Voltage-Driven Fluorine Motion for Novel Organic Spintronic Memristor.

Integrating tunneling magnetoresistance (TMR) effect in memristors is a long-term aspiration because it allows to realize multifunctional devices, such as multi-state memory and tunable plasticity for synaptic function. However, the reported TMR in different multiferroic tunnel junctions is limited to 100%. This work demonstrates a giant TMR of -266% in La0.6Sr0.4MnO3(LSMO)/poly(vinylidene fluoride)(PVDF)/Co memristor with thin organic barrier. Different from the ferroelectricity-based memristors, this work discovers that the voltage-driven florine (F) motion in the junction generates a huge reversible resistivity change up to 106% with nanosecond (ns) timescale. Removing F from PVDF layer suppresses the dipole field in the tunneling barrier, thereby significantly enhances the TMR. Furthermore, the TMR can be tuned by different polarizing voltage due to the strong modification of spin-polarization at the LSMO/PVDF interface upon F doping. Combining of high TMR in the organic memristor paves the way to develop high-performance multifunctional devices for storage and neuromorphic applications.

Kinetics vs. thermodynamics: walking on the line for a five-fold increase in MnSi Curie temperature.

Green and digital transitions will induce tremendous demand for metals and semiconductors. This raises concerns about the availability of materials in the rather near future. Addressing this challenge requires an unprecedented effort to discover new materials that are more sustainable and also to expand their functionalities beyond conventional material limits. From this point of view, complex systems combining semiconductor and magnetic properties in a single material lay the foundations for future nanoelectronics devices. Through a combination of out-of-stable equilibrium processes, we achieved fine control over the crystallisation of non-stoichiometric MnSix (x = 0.92). The Curie temperature shows non-monotonous evolution with crystallisation. At the earliest and final stages, the Curie temperature is comparable with stoichiometric MnSi (TC = 30 K). At the intermediate stage, while the material is crystalline and remains non-stoichiometric, a remarkable fivefold increase in Curie temperature (TC = 150 K) is observed. This finding highlights the potential for controlling the metastability of materials as a promising and relatively unexplored pathway to enhance material properties, without relying on critical materials such as rare earth elements.

Room Temperature Spin-to-Charge Conversion in Amorphous Topological Insulating Gd-Alloyed BixSe1-x/CoFeB Bilayers.

Disordered topological insulator (TI) films have gained intense interest by benefiting from both the TI's exotic transport properties and the advantage of mass production by sputtering. Here, we report on the clear evidence of spin-charge conversion (SCC) in amorphous Gd-alloyed BixSe1-x (BSG)/CoFeB bilayers fabricated by sputtering, which could be related to the amorphous TI surface states. Two methods have been employed to study SCC in BSG (tBSG = 6-16 nm)/CoFeB(5 nm) bilayers with different BSG thicknesses. First, spin pumping is used to generate a spin current in CoFeB and detect SCC by the inverse Edelstein effect (IEE). The maximum SCC efficiency (SCE) is measured to be as large as 0.035 nm (IEE length λIEE) in a 6 nm thick BSG sample, which shows a strong decay when tBSG increases due to the increase of BSG surface roughness. The second method is THz time-domain spectroscopy, which reveals a small tBSG dependence of SCE, validating the occurrence of a pure interface state-related SCC. Furthermore, our angle-resolved photoemission spectroscopy data show dispersive two-dimensional surface states that cross the bulk gap until the Fermi level, strengthening the possibility of SCC due to the amorphous TI states. Our studies provide a new experimental direction toward the search for topological systems in amorphous solids.

Surface Modification of Brass via Ultrashort Pulsed Direct Laser Interference Patterning and Its Effect on Bacteria-Substrate Interaction.

In recent decades, antibiotic resistance has become a crucial challenge for human health. One potential solution to this problem is the use of antibacterial surfaces, i.e., copper and copper alloys. This study investigates the antibacterial properties of brass that underwent topographic surface functionalization via ultrashort pulsed direct laser interference patterning. Periodic line-like patterns in the scale range of single bacterial cells were created on brass with a 37% zinc content to enhance the contact area for rod-shaped Escherichia coli (E. coli). Although the topography facilitates attachment of bacteria to the surface, reduced killing rates for E. coli are observed. In parallel, a high-resolution methodical approach was employed to explore the impact of laser-induced topographical and chemical modifications on the antibacterial properties. The findings reveal the underlying role of the chemical modification concerning the antimicrobial efficiency of the Cu-based alloy within the superficial layers of a few hundred nanometers. Overall, this study provides valuable insight into the effect of alloy composition on targeted laser processing for antimicrobial Cu-surfaces, which facilitates the thorough development and optimization of the process concerning antimicrobial applications.

