Alignment-Free Sensing Module for Absolute and Incremental Lines in Linear Positioning System Based on Tunneling-Magnetoresistance Sensors.

An alignment-free sensing module for the positioning system based on tunneling magnetoresistive (TMR) sensors with an absolute-incremental-integrated scale is demonstrated. The sensors of the proposed system for both lines consist of identical layer stacks; therefore, all sensors can be fabricated in identical processes from thin film deposition to device patterning on a single substrate. Consequently, the relative position of the sensors can be predefined at the lithography stage and the alignment error between sensors caused by the manual installation is completely eliminated. Different from the existing sensing scheme for incremental lines, we proposed to utilize the magnetic tunnel junctions with a perpendicular anisotropy reference layer and an in-plane anisotropy sensing layer. The sensors are placed parallel to the scale plane with magnetization of the sensing layer in the plane, which show the capability of polarity detection for the absolute line and reveal sinusoidal output signal for the incremental line. Furthermore, due to the large signal of TMR, the working distance can be further improved compared with conventional sensors. In addition, the cost of the positioning system is expected to be lowered, since all the sensors are fabricated in the same process without extra installation. Our design may pave a new avenue for the positioning system based on a magnetic detection scheme.

Manipulating exchange bias by spin-orbit torque.

Exchange bias, a shift in the hysteresis loop of a ferromagnet arising from interfacial exchange coupling between adjacent ferromagnetic and antiferromagnetic layers, is an integral part of spintronic devices. Here, we show that spin-orbit torque generated from spin current, a promising approach to switch the ferromagnetic magnetization of next-generation magnetic random access memory, can also be used to manipulate the exchange bias. Applying current pulses to a Pt/Co/IrMn trilayer causes concurrent switching of ferromagnetic magnetization and exchange bias, but with different underlying mechanisms. This implies that the ferromagnetic magnetization and exchange bias can be manipulated independently. Our work demonstrates that spin-orbit torque in ferromagnet/antiferromagnet heterostructures facilitates independent manipulations of distinct magnetic properties, motivating innovative designs for future spintronics devices.

Self-Assembled Epitaxial Core-Shell Nanocrystals with Tunable Magnetic Anisotropy.

Epitaxial core-shell CoO-CoFe2 O4 nanocrystals are fabricated by using pulsed laser deposition with the aid of melted material (Bi2 O3 ) addition and suitable lattice mismatch provided by substrates (SrTiO3 ). Well aligned orientations among nanocrystals and reversible core-shell sequence reveal tunable magnetic anisotropy. The interfacial coupling between core and shell further engineers the nanocrystal functionality.

Application of strong transverse magneto-optical Kerr effect on high sensitive surface plasmon grating sensors.

A high sensitive sensor is demonstrated by exploiting strong transverse magneto-optical Kerr effect on a ferromagnetic surface plasmon grating. The surface plasmon grating, made of a hybridized Au/Fe/Au layer, exhibits a very dispersive Kerr parameter variation near the surface plasmon polariton (SPP) wavelength via coherent scattering of the SPP on the grating structure. Interrogating this Kerr parameter can be utilized for detecting chemical or biological objects in a fluid medium. The experiment results show the minimal detectable mass concentration of sodium chloride in a saline solution is 4.27 × 10(-3) %, corresponding to a refractive index change of 7.60 × 10(-6) RIU. For an avidin-biotin interaction experiment, the sensitivity of avidin detection in PBS solution is 1.97 nM, which is limited by the index fluctuation of flowing media during measurement.

Ultrafast carrier dynamics in Cu(In,Ga)Se2 thin films probed by femtosecond pump-probe spectroscopy.

Ultrafast carrier dynamics in Cu(In,Ga)Se2 films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former.

Large scale single-crystal Cu(In,Ga)Se2 nanotip arrays for high efficiency solar cell.

In this paper, we demonstrated direct formation of large area Cu(In,Ga)Se(2) nanotip arrays (CIGS NTRs) by using one step Ar(+) milling process without template. By controlling milling time and incident angles, the length of CIGS NTRs with adjustable tilting orientations can be precisely controlled. Formation criteria of these CIGS NTRs have been discussed in terms of surface curvature, multiple components, and crystal quality, resulting in a highly anisotropic milling effect. The CIGS NTRs have very low reflectance <0.1% at incident wavelengths between 300 to 1200 nm. Open circuit voltage and short circuit current of CIGS NTRs solar cell were measured to be ∼390 mV and ∼22.56 mA/cm(2), yielding the filling factor and the efficiency of 59 and 5.2%, respectively. In contrast to CIGS thin film solar cell with efficiency of 3.2%, the nanostructured CIGS NTRs can have efficiency enhancement of ∼160% due to the higher light absorption ability because of the nanostructure. The merits of current approach include the latest way via template-free direct creating process of nanostructured CIGS NTRs with controllable dimensionality and large scale production without postselenization process.

