Thin NiO Research Articles

Spin conduction and interfacial effects in La2/3Sr1/3MnO3/NiO/Pt heterostructures

In this work we report on the generation and transport of pure spin currents in La2/3Sr1/3MnO3(LSMO)/NiO/Pt heterostructures. Pure spin currents, generated by means of spin pumping in the LSMO layer, are transmitted through the NiO layer and detected by measuring the transverse voltage signal generated by the inverse spin Hall effect (ISHE) in the Pt layer. The spin current transmission is studied as a function of the NiO thickness and temperature. NiO layers grown at room temperature are polycrystalline and their microstructural and magnetic features clearly depend on thickness. Scanning transmission electron microscopy techniques revealed that homogeneous conformal coating of the LSMO layer is only achieved for NiO layer thicknesses above about 2 nm. Below this critical thickness the NiO layer is discontinuous and its main role would be to disturb the LSMO/Pt interface causing it to behave as something similar to an insulating barrier. Magnetic measurements indicate that NiO layers are paramagnetic (PM) well below room temperature. In agreement with these observations, the amplitude of the ISHE voltage signal decreases monotonically on lowering temperature and makes evident that there are two spin conduction regimes as a function of the NiO layer thickness with different spin diffusion lengths. For thin discontinuous NiO layers a spin diffusion length of λSDL≈0.8 nm is found, which is slightly larger than typical values found for insulating barriers, but definitely smaller than λSDL≈3.8 nm found for NiO layer thickness above 2 nm. The latter being similar to λSDL previously reported for epitaxial NiO layers. Our findings indicate that inserting a PM NiO insulating layer of optimal thickness improves the spin transparency of the LSMO/Pt interface. Additionally, the results suggest that spin conduction through the NiO layer is likely driven by magnetic correlations and short-range thermal magnons.

Effect of surface orientation on the corrosion behavior of Ni-based single-crystal alloy in a simulated marine atmosphere at 750 °C

DD5 single-crystal alloy samples with different surface orientations were obtained by machining the alloy ingot perpendicular (PP) and parallel (P), respectively, to its solidification direction (SD). The corrosion behaviors of the PPSD and PSD alloy samples were investigated in the atmosphere of moist air + NaCl aerosol at 750 °C. Results showed that during initial corrosion, γ′ phase developed a thin NiO layer with Al internal corrosion product. However, a NiO nodule with an internal Cr2O3 layer was formed on γ phase. As corrosion progressed, the corrosion product gradually grew to a NiO/(Al, Cr)2O3 bilayer, accompanied by internal corrosion consisting of Al2O3. It was also found that the PSD sample with a lower fraction and smaller size of γ’ phases exhibited better corrosion resistance than the PPSD sample. The effect of alloy surface orientation on the corrosion mechanism was discussed.

Enhanced Spin–Orbit Torque Efficiency by the Insertion of a Thin NiO Underlayer in Pt/Co/Ta Films

Enhanced Spin–Orbit Torque Efficiency by the Insertion of a Thin NiO Underlayer in Pt/Co/Ta Films

Selective Detection of H2 Gas in Gas Mixtures Using NiO‐Shelled Pd‐Decorated ZnO Nanowires

AbstractHydrogen (H2) gas is a green fuel, but its leakage during storage and transportation can lead to disasters due to its explosive nature. Here, a sensitive and selective H2 gas sensor is developed that can detect H2 in H2/CO and H2/NO2 gas mixtures. First, ZnO nanowires (NWs) are grown using vapor–liquid–solid growth. This is followed by atomic layer deposition‐mediated growth of Pd nanoparticles (NPs) on the ZnO NWs and uniform deposition of a thin NiO shell layer (12 nm in thickness) over the Pd‐decorated ZnO NWs. Characterization of the synthesized samples by different methods confirms the desired chemical composition, morphology, and phases. H2 gas sensing studies reveals the highly sensitive and selective response of the optimized gas sensor to H2 at 200 °C. In the presence of H2/CO and H2/NO2 gas mixtures, the NiO‐shelled Pd‐decorated ZnO NW sensor displays good selectivity for H2, but not the Pd‐decorated ZnO NW gas one. The NiO‐shelled Pd‐decorated ZnO NW gas sensor efficiently detected H2 also in the presence of 40% relative humidity and displays good stability even after 1 month. The present results can open the doors to the fabrication of highly selective H2 gas sensors using the described rationale design.

