Ni Sublayers Research Articles

Modification of magnetoelectric properties of thin film composite multiferroics of Fe10Ni90/PVDF type with using nanostructured Fe10Ni90 layer

In this work, we study the effectiveness of using nanostructuring of a magnetostrictive Fe10Ni90 layer in a film composite multiferroic of the [Fe10Ni90/Cu]p/Fe10Ni90/PVDF type in order to obtain the maximum magnetoelectric effect (ME) while maintaining the high magnetosensitivity characteristic of thin Fe10Ni90 films. In this case, such nanostructuring parameters as the number of periods p and the thickness LFeNi of the Fe–Ni sublayers were varied. The analysis of the dependences of the ME coefficient αE on the indicated parameters carried out in this work made it possible to identify the optimal configuration of the multiferroic. Thus, the work shows that in the non-resonant mode the highest ME coupling coefficient (αE = 4.2 V/cm∙Oe) is achieved for a structure with p = 4 and LFeNi = 200 nm in a saturation field of ∼50 Oe, and a close to linear change in αE is realized in the field range 6–15 Oe. At the same time, this structure retains a low coercive field Hc ∼ 4–5 Oe and pronounced uniaxial magnetic anisotropy in the film plane. These facts indicate the absence in the magnetic component of the multiferroic structure of an undesirable “supercritical” magnetic state, which appears in unstructured Fe10Ni90 films with equivalent thicknesses of the ferromagnetic layer and sharply worsens its magnetosensitivity. Thus, in the case of a homogeneous magnetic layer, the thickness of which is equivalent to the total thickness of the magnetic phase of a nanostructured multiferroic (1000 nm), a significantly lower value of αE = 1.6 V/cm∙Oe is achieved. The totality of the data obtained indicates the validity of using the nanostructuring method when optimizing the properties of film composite multiferroics. A detailed analysis of the dependences αE(p) and αE (LFeNi) together with the results of computer modeling made it possible to determine the main factors influencing the value of αE. In particular, it is shown that an increase in αE is primarily caused by an increase in the total volume of the magnetostrictive phase in the composite, as well as a weakening of the damping effect of the substrate, which occurs with an increase in the number of periods and the thickness of individual sublayers in the structure.

Investigation of Cap and Buffer Layer Effect in Co/Ni Thin Films by Ferromagnetic Resonance Technique

In this study, the magnetic properties of Si(100)/X5/(Co0.3/Ni0.5)3/Y5 (X: Pt, Cu and Y: Pt, Cu, all thicknesses are nm) multilayers were investigated using ferromagnetic resonance technique (FMR). In sample sets all layers (buffer, cap, and Co) were grown by magnetron sputtering while Ni sub-layers were grown by molecular beam epitaxy (MBE) at high vacuum. The effective magnetic anisotropy is 300 mT when copper is used as the buffer and cap layer, 290 mT when the buffer layer is copper, and the cap layer is Pt. On the other hand, it is seen that the effective magnetic anisotropy is 350 mT when Pt is used as buffer and cap layer, and 150 mT when Pt buffer and Cu cap layer are used. Furthermore, magnetic easy axis is out of plane when the Pt buffer layer is used, while the magnetic easy axis is parallel to the plane when the Cu buffer layer is used. The results show that the buffer and cap layers of Co/Ni thin films, which are frequently used in the field of spintronics influence the magnetic properties.

Effect of Si addition on the microstructure and oxidation behaviour of formed aluminide coating on HH309 steel by cast-aluminizing