Current-induced Néel order switching facilitated by magnetic phase transition.

Terahertz (THz) spin dynamics and vanishing stray field make antiferromagnetic (AFM) materials the most promising candidate for the next-generation magnetic memory technology with revolutionary storage density and writing speed. However, owing to the extremely large exchange energy barriers, energy-efficient manipulation has been a fundamental challenge in AFM systems. Here, we report an electrical writing of antiferromagnetic orders through a record-low current density on the order of 106 A cm-2 facilitated by the unique AFM-ferromagnetic (FM) phase transition in FeRh. By introducing a transient FM state via current-induced Joule heating, the spin-orbit torque can switch the AFM order parameter by 90° with a reduced writing current density similar to ordinary FM materials. This mechanism is further verified by measuring the temperature and magnetic bias field dependences, where the X-ray magnetic linear dichroism (XMLD) results confirm the AFM switching besides the electrical transport measurement. Our findings demonstrate the exciting possibility of writing operations in AFM-based devices with a lower current density, opening a new pathway towards pure AFM memory applications.

High thermal durability of Ru-based synthetic antiferromagnet by interfacial engineering with Re insertion.

Synthetic antiferromagnets (SAFs), composed of Ru spacer with a Re insertion layer, reveal superior thermal stability up to 450 °C annealing, making the back-end of line process a wider manufacturing window and tolerance to integrate the perpendicular magnetic tunneling junctions (P-MTJs) into CMOS process. The coupling strength decays significantly for SAFs with single Ru spacer after annealing above 400 °C. Due to the characteristics of refractory metals, Re can behave as a diffusion barrier during annealing. Furthermore, the Re spacer can still keep reasonable RKKY coupling strength. Therefore, the SAFs with Ru/Re composite spacers exhibit higher RKKY coupling strength than Ru spacers after 450 °C annealing. In addition, we discovered the different enhancements for the upper and lower interfacial Re insertion, which was attributed to the varied defect formation at interfaces. The stacking fault was formed at the upper Ru/Co interface in as-deposited state. When Re was inserted at the upper interface, the diffusion between Co and Ru was significantly suppressed and the stacking fault can be eliminated during annealing, leading to enhanced interlayer coupling. Through the interfacial engineering, we may have more degrees of freedom to tune the SAF performance and thus enhance process compatibility of P-MTJ to the CMOS process.

Efficiency Enhancement of Cu(In,Ga)(S,Se)2 Solar Cells by Indium-Doped CdS Buffer Layers.

Improving power conversion efficiency of photovoltaic devices has been widely investigated; however, most research studies mainly focus on the modification of the absorber layer. Here, we present an approach to enhance the efficiency of Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cells simply by tuning the CdS buffer layer. The CdS buffer layer was deposited by chemical bath deposition. Indium doping was done during the growth process by adding InCl3 into the growing aqueous solution. We show that the solar cell efficiency is increased by proper indium doping. Based on the characteristics of the single CdS (with or without In-doping) layer and of the CIGSSe/CdS interface, we conclude that the efficiency enhancement is attributed to the interface-defect passivation of heterojunction, which significantly improves both open circuit voltage and fill factor. The results were supported by SCAPS simulations, which suggest that our approach can also be applied to other buffer systems.

Engineering a Ga-Gradient by One-Step Sputtering to Achieve Over 15% Efficiency of Cu(In,Ga)Se2 Flexible Solar Cells without Post-selenization.

A one-step sputtering process using a quaternary target has been demonstrated to be a simple route to form Cu(In,Ga)Se2 (CIGSe) absorber without post-selenization; however, the lack of a Ga-grading structure in the CIGSe absorber confines its efficiency. Here, we demonstrate a one-step cosputtering process to control the Ga profile in the CIGSe absorber on flexible stainless steel substrates. Special attention was paid to the formation of second phases and their effects on the cell performance. Although the normal Ga-grading and efficiency enhancement could be achieved by cosputtering of CIGSe and Ga2Se3 targets, high-energy ion bombardment during the sputtering process might cause the decomposition of the Ga2Se3 target, leading to the formation of Ga2O3 in the CIGSe absorber, which gradually degraded the device performance. We replaced the Ga2Se3 target with a stoichiometric CuGaSe2 target for cosputtering, which can further enhance the cell efficiency due to the elimination of Ga2O3. However, when the Ga content at the back side of CIGSe is further increased by raising the deposition power of the CuGaSe2 target, the phase separation of CuGaSe2 may take place, resulting in the formation of Cu2-XSe and CuGaSe2 at the back side of the CIGSe absorber; therefore, the recombination at the back side is increased. By cosputtering a CIGSe target with a Cu-deficient CuGaSe2 target, we can suppress the formation of second phases and achieve designable normal grading, leading to the highest efficiency of 15.63% without post-selenization on flexible substrates.