Interfacial engineering of ferromagnetism in wafer-scale van der Waals Fe4GeTe2 far above room temperature.

Despite recent advances in exfoliated vdW ferromagnets, the widespread application of 2D magnetism requires a Curie temperature (Tc) above room temperature as well as a stable and controllable magnetic anisotropy. Here we demonstrate a large-scale iron-based vdW material Fe4GeTe2 with the Tc reaching ~530 K. We confirmed the high-temperature ferromagnetism by multiple characterizations. Theoretical calculations suggested that the interface-induced right shift of the localized states for unpaired Fe d electrons is the reason for the enhanced Tc, which was confirmed by ultraviolet photoelectron spectroscopy. Moreover, by precisely tailoring Fe concentration we achieved arbitrary control of magnetic anisotropy between out-of-plane and in-plane without inducing any phase disorders. Our finding sheds light on the high potential of Fe4GeTe2 in spintronics, which may open opportunities for room-temperature application of all-vdW spintronic devices.

Large Perpendicular Magnetic Anisotropy in Ta/CoFeB/MgO on Full-Coverage Monolayer MoS2 and First-Principles Study of Its Electronic Structure.

A perpendicularly magnetized spin injector with a high Curie temperature is a prerequisite for developing spin optoelectronic devices on two-dimensional (2D) materials working at room temperature (RT) with zero applied magnetic field. Here, we report the growth of Ta/CoFeB/MgO structures with large perpendicular magnetic anisotropy (PMA) on full-coverage monolayer (ML) molybdenum disulfide (MoS2). A large perpendicular interface anisotropy energy of 0.975 mJ/m2 has been obtained at the CoFeB/MgO interface, comparable to that observed in magnetic tunnel junction systems. It is found that the insertion of MgO between the ferromagnetic (FM) metal and the 2D material can effectively prevent the diffusion of the FM atoms into the 2D material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical role in establishing the large PMA. First-principles calculations on a similar Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band structure, from a direct band gap with 3ML-MgO to an indirect band gap with 7 ML-MgO. The proximity effect induced by Fe results in splitting of 10 meV in the valence band at the Γ point for the 3ML-MgO structure, while it is negligible for the 7 ML-MgO structure. These results pave the way to develop RT spin optoelectronic devices based on 2D transition-metal dichalcogenide materials.

Current-Induced Spin Torques on Single GdFeCo Magnetic Layers.

Spintronics exploit spin-orbit coupling (SOC) to generate spin currents, spin torques, and, in the absence of inversion symmetry, Rashba and Dzyaloshinskii-Moriya interactions. The widely used magnetic materials, based on 3d metals such as Fe and Co, possess a small SOC. To circumvent this shortcoming, the common practice has been to utilize the large SOC of nonmagnetic layers of 5d heavy metals (HMs), such as Pt, to generate spin currents and, in turn, exert spin torques on the magnetic layers. Here, a new class of material architectures is introduced, excluding nonmagnetic 5d HMs, for high-performance spintronics operations. Very strong current-induced torques exerted on single ferrimagnetic GdFeCo layers, due to the combination of large SOC of the Gd 5d states and inversion symmetry breaking mainly engineered by interfaces, are demonstrated. These "self-torques" are enhanced around the magnetization compensation temperature and can be tuned by adjusting the spin absorption outside the GdFeCo layer. In other measurements, the very large emission of spin current from GdFeCo, 80% (20%) of spin anomalous Hall effect (spin Hall effect) symmetry is determined. This material platform opens new perspectives to exert "self-torques" on single magnetic layers as well as to generate spin currents from a magnetic layer.

Toxicity of TiO2 Nanoparticles: Validation of Alternative Models.