Cast-aluminizing (CA) of HH309 stainless steel by Al-alloy sheets containing various amounts of silicon was studied in this research. To do this, the Al-alloy sheets were inserted at the bottom of a copper mould. Then, molten HH309SS was cast into the mould. The heat transfer from the steel melt enabled the melting of Al-alloy and the high-temperature reactions with the substrate elements to result in the intermetallic compounds (IMCs) at the HH309/Al interface. FE-SEM and XRD results indicated that the (Fe,Cr,Ni)2Al5 and (Fe,Cr,Ni)Al3 phases were detectable when pure Al sheet was used. In contrast, when the sheet contained silicon, a new intermetallic layer Al7(Fe,Cr)2Si appeared at the interface. To create the aluminide coating on the cast-aluminized specimens, they were annealed at 1000 °C for 2h. The multilayer coating was formed by inter-diffusion between aluminum and substrate elements during the heat treatment. This multilayer coating included (Fe,Cr,Ni)2Al5 + (Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sub-layers. To investigate the oxidation behaviour of the CA specimens with various Si contents, the isothermal oxidation tests were conducted at 1000 °C for 75 h. The oxidation rate constant, kp, was inversely proportional to the Si content in the aluminide layers. The kp values for CA samples with pure Al, 5, 8 and 12 wt% Si sheets were 1.5 × 10−11, 1.175 × 10−11, 6.39 × 10−12 and 4.36 × 10−12 g2 cm−4 s−1, respectively. The addition of Si significantly improved the high-temperature oxidation resistance of CA samples due to the formation of a combined Al2O3-SiO2 scale on the surface.

Characterization, growth kinetics and high-temperature oxidation behavior of aluminide coating formed on HH309 stainless steel by casting and subsequent heat treatment

In the present work, a novel technique has been introduced to obtain an aluminide coating by casting process and subsequent heat treatment. To do so, the aluminum sheet was placed at the bottom of a copper mold, then HH309 SS melt was poured into the mold. This technique was named Cast-Aluminizing (CA). The CA samples were heat-treated at the temperature range of 900–1050 °C for 0.5–5 h. The FE-SEM, XRD, and EDS were utilized to characterize the microstructure, phase analysis and chemical composition of cast-aluminized samples, respectively. Results showed that (Fe,Cr,Ni)Al3 and (Fe,Cr,Ni)2Al5 layers were formed at the Al/HH309 interface. FE-SEM analysis demonstrated a multi-layer aluminide coating on the heat-treated specimens. This coating consisted of (Fe,Cr,Ni)2Al5+(Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sub-layers. The growth kinetics investigation showed that the thickness of layers increased with the increase of the annealing temperature and time. The growth rate of layers obeyed a parabolic law. The activation energies for the growth of (Fe,Cr,Ni)2Al5+(Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers were about 203, 250 and 247 kJ/mol, respectively. Microhardness measurements revealed that (Fe,Cr,Ni)2Al5+(Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers had a hardness of about 820–1040, 580–710 and 380–470 HV, respectively. The resistance to oxidation of cast-aluminized and heat-treated (CA + HT) samples in the air at 1000 °C was studied. The CA + HT samples exhibited higher oxidation resistance than uncoated samples due to the formation of a protective Al2O3 layer on the surface.

Effect of deposition technique of Ni on the perpendicular magnetic anisotropy in Co/Ni multilayers

The perpendicular magnetic anisotropy (PMA) of Si/Pt 3.5/(Co 0.3/Ni 0.6)n /Co 0.3/ Pt 3 (all thicknesses are nm) multilayers were investigated for two different sample sets by using ferromagnetic resonance (FMR) and magnetooptic Kerr effect (MOKE) techniques. In the first sample set all layers (buffer, cap, Co and Ni) were grown by magnetron sputtering technique while in the second sample set Ni sub-layers were grown by molecular beam epitaxy (MBE) at high vacuum. Apart from deposition technique of Ni, all other parameters like thicknesses and growth rates of each layers are same for both sample sets. Multilayers in these two sample sets display PMA in the as grown state until a certain value of bilayer repetition (n) and the strength of PMA decreases with increasing n. Magnetic easy axis's of the multilayered samples switched from film normal to the film plane when n is 9 and 5 for the first and second sample sets, respectively. The reason for that, PMA was decreased due to increasing roughness with increasing n. This was confirmed by X Ray Reflectivity (XRR) measurements for both sample sets. Moreover, in the first sample set coercive field values are smaller than the second sample set, which means magnetic anisotropy is lower than the latter one. This stronger PMA is arising due to existence of stronger Pt (111) and Co/Ni (111) textures in the second sample set.