Reducing and tuning the work function of field emission nanocomposite CNT/NiO cathodes by modifying the chemical composition of the oxide.

This work presents for the first time the possibility of reducing and tuning the work function of field emission cathodes coated with metal oxides by changing the chemical composition of oxide coatings using an example of heat-treated CNT/NiO nanocomposite structures. These cathodes are formulated using carbon nanotube (CNT) arrays that are coated with ultrathin layers of nickel oxide (CNT/NiO) by atomic layer deposition (ALD). It was found that NiO at thicknesses of several nanometers grown on CNTs heat treated at a temperature of 350 °C can change its stoichiometric composition towards the formation of oxygen vacancies, since the Ni3+/Ni2+ peak area ratio increases and the position of the Ni-O peak binding energies shifts as observed using X-ray photoelectron spectroscopy (XPS). According to the secondary electron cut-off, the work function was 4.95 for pristine CNTs and it was found that the work function of deposited NiO layers on CNTs decreased after heat treatment. The decrease in work function occurs as a result of changes in the chemical composition of the oxide film. For the heat-treated CNT/NiO composites, the work function was 4.30 eV with a NiO layer thickness of 7.6 nm, which was less than that for a NiO thin film close to the stoichiometric composition, which had a work function of 4.48 eV. The field emission current-voltage characteristics showed that the fields for producing an emission current density of 10 μA cm-2 were 5.54 V μm-1 for pure nanotubes and 4.32 V μm-1 and 4.19 V μm-1 for NiO-coated CNTs (3.8 and 7.6 nm), respectively. The present study has shown that heat treatment of deposited thin NiO layers on field cathodes is a promising approach to improve the efficiency of field emission cathodes and is a new approach in vacuum nanoelectronics that allows tuning the work function of field emission cathodes.

The noncollinear interlayer coupling in NiFe/NiO/NiFe trilayers

The interlayer coupling and magnetization reversal behaviors in NiFe/NiO/NiFe trilayers were investigated using polarized neutron reflectivity and Monte Carlo (MC) simulations. Our results reveal that the shape of NiFe loops transitions from square to tilted as the NiO thicknesses decrease, indicating changes in the direction of NiFe layer’s easy axis. This phenomenon can be attributed to variations occurring at NiO/NiFe interfaces for different NiO layer thicknesses. With thin NiO layer, interdiffusion at the NiO/NiFe interfaces leads to frustrated coupling, resulting in a noncollinear interlayer coupling. This observation is supported by MC simulations. Conversely, hardly any coupling frustration is observed for the sample with a thick NiO layer. Our findings propose a novel way to tailor the interlayer coupling through interface engineering.

Effects of salt and ash deposits on corrosion behaviour of Ni-25Cr in Ar-60CO2-20H2O gas at 650 oC

ABSTRACT A Ni-25Cr (wt.%) alloy coated with the salts and mixtures of these salts with an industrial coal ash was exposed to Ar-60CO2-20 H2O at 650 °C for 300 h. The alloy without any deposit developed a uniform internal oxidation zone (IOZ) and an external metallic nickel layer surmounted by a thin NiO scale. Salt deposits changed the scale constituents and morphology. Predominantly, a duplex scale of NiO over a Cr2O3 layer grew with an underlying IOZ containing large Cr-rich oxide precipitates. Under a deposit of ash plus salts, the basic reaction morphology was the same as in the gas-only case, but the oxide layer was non-uniform. Local voids were formed within the alloy beneath chloride plus ash deposits. Under ash with sulphates, the alloy formed both partially protected areas and non-protective multi-layered scales. Spherical ash particles were enveloped by NiO in both salt-ash deposits.

Disentangling electrical switching of antiferromagnetic NiO using high magnetic fields

Recent demonstrations of the electrical switching of antiferromagnets (AFs) have given an enormous impulse to the field of AF spintronics. Many of these observations are plagued by non-magnetic effects that are very difficult to distinguish from the actual magnetic ones. Here, we study the electrical switching of thin (5 nm) NiO films in Pt/NiO devices using magnetic fields up to 15 T to quantitatively disentangle these magnetic and non-magnetic effects. We demonstrate that these fields suppress the magnetic components of the electrical switching of NiO, but leave the non-magnetic components intact. Using a monodomainization model the contributions are separated, showing how they behave as a function of the current density. These results show that combining electrical methods and strong magnetic fields can be an invaluable tool for AF spintronics, allowing for implementing and studying electrical switching of AFs in more complex systems.

