Vertical Composition Gradient Research Articles

Study of intricate extraordinary Hall effect and magnetization switching behavior in graded GdFeCo ferrimagnet

Abstract This study delves into the unique properties of the GdFeCo Hall bar with a vertical composition gradient. Our exploration includes a detailed analysis of the temperature-dependent extraordinary Hall effect (EHE) response and magnetization-switching behavior. The findings reveal a FeCo-rich state at room temperature, characterized by an asymmetric drop at 20 Oe and a magnetic compensation temperature (Tcomp) around 150 K. The presence of triple hysteresis loops at 280 K and 270 K, along with unexpected changes in the EHE resistance difference (ΔRXY) at temperatures distant from Tcomp, hint at complex compositional effects similar to the artificial skyrmion-like Hall effect. The temperature points for zero ΔRXY values at ±30 kOe and ±4 kOe show a difference of 18 K, suggesting a spin-flop effect at compensation. Detailed analysis near Tcomp uncovers multiple loops, indicating coexisting Gd and FeCo sublattices with varied compositions. The magnetization switching experiments demonstrate field-driven switching and a limited role of electrical current in the system. These unique findings enhance our understanding of compositionally controlled ferrimagnets for spintronic applications.

Temperature-dependence of anomalous Hall effect in Ta-(GdFeCo)δ-Ta films with vertical composition gradients

Ferrimagnetic rare-earth (RE) – transition-metal (TM) alloys are considered small ferromagnets with antiferromagnetic exchange in sublattices, which makes them a preferable choice for future ultrafast memory-related applications. Here, we explore the magnetic properties of two GdFeCo thin films with tailored vertical composition gradients, achieved by gradually varying Gd concentration across the film thickness. The temperature-dependent magnetic behavior of these films is investigated using anomalous Hall effect (AHE) measurements and SQUID magnetometry. Both films exhibit perpendicular magnetic anisotropy with distinct coercivities, indicating variations in their magnetic properties. The films show RE-dominant behavior at 300 K. Moreover, for the positive composition gradient (+δ) film, the compensation temperature is around 350 K, while the film with a negative gradient (−δ) exhibits a compensation temperature above 400 K. Furthermore, the spin-flop behavior and AHE sign reversal provide insights into the sublattice dominance and exchange interactions within the films. The magnitude of ΔRXY (the difference in AHE resistance at different magnetic fields) is shown to be sensitive to the composition gradient, indicating stronger perpendicular magnetic anisotropy for the “−δ” sample. This study demonstrates the potential of composition engineering to tailor the magnetic characteristics of ferrimagnetic materials for specific spintronics applications.

Efficient Spin-Orbit Torque Switching in a Perpendicularly Magnetized Heusler Alloy MnPtGe Single Layer.

Electrically manipulating magnetic moments by spin-orbit torque (SOT) has great potential applications in magnetic memories and logic devices. Although there have been rich SOT studies on magnetic heterostructures, low interfacial thermal stability and high switching current density still remain an issue. Here, highly textured, polycrystalline Heusler alloy MnxPtyGe (MPG) films with various thicknesses are directly deposited onto thermally oxidized silicon wafers. The perpendicular magnetization of the MPG single layer can be reversibly switched by electrical current pulses with a magnitude as low as 4.1 × 1010Am-2, as evidenced by both the electrical transport and the magnetic optical measurements. The switching is shown to arise from inversion symmetry breaking due to the vertical composition gradient of the films after sample annealing. The SOT effective fields of the samples are analyzed systematically. It is found that the SOT efficiency increases with the film thickness, suggesting a robust bulk-like behavior in the single magnetic layer. Furthermore, a memristive characteristic has been observed due to a multidomain switching property in the single-layer MPG device. Additionally, deterministic field-free switching of magnetization is observed when the electric current flows orthogonal to the direction of the in-plane compositional gradient due to the in-plane symmetry breaking. This work proves that the MPG is a good candidate to be utilized in high-density and efficient magnetoresistive random access memory devices and other spintronic applications.

