauge theory, we show that thermal fluctuations cause a transition from the superconducting state at low temperature to a resistive state above T_{c}, accompanied by the proliferation of unbound vortices. Remarkably, while the proliferation of vortex excitations causes dissipation of homogeneous in-plane currents, we find that counterflow currents, flowing in the opposite direction within a bilayer, remain dissipationless. The presence of a dissipationless current channel above T_{c} is attributed to the inhibition of vortex motion by local superconducting coherence within a single bilayer, in the presence of counterflow currents. Our theory presents a possible scenario for the pseudogap phase in bilayer cuprates.

Effects of electric field and light on resistivity switching of Eu0.7Sr0.3MnO3 thin films.

Based on the excellent piezoelectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystals, a hole-doped manganite film/PMN-PT heterostructure has been constructed to achieve electric-field and light co-control of physical properties. Here, we report the resistivity switching behavior of Eu0.7Sr0.3MnO3/PMN-PT(111) multiferroic heterostructures under different in-plane reading currents, temperatures, light stimuli and electric fields, and discuss the underlying coupling mechanisms of resistivity change. The transition from the electric-field induced lattice strain effect to polarization current effect can be controlled effectively by decreasing the in-plane reading current at room temperature. With the decrease of temperature, the interfacial charge effect dominates over the lattice strain effect due to the reduced charge carrier density. In addition, light stimulus can lead to the delocalization of eg carriers, and thus enhance the lattice strain effect and suppress the interfacial charge effect. This work helps to understand essential physics of magnetoelectric coupling and also provides a potential method to realize energy-efficient multi-field control of manganite thin films.

Interaction phenomena of electrically coupled cells under local reactant starvation in automotive PEMFC stacks

The local current density distribution of polymer electrolyte membrane fuel cells (PEMFCs) can be distorted by various error states. Differences in current density distributions (CDDs) of adjacent cells in a stack are equilibrized by in-plane currents within the sandwiched bipolar plates. Degradation stressors such as detrimental differences in local cell voltage and current density maxima can thus be generated. A novel method was therefore developed to intentionally manipulate CDD profiles by integrating local artificial starvation into only one fuel cell in an assembly. This technique is applied to automotive-sized PEMFCs single cells as well as in 20 cell short-stack to analyze such voltage and current redistribution phenomena. A drastic distortion of local cell voltage is only observed for stacks, which is explained by a supplementary simulation. The local voltage distribution of an electrically coupled fuel cell is therefore calculated by combining CDD measurements with a spatially resolved polarization curve model. The capabilities and limits of a multipoint cell voltage monitoring measurement device are discussed on this basis. The inspected correlation between these two independent online measurement techniques allows to localize such error states with considerable accuracy during operation of automotive sized PEMFC stacks.

Generation of Terahertz Radiation via the Transverse Thermoelectric Effect.

Terahertz (THz) radiation is a powerful tool with widespread applications ranging from imaging, sensing, and broadband communications to spectroscopy and nonlinear control of materials. Future progress in THz technology depends on the development of efficient, structurally simple THz emitters that can be implemented in advanced miniaturized devices. Here, it is shown how the natural electronic anisotropy of layered conducting transition metal oxides enables the generation of intense terahertz radiation via the transverse thermoelectric effect. In thin films grown on off-cut substrates, femtosecond laser pulses generate ultrafast out-of-plane temperature gradients, which in turn launch in-plane thermoelectric currents, thus allowing efficient emission of the resulting THz field out of the film structure. This scheme is demonstrated in experiments on thin films of the layered metals PdCoO2 and La1.84 Sr0.16 CuO4 , and model calculations that elucidate the influence of the material parameters on the intensity and spectral characteristics of the emitted THz field are presented. Due to its simplicity, the method opens up a promising avenue for the development of highly versatile THz sources and integrable emitter elements.

