Ni Layer Thickness Research Articles

Magnetic Correlations in Some FeNi/Cu(100) Multilayers

We present a first-principles electronic structure based study of magnetic correlations in the paramagnetic phase of some Fe and Ni multilayers on Cu(100) substrates. We have used the screened Korringa–Kohn–Rostoker method to calculate the electronic structure within the spin-density functional theory and used the disordered local moments model to describe the paramagnetic phase above the Curie temperature. We find that in the Ni/Cu(100) films, there are no local moments formation for any thickness. For the multilayers of Fe and Ni, we find that there is some, although very small, local moment formation in the interior of the Ni film if the Ni layer is more than 6 monolayers thick. However, the Ni layers facilitate the magnetic correlation between the Fe layers. The Curie temperatures of the multilayers also depend upon the Ni layer thickness.

Ultrathin epitaxial Ni-silicide contacts on (1 0 0) Si and SiGe: Structural and electrical investigations

The formation of epitaxial NiSi2 and NiSiGe layers is investigated. On Si(100) epitaxial NiSi2 is obtained at T>400^oC with a Ni layer thickness below 3nm, while polycrystalline NiSi layer is produced with a 5nm thick Ni layer. With increasing silicidation temperature, epitaxial NiSi2 layers show a higher single crystalline quality, a lower resistivity and a lower Schottky barrier height to electrons. We highlight the extremely low contact resistivity of epitaxial NiSi2, which is about one order of magnitude lower than that of NiSi on both, As and B doped SOI. Epitaxial NiSiGe is also obtained on (100) SiGe employing an Al interlayer mediated epitaxy growth method. The Al atoms from the original 3nm interlayer diffused towards the surface during annealing. The incorporation of some Al increases the transition temperature from the Ni-rich germano-silicide phase to the mono-germano-silicide phase. The formed epitaxial NiSi0.7Ge0.3 shows an orthorhombic structure with a (101) base plane rotated by 45^o with respect to the (100) Si0.7Ge0.3 substrate.

Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies

The magnetization reversal processes are discussed for exchange-coupled ferromagnetic hard/soft bilayers made from Co0.66Cr0.22Pt0.12 (10 and 20 nm)/Ni (from 0 to 40 nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect, consisting of a shift of the in-plane minor hysteresis loops along the field axis, was observed at room temperature after in-plane saturation. This effect was associated with specific ferromagnetic domain configurations experimentally determined by polarized neutron reflectivity. On the other hand, perpendicular exchange bias effect was revealed from the out-of-plane hysteresis loops and it was attributed to residual domains in the magnetically hard layer.

Graphene ultrathin film electrode for detection of lead ions in acetate buffer solution

Few-layer graphene ultrathin films were synthesized via solid-state carbon diffusion from amorphous carbon (a-C) thin layers sputtering coated on Si substrates with or without a SiO2 layer, which an a-C layer was covered by a nickel (Ni) layer as a catalyst. When the Ni/a-C bilayer coated samples were heated at 1000°C the carbon (C) atoms from the a-C layers diffused into the top Ni layers to form a C rich surface. Upon rapid cooling, the C atoms accumulated on the surface of the Ni layers and formed graphene ultrathin films through nucleation and growth processes. The formation of graphene ultrathin films was confirmed by Raman spectroscopy, high resolution transmission electron microscopy (HR-TEM), electron diffraction, field-emission scanning electron microscopy (FE-SEM) and 4-point probe. The synthesized graphene ultrathin films were used as working electrodes for detection of trace heavy metal ions (Pb2+, as low as 7nM) in acetate buffer solutions (pH 5.3) using square wave anodic stripping voltammetry (SWASV). The effects of substrate surface condition and Ni layer thickness on the structure and electrochemical properties of graphene ultrathin film electrodes were investigated in detail. Compared to conventional diamond-like carbon (DLC) electrodes, the graphene electrodes developed in this study had better repeatability, higher sensitivity and higher resistance to passivation caused by surface active species.