Controllable unidirectional magnetoresistance in ferromagnetic films with broken symmetry

Spin-related unidirectional magnetoresistance (UMR) exhibits nonreciprocal transport to either charge current or magnetization reversal, providing a potential application for direct electrical readout of in-plane magnetization using simple two-terminal geometry. To achieve such a UMR, it is usually believed that an adjacent heavy metal or other materials with strong spin-orbit coupling (SOC) as the spin polarizer is indispensable, leading to most efforts on multilayer structures rather than single-layer ferromagnetic films. Here, we report the observation of UMR in a single CoPt film by introducing a vertical composition gradient to break the structural inversion symmetry. Moreover, unlike conventional heavy-metal/ferromagnetic-metal bilayer films, the UMR in the CoPt film with a positive Co composition gradient is controllable as a function of current amplitude, which shows the decrease, sign reversal, and then enhancement with the increment of current density. These results reveal two competing UMR mechanisms with opposite signs in electrical current dependency, i.e., bulk Rashba effect induced by composition gradients and anomalous Nernst effect driven by a vertical temperature gradient. Our work provides a promising way to manipulate the UMR in a single ferromagnetic film with the combination of charge-spin conversion and magnetothermal effect.

Masked-assisted radial-segmented target pulsed-laser deposition: A novel method for area-selective deposition using pulsed-laser deposition

We introduce a novel technique, masked-assisted radial-segmented target pulsed-laser deposition (MARS-PLD) for unprecedented capabilities in area-selective physical vapor deposition. The MARS-PLD setup consists of a conventional PLD chamber with mechanical feedthrough for a laterally movable mask or mask set. By this means and, in principle, the arbitrary choice of a shadow mask layout, any desired area on a substrate can be masked in order to create multinary lateral and vertical material composition gradients using radially segmented targets already described in the literature [Kneiß et al., ACS Comb. Sci. 20, 643–652 (2018)]. To illustrate the capabilities of this method, we fabricated material gradients in (Mg,Zn)O thin films with a nearly linear spatial variation of the cation composition of 15at.%mm−1. Additionally, we fine-tuned our setup to fabricate a material gradient on a predefined two-dimensional lateral pattern to demonstrate the versatile capabilities of the MARS-PLD technique.

Nanoimprint Lithography‐Dependent Vertical Composition Gradient in Pseudo‐Planar Heterojunction Organic Solar Cells Combined with Sequential Deposition

AbstractAlthough suitable vertical phase separation morphology in organic solar cells (OSCs) can be obtained by the donor/acceptor sequential deposition (SD) method, the lack of precisely adjusting vertical composition gradient and molecular crystallinity is a key limitation. Here, nanoimprint lithography (NIL) combined with SD dual‐functionalized regulation strategy is first used to fabricate high‐performance pseudo‐planar heterojunction (PPHJ) OSCs, which is conducive to constructing vertical bi‐continuous donor/acceptor network to provide sufficient charge separation interface area and orderly charge transport channels. PM6 donor with regular periodic nanograting structure and improved crystallinity is formed via NIL, effectively avoiding the erosion problem ascribed from the subsequent depositing of the Y6 acceptor. Furthermore, the finite‐different time‐domain (FDTD) measurement is employed to confirm the vertical composition gradient of the donor/acceptor, revealing a strong regular light absorption and reduced voltage loss. As a result, the best‐imprinted device enables a power conversion efficiency as high as 17.36%, which is higher than the control SD‐based device (15.46%). It is the first time to obtain high‐quality PM6 nanograting by NIL, which can provide an avenue to form favorable phase separation morphology and adjust the vertical composition gradient for the high‐performance PPHJ OSCs.

Improving the Molecular Packing Order and Vertical Phase Separation of the P3HT:O-IDTBR Blend by Extending the Crystallization Period of O-IDTBR.

The morphology with strong molecular packing order and gradient vertical composition distribution associated with efficient charge transport and collection is critical to achieve high performance in nonfullerene solar cells. However, the rapid solidification process of the active layer upon the fast removal of solvent usually results in a kinetically trapped state with undesired morphology. Herein, we proposed a strategy to extend the crystal growth time of the acceptor via a high-boiling-point additive that selectively dissolved the acceptor. This was enabled by adding dibenzyl ether (DBE) to the poly(3-hexylthiophene) (P3HT):O-IDTBR blend in chlorobenzene (CB) solution. The combination of the kinetic study by time-resolved ultraviolet-visible (UV-vis) absorption spectra and detailed morphological characterization allows us to correlate the crystallization kinetics with the microstructural transition. The results show that the crystal growth time of O-IDTBR increases from 3 to 60 s upon the addition of 0.75% DBE, leading to further evolution of the molecular order of O-IDTBR during the DBE-dominated drying period. Meanwhile, O-IDTBR has more time to migrate toward the substrate owing to the larger surface energy. In addition, the onset of the crystallization process of P3HT is brought forward from 8 to 6 s due to the reduced solvent quality, which favors P3HT to crystallize into a fibril network. As a result, an optimized morphology that features the enhanced molecular packing order of P3HT and O-IDTBR as well as the vertical compositional gradient of O-IDTBR is obtained. Devices based on the optimized blend show more balanced charge transport and suppressed bimolecular recombination, giving rise to an improved power conversion efficiency (PCE) from 4.29 ± 0.04 to 7.30 ± 0.12%.

