Ni Overlayers Research Articles

Low-voltage magnetoelectric coupling in membrane heterostructures.

Strain-mediated magnetoelectric (ME) coupling in ferroelectric (FE)/ferromagnetic (FM) heterostructures offers a unique opportunity for both fundamental scientific research and low-power multifunctional devices. Relaxor-FEs, such as (1 − x)Pb(Mg1/3Nb2/3)O3-(x)PbTiO3 (PMN-xPT), are ideal FE layer candidates because of their giant piezoelectricity. However, thin films of PMN-PT suffer from substrate clamping, which substantially reduces piezoelectric in-plane strains. Here, we demonstrate low-voltage ME coupling in an all-thin-film heterostructure that uses the anisotropic strains induced by the (011) orientation of PMN-PT. We completely remove PMN-PT films from their substrate and couple with FM Ni overlayers to create membrane PMN-PT/Ni heterostructures showing 90° Ni magnetization rotation with 3 V PMN-PT bias, much less than the bulk PMN-PT ~100-V requirement. Scanning transmission electron microscopy and phase-field simulations clarify the membrane response. These results provide a crucial step toward understanding the microstructural behavior of PMN-PT thin films for use in piezo-driven ME heterostructures.

Magnetoelectric Coupling by Piezoelectric Tensor Design

Strain-coupled magnetoelectric (ME) phenomena in piezoelectric/ferromagnetic thin-film bilayers are a promising paradigm for sensors and information storage devices, where strain manipulates the magnetization of the ferromagnetic film. In-plane magnetization rotation with an electric field across the film thickness has been challenging due to the large reduction of in-plane piezoelectric strain by substrate clamping, and in two-terminal devices, the requirement of anisotropic in-plane strain. Here we show that these limitations can be overcome by designing the piezoelectric strain tensor using the boundary interaction between biased and unbiased piezoelectric. We fabricated 500 nm thick, (001) oriented [Pb(Mg1/3Nb2/3)O3]0.7-[PbTiO3]0.3 (PMN-PT) unclamped piezoelectric membranes with ferromagnetic Ni overlayers. Guided by analytical and numerical continuum elastic calculations, we designed and fabricated two-terminal devices exhibiting electric field-driven Ni magnetization rotation. We develop a method that can apply designed strain patterns to many other materials systems to control properties such as superconductivity, band topology, conductivity, and optical response.

Fabry–Perot magnonic ballistic coherent transport across ultrathin ferromagnetic lamellar bcc Ni nanostructures between Fe leads

We propose thermodynamically stable systems of ultrathin lamellar bcc Ni nanostructures between bcc Fe leads, Fe[Ni(n)]Fe, based on the available literature for bcc Ni overlayers on Fe(001) surfaces, and establish the necessary criteria for their structural and ferromagnetic order, for thicknesses n ≤ 6 bcc Ni monatomic layers. The system is globally ferromagnetic. A theoretical model is presented to investigate and understand the ballistic coherent scattering of Fe spin-waves, incident from the leads, at the ferromagnetic bcc Ni nanostructure. The NiNi and NiFe exchange are computed using the Ising effective field theory (EFT), and the magnetic ground state of the system is constructed in the Heisenberg representation. We compute the spin-wave eigenmodes localized on the bcc Ni nanostructure, using the phase field matching theory (PFMT), illustrating the effects of symmetry breaking on the confinement of localized spin excitations. The reflection and transmission scattering properties of spin-waves incident from the Fe leads, across the embedded Ni nanostructures are investigated within the framework of the same PFMT methodology. A highly refined Fabry–Perot magnonic ballistic coherent transmission spectra is observed for these Fe[Ni(n)]Fe systems.

Electroless nickel plating on polymer particles

Near-monodisperse, micrometer-sized polypyrrole–palladium (PPy–Pd) nanocomposite-coated polystyrene (PS) particles have been coated with Ni overlayers by electroless plating in aqueous media. Good control of the Ni loading was achieved for 1.0μm diameter PPy–Pd nanocomposite-coated PS particles and particles of up to 20μm in diameter could also be efficiently coated with the Ni. Laser diffraction particle size analysis studies of dilute aqueous suspensions indicated that an additional water-soluble colloidal stabilizer, poly(N-vinyl pyrrolidone), in the electroless plating reaction media was crucial to obtain colloidally stable Ni-coated composite particles. Elemental microanalysis indicated that the Ni loading could be controlled between 61 and 78wt% for the 1.0μm-sized particles. Scanning/transmission electron microscopy studies revealed that the particle surface had a flaked morphology after Ni coating. Spherical capsules were obtained after extraction of the PS component from the Ni-coated composite particles, which indicated that the shell became rigid after Ni coating. X-ray diffraction confirmed the production of elemental Ni and X-ray photoelectron spectroscopy studies indicated the existence of elemental Ni on the surface of the composite particles.

Surface structure of nickel oxide layers on a Rh(111) surface

The formation of nickel oxide nanolayers by oxidizing Ni overlayers on Rh(111) has been investigated and their structures are reported as a function of the nickel coverage and oxygen pressure. Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and diffraction (XPD), and high-resolution electron energy loss spectroscopy (HREELS) have been applied to characterize the structure and stoichiometry of the nickel oxide nanolayers. Several different phases have been observed depending on the strain state of the metallic Ni overlayers. For the pseudomorphic Ni monolayer, two distinctly different oxide phases with (6×1)-Ni5O5 and (2√3×2)-Ni8O10 structures have been identified at oxygen-poor (p=5×10−8mbar) and oxygen-rich (p≥1×10−6mbar) conditions, respectively. Above one monolayer, where the Ni layers are relaxed, bulk-like NiO(100) films form at the O-rich conditions, whereas chemisorbed-type p(2×2)ONi(111) layers develop in the O-poor regime. X-ray photoelectron diffraction analysis has provided additional insight into the relaxation mechanism and the detailed atomic structure of the Ni-oxide nanolayers.

Evaluating Overpotentials in GDC Electrodes for H2/H2O Reactions in Solid Oxide Electrochemical Cells

Thin-film GDC (Ce0.8Gd0.2O2-x 300 nm thick) electrodes with porous Ni or Au metal overlayers (300 nm thick) were sputter-deposited on polycrystalline YSZ substrates and tested in single-chamber cells. With a back-side Pt-counter electrode, the electrochemical cells were characterized by linear-sweep voltammetry and impedance spectroscopy between 650 to 750 ºC in mixtures of H2/H2O/Ar diluent. To complement the electro-chemical tests, ambient-pressure XPS at the Advanced Light Source was performed on active cells in H2/H2O mixtures to measure surface potential and oxidation states. The GDC electrodes showed lower resistance to H2O electrolysis than H2 oxidation with Ni and Au overlayers. XPS measurements indicated large overpotentials across the YSZ/GDC interface particularly for H2 oxidation on the GDC electrode. Increasing PH2/PH2O (up to 10) resulted in higher rates for not only H2 oxidation, but also H2O electrolysis, suggesting that H2O electrolysis activity is enhanced by increased reduction of the GDC surface.

Effects of substrate on the melting behavior of Cd arachidate Langmuir–Blodgett films

Nineteen monolayered Cd arachidate films were deposited on float glass substrate coated with Si and Ni over-layers. Two layers have been chosen with very different surface free energies. Melting behavior of films were studied using variable temperature X-ray specular reflectivity and Fourier transform infrared spectroscopy measurements. In conformity with earlier studies, melting of the multilayer precede by a transition from distorted hexagonal to hexaticlike phase. However, the transition temperature to hexaticlike phase as well as the melting temperature depend significantly on the type of layer. Both the transition temperatures are higher for the multilayer deposited on Ni layer as compared to those for the film deposited on Si layer. These results can be understood in terms of different surface free energies of Ni and Si layers. Further, in case of Ni layer, transition to hexaticlike phase is relatively sharper. Even in the molten state there is a significant difference in the structure of the Cd arachidate film on two layers; packing density of molecules in molten state is lower in case of substrate with higher surface free energy. These results suggest that the surface free energy of substrate plays a significant role in melting behavior of Langmuir–Blodgett films.

Overlayer thickness determination by XPS using the multiline approach

AbstractAn important analytical application of x‐ray photoelectron spectroscopy is the possibility of determining the overlayer thickness in the nanometer range. The relevant procedure should be based on a realistic theoretical model relating the photoelectron signal intensities, measured for the overlayer and for the substrate, to the overlayer thickness. In the most frequently used procedure, we derive the thickness from the ratio of intensity measured for the overlayer and intensity measured for the substrate. We may expect that the resulting thickness depends on the pair of intensities selected for analysis. This is due to the fact that each calculated intensity has a certain systematic error associated with accuracy of parameters used in calculations.We consider here a possibility of using more than two intensities, for a given element, in the procedure of thickness determination. In this way, we expect that the analysis is more accurate due to the fact that the systematic errors are averaged. In the past, similar approach has been successfully applied to the procedure of surface composition determination. To test the proposed multiline approach, the Au 4f, Ni 2p, and Ni 3p intensities were measured for a series of Ni overlayers deposited on the Au substrate. It has been found that the proposed approach well performs at relatively large overlayer thicknesses. It provides values that are averages of values obtained from other methods. At small overlayer thicknesses, we observe a scatter of thicknesses obtained from different methods which can be due to the discontinuity of the overlayer. Copyright © 2008 John Wiley & Sons, Ltd.

Ab initiostudy of Co and Ni under uniaxial and biaxial loading and in epitaxial overlayers

A detailed theoretical study of structural and magnetic behaviors of cubic cobalt and nickel along the bcc-fcc (Bain) transformation paths as well as of hcp cobalt and nickel loaded uniaxially along the [0001] direction at various atomic volumes is presented. The total energies are calculated by spin-polarized full-potential linearized augmented plane-wave method within the generalized gradient approximation and are displayed in contour plots as functions of tetragonal or hcp $c/a$ ratio and atomic volume; the borderlines between the ferromagnetic and nonmagnetic phases are shown. Stability of possible ferromagnetic phases of bcc nickel is analyzed. The calculated contour plots are used to explain and predict the lattice parameters and magnetic states of tetragonal and hcp cobalt and nickel overlayers on various (001) or (111) substrates, respectively. In case of tetragonally deformed structures, the stresses needed to keep the thin films coherent with the substrates are also determined and all Co and Ni overlayers on (001) cubic substrates are predicted to be ferromagnetic. The agreement of available experimental data for Co and Ni overlayers with the results of bulk calculations is remarkable and suggests that the geometrical effect of the substrate, i.e., imposing the lattice dimensions of the substrate in the plane of the film to the film material, is one of the most important factors determining the structure and properties of the film. In this way, the lattice parameters of Co and Ni overlayers may be very well understood in terms of properties of appropriately deformed bulk Co and Ni.

Electronic structure modifications and HER of annealed electrodeposited Ni overlayers on Mo polycrystalline surface

The nature, the chemical state and electrocatalytic activity of Ni overlayers electrochemically prepared on a polycrystalline Mo surface were studied by means of X-ray and UV photoelectron spectroscopies (XPS, UPS) and steady state potentiostatic polarization measurements for the H2 evolution reaction in 1 M NaOH solution. Upon cathodic polarization, a Ni(OH)2-α phase adlayer is formed by precipitation which is gradually reduced by annealing under UHV (ultra-high vacuum) conditions forming a new Ni–Mo intermetallic phase at temperatures as low as 670 K. This was confirmed both by following the Ni 2p core-level shift and the appearance of new features in the UV spectrum thus denoting the modification of the electronic structure of Ni due to the formation of a common strongly hybridised Ni–Mo electronic state. The steady state polarization measurements revealed that sub-monolayer deposition of Ni on the Mo surface (0.8 ML) with subsequent heating at 870 K results in the threefold enhancement of its electrocatalytic activity for the hydrogen evolution reaction (HER) with respect to the Mo polycrystalline foil and the corresponding increase in its desorption kinetic constants in comparison to both Ni and Mo polycrystalline surfaces. As has been recorded for the first time by UPS, in the case of Ni–Mo, this can be attributed to the downshift of the d-band center of the Ni–Mo intermetallic phase, thus resulting in a weaker bonding strength of Had with respect to its adsorption on the Ni foil.

Core-level spectroscopy of the Ni/W(1 1 0) interface: Correlation of W interfacial core-level shifts with first-layer Ni phases

We have measured W 4f 7/2 core-level photoemission spectra from W(1 1 0) in the presence of Ni overlayers, from ∼0.2 to ∼3 monolayers. Interfacial core-level shifts associated with first-layer Ni phases have been identified: −230 ± 15 meV for the 1 × 1 pseudomorphic phase and −70 ± 7 meV for the 7 × 1 close-packed commensurate phase. At higher Ni coverages the interfacial core-level shift is −100 ± 10 meV. These shifts are analyzed using the partial-shift model of Nilsson et al. [Phys. Rev. B 38 (1988) 10357]; the analysis indicates that the difference in binding energies between the 1 × 1 and 7 × 1 phases has a large contribution from structural differences between the two phases.

Tailoring the Surface Reactivity: Comparison of Pd/Nb(110) and Rh/Nb(110)

Ni, Pd and Pt overlayers deposited on many metallic surfaces show properties resembling those of noble metals. We pose the question whether a similar trend might occur also for other transition-metal overlayers. To this goal, we perform first-principles density-functional theory calculations for Pd(111), Rh(111) surfaces, Pd and Rh epitaxial monolayers deposited on Nb(110), and for CO chemisorption on these systems. Density functional calculations indicate that the behavior of the two overlayers is quite different. Whereas the Rh overlayer on Nb(110) resembles the Rh(111) surface, for the Pd overlayer the electronic structure around the Fermi level is strongly affected by hybridization with Nb electrons, which accounts for unique properties of the overlayer. We expect that the latter mechanism may be of importance just for Pd, Pt, Ni and not for other transition metals with lower d-electron occupation.

Ni deposition on the pentagonal surface of an icosahedral Al–Pd–Mn quasicrystal

We have evaporated Ni on the pentagonal surface of an icosahedral Al–Pd–Mn quasicrystal kept at room temperature. At the initial stage of growth, Ni intermixes with the substrate surface. Subsequently, Al from the quasicrystal matrix migrates to growing layers. The modified chemical composition in an initially icosahedral region near the surface induces a structural transformation. An Al–Pd–Mn alloy is formed which consists of five cubic domains with dimensions in the nm-range exposing their (1 1 0) faces parallel to the surface. These domains are azimuthally rotated by 2π/5 with respect to each other and aligned with symmetry directions of the icosahedral substrate. Al–Mn–Ni, Al–Ni, and Ni overlayers adopt both structure and orientation of these domains which stabilises Ni in a novel body-centred cubic phase. Ni-rich overlayers exhibit out-of-plane magnetic ordering.

Thin layers of Fe, Co and Ni on V 2O 3[formula omitted] and V 2O 3 (0 0 0 1): A comparison of the interfacial magnetic interactions

Magnetization reversal of thin Fe, Co and Ni layers deposited on single-crystalline layers of V 2O 3 is shown to be governed by various contributions to the magnetic anisotropy of the ferromagnetic layers, which reflect the sensitivity of the systems to the crystallographic and magnetic moment orientation or the structural morphology of the interfaces. The complex behaviour observed is related to the rich phase diagram of the oxide, which undergoes a transition from a paramagnetic metallic to an insulating antiferromagnetic phase as the temperature is decreased to below ∼150 K. Focussing on the anisotropy contributions that result from the exchange interaction across the interface with antiferromagnetic V 2O 3, we have found a remarkable behaviour of the Fe overlayers which grow in a textured BCC(1 1 0) structure. The exchange anisotropy, probed as a bias of the ferromagnetic hysteresis loops for field-cooled samples, is absent for the interface with the ( 1 1 2 ¯ 0 ) surface of antiferromagnetic V 2O 3, but exists for the interface with the V 2O 3 (0 0 0 1) surface. This points to a very different electronic interaction at the two interfaces. The behaviour is at variance with what is observed for the Co and Ni overlayers which show an exchange anisotropy for both surfaces of the oxide. Measurements of X-ray magnetic circular dichroism reveal the presence of a magnetically dead layer in Fe at the interface with V 2O 3 ( 1 1 2 ¯ 0 ) . It is likely to disrupt the exchange coupling, and thus may be at the origin of the missing exchange anisotropy of Fe on this surface. The different evolution of the spin magnetic moment of Fe with the overlayer thickness suggests a different growth morphology on the two surfaces of the oxide. As a result of the interaction with Fe, the V 2O 3 layers acquire a small magnetic polarization at the interface at room temperature for both orientations.

The manufacturing of an electroplated Ni layer on textured Cu substrate for Cu-based HTScoated conductors

A sharp cube textured Ni overlayer on Cu substrates has been developed for themanufacturing of long-length RABiTS-based coated conductor tapes. Using a low-cost,non-vacuum and easily scalable technique of electroplating, smooth, crack-freeand continuous Ni overlayers were deposited on cube textured Cu substrateswithout any intermediate layers. In addition, sharp cube textured Sm-dopedCeO2 buffer layers have been grown on the Ni-plated Cu substrates using pulsed laserdeposition and found to exhibit in-plane and out-of-plane FWHM values of6.50° and 5.25°, respectively. This electroplating process promises an efficient route for manufacturingCu-based HTS coated conductors.

Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces III. Ni on TiO2 and Cu on TiO2

The electronic states of the Ni overlayers on TiO2(001) surface and Cu overlayers on TiO2(110) surface have been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. In the Ni/TiO2(001) system, an in-gap emission from the reduced Ti3+ 3d1 state appears at 1.2 eV below EF for the initial Ni deposition. Upon further deposition, the emission grows and is evolved to a spectrum of Ni metal, with forming no Fermi edge, suggesting the formation of small Ni clusters on TiO2(001). In the Cu/TiO2(110) system, Cu grows in a Stranski–Krastanov mode. Two in-gap emissions from the reduced Ti3+ 3d1 state and oxidized Cu+ 3d10 or Cu2+ 3d9 state are observed at 0.8 and 2.8 eV below EF, respectively, for the initial Cu deposition, giving convincing evidence that there is significant charge transfer between Cu and the substrate. Upon further deposition, the emission in the band-gap region is evolved to a spectrum of Cu metal, though delayed completion of the Fermi edge is observed. Such delay can be explained by finite cluster size effects.

Stability and magnetism of Ni–Fe alloyed overlayers on Fe(001)

The surface ordered alloy formation of Ni/Fe(001) overlayer system is explored, using the tight-binding linear muffin-tin orbital (TB-LMTO) method. The alloy formation for one- and two- surface layer of Ni–Fe on Fe(001) are considered by calculating the total and surface formation energies for different configurations of Ni overlayers on Fe(001). The preferred ground states for different configurations are determined and Ni (alloyed with Fe) is found to remain at the Fe surface. The magnetic profile for these configurations is found comparable to the experimental predictions.

Optical reflectance anisotropy of Al(110): Experiment andab initiocalculation

The reflectance anisotropy of a clean Al(110)-vacuum interface has been measured experimentally, and compared with the results of an ab-initio self-consistent full-potential linear augmented plane-wave calculation. Good agreement is found in the interband regime, while a discrepancy in the Drude regime is consistent with anisotropy in the intraband contribution. The calculated surface dielectric anisotropy and reflectance anisotropy of Al(110) are not significantly affected by surface relaxation, suggesting that optical transitions between surface states and surface resonances make only a minor contribution to the surface dielectric response. The measured reflectance anisotropy of Al(110) is found to be only weakly affected by oxidation and by the deposition of Ni overlayers, providing further evidence in support of this conclusion.

Thermal stability of electrodeposited nickel on vanadium: evidence for oxygen diffusion and intermetallic phase formation

The thermal stability of Ni overlayers electrochemically prepared on a polycrystalline vanadium (V) surface, was studied by means of X-ray and UV photoelectron spectroscopies (XPS, UPS). The results showed that upon cathodic polarization, a mixed Ni–Ni(OH) 2 adlayer is formed, while V surface becomes partially oxidized into vanadium oxide (V 2O x ). The applied potential during nickel electrodeposition is crucial for both Ni–Ni(OH) 2 composition and V 2O x film thickness. Annealing the Ni–Ni(OH) 2/V 2O x interface under UHV conditions leads to the gradual reduction of the films and the formation of a new Ni–V intermetallic phase. A solid state reaction accompanies the decomposition of nickel and vanadium oxides resulting in oxygen migration from the Ni adlayer to the V support. The electronic structure of Ni–V intermetallic phase formed after oxide decomposition is compared to previously reported results for Ni alloys and intermetallic compounds.

Ni overlayers on biaxially textured Ni-alloy and Cu substrates by DC sputtering

Use of many Ni-based and Cu-based alloys requires deposition of a Ni overlayerprior to the deposition of the seed layer in order to obtain better epitaxy. In thiswork, Ni overlayers have been deposited by DC sputtering on biaxially texturedNiW and Cu substrates. Films were characterized by x-ray diffraction and atomicforce microscopy as a function of temperature, film thickness and depositionrate. The optimum temperature range for deposition was found to be around400–500 °C, which results in relatively smooth films with a good cube texture on both substrates.