Study on the microstructure and mechanical properties of multilayer Cu/Ni composite processed by accumulative roll bonding

In the present research, Cu/Ni multilayer composite was produced by accumulative roll bonding (ARB) process. The microstructure evolution and mechanical properties of the multilayered Cu/Ni composites were evaluated within different cycles of ARB process. It was observed that as the number of ARB cycles increased, the thickness of Ni layers was decreased and synchronous necking and fracturing occurred. After six ARB cycles, a multilayer Cu/Ni composite with homogeneously distributed fragmented nickel layers in Cu matrix was achieved. It was found that with increasing strain during ARB cycles strength and elongation of these composites increased. The observations of fracture surfaces after tensile tests revealed that rupture mode of Cu/Ni multilayer composites is ductile rupture with shear dimples.

Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers

Ni/NiO multilayers were grown by magnetron sputtering at room temperature, with the aid of the natural oxidation procedure. That is, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (∼1.2 nm) is formed. Simulated x-ray reflectivity patterns reveal that layering is excellent for individual Ni-layer thickness larger than 2.5 nm, which is attributed to the intercalation of amorphous NiO between the polycrystalline Ni layers. The magnetization of the films, measured at temperatures 5–300 K, has almost bulk-like value, whereas the films exhibit a trend to perpendicular magnetic anisotropy (PMA) with an unusual significant positive interface anisotropy contribution, which presents a weak temperature dependence. The power-law behavior of the multilayers indicates a non-negligible contribution of higher order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the temperature dependence of the magnetization in the spin-wave regime show that the magnetization in the multilayers decreases faster as a function of temperature than the one of bulk Ni. Finally, when the individual Ni-layer thickness decreases below 2 nm, the multilayer stacking vanishes, resulting in a dramatic decrease of the interface magnetic anisotropy and consequently in a decrease of the perpendicular magnetic anisotropy.

The Electrochemical Effects of Immersion Au on Electroless Nickel and Its Consequences on the Hermetic Reliability of a Semiconductor Device

We have analyzed the effects of the thicknesses of immersion Au (i-Au) and electroless nickel (e-Ni) on the reliability of a semiconductor device that has special constraints on the absolute thicknesses of Ni and Au layers. The electrochemical reactions involved in the deposition of i-Au over Ni involve a substitution process by which Ni atoms are replaced by Au atoms. We demonstrate that, in some cases, this reaction changes the microstructure of the Ni near the perimeter of the pad where the Ni layer overlaps the passivation layer of a semiconductor die, forming a mechanical seal to the passivation. This seal is extremely important for hermetic applications where one wants to keep moisture and contaminants away from the active pad area of the semiconductor die. We have found that extended exposure of this Ni-to-passivation interface to the Au plating chemicals damages this interface, leading to reliability issues. In this paper, we analyze the phenomena by changing the thicknesses of Ni and Au layers and by observing its effects on metal-oxide-semiconductor field-effect transistors that are subjected to accelerated reliability stress testing in an autoclave.

Experiment and Theoretical Study of Critical Behavior in Magnetic Multilayers

Ni/NM multilayers (with noble metal NM=Au,Ag and Cu) were prepared by the electron beam evaporation method under ultra high vacuum conditions. The magnetic properties of Ni/NM multilayers are examined as a function of Ni layer thickness t Ni. The temperature dependence of the spontaneous magnetization M(T) is well described by a T 3/2 law in all multilayers. A spin-wave theory has been used to explain the temperature dependence of the magnetization and the approximate values for the bulk Exchange interaction J b and surface exchange interaction J s for various Ni layer thicknesses have been obtained. In the other hand we have used the high-temperature series expansion technique, to analyze the phase transition and the critical phenomena of a ferromagnetic a two-component multilayer, through three models: Ising, XY and Heisenberg. The critical reduced temperature τ c (ν) is studied as function of the thickness of constituents in the unit cell of the multilayer. In the two-component multilayer τ c (ν) is studied as function of the exchange interaction in each material and within the interface J s ,J b and J ⊥, respectively. A critical value of the surface exchange interaction in the film and interface exchange interaction in the multilayer above which the surface and the interface magnetism appears is obtained. The dependence of the reduced critical temperature on the thickness of the film and the unit cell of multilayer has been investigated. The effects of an amorphous magnetic surface on the critical properties of the film of simple cubic lattice have been studied. A number of characteristic behaviors, such as the possibility of the existence of a critical length of the unit cell thickness at which the temperature of the multilayer remains insensitive to the exchange coupling within interface, are reported. In a defined range of the exchange interactions, the values of γ are comparable to the universal ones and are independent of the film thickness. The asymmetry of the structure and the competition of the effects of the exchange coupling are important for the magnetic properties of the system.