Controlled Growth of Wafer-Scale Transition Metal Dichalcogenides with a Vertical Composition Gradient for Artificial Synapses with High Linearity.

Artificial synapses are promising for dealing with large amounts of data computing. Great progress has been made recently in terms of improving the on/off current ratio, the number of states, and the energy efficiency of synapse devices. However, the nonlinear weight update behavior of a synapse caused by the uncertain direction of the conductive filament leads to complex weight modulation, which degrades the delivery accuracy of information. Here we propose a strategy to improve the weight update behavior of synapses using chemical-vapor-deposition-grown transition metal dichalcogenides (TMDCs) with a vertical composition gradient, where the sulfur concentration decreases gradually along the thickness direction of TMDCs and thus forms a certain direction of the conduction filament for synapse devices. It is worth noting that the devices show an excellent linear conductance of potentiation and depression with a high linearity of 0.994 (surpassing most state-of-the-art synapses), have a large number of states, and are able to fabricate synapse arrays with wafer-scale. Furthermore, the devices based on the TMDCs with the vertical composition gradient exhibit an asymmetric feature of potentiation and depression behaviors with high linearity and follow the simulated linear Leaky ReLU function, resulting in a high recognition accuracy of 94.73%, which overcomes the unreliability issue in the Sigmoid function due to the vanishing gradient phenomenon. This study not only provides a universal method to grow TMDCs with a vertical composition gradient but also contributes to exploring highly linear synapses toward neuromorphic computing.

Field-Free Magnetization Switching in a Ferromagnetic Single Layer through Multiple Inversion Asymmetry Engineering.

A simple, reliable, and self-switchable spin-orbit torque (SOT)-induced magnetization switching in a ferromagnetic single layer is needed for the development of next generation fully electrical controllable spintronic devices. In this work, field-free SOT-induced magnetization switching in a CoPt single layer is realized by broken multiple inversion symmetry through simultaneously introducing both oblique sputtering and a vertical composition gradient. A quantitative analysis indicates that multiple inversion asymmetries can produce dynamical bias fields along both z- and x-axes, leading to the observed field-free deterministic magnetization switching. Our study provides a method to accomplish fully electrical manipulation of magnetization in a ferromagnetic single layer without the external magnetic field and auxiliary heavy metal layer, enabling flexible design for future spin-orbit torque-based memory and logic devices.

Full‐Scale Field‐Free Spin‐Orbit Torque Switching in HoCo Structure with a Vertical Composition Gradient

AbstractCurrent‐induced magnetization switching without the assistance of an in‐plane magnetic field by using the spin orbit torque (SOT) has attracted lots of attention for the novel energy‐efficient and nonvolatile spintronics devices. In this study, a full‐scale field‐free SOT switching in perpendicularly magnetized Ru/Pt/HoCo/Ru multilayers with a designed vertical composition gradient is reported. Through annealing the multilayers at 150 °C for 0–40 min, a structural change from a multilayer to an alloying structure with mixed distribution of Ho and Co is found. Simultaneously, the field‐free switching gradually turns from a full‐scale to a partial switching with the structural change. The multiple mechanisms brought by fabricating the HoCo layers with vertical composition gradient (including the non‐uniform Dzyaloshinskii–Moriya interaction (DMI), the defects, the inevitable lateral gradient, or the exchange bias effect along the thickness direction) lead to the deterministic field‐free switching. In addition, a detailed study on the stack of Ru/gradient HoCo/Ru shows that HoCo itself can also produce a certain amount of SOT to realize a partial switching in the absence of the in‐plane field. This work provides a route via engineering rare‐earth‐transition‐metal ferrimagnet to manipulate DMI and SOT for fully electrically controlled spintronics devices.