Significance of nickel particles on reducing friction and wear of polyimide subjected to harsh boundary lubrication conditions

Nickel (Ni) particles were compounded into polyimide (PI) and PI reinforced with carbon fiber (CF). Role of Ni particles on tribological performance of the PI-based materials sliding against a bearing steel were investigated under boundary and mixed lubrication regimes. Remarkably, PI composites filled with Ni exhibit ultra-low friction and wear. Interestingly, friction- and wear-reduction roles of Ni are much more significant than those of CF. The results provide direct evidence that an extremely thin NiO glaze layer grows on the steel counterface, above which a uniform C-based tribofilm is formed. It is assumed that the NiO layer can catalyze dehydrogenation of oil molecules and promote tribofilm formation. The present work proposes a design concept of high-performance tribofilm for improving boundary lubrication of motion components.

Oxidation behavior of near nanostructured coating developed by the HVOF process

AbstractMicrostructured and near nanostructured Cr3C2–Ni–Cr–B–Si coatings were deposited on Inconel 718 superalloy by high‐velocity oxygen fuel thermal spray technique. The oxidation behavior of microstructured and near nanostructured Cr3C2–Ni–Cr–B–Si coated superalloy at 900°C in air for 50 cycles under cyclic heating and cooling conditions was studied. The kinetics of oxidation of both nanostructured and microstructured coated superalloy was analyzed by weight change measurement. It was noticed that the uncoated and coated alloy obey the parabolic rate law of oxidation. X‐ray diffraction, field‐emission scanning electron microscopy/energy dispersive x‐ray analysis, and elemental mapping techniques were used to analyze the oxidized samples of coated and uncoated samples. In the coated superalloy, iron, silicon, and titanium were oxidized in the inter‐splat region, whereas the splats which consist mainly of Ni and Cr remain unoxidized. Because of the formation of compact and adhesive thin NiO and Cr2O3 scale on the surface of the coating during oxidation, the developed nanostructured coated superalloy shows better oxidation resistance than the microstructured one.

Fabrication of nanocrystalline Ga2O3-NiO heterojunctions for large-area low-dose X-ray imaging

The Ga2O3 photoconductor possesses a wide bandgap, a relatively large density and a high absorption coefficient for low-energy X-ray. Therefore, it is expected to realize a highly sensitive low-energy X-ray detector. Realizing p-n heterojunctions using Ga2O3 thin film is essential for large-area low-dose X-ray imaging. Here, a p-n diode consisting of nanocrystalline Ga2O3 thin film and NiO thin film is successfully prepared by electron beam evaporation, exhibiting a large current rectification ratio of 6.0 × 104. The p-n heterojunction shows a large valence band offset of 1.7 eV, a large conduction band offset of 0.84 eV and a built-in potential of 0.2 eV near the interfacial region, benefiting for separation and migration of photogenerated carriers. In addition, the photoconductive gain mechanism of Ga2O3/NiO heterojunction based X-ray detectors with optimized NiO thickness is investigated, achieving a high internal gain (3.3 × 103), a high sensitivity (3.4 × 104 μCGyair−1 cm−2), a low detection limit (42 μGyair s−1) and a short response time (2.2 ms). The optimized detectors with 10 × 10 pixels show a uniform X-ray response, demonstrating their promising use in large-area X-ray imaging applications.

Investigation on high-temperature stability of Ni60A alloy coating on copper substrate fabricated by plasma cladding

To investigate the service stability, the plasma cladding Ni60A coating on copper surface was heat-treated at 600 °C for a long time in air. The evolution of the surface morphology, microstructure, hardness and wear resistance of the coating was systematically investigated. After heat treatment, a thin NiO oxide layer formed on the coating surface, but its thickness did not increase with the extension of heat treatment time, indicating the good oxidation resistance of the coating. Heat treatment had no obvious effect on the phase composition and distribution of the coating. Only the crystal grains of γ-(Cu, Fe, Ni) solid solution and part of Cr23C6 reinforcing phase tended to grow up under the action of heat. The coarsening of the microstructure led to a slight decrease in hardness and wear resistance. The thin NiO oxide layer had less protective effect on the coating surface, because it was easily damaged during the friction process. Compared with the coating before heat treatment, the wear resistance after heat treatment for 30 days was only reduced by 1.3 times, which was still 3.95 times that of the copper substrate, indicating that the coating can basically maintain good stability in a high-temperature air environment.