Synthesis of graphene on SiC substrate via Ni-silicidation reactions

In this work, the features of graphene layers are studied with the aim of preparing the thinnest layers possible. The graphene layers were prepared by the annealing of Ni/SiC structures. The main advantage of this process is a relatively low temperature compared with the method of graphene epitaxial growth on SiC and short annealing times compared with the chemical vapor deposition method. We prepared graphene layers from several Ni/SiC structures in which the Ni layer thickness ranged from 1 to 200nm. The parameters of the annealing process (temperature, rate of temperature increase, annealing time) were modified during the experiments. The formed graphene layers were analyzed by means of Raman spectroscopy. From the spectra, the basic parameters of graphene, such as the number of carbon layers and crystallinity, were determined. The annealing of the Ni(200nm)/SiC structure at 1080°C for 10s, produced graphene in the form of 3–4 carbon monolayers. The value was verified by X-ray Photoelectron Spectroscopy (XPS). Good agreement was achieved in the results obtained using Raman spectroscopy and XPS.

Influence of the microstructure on the oxidation of Ni thin films

The influence of the initial grain structure on the oxidation of 100nm thick Ni layers is investigated at 500°C. Ni films were either formed by fine (diameter<70nm) or large grains (diameter<500nm) with enhanced Ni(111) crystallite size. The oxide growth rate was determined by gravimetric methodologies. Results of the oxide composition, grain morphology, crystallinity and electrical resistance as a function of the oxidation time are also presented.

Microstructure and bonding strength of diffusion welding of Mo/Cu joints with Ni interlayer

The possibility and appropriate processing parameters of diffusion bonding of Mo and Cu using a Ni interlayer was investigated. The microstructure of the bonded joints was studied by SEM, EPMA and XRD, and the main factors affecting diffusion bonding process were analyzed. The results showed that the solid solutions formed in both Mo–Cu and Cu–Ni interfaces during the diffusion bonding process, and no intermetallic compound appeared. The thickness of Ni layer decreased with the bonding temperature raising. The fracture of the joints had taken place in the Mo/Ni interface rather than the Ni/Cu interface. With the increasing of bonding temperature or holding time, the tensile strengths of the joints increased firstly and then decreased. The Mo/Ni/Cu joint bonded at 800°C for 30min exhibited a maximum value of tensile strength of 97MPa.

Diameter-Reduced Islands for Nanofabrication Toward Bit Patterned Magnetic Media

Thin films deposited on a flat substrate, if the adhesion is not strong, can be made to ball up and form discrete islands upon heating to elevated temperatures due to the surface energy difference. We have applied this useful process to electron beam lithography (EBL) and nano-imprinting lithography (NIL). By combining the ball-up processes of Ni thin film with e-beam lithography followed by reactive ion etching, Si nano islands and vertical nanopillars as small as 10 nm in diameter have been realized. There is a strong correlation between the initial thickness of Ni mask layer and final Ni island diameter obtained after ball-up. The smallest island diameter is obtained using ~ 5-nm initial Ni layer thickness. Below 3-nm initial layer thickness, the intended ball-up reaction does not occur. With more than ~ 15-nm initial metal film thickness, the Ni layer is broken up into nonspherical and highly irregular structures. [Co/Pd] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> multilayer magnetic thin films deposited on prepatterned substrate by nano imprinting lithography with versus without island ball-up process have been investigated for bit patterned media studies. The coercivity of magnetic islands can be enhanced by island diameter reduction using the ball up process.

Growth and Magnetism of Natural Multilayers

. In this work, we present a simple method to fabricate high quality Ni/NiO multilayers with the use of a single magnetron sputtering head. Namely, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (~ 1nm) is formed by natural oxidation. The process is reproducible and the result is the formation of a multilayer with excellent layering. Magnetization hysteresis loops recorded at 5 K and room temperature reveal a tendency for perpendicular magnetic anisotropy as the thickness of the individual Ni layers decreases. It is shown that the Ni/NiO interface has sizeable positive surface/interface anisotropy, i.e. it favors the development of perpendicular magnetic anisotropy. This is rather unusual for a Ni-based multilayered system and may render Ni/NiO multilayers useful for magneto-optical recording applications.

Enhanced electrochemical activity using vertically aligned carbon nanotube electrodes grown on carbon fiber

Vertically aligned carbon nanotubes were successfully grown on flexible carbon fibers by plasma enhanced chemical vapor deposition. The diameter of the CNT is controllable by adjusting the thickness of the catalyst Ni layer deposited on the fiber. Vertically aligned nanotubes were grown in a Plasma Enhanced Chemical Deposition system (PECVD) at a temperature of 630 oC, d.c. bias of -600 V and 160 and 68 sccm flow of ammonia and acetylene, respectively. Using cyclic voltammetry measurements, an increase of the surface area of our electrodes, up to 50 times higher, was observed in our samples with CNT. The combination of VACNTs with flexible carbon fibers can have a significant impact on applications ranging from sensors to electrodes for fuel cells.

Nickel aluminide coating produced on γ-TiAl alloy by high activity aluminising process

Ni aluminide diffusion coatings on the surface of γ-TiAl alloy were produced by electroplating a Ni layer followed by a single step high activity aluminising carried out in Ar+H2 atmosphere with a mixture of Al, NH4Cl and Al2O3 powders at 1000°C for 5 h. The effect of initial thickness for Ni layer on microstructure of produced Ni aluminide coating was highlighted. The thickness of initial Ni layer was changed to 4–20 μm. In the case of the Ni layer with thickness of 4 μm, only a little amount of NiAl phase was formed in a TiAl3 matrix. However, the microstructure of coating, in the case of the Ni layer with thickness of 8 μm, consisted of an outer layer of two phases (NiAl+TiAl3), an intermediate layer of TiAl3 and an interdiffusion layer. For thicker initial Ni layers (16 and 20 μm), beside the latter coating microstructure, a continuous surface layer of NiAl phase was observed. Isothermal oxidation tests on these aluminide coatings reveal that the oxidation resistance of the aluminide coatings increases with increase in initial thickness of Ni layer.

A phenomenological model for lifetime design of Ni2Al3/Ni hybrid coating formed on creep resistant ferritic steels

The phase layer transformation kinetics in the Ni2Al3/Ni hybrid coating formed on creep resistant steel P92 has been studied via a series of prolonged isothermal annealing experiments at 650 °C. All the intermediate phase layers of NiAl, Ni5Al3 and Ni3Al formed in the coating by interdiffusion during isothermal annealing process. The phase layers of NiAl and Ni3Al formed at the very beginning of isothermal annealing at the interface between Ni2Al3 and Ni, but the Ni5Al3 phase layer formed at the interface between the NiAl and Ni3Al phase layer only at an annealing time at which the outer Ni2Al3 phase layer was completely consumed. The growth and consumption of the Ni2Al3 and NiAl phase layers and the growth of the Ni3Al5 phase layer were all parabolic, but the growth of the Ni3Al phase layer obeyed the power rate law d = kt1/n. The growth kinetics of an intermediate phase layer was found to be faster than the kinetics of its subsequent consumption. The rate constants in both the growth and consumption kinetics need to be determined for