High-Performance Tin-Lead Mixed-Perovskite Solar Cells with Vertical Compositional Gradient.

Sn-Pb mixed perovskites with bandgaps in the range of 1.1-1.4eV are ideal candidates for single-junction solar cells to approach the Shockley-Queisser limit. However, the efficiency and stability of Sn-Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn2+ . Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF2 to suppress the oxidation of Sn2+ . Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn-Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.

Baseplate Temperature‐Dependent Vertical Composition Gradient in Pseudo‐Bilayer Films for Printing Non‐Fullerene Organic Solar Cells

AbstractNumerous previous reports on the sequential deposition (SD) technique have demonstrated that this approach can achieve a p‐i‐n active layer architecture with an ideal vertical composition gradient, which is one of the critical factors that can influence the physical processes that determine the photovoltaic performance of organic solar cells. Herein, a commonly used photovoltaic system comprised of PM6 as a donor and Y6 as an acceptor is investigated with respect to sequential blade‐processing deposition to comprehensively explore the morphology characteristics as a function of baseplate temperature. A systematic study of the temperature‐dependent blend morphology elucidates the SD‐processed configuration merits and device physics behind temperature‐controlled degree of vertical composition gradient, and constructs the temperature‐microstructure‐property relationship for the corresponding photovoltaic parameters. The result shows, as the temperature increases, the morphology of the active layer has undergone a distinct evolution from the pseudo‐bulk heterojunction to a pseudo‐planar heterojunction and then to a pseudo‐planar bilayer, leading to a non‐monotonic correlation between baseplate temperature and device performance. This investigation not only reveals the importance of precisely controlling baseplate temperature for gaining vertical morphology control, but also provides a path toward rational optimization of device performance in the lab‐to‐fab transition.

Vertical compositional variations of liquid hydrocarbons in Titan’s alkanofers

Context.According to clues left by theCassinimission, Titan, one of the two Solar System bodies with a hydrologic cycle, may harbor liquid hydrocarbon-based analogs of our terrestrial aquifers, referred to as “alkanofers”.Aims.On the Earth, petroleum and natural gas reservoirs show a vertical gradient in chemical composition, established over geological timescales. In this work, we aim to investigate the conditions under which Titan’s processes could lead to similar situations.Methods.We built numerical models including barodiffusion and thermodiffusion (Soret’s effect) in N2+CH4+C2H6liquid mixtures, which are relevant for Titan’s possible alkanofers. Our main assumption is the existence of reservoirs of liquids trapped in a porous matrix with low permeability.Results.Due to the small size of the molecule, nitrogen seems to be more sensitive to gravity than ethane, even if the latter has a slightly larger mass. This behavior, noticed for an isothermal crust, is reinforced by the presence of a geothermal gradient. Vertical composition gradients, formed over timescales of between a fraction of a mega-year to several tens of mega-years, are not influenced by molecular diffusion coefficients. We find that ethane does not accumulate at the bottom of the alkanofers under diffusion, leaving the question of why ethane is not observed on Titan’s surface unresolved. If the alkanofer liquid was in contact with water-ice, we checked that N2did not, in general, impede the clathration of C2H6, except in some layers. Interestingly, we found that noble gases could easily accumulate at the bottom of an alkanofer.

Field-free spin-orbit torque-induced switching of\xa0perpendicular magnetization in a ferrimagnetic layer with a vertical composition gradient

Current-induced spin-orbit torques (SOTs) are of interest for fast and energy-efficient manipulation of magnetic order in spintronic devices. To be deterministic, however, switching of perpendicularly magnetized materials by SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do so involve the application of an in-plane bias magnetic field, or incorporation of in-plane structural asymmetry in the device, both of which can be difficult to implement in practical applications. Here, we report bias-field-free SOT switching in a single perpendicular CoTb layer with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which breaks the in-plane symmetry during the switching process. Micromagnetic simulations are in agreement with experimental results, and elucidate the role of g-DMI in the deterministic switching processes. This bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI provides a strategy for efficient and compact SOT device design.