Temperature dependence of spin pumping in YIG/NiO(x)/W multilayer

We report the temperature dependence of the spin pumping effect for Y3Fe5O12 (YIG, 0.9 μm)/NiO (t NiO)/W (6 nm) (t NiO = 0 nm, 1 nm, 2 nm, and 10 nm) heterostructures. All samples exhibit a strong temperature-dependent inverse spin Hall effect (ISHE) signal I c and sensitivity to the NiO layer thickness. We observe a dramatic decrease of I c with inserting thin NiO layer between YIG and W layers indicating that the inserting of NiO layer significantly suppresses the spin transport from YIG to W. In contrast to the noticeable enhancement in YIG/NiO (t NiO ≈ 1–2 nm)/Pt, the suppression of spin transport may be closely related to the specific interface-dependent spin scattering, spin memory loss, and spin conductance at the NiO/W interface. Besides, the I c of YIG/NiO/W exhibits a maximum near the T N of the AF NiO layer because the spins are transported dominantly by incoherent thermal magnons.

Cyclic Oxidation Properties of the Nanocrystalline AlCrFeCoNi High-Entropy Alloy Coatings Applied by the Atmospheric Plasma Spraying Technique

Microcrystalline and nanocrystalline AlCrFeCoNi high-entropy alloy (HEA) coatings were applied on Inconel 718 superalloy using the atmospheric plasma spraying (APS) process. The high-temperature oxidation behavior of the microcrystalline and nanocrystalline AlCrFeCoNi HEA-coated superalloy was examined at 1100 °C under the air atmosphere for 50 cycles under cyclic heating and cooling (1 h for each cycle). The oxidation kinetics of both nanocrystalline- and microcrystalline-coated superalloys were accordingly analyzed by weight change measurements. We noted that the uncoated and coated samples followed the parabolic rate law of the oxidation. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDS), elemental mapping and X-ray photoelectron spectroscopy (XPS) were used to analyze the oxidized coated and uncoated samples. In the HEA-coated superalloy, Fe, Ni, Co and Al were oxidized in the inter-splat region, whereas the splats, which consisted mainly of Ni and Cr, remained unoxidized. Due to the formation of compact and adhesive thin NiO, CoO oxides and spinels together with the Al2O3 oxide scale on the surface of the coating during oxidation, the developed nanocrystalline HEA coating showed better oxidation resistance compared with the microcrystalline HEA coating.

EFFECT OF MO DURING THE OXIDATION OF NI20CR-XMO ALLOYS AT 570°C

The effect of Mo addition on the oxidation behavior of the Ni20Cr-xMo alloys at 570°C for 100 h was investigated. The results revealed that oxidation performance of the Ni20Cr alloys was increased by Mo addition. During the oxidation, the Mo-free alloy formed thin NiO scale on the outer scale and thick continous Cr2O3 scale accompanied with inward Cr2O3 penetrations and a Cr-depleted zone. There were also formations of mix Ni-Cr oxides in between NiO and Cr2O3 scale. While, the oxide scales formed on the Mo-containing alloys consisted of a duplex structure with a thick NiO scale on the outer scale and Cr(Mo)-Cr-Mo oxides on the inner scale. A porous oxide structure was also formed in the inner scale. The presence of Mo resulted in the formation of NiO scale and less the inward diffusion into Cr2O3 scale. But, more volatile oxides and porous oxide structures would form with higher Mo addition resulted in lower oxidation resistance for the Ni20Cr alloys.

Thin p-type NiO nanosheet modified peanut-shaped monoclinic BiVO4 for enhanced charge separation and photocatalytic activities

Typical NiO nanosheet coated BVO type-II composite heterostructure can effectively promote the space separation of charge and improve the photocatalytic performance.