In-Situ observation of local atomic structure of Al-Cu-Fe quasicrystal formation.

The phase and local environment, neighbouring atoms and coordination numbers (CN), for an Al-Cu-Fe multilayer were studied during heating (to 800 °C) and cooling (to room temperature) processes using in-situ X-Ray diffraction (XRD) and in-situ X-ray absorption spectroscopy (XAS) techniques to investigate the formation of Al-Cu-Fe quasicrystals (QCs). In-situ XRD clarified the transition of the ω-Al7Cu2Fe phase to a liquid state at the high temperature which transformed into the QC phase during cooling. The in-situ XAS showed a relatively small shift in distance between Cu-Al and Fe-Al during the phase evolution from RT to 700 °C. The distance between Cu-Cu, however, showed a significant increase from ω-phase at 700 °C to the liquid state at 800 °C, and this distance was maintained after QC formation. Furthermore, the CN of Fe-Al was changed to N = 9 during cooling. Through our observations of changes in CN, atomic distances and the atomic environment, we propose the local structural ordering of the quasicrystalline phase originated from a liquid state via ω-phase. In this study, we give a clear picture of the atomic environment from the crystalline to the quasicrystalline phase during the phase transitions, which provides a better understanding of the synthesis of functional QC nanomaterials.

Scalable Epitaxial Growth of WSe2 Thin Films on SiO2/Si via a Self-Assembled PtSe2 Buffer Layer.

The growth of large-area epitaxial transition metal dichalgogenides (TMDCs) are of central importance for scalable integrated device applications. Different methods have been developed to achieve large-sized high quality films. However, reliable approaches for centimeter-sized or even wafer-level epitaxial growth of TMDCs are still lacking. Here we demonstrate a new method to grow inch-sized epitaxial WSe2 films on SiO2/Si substrates at a much lower temperature with high repeatability and scalability. High quality crystalline films are achieved through direct selenization of a tungsten film with platinum as the underlayer. The self-assembled PtSe2 buffer layer, formed during selenization, assists epitaxial growth of WSe2. Using fabricated WSe2 films, excellent performance memory devices are demonstrated. As a member of the TMDC family, our findings based on WSe2 may also be applied to other TMDC materials for large-scale production of high quality TMDC films for various applications.

Publisher Correction: Symmetric and Asymmetric Magnetic Tunnel Junctions with Embedded Nanoparticles: Effects of Size Distribution and Temperature on Tunneling Magnetoresistance and Spin Transfer Torque.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Magnetic Interaction of Multifunctional Core-Shell Nanoparticles for Highly Effective Theranostics.

The controlled size and surface treatment of magnetic nanoparticles (NPs) make one-stage combination feasible for enhanced magnetic resonance imaging (MRI) contrast and effective hyperthermia. However, superparamagnetic behavior, essential for avoiding the aggregation of magnetic NPs, substantially limits their performance. Here, a superparamagnetic core-shell structure is developed, which promotes the formation of vortex-like intraparticle magnetization structures in the remanent state, leading to reduced dipolar interactions between two neighboring NPs, while during an MRI scan, the presence of a DC magnetic field induces the formation of NP chains, introducing increased local inhomogeneous dipole fields that enhance relaxivity. The core-shell NPs also reveal an augmented anisotropy, due to exchange coupling to the high anisotropy core, which enhances the specific absorption rate. This in vivo tumor study reveals that the tumor cells can be clearly diagnosed during an MRI scan and the tumor size is substantially reduced through hyperthermia therapy by using the same FePt@iron oxide nanoparticles, realizing the concept of theranostics.

Symmetric and Asymmetric Magnetic Tunnel Junctions with Embedded Nanoparticles: Effects of Size Distribution and Temperature on Tunneling Magnetoresistance and Spin Transfer Torque.

The problem of the ballistic electron tunneling is considered in magnetic tunnel junction with embedded non-magnetic nanoparticles (NP-MTJ), which creates additional conducting middle layer. The strong temperature impact was found in the system with averaged NP diameter d av < 1.8 nm. Temperature simulation is consistent with experimental observations showing the transition between dip and classical dome-like tunneling magnetoresistance (TMR) voltage behaviors. The low temperature approach also predicts step-like TMR and quantized in-plane spin transfer torque (STT) effects. The robust asymmetric STT respond is found due to voltage sign inversion in NP-MTJs with barrier asymmetry. Furthermore, it is shown how size distribution of NPs as well as quantization rules modify the spin-current filtering properties of the nanoparticles in ballistic regime. Different quantization rules for the transverse component of the wave vector are considered to overpass the dimensional threshold (d av ≈ 1.8 nm) between quantum well and bulk-assisted states of the middle layer.