There are many studies concerning titanium dioxide (TiO2) nanoparticles (NP) toxicity. Nevertheless, there are few publications comparing in vitro and in vivo exposure, and even less comparing air-liquid interface exposure (ALI) with other in vitro and in vivo exposures. The identification and validation of common markers under different exposure conditions are relevant for the development of smart and quick nanotoxicity tests. In this work, cell viability was assessed in vitro by WST-1 and LDH assays after the exposure of NR8383 cells to TiO2 NP sample. To evaluate in vitro gene expression profile, NR8383 cells were exposed to TiO2 NP during 4 h at 3 cm2 of TiO2 NP/cm2 of cells or 19 µg/mL, in two settings-submerged cultures and ALI. For the in vivo study, Fischer 344 rats were exposed by inhalation to a nanostructured aerosol at a concentration of 10 mg/m3, 6 h/day, 5 days/week for 4 weeks. This was followed immediately by gene expression analysis. The results showed a low cytotoxic potential of TiO2 NP on NR8383 cells. Despite the absence of toxicity at the doses studied, the different exposures to TiO2 NP induce 18 common differentially expressed genes (DEG) which are involved in mitosis regulation, cell proliferation and apoptosis and inflammation transport of membrane proteins. Among these genes, we noticed the upregulation of Ccl4, Osm, Ccl7 and Bcl3 genes which could be suggested as early response biomarkers after exposure to TiO2 NP. On the other hand, the comparison of the three models helped us to validate the alternative ones, namely submerged and ALI approaches.

Transport properties of polycrystalline SnTe prepared by saturation annealing.

Because the binary chalcogenide SnTe is an interesting Pb-free alternative to the state-of-the-art thermoelectric material PbTe, significant efforts were devoted to the optimization of its thermoelectric properties over the last few years. Here, we show that saturation-annealing treatments performed at 823, 873 or 973 K under Sn-rich conditions provide a successful strategy to prepare polycrystalline samples with a controlled concentration of Sn vacancies. Both scanning transmission electron microscopy and Mössbauer spectroscopy demonstrate the absence of Sn-rich areas at the grain boundaries in the saturation-annealed samples. Transport property measurements, performed over a wide range of temperatures (5-800 K), show that this technique enables achieving thermoelectric performances at 800 K similar to those obtained using Sn self-compensation. The three saturation annealing temperatures result in comparable transport properties across the entire temperature range due to similar hole concentrations ranging between 1.0 and 1.5 × 1020 cm-3 at 300 K. As equally observed in samples prepared by other synthetic routes, the temperature dependence of the Hall mobility evidences that charge transport is strongly affected by point-defect scattering caused by the random distribution of Sn vacancies.

Crystalline and Electronic Structures of the Al1+xV2Sn2-x (x = 0.19) Intermetallic Compound.

A new ternary phase with a composition Al1+xV2Sn2-x (x = 0.19) has been found during investigation of the Al-V-Sn ternary system. Single-crystal X-ray diffraction measurements reveal that this ternary phase crystallizes with an orthorhombic structure with a = 5.5931(1) Å, b = 18.8017(5) Å, and c = 6.7005(2) Å (space group Cmce). This compound is thus isostructural to the GaV2Sn2 structure type, showing a layered structure composed of vanadium cluster bands formed with pentagonal faces intercalated by Sn atom layers. High-resolution transmission electron microscopy measurements confirm the orthorhombic structure. Regarding lattice perfection, no dislocation could be identified within the probed Al1.19V2Sn1.81 single-crystal lamella. Ab initio calculations reveal a reduction of the density of states at the Fermi level, which could be attributed to both a Hume-Rothery effect combined with strong spd hybridization.

Evidence of a strong perpendicular magnetic anisotropy in Au/Co/MgO/GaN heterostructures.

We report a strong perpendicular magnetic anisotropy (PMA) in Au/Co/MgO/GaN heterostructures from both experiments and first-principles calculations. The Au/Co/MgO heterostructures have been grown by molecular beam epitaxy (MBE) on GaN/sapphire substrates. By carefully optimizing the growth conditions, we obtained a fully epitaxial structure with a crystalline orientation relationship Au(111)[1̄10]//Co(0001)[112̄0]//MgO(111)[101̄]//GaN(0002)[112̄0]. More interestingly, we demonstrate that a 4.6 nm thick Co film grown on MgO/GaN still exhibits a large perpendicular magnetic anisotropy. First-principles calculations performed on the Co (4ML)/MgO(111) structure showed that the MgO(111) surface can strongly enhance the magnetic anisotropy energy by 40% compared to a reference 4ML thick Co hcp film. Our layer-resolved and orbital-hybridization resolved anisotropy analyses helped to clarify that the origin of the PMA enhancement is due to the interfacial hybridization of O 2p and Co 3d orbitals at the Co/MgO interface. The perpendicularly magnetized Au/Co/MgO/GaN heterostructures are promising for efficient spin injection and detection in GaN based opto-electronics without any external magnetic field.