Unveiling the Mechanism of Bulk Spin‐Orbit Torques within Chemically Disordered FexPt1‐x Single Layers

AbstractThe recent discovery of spin‐orbit torques (SOTs) within magnetic single‐layers has attracted attention. However, it remains elusive as to how to understand and how to tune the SOTs. Here, utilizing the single layers of chemically disordered FexPt1‐x, the mechanism of the “unexpected” bulk SOTs is unveiled by studying their dependence on the introduction of a controlled vertical composition gradient and temperature. The bulk dampinglike SOT is found to arise from an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation. The torque can be strong only in the presence of a vertical composition gradient. The SOT efficiency per electric field is insensitive to temperature but changes sign upon reversal of the orientation of the composition gradient, which is analog to the strain behaviors. These characteristics suggest that the imbalanced internal spin current originates from a bulk spin Hall effect and that the associated inversion asymmetry that allows for a non‐zero net torque is most likely a strain non‐uniformity induced by the composition gradient. The fieldlike SOT is a relatively small bulk effect compared to the dampinglike SOT. This study points to the possibility of developing low‐power single‐layer SOT devices by strain engineering.

Overview of Ge-incorporated kesterite solar cells

<p indent=0mm>Kesterite (CZTS(e)) solar cell has attracted tremendous interest for its feature: Earth-abundant composition, convenient fabrication process, environmental friendliness and low cost. However, the severe large voltage deficient in CZTS(e) solar cell remains a challenge for further enhancement of the power conversion efficiency (PCE). In this prospect, three mechanisms are proposed: Cu/Zn disorder, potential interfacial losses and Sn-related deep-level defects. Sn-related deep-level defects are the most detrimental and hardest factor to manipulate due to the low energy of the formation of atomic antisite and multivalency nature of Sn. Adding extrinsic elements into the absorber layer is of an important method to passivate Sn-related defects. Among them, incorporation of Ge (CZTS:Ge) shows great potential. Comparing to Sn, Ge is more likely to be stabilized at +4 oxidation state. DFT calculation suggests that the deep-level defect density of <sc>CZTS(e):Ge</sc> is much smaller than that of CZTS(e). The maximum trap-limited conversion efficiency can be promoted to 24.1% with Ge incorporation compared with 20.9% in intrinsic CZTS(e). Early works show that CZTS(e):Ge solar cells present relative higher <italic>V</italic>_oc and has reached 12.3% power conversion efficiency (PCE), promising to champion the efficiency of CZTS(e) solar cell. Up to date, the benefits from Ge incorporation can be summarized in three aspects. First, Ge incorporation can induce the formation of Ge-Se low-temperature liquid phase as crystallization flux during annealing, which finally enlarges grain size and promote morphology. In the meantime, Sn-Se phases formation can be minimized through a modified reaction pathway brought by Ge doping. Second, Sn-related deep-level defects experimentally observed will be passivated when Ge is incorporated. Third, the energy difference between kesterite, stannite and primitive mixed CuAu is enlarged in Cu₂ZnGeS₄, indicating that Ge may stabilize kesterite in absorber and suppress band tailing caused by atomic disorder (or phase instability). To break through current efficiency bottleneck of CZTS(e) solar cells, we prospect the futural directions in constructing CZTS(e):Ge. First, Zn composition in absorber should be handled with great care to suppress the formation of Ge_Zn-Cu_Zn further. Next, modulation of the gradient band gap, achieved by vertical Ge-Sn compositional gradient in absorber, is also an important aspect. How to avoid the fast diffusion of Ge to form the vertical Ge-Sn gradient will be of the most essential question. Vertical control of independent Sn composition or Ge incorporation in the post-selenization process may be two potential solutions. Furthermore, the nature of the wide band gap of pure sulfide CZTS(e):Ge makes itself suitable to fabricate the top cell in tandem solar cell. Finally, interface modifications and band engineering in CZTS(e) solar cells will also benefit in constructing CZTS(e):Ge solar cells.

High‐Performance Quasi‐2D Perovskite/Single‐Walled Carbon Nanotube Phototransistors for Low‐Cost and Sensitive Broadband Photodetection

Quasi‐2D Ruddlesden–Popper perovskites (RPPs) have emerged as promising functional materials for optoelectronics due to the highly tunable optoelectronic properties and have been considered as alternative candidates for semiconductors in photodetectors. However, RPPs normally show low carrier mobilities, which is unfavorable for the performance of photodetectors. Herein, a solution‐processed hybrid phototransistor based on RPP/single‐walled carbon nanotube (SWCNT) heterostructure is reported. A vertical composition gradient is formed upon RPP film preparation, which leads to a gradient type‐II heterojunction in the film. More importantly, the high carrier mobility of SWCNTs can result in ultrahigh responsivity and detectivity of 2 × 106 A W−1 and 7 × 1014 Jones, respectively. In addition, high on/off ratio of ≈103 can be achieved in the phototransistors. Herein, the high potential of RPP/SWCNT hybrid phototransistors as next‐generation photodetectors with superior sensitivity is demonstrated.