Measurement Geometry and Hydrostatic Pressure‐Dependent Magnetoresistance in All‐Oxide‐Based Synthetic Antiferromagnets

AbstractThe electronic structure of constituent layers and the spin channel of propagating electrons are critical factors that affect the magnitude and sign of magnetoresistance (MR) in synthetic antiferromagnets (SAFMs), which are important for spintronic applications. However, for all‐oxide‐based SAFMs, where there is strong coupling between multiple degrees of freedom, spin transport becomes more complex and remains elusive. Here, using ultrathin half‐metallic manganite/doped ruthenate SAFMs as a model system, three sign reversals of MR are demonstrated accompanied by the crossover between underlying spin‐dependent transport mechanisms. Electron tunneling produces normal MR in the current‐perpendicular‐to‐plane (CPP) geometry at low temperatures, whereas carrier confinement causes inverse MR in the current‐in‐plane (CIP) geometry. Strikingly, CPP MR can undergo a temperature‐driven sign reversal due to resonant tunneling via localized states in the spacer. Moreover, hydrostatic pressure can modulate the interlayer exchange coupling and induce an asymmetric interfacial response to dramatically facilitate electron tunneling, driving a controllable sign reversal of CIP MR. These results provide new insights into understanding and optimization of MR in all‐oxide‐based SAFMs.

Manipulation and Optical Detection of Artificial Topological Phenomena in 2D Van der Waals Fe5 GeTe2 /MnPS3 Heterostructures.

2D ferromagnet is a good platform to investigate topological effects and spintronic devices owing to its rich spin structures and excellent external-field tunability. The appearance of the topological Hall Effect (THE) is often regarded as an important sign of the generation of chiral spin textures, like magnetic vortexes or skyrmions. Here, interface engineering and an in-plane current are used to modulate the magnetic properties of the nearly room-temperature 2D ferromagnet Fe5 GeTe2 . An artificial topology phenomenon is observed in the Fe5 GeTe2 /MnPS3 heterostructure by using both anomalous Hall Effect and reflective magnetic circular dichroism (RMCD) measurements. Through tuning the applied current and the RMCD laser wavelength, the amplitude of the humps and dips observed in the hysteresis loops can be modulated accordingly. Magnetic field-dependent hysteresis loops demonstrate that the observed artificial topological phenomena are induced by the generation and annihilation of the magnetic domains. This work provides an optical method for investigating the topological-like effects in magnetic structures and proposes an effective way to modulate the magnetic properties of magnetic materials, which is important for developing magnetic and spintronic devices in van der Waals magnetic materials.

Detectability of delamination in laminated CFRPs with diverse stacking sequences using eddy current method with T-R pancake coil

A T-R probe composed of two non-coaxial pancake-based coils is used to identify delamination at different depths of laminated CFRPs with diverse stacking sequences. This type of probe is capable of inspecting delamination, since an interactional in-plane currents across the adjacent and non-adjacent layers of CFRPs stacked by different fibers can be induced using pancake-type coil. For optimizing the probe performance, a frequency sweep method is conducted to obtain the appropriate working frequency. To fabricate the imperfect samples containing the delamination-based damage, a tailored Teflon film is inserted into the appointed interfaces of 16-layer CFRPs. The physical mechanism of delamination detection using pancake coil is analyzed by using FE method. After then, a set of ECT experiments for various CFRPs are performed to verify the sensitivity to delamination detected by pancake coil. Correspondingly, both shape and location of the delamination are compared with those obtained from a well-established UT device. Experimental results indicate that the sensitivity to delamination is high at the interface formed by different stacked fibers, but it lacks sensitivity to delamination in unidirectional one due to the same aligned fibers, which are qualitatively consistent with the numerical results. Unexpectedly, delamination at the interfaces between adjacent layers stacked by the same fibers of the other two CFRPs can still be distinguished from the non-damage region, while this phenomenon fails to explain by the numerical analysis. Quantitatively, the visualized damage region agrees with the UT C-scan results.