Scaffold-Free Liver-On-A-Chip with Multiscale Organotypic Cultures.

The considerable advances that have been made in the development of organotypic cultures have failed to overcome the challenges of expressing tissue-specific functions and complexities, especially for organs that require multitasking and complex biological processes, such as the liver. Primary liver cells are ideal biological building blocks for functional organotypic reconstruction, but are limited by their rapid loss of physiological integrity in vitro. Here the concept of lattice growth used in material science is applied to develop a tissue incubator, which provides physiological cues and controls the 3D assembly of primary cells. The cues include a biological growing template, spatial coculture, biomimetic radial flow, and circulation in a scaffold-free condition. The feasibility of recapitulating a multiscale physiological structural hierarchy, complex drug clearance, and zonal physiology from the cell to tissue level in long-term cultured liver-on-a-chip is demonstrated. These methods are promising for future applications in pharmacodynamics and personal medicine.

Sputtered Inx(O,S)y Buffer Layers for Cu(In,Ga)Se2 Thin-Film Solar Cells: Engineering of Band Alignment and Interface Properties.

We propose a simple approach to engineering the sputtered Inx(O,S)y/Cu(In,Ga)Se2 heterojunction, in terms of band alignment and interface properties. The band alignment was tailored by tuning the base pressure of the sputtering deposition to incorporate oxygen into deposited In2S3 layers (termed as Inx(O,S)y). The interface properties were improved by optimizing the air-annealing temperature on Inx(O,S)y/Cu(In,Ga)Se2 stacked layers. Increasing the base pressure raises the O/(S + O) ratio contained in deposited Inx(O,S)y films and thus widens the band gaps. This could effectively tailor the conduction band offset (ΔEC) at the Inx(O,S)y/Cu(In,Ga)Se2 interface from a cliff (-0.25 eV) to a nearly flat band (0.07 eV) alignment. On the other hand, the extra air annealing at 235 °C did not significantly change the band alignment but did ameliorate the interface properties by reducing the Cu content at the Cu(In,Ga)Se2 surface and diminish the interface defect density induced by sputtering damages. The former might enhance the type of inversion and increase the hole barrier at the interface, preventing the detrimental recombination behavior. The latter could effectively strengthen the junction quality. Consequently, our approach substantially enhances the cell efficiency from 2.30% to 11.04%.

Initialization-Free Multilevel States Driven by Spin-Orbit Torque Switching.

By engineering multidomain formation in Co/Pt multilayers, it is demonstrated how multilevel storage can be achieved by spin-orbit torque switching. It is rather remarkable that, by modulating the writing pulse conditions, the final magnetization states can be controlled, independent of the initial configurations. The initialization-free multilevel memory advances the spin-orbit-torque magnetic random access memory to higher storage density for practical applications.

Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers.

Magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the ΔM method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices.

Room-Temperature Chemical Solution Treatment for Flexible ZnS(O,OH)/Cu(In,Ga)Se2 Solar Cell: Improvements in Interface Properties and Metastability.

We demonstrate an effective room-temperature chemical solution treatment, by using thioacetamide (S treatment) or thioacetamide-InCl3 (In-S treatment) solution, on Cu(In,Ga)Se2 (CIGSe) surface to engineer the ZnS(O,OH)/CIGSe interface and junction quality, leading to enhanced efficiency and minimized metastability of flexible solar cells. The control device without treatment reveals a relatively low efficiency of 8.15%, which is significantly improved to 9.74% by In-S treatment, and 10.39% by S treatment. Results of X-ray photoelectron spectroscopy suggest that S is incorporated into CIGSe surface forming CIGSSe by S treatment, whereas a thin In-S layer is formed on CIGSe surface by In-S treatment with reduced amount of S diffusing into CIGSe. PL spectra and TRPL lifetime further reveal that S incorporation into CIGS surface may substitute the OSe and/or directly occupy the vacant anion site (VSe), resulting in the effective passivation of the recombination centers at CIGSe surface. Moreover, reducing the concentrations of VSe may thereby decrease the density of (VCu-VSe) acceptors, which can minimize the metastability of ZnS(O,OH)/CIGSe solar cells. With S treatment, the light soaking (LS) time of ZnS(O,OH)/CIGSe device is reduced approximately to one-half of control one. Our approach can be potentially applied for alternative Cd-free buffer layers to achieve high efficiency and low metastability.