Spray deposited thin uniform NiO/Spiro-OMeTAD composite hole transport layer with top carbon nanotube electrode

Thin Spiro-OMeTAD and NiO nanoparticles layers, as well as their composite layer was formed by layer-by-layer spray deposition as hole-transport layer (HTL), with followed carbon nanotubes (CNT) deposition to form Ti/TiO2/HTL/CNT structures. Layers’ uniformity was estimated by Raman intensity maps, AFM and current-voltage characteristics of the CNT layer and between CNT and Ti contacts. The possibility of formation of thin, less than 100 nm, pinhole-free uniform composite NiO/Spiro-OMeTAD layer by spray-deposition was shown, which manifests itself as continuous HTL even after top CNT layer deposition.

Interface design and the strengthening-ductility behavior of tetra-needle-like ZnO whisker reinforced Sn1.0Ag0.5Cu composite solders prepared with ultrasonic agitation

To develop a lead-free solder with high strength and ductility, a novel interface was designed between the tetra-needle-like ZnO whisker (T-ZnOw) and Sn1.0Ag0.5Cu solder matrix. First, surface modification of T-ZnOw was conducted using a pyrolysis method based on self-assembly to deposit NiO nanoparticles densely and evenly on the whisker surface. The interfacial relationship between T-ZnOw (100) and NiO (101) planes was semi-coherent with a misfit of 0.145. In-situ transmission electron microscopy was used to explore the surface modification mechanism. Next, a composite solder of 0.3mass% NiO/T-ZnOw reinforced Sn1.0Ag0.5Cu was prepared with ultrasonic agitation, and the introduced reinforcement was uniformly distributed in the solder matrix. A thin NiO transition layer existed between the reinforcement and matrix, indicating the formation of the Sn1.0Ag0.5Cu/NiO/T-ZnOw sandwich structure interface. No gaps, pores, or new phases were formed at the interface, and atomic interdiffusion occurred at the interface to form effective bonding. The composite solder shows improved wettability compared with plain solder. The ultimate tensile strength (UTS) and elongation of the composite solder were 32.5 MPa and 28.1%, respectively, being 19.0% and 35.1% higher than those of the plain Sn1.0Ag0.5Cu solder. The strengthening-ductility mechanisms primarily included load transfer, grain refinement, and misfit dislocation effects.

Very thin (111) NiO epitaxial films grown on c-sapphire substrates by pulsed laser deposition technique

(111) NiO epitaxial layers are grown on c-sapphire substrates by pulsed laser deposition technique. Structural and morphological properties of the films are studied using in-plane as well as out-of-plane high resolution x-ray diffraction and atomic force microscopy techniques as functions of growth temperature, oxygen pressure and the pulses count of the laser. The study shows that continuous epitaxial films of thickness as low as 3 nm with high crystalline quality, smooth surface and interface morphology can be grown by this technique. The study also reveals the co-existence of 60°-rotated (111) triangular domains of NiO in the film. The study also evidences the presence of a very low density of 60° dislocations in these films. Density of screw and edge dislocations are also estimated to be quite low. It has been found that growth-temperature, oxygen partial pressure and the film thickness can influence differently the density of various dislocation types. These parameters are also found to affect significantly the strain developed in the films.

Fully Spin-Transparent Magnetic Interfaces Enabled by the Insertion of a Thin Paramagnetic NiO Layer

Spin backflow and spin-memory loss have been well established to considerably lower the interfacial spin transmissivity of metallic magnetic interfaces and thus the energy efficiency of spin-orbit torque technologies. Here, we report that spin backflow and spin-memory loss at Pt-based heavy metal-ferromagnet interfaces can be effectively eliminated by inserting an insulating paramagnetic NiO layer of optimum thickness. The latter enables the thermal magnon-mediated essentially unity spin-current transmission at room temperature due to considerably enhanced effective spin-mixing conductance of the interface. As a result, we obtain dampinglike spin-orbit torque efficiency per unit current density of up to 0.8 as detected by the standard technology ferromagnet FeCoB and others, which reaches the expected upper-limit spin Hall ratio of Pt. We establish that Pt/NiO and Pt-Hf/NiO are two energy-efficient, integration-friendly, and high-endurance spin-current generators that provide >100 times greater energy efficiency than sputter-deposited topological insulators BiSb and BiSe. Our finding will benefit spin-orbitronic research and advance spin-torque technologies.