Q-lipid-containing membranes show high in-plane conductivity using a membrane-on-a-chip setup

The light-driven reactions of photosynthesis as well as the mitochondrial power supply are located in specialized membranes containing a high fraction of redox-active lipids. In-plane charge transfer along such cell membranes is currently thought to be facilitated by the diffusion of redox lipids and proteins. Using a membrane on-a-chip setup, we show here that redox-active model membranes can sustain surprisingly high currents (mA) in-plane at distances of 25μm. We also show the same phenomenon in free-standing monolayers at the air-water interface once the film is compressed such that the distance between redox centers is below 1nm. Our data suggest that charge transfer within cell walls hosting electron transfer chains could be enabled by the coupling of redox-lipids via simultaneous electron and proton in-plane hopping, similar to conductive polymers. This has major implications for our understanding of the role of lipid membranes, suggesting that Q-lipid-containing membranes may be essential for evolving the complex redox machineries of life.

Enhanced spin current by anti-reflective magnetic tunnel junctions

Charge current and in-plane spin current are calculated in magnetic tunnel junctions (MTJs). The transmission function is investigated using nonequilibrium Green’s function formalism in a single-barrier MTJ, a resonant-tunneling MTJ and an anti-reflective MTJ (A-MTJ). The results show an enhancement in magnitude of the charge and in-plane spin currents for an A-MTJ compared to the other MTJs due to the presence of anti-reflective regions.

Efficient perpendicular magnetization switching by a magnetic spin Hall effect in a noncollinear antiferromagnet

Current induced spin-orbit torques driven by the conventional spin Hall effect are widely used to manipulate the magnetization. This approach, however, is nondeterministic and inefficient for the switching of magnets with perpendicular magnetic anisotropy that are demanded by the high-density magnetic storage and memory devices. Here, we demonstrate that this limitation can be overcome by exploiting a magnetic spin Hall effect in noncollinear antiferromagnets, such as Mn3Sn. The magnetic group symmetry of Mn3Sn allows generation of the out-of-plane spin current carrying spin polarization collinear to its direction induced by an in-plane charge current. This spin current drives an out-of-plane anti-damping torque providing the deterministic switching of the perpendicular magnetization of an adjacent Ni/Co multilayer. Due to being odd with respect to time reversal symmetry, the observed magnetic spin Hall effect and the resulting spin-orbit torque can be reversed with reversal of the antiferromagnetic order. Contrary to the conventional spin-orbit torque devices, the demonstrated magnetization switching does not need an external magnetic field and requires much lower current density which is useful for low-power spintronics.

Tunable skyrmion–edge interaction in magnetic multilayers by interlayer exchange coupling

Magnetic skyrmions are appealing for applications in emerging topological spintronic devices. However, when magnetic skyrmions in a nanowire are driven by an in-plane current, a transverse Magnus force deflects their trajectories from the current direction, which tends to push the skyrmion toward the edge. If the current density is exceedingly large, the skyrmion will be annihilated around the edge, leading to a greatly reduced propagation distance and a maximum speed of the skyrmion, which is detrimental to skyrmion-based spintronic applications. Here, we prepare a magnetic multilayer Ta/[Pt/Co]3/Ru/[Co/Pt]3 and tailor the interlayer exchange coupling strength by varying the thickness of the Ru layer. Based on the magneto-optic Kerr effect microscope, we find that the skyrmion–edge interaction is tunable by the interlayer exchange coupling strength, namely, the strength of the repulsive potential from the film edge is tailored by the interlayer exchange coupling strength. Our results unveil the significant role of the interlayer exchange coupling in skyrmion dynamics.