Evidence of Pure Spin-Current Generated by Spin Pumping in Interface-Localized States in Hybrid Metal-Silicon-Metal Vertical Structures.

Due to the difficulty of growing high-quality semiconductors on ferromagnetic metals, the study of spin diffusion transport in Si was limited to lateral geometry devices. In this work, by using an ultrahigh-vacuum wafer-bonding technique, we have successfully fabricated metal-semiconductor-metal CoFeB/MgO/Si/Pt vertical structures. We hereby demonstrate pure spin-current injection and transport in the perpendicular current flow geometry over a distance larger than 2 µm in n-type Si at room temperature. In those experiments, a pure propagating spin current is generated via ferromagnetic resonance spin pumping and converted into a measurable voltage by using the inverse spin Hall effect occurring in the top Pt layer. A systematic study varying both Si and MgO thicknesses reveals the important role played by the localized states at the MgO-Si interface for the spin-current generation. Proximity effects involving indirect exchange interactions between the ferromagnet and the MgO-Si interface states appears to be a prerequisite to establishing the necessary out-of-equilibrium spin population in Si under the spin-pumping action.

Tunable Localized Surface Plasmon Resonance and Broadband Visible Photoresponse of Cu Nanoparticles/ZnO Surfaces.

Plasmonic Cu nanoparticles (NP) were successfully deposited on ZnO substrates by atomic layer deposition (ALD) owing to the Volmer-Weber island growth mode. An evolution from Cu NP to continuous Cu films was observed with an increasing number of ALD cycles. Real and imaginary parts of the NP dielectric functions, determined by spectroscopic ellipsometry using an effective medium approach, evidence a localized surface plasmon resonance that can be tuned between the visible and near-infrared ranges by controlling the interparticle spacing and size of the NP. The resulting Cu NP/ZnO device shows an enhanced photoresponse under white light illumination with good responsivity values, fast response times, and stability under dark/light cycles. The significant photocurrent detected for this device is related to the hot-electron generation at the NP surface and injection into the conduction band of ZnO. The possibility of tuning the plasmon resonance together with the photoresponsivity of the device is promising in many applications related to photodetection, photonics, and photovoltaics.

Quenching of Spin Polarization Switching in Organic Multiferroic Tunnel Junctions by Ferroelectric "Ailing-Channel" in Organic Barrier.

The ferroelectric control of spin-polarization at ferromagnet (FM)/ferroelectric organic (FE-Org) interface by electrically switching the ferroelectric polarization of the FE-Org has been recently realized in the organic multiferroic tunnel junctions (OMFTJs) and gained intensive interests for future multifunctional organic spintronic applications. Here, we report the evidence of ferroelectric "ailing-channel" in the organic barrier, which can effectively pin the ferroelectric domain, resulting in nonswitchable spin polarization at the FM/FE-Org interface. In particular, OMFTJs based on La0.6Sr0.4MnO3/P(VDF-TrFE) ( t)/Co/Au structures with different P(VDF-TrFE) thickness ( t) were fabricated. The combined advanced electron microscopy and spectroscopy studies clearly reveal that very limited Co diffusion exists in the P(VDF-TrFE) organic barrier when the Au/Co electrode is deposited around 80K. Pot-hole structures at the boundary between the P(VDF-TrFE) needle-like grains are evidenced to induce "ailing-channels" that hinder efficient ferroelectric polarization of the organic barrier and result in the quenching of the spin polarization switching at Co/P(VDF-TrFE) interface. Furthermore, the spin diffusion length in the negatively polarized P(VDF-TrFE) is measured to be about 7.2 nm at 20K. The evidence of the mechanism of ferroelectric "ailing-channels" is of essential importance to improve the performance of OMFTJ and master the key condition for an efficient ferroelectric control of the spin polarization of "spinterface".