All-Inorganic CsPbBr3 Perovskite Films Prepared by Single Source Thermal Ablation.

Hybrid organo-lead halide perovskites are becoming the benchmark material for next generation photovoltaics and a very important player for other applications such as photodetectors and light emitting diodes. Nevertheless, the most important issue hindering the large-scale application of these materials remains their intrinsic instability due to the organic cation. Although the substitution with inorganic cesium (Cs) enhances stability, in most cases solution deposition methods of fully inorganic perovskites result in high surface roughness and poor surface coverage. This work reports on the evaporation of the CsPbBr3 precursor by Single Source Thermal Ablation, showing that just after deposition films consist of a mixture of CsPbBr3, CsPb2Br5, and Cs4PbBr6 due to a vertical composition gradient. We point out that mild post deposition treatments lead to the conversion of CsPb2Br5 and Cs4PbBr6 into CsPbBr3 due to its higher thermodynamic stability. Conversion results into smooth and pinhole-free CsPbBr3 films with good light absorption and emission properties. We demonstrate the suitability of obtained films for planar devices by preparing perovskite-based pure-green light emitting diodes, thus promoting Single Source Thermal Ablation as a promising alternative deposition technique for all-inorganic perovskite-based devices.

Spin-Orbit Torque in a Perpendicularly Magnetized Ferrimagnetic Tb - Co Single Layer

Spin-orbit torque (SOT) has been studied extensively in a heavy-metal (HM)/ferromagnet (FM) bilayer structure, where a HM is an essential ingredient because a spin current is generated via the spin Hall effect within the HM layer and/or the Rashba-Edelstein effect from the HM/FM interface. Here, we report the observation of SOT in a ferrimagnetic $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ single layer with perpendicular magnetic anisotropy without a HM layer. Using harmonic Hall voltage measurements, we investigate the SOT-induced dampinglike effective field $({B}_{\mathrm{DL}})$ in a $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer; the sign of ${B}_{\mathrm{DL}}$ is opposite from that of a $\mathrm{Pt}/\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ bilayer, and the magnitude of ${B}_{\mathrm{DL}}$ increases as the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ composition approaches its magnetization compensation point. Moreover, we analyze the elemental composition of $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ as a function of film thickness using scanning transmission electron microscopy and electron energy-loss spectroscopy, indicating that the sign and magnitude of the SOT are virtually insensitive to the vertical composition gradient within the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer. Our results demonstrate that the bulk spin-orbit interaction within the $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ layer itself plays a major role in generating SOT in a $\mathrm{Tb}\text{\ensuremath{-}}\mathrm{Co}$ single layer.

Direct characterization of vertical molecular distributions of organic bulk heterojunction structure by photoemission spectroscopy combined with argon gas cluster ion beam sputtering

The characterization of the vertical molecular distribution of organic bulk heterojunction (BHJ) structures is crucial to the development of high-performance organic photovoltaic (OPV) devices. Herein, we report a novel and direct method for the characterization of the vertical composition gradient of a BHJ structure. Ar gas cluster ion beam (GCIB) sputtering provided a uniform sputtering yield that preserved the chemical structure of the organic semiconducting materials. The combination of X-ray photoelectron spectroscopy (XPS) and Ar GCIB sputtering facilitated the accurate analysis of the vertical molecular distribution of a regioregular poly(3-​hexylthiophene) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) BHJ structure. The J-V characteristics and depth profiles of the as-deposited, ethanol-annealed, and chloroform-annealed devices were examined to demonstrate the usefulness of Ar GCIB sputtering in optimizing the BHJ morphology. Chloroform-annealed device exhibited both the best performance and the most homogeneous morphology among the three prepared samples, and the thickness of the P3HT-rich region was substantially reduced from 20 to 10 nm. Consequently, these results provide important information about the correlation between the vertical molecular distribution of a BHJ blend structure and power conversion efficiency of an OPV device.