Gold-Metal (Ni, Cu) Deposited on the Titania Nanotubes as Electrocatalysts for Borohydride Oxidation

In the present work the optimization of the preparation of titania nanotube supported Au-Ni and Au-Cu catalysts (denoted as Au(Ni)/TiO2-NTs and Au(Cu)/TiO2-NTs) via the galvanic displacement technique is presented. The catalysts were prepared according the following procedures: i) preparation of TiO2 nanotube arrays by anodization of titanium sheets in an aqueous sulfuric acid solution containing some NH4F; ii) deposition of an electroless Ni or Cu sublayers on the synthesized TiO2 nanotube arrays [1-3] and iii) subsequent immersion of the Ni/TiO2-NTs or Cu/NiTiO2-NTs electrodes into the gold-containing solution for various time periods. The morphology and composition of the catalysts were examined by means of Field Emission Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, X-ray diffraction and Inductively Coupled Plasma Optical Emission Spectroscopy. The electrochemical behavior and electrocatalytic activity of the fabricated catalysts towards borohydride oxidation were measured by means of cyclic voltammetry. The electroactive area of gold surface created on fabricated catalysts was determined from cyclic voltammograms of Au/TiO2-NTs electrodes recorded in a deaerated 0.5 M H2SO4 solution at a scan rate of 10 mV s-1.The prepared Au(Ni)/TiO2-NTs and Au(Cu)/TiO2-NTs electrodes exhibited a high catalytic efficiency towards the oxidation of borohydride when compared to that of bulk Au electrode. A higher catalytic activity of the prepared Au(Ni)/TiO2-NTs and Au(Cu)/TiO2-NTs catalysts under various conditions may be related to production of smaller gold nanoparticles and their fine and uniform dispersion on the surface.The Au-Ni and Au-Cu catalysts deposited on the titania nanotube surface seem to be a promising anodic material for direct borohydride fuel cells. Acknowledgement This research was funded by a grant (No. ATE-08/2012) from the Research Council of Lithuania.

Characterization of the surface topography and nano-hardness of Cu/Ni multilayer structures

Abstract This article describes the results of a study of Cu/Ni multilayer coatings applied on a monocrystalline Si(100) silicon substrate by the deposition magnetron sputtering technique. Composed of 100 bilayers each, the multilayers were differentiated by the Ni sublayer thickness (1.2 to 3 nm), while maintaining the constant Cu sublayer thickness (2 nm). The multilayer coatings were characterized by assessing their surface topography using atomic force microscopy and their mechanical properties with nano-hardness measurements by the Berkovich method. The tests showed that the hardness of multilayers was substantially influenced by the thickness ratio of Cu and Ni sublayers and by surface roughness. The highest hardness and, at the same time, the lowest roughness was exhibited by a multilayer structure with a Cu-to-Ni sublayer thickness ratio of 2:1.5.

The influence of sublayer material on surface properties of electrodeposited nickel with periodical structures

Surface topography, hardness and microstructure of nickel coatings electrodeposited on Si master with a periodical structure have been studied depending on vacuum deposited sublayer material (Ni, Cu or Ag). It is shown that the quality of replication of silicon master in a nickel shim electrodeposited simultaneously on different sublayers is dependent on the material used. All types of the analyzed coatings enabled transfer of lateral dimensions and showed good replication quality of the tested periodical structures (2 μm period), while the structure replicated using the coatings deposited on the Ni sublayer exhibited the worst roughness. The hardness of the electrodeposited layers was found to be dependent on the sublayer material used as well as on the side of deposit. Despite the fact that the backside of nickel shim had the same hardness for all the sublayers used, it was found that the hardness of the working surface with periodical structure (the side that has been in contact with the vacuum deposited layer) is dependent on the sublayer material: the Ni and Cu sublayers increased the hardness of Ni coating, while for the nickel shim deposited on Ag sublayer the hardness was reduced.