Crystal Structure and Transport Properties of the Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 2, 3).

We report on a detailed investigation of the crystal structure and transport properties in a broad temperature range (2-723 K) of the homologous compounds (PbSe)5(Bi2Se3)3m for m = 2, 3. Single-crystal X-ray diffraction data indicate that the m = 2, 3 compounds crystallize in the monoclinic space groups C2/m (No. 12) and P21/m (No. 11), respectively. In agreement with diffraction data, high-resolution transmission electron microscopy analyses carried out on single crystals show that the three-dimensional crystal structures are built from alternating Pb-Se and m Bi-Se layers stacked along the a axis in both compounds. Scanning electron microcopy and electron-probe microanalyses reveal deviations from the nominal stoichiometry, suggesting a domain of existence in the pseudo binary phase diagram at 873 K. The complex atomic-scale structures of these compounds lead to very low lattice thermal conductivities κL that approach the glassy limit at high temperatures. A comparison of the κL values across this series unveiled an unexpected increase with increasing m from m = 1 to m = 3, in contrast to the expectation that increasing the structural complexity should tend to lower the thermal transport. This result points to a decisive role played by the Pb-Se/Bi-Se interfaces in limiting κL in this series. Both compounds behave as heavily doped n-type semiconductors with relatively low electrical resistivity and thermopower values. As a result, moderate peak ZT values of 0.25 and 0.20 at 700 K were achieved in the m = 2, 3 compounds, respectively. The inherent poor ability of these structures to conduct heat suggests that these homologous compounds may show interesting thermoelectric properties when properly optimized by extrinsic dopants.

Bacterial adhesion on biomedical surfaces covered by yttria stabilized zirconia.

The aim of this study was to compare the bacterial adhesion of Staphylococcus spp. on Ti-6Al-4V with respect to Ti-6Al-V modified alloys with a set of Cubic yttria stabilized zirconia (YSZ) and Ag-YSZ nanocomposite films. Silver is well known to have a natural biocidal character and its presence in the surface predicted to enhance the antimicrobial properties of biomedical surfaces. Microbial adhesion tests were performed using collection strains and twelve clinical strains of Staphylococcus aureus and Staphylococcus epidermidis. The adherence study was performed using a previously published protocol by Kinnari et al. Both collection strains and clinical isolates have shown lower bacterial adhesion to materials modified with respect to the alloy Ti-6Al-4V and the modification with silver reduced the bacterial adhesion for most of all the strains studied. Moreover the percentage of dead bacteria have been evaluated, demonstrating increased proportion of dead bacteria for the modified surfaces. Nanocrystalline silver dissolves releasing both Ag(+) and Ag(0) whereas other silver sources release only Ag(+). We can conclude that YSZ with nanocrystalline silver coating may lead to diminished postoperative infections and to increased corrosion and scratch resistance of YSZ incorporating alloys Ti-6Al-4V.

Local modification of the microstructure and electrical properties of multifunctional Au-YSZ nanocomposite thin films by laser interference patterning.

Nanocomposite films consisting of gold nanoparticles embedded in an yttria-stabilized zirconia matrix (Au-YSZ) have been synthesized with different gold loadings by reactive magnetron sputtering followed by ex situ annealing in air or laser interference patterning (LIP) treatment. It is shown that the electrical conductivity of the nanocomposite films can be modified to a large extent by changing the gold loading, by thermal annealing, or by LIP. The structural and microstructural analyses evidenced the segregation of metallic gold in crystalline form for all synthesis conditions and treatments applied. Thermal annealing above 400 °C is observed to trigger the growth of pre-existing nanoparticles in the volume of the films. Moreover, pronounced segregation of gold to the film surface is observed for Au/(Au + Zr + Y) ratios above 0.40, which may prevent the use of thermal annealing to functionalize gold-rich Au-YSZ coatings. In contrast, significant modifications of the microstructure were detected within the interference spot (spot size close to 2 × 2 mm) of LIP treatments only for the regions corresponding to constructive interference. As a consequence, besides its already demonstrated ability to modify the friction behavior of Au-YSZ films, the LIP treatment enables local tailoring of their electrical resistivity. The combination of these characteristics can be of great interest for sliding electrical contacts.