In-plane current induced nonlinear magnetoelectric effects in single crystal films of barium hexaferrite

This report is on the observation and analysis of nonlinear magnetoelectric effects (NLME) for in-plane currents perpendicularly to the hexagonal axis in single crystals and liquid phase epitaxy grown thin films of barium hexaferrite. Measurements involved tuning of ferromagnetic resonance (FMR) at 56–58 GHz in the multidomain and single domain states in the ferrite by applying a current. Data on the shift in the resonance frequency with input electric power was utilized to estimate the variations in the magnetic parameter that showed a linear dependence on the input electric power. The NLME tensor coefficients were determined form the estimated changes in the magnetization and uniaxial anisotropy field. The estimated NLME coefficients for in-plane currents are shown to be much higher than for currents flowing along the hexagonal axis. Although the frequency shift of FMR was higher for the single domain resonance, the multi-domain configuration is preferable for device applications since it eliminates the need for a large bias magnetic field. Thus, multidomain resonance with current in the basal plane is favorable for use in electrically tunable miniature, ferrite microwave signal processing devices requiring low operating power.

Enhancement of the Walker limit by bulk Dzyaloshinskii-Moriya interaction

In this work, dynamics of chiral domain walls in long and narrow magnetic heterostructures based on non-centrosymmetric chiral magnets with bulk Dzyaloshinskii-Moriya interactions (DMI) and perpendicular magnetic anisotropy is systematically investigated. The driving forces can be out-of-plane magnetic fields and in-plane currents, correspondingly both steady and precessional flows are considered. Their dividing points (the Walker critical field and current density) are obtained as functions of bulk DMI strength ($D_{\mathrm{b}}$) and the ratio ($\kappa$) of total (crystalline plus shape) anisotropy in the hard axis over that in the easy one. When far beyond Walker breakdown, the dependence curve of wall velocity on external in-plane bias field takes parabolic shape around the compensation point where the total in-plane field disappears. The center shift is determined by $D_{\mathrm{b}}$, $\kappa$, and the wall's topological charge, thus can be used to measure the bulk DMI strength in chiral magnets.

Optimization of Tungsten β -Phase Window for Spin-Orbit-Torque Magnetic Random-Access Memory

Switching induced by spin-orbit torque (SOT) is being vigorously explored, as it allows the control of magnetization using an in-plane current, which enables a three-terminal magnetic-tunnel-junction geometry with isolated read and write paths. This significantly improves the device endurance and the read stability, and allows reliable subnanosecond switching. Tungsten in the $\ensuremath{\beta}$ phase, $\ensuremath{\beta}$-$\mathrm{W}$, has the largest reported antidamping SOT charge-to-spin conversion ratio $({\ensuremath{\theta}}_{\mathrm{AD}}\ensuremath{\approx}\ensuremath{-}60\mathrm{%})$ for heavy metals. However, $\ensuremath{\beta}$-$\mathrm{W}$ has a limitation when one is aiming for reliable technology integration: the $\ensuremath{\beta}$ phase is limited to a thickness of a few nanometers and enters the $\ensuremath{\alpha}$ phase above 4 nm in our samples when industry-relevant deposition tools are used. Here, we report our approach to extending the range of $\ensuremath{\beta}$-$\mathrm{W}$, while simultaneously improving the SOT efficiency by introducing $\mathrm{N}$ and $\mathrm{O}$ doping of $\mathrm{W}$. Resistivity and XRD measurements confirm the extension of the $\ensuremath{\beta}$ phase from 4 nm to more than 10 nm, and transport characterization shows an effective SOT efficiency larger than $\ensuremath{-}44.4\mathrm{%}$ (reaching approximately $\ensuremath{-}60\mathrm{%}$ for the bulk contribution). In addition, we demonstrate the possibility of controlling and enhancing the perpendicular magnetic anisotropy of a storage layer ($\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$). Further, we integrate the optimized $\mathrm{W}(\mathrm{O},\mathrm{N})$ into SOT magnetic random-access memory (SOT-MRAM) devices and project that, for the same thickness of SOT material, the switching current decreases by 25% in optimized $\mathrm{W}(\mathrm{O},\mathrm{N})$ compared with our standard $\mathrm{W}$. Our results open the path to using and further optimizing $\mathrm{W}$ for integration of SOT-MRAM technology.