Anisotropic strain-stress state and intermixing in epitaxial Mo(110)/Ni(111) multilayers: An x-ray diffraction study

The present study deals with the analysis of elastic strains and stresses in high-quality heteroepitaxial Mo/Ni superlattices with periods Λ lying in the range 4.8–27.6 nm. The strain-stress state in this lattice-mismatched system grown under energetic deposition conditions (ion beam sputtering) is rather complex, resulting from three contributions: (i) intrinsic (growth) stress due to atomic peening, (ii) coherency stresses of opposite sign in the two elemental layers due to the observed Nishiyama–Wassermann epitaxial relationship Ni[11¯0](111)∥Mo[001](110), and (iii) interfacial mixing. The measurement of the lattice parameters of Mo and Ni sublayers in various crystallographic directions was performed by x-ray diffraction, using the sin2 ψ method adapted for epitaxial layers. A large anisotropy of elastic strain and associated in-plane coherency stresses is revealed in the Mo sublayers, while for Ni sublayers no such behavior could be detected due to the superimposition of growth variants with threefold symmetry. Postgrowth ion irradiation with Ar ions at very low dose (∼0.2 dpa) was employed as a powerful tool to modify the intrinsic stress, thus providing additional data to be implemented in a triaxial strain-stress model, which enabled us to separate the different stress sources (intrinsic and coherency stresses) as well as to quantify the intermixing occurring during growth. This model, which has been successfully applied previously to Mo thin films, yields in the case of multilayer systems to the determination of the “stress-free and defect-free” lattice parameter, a0, i.e., solely linked to chemical mixing. The linear dependence of a0 with Λ observed in both sublayers reveals an interface-mediated chemical mixing mechanism, the extent of this interfacial mixing being much more pronounced in Mo sublayers than in Ni ones.

Plastic deformation in bi-metal multilayer nanowires

Bi-metal multilayer nanowires attract much more attention in recent years because of potential applications in NEMS. In this work, the tensile loading process is simulated by molecular dynamics method on the Cu–Ni bi-metal multilayer nanowires to investigate their mechanical behaviors. According to the stress–strain curves as well as accompanying microstructural evolution, we focus on analyzing the influence of coherent interfaces on the plastic deformation of the nanowires. These results show that the interfacial stress has considerable effect on the plastic behaviors, which is extraordinarily sensitive to the local regions. Specifically, the interfacial stress may accelerate the destructive lattice distortion in Cu sublayers, while it will retard the plastic deformation in Ni sublayers. In such a case, the plastic deformation takes place in Cu sublayers more easily than in Cu nanowires, and then propagates into Ni sublayers. This should be emphasized in the design of NEMS.

Stress and Intermixing in Epitaxial Ni(111)/Mo(110) Superlattices

The stress state and intermixing in epitaxial Ni/Mo multilayers grown on (11 2 0) sapphire substrates are investigated using X-ray Diffraction (XRD). Two deposition techniques were used, namely ion beam sputtering (IBS) and magnetron sputtering (MS), to vary the energy of the deposited species. In both cases, high-quality superlattices with a Nishiyama-Wasserman epitaxial relationship Ni [110] (111) // Mo [001] (110) were obtained. The residual stress state appears rather complex, resulting from two contributions: a growth-stress whose magnitude and sign depend on growth conditions and coherency stresses of opposite signs in the two elemental sublayers (tensile for Ni and compressive for Mo). Post-growth ion irradiation at low fluences was used to induce structural changes in a controlled way. For the case of IBS, it resulted in partial stress relaxation, as the growth stress could be almost fully relaxed, while the coherency stresses remained unchanged. For the case of MS, a distinct behavior was found: a stress increase of the tensile component of Mo-sublayers was observed, while a stress reduction of the compressive component was noticed. We attribute this phenomenon to ion irradiation induced intermixing. For the Ni sublayers, this intermixing leads to a stress relaxation. The modeling of the stress evolution during ion irradiation was performed using a triaxial stress analysis which enabled us to determine the ‘stress-free and defect-free lattice parameter’, solely linked to chemical effect.

Influence of Mo in the Ni sublayers on the magnetization of Ni/Ti neutron supermirrors

For a polarized neutron beam in a neutron guide with a polarization parallel to the side walls of the guide, depolarization of the beam can occur at the top and bottom walls due to the in-plane magnetization of the Ni/Ti supermirrors. We alloyed the Ni-sublayers with Mo in order to suppress the magnetization of the layers. Magnetization measurements of the NiMo/Ti supermirrors indicate a Mo-dependency on the in-plane magnetization. For increasing Mo concentration the magnetization is decreased. Polarized neutron reflectivity measurements show no depolarizing for in-plane magnetization parallel to the neutron spin as normally is the case in the side walls of a neutron guide. A polarized neutron beam with polarization perpendicular to the magnetization of the layers is partially depolarized.

Novel monochromator design

Multilayer monochromators can be used in neutron scattering experiments and for synchrotron radiation. Sharp interfaces between the sublayers give higher-order contributions. We created multilayers, consisting of Ni and Ti sublayers, with non-sharp scattering length density profiles at the interfaces to reduce this contributions. The smoothened shape functions have been achieved by introducing intermediate sublayers at the interfaces. The influence of annealing has been investigated with respect to roughness and smoothness of the shape function. Decrease in total reflected intensity for higher-order terms has been observed in reflectivity measurements.

Solid-state reaction in Ti/Ni multilayers studied by magneto-optical and optical spectroscopies, and X-ray diffraction

Comparative study of the solid-state reaction (SSR) in a series of Ti/Ni multilayered films (MLF) with a bilayer period of 0.65-22.2 nm and a constant Ti to Ni sublayer thickness ratio has been performed by using the experimental and computer-simulated magneto-optical (MO) and optical spectroscopies as well as X-ray diffraction (XRD). It was shown that alloyed-like regions in an amorphous structure is spontaneously formed near the interfaces between pure elements during the film deposition. The thickness of this region was estimated as 2-3.8 nm on the basis of the MO and optical studies. The SSR in the Ti/Ni MLF caused by an annealing at 580 K for 60 min increases the thickness of these interfacial amorphous regions. It was shown that SSR takes place mainly in the Ti/Ni MLF with relatively “thick” sublayers. The existence of a threshold nominal Ni-sublayer thickness for observing the equatorial Kerr effect of about 3.0 and 4.5 nm for the as-deposited and annealed Ti/Ni MLF, respectively, is explained by formation of the nonmagnetic alloyed regions between pure components during the film deposition as a result of the SSR. For the case of Ti/Ni MLF, the MO and optical approaches turn out to be more sensitive in determining the thickness of the reacted zone, while XRD is more useful for the structural analysis. It was also shown that the very thin nonreacted Ni sublayers have different MO properties (and hence electronic structure) from the bulk.

Corrosion behaviour of epoxy coatings electrodeposited on galvanized steel and steel modified by Zn–Ni alloys

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on hot-dip galvanized steel and steel modified by Zn–Ni alloys were investigated during exposure to 3% NaCl solution. Zn–Ni alloys were electrodeposited on steel by direct and pulse current. From the time dependence of pore resistance, coating capacitance and relative permittivity of epoxy coating, diffusion coefficient of water through epoxy coating, D(H 2O) and thermal stability, it was shown that Zn–Ni sublayers significantly improve the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of investigated time for epoxy coatings on steel modified by Zn–Ni alloys, indicating the great stability of these protective systems, due to the existence of the inner oxide phase layer and the outer layer consisting of basic salts.

Magneto-optical spectroscopy study of the solid-state reaction in Ti/Ni multilayered films

A comparative study of the solid-state reaction (SSR) in a series of Ti/Ni multilayered films (MLF) with a bilayer period of 0.65–22.2 nm and constant Ti to Ni sublayer thickness ratio has been performed by magneto-optical (MO) spectroscopy as well as x-ray diffraction (XRD). The spectral and sublayer-thickness dependences of the MO properties of the Ti/Ni MLF were explained on the basis of the electromagnetic theory. A threshold nominal Ni-sublayer thickness of about 3 nm in the as-deposited Ti/Ni MLF is necessary for observing the equatorial Kerr effect. Such a fact is explained by the formation, by solid-state reaction, of nonmagnetic alloyed regions between pure components. The SSR in the Ti/Ni MLF caused by the low temperature annealing leads to a formation of an amorphous Ti/Ni alloy and takes place mainly in Ti/Ni MLF with “thick” sublayers. In the case of Ti/Ni MLF, MO turns out to be more sensitive in determining the thickness of the reacted zone, while XRD is more useful for the structural analysis. It was also suggested that the very thin nonreacted Ni sublayers have MO properties different from the bulk.

Interdependence of elastic strain and segregation in metallic multilayers: An x-ray diffraction study of (111) Au/Ni multilayers

This study concerns the analysis of elastic strains in Au/Ni multilayers whose periods lie in the range 1–5 nm. Lattice parameter measurements have been performed by x-ray diffraction in various directions. The results are interpreted in terms of elastic strain and interfacial mixing. It is shown that considerable strains (several percent) are encountered within these very thin layers. In the Au sublayers, coherence is never observed with the underlying Ni film and the residual strain relaxes progressively as the Au film thickens. In the Ni sublayers, interfacial mixing controls the lattice parameter and the elastic strain is a function of this mixing. Out of equilibrium mixing of Au into Ni is ascribed to dynamic segregation during the growth of Ni on Au. All these results bring new insights on the still open question of stress relaxation mechanisms in ultrathin films.

AMORPHIZATION REACTION PROCESS IN Ni/AMORPHOUS Si MULTILAYER

Solid state amorphization reaction process in Ni/amorphous Si multilayers has been quanlitatively studied by using in situ X-ray diffraction. An amorphous formation and growth model is suggested for elucidating the solid state reaction in the Ni/amorphous Si multilayers. Thermodynamic and kinetic interpretations for the amorphization reaction at grain boundaries in Ni sublayers are presented.

Structure and Magnetic Properties of Ni/Ti Multilayers

We report a study of structure and magnetic properties of Ni/Ti multi- layers with different modulation wavelength, 10 nm < a < 40 nm, and con- stant sublayers thickness ratio dNi/dTi = 1. Specimens were characterized with X-ray diffraction, vibrating sample magnetometer and torque meter in as-deposited state and in successive stages of isothermal annealing at 423 K. Hysteresis curves of as-deposited samples revealed the typical feature of fer- romagnetic films with predominant shape anisotropy. During annealing very pronounced changes in the hysteresis loops were observed. An analysis of the loops indicates the occurrence of distinct changes in magnetic anisotropy of Ni sublayers: their effective shape anisotropy is drastically reduced due to a development of the perpendicular anisotropy and/or a modification of the microstructure. The effect can possibly be applied to achieve desirable varia- tion of perpendicular anisotropy in multilayers which consist of Ni sublayers. PACS numbers: 75.50.Kj, 75.70.—i The transition of polycrystalline metallic multilayers (Mls) to amorphous

Structure and magnetic properties of Fe1−xNix/Cu Invar superlattices

We have prepared by sputtering techniques a series of fcc [Fe1−xNix/Cu]×10 superlattices with sublayer thicknesses of 3 nm and with Ni concentrations x ranging from 0.26 to 0.54. The use of MgO single-crystal substrates and Cu sublayers in the superlattice growth ensures a well-defined fcc crystal structure in the Fe–Ni sublayers with a Ni concentration as low as 26 at. % and down to liquid helium temperatures. The magnetization of the Fe–Ni sublayers in the superlattices starts to deviate from the well-known Slater–Pauling curve at 40 at. % Ni, and continues to drop until the fcc–bcc transition is completed. A strong dependence of the magnetization on temperature was also observed for the Fe–Ni sublayers in the Invar range, consistent with the behavior of bulk Fe–Ni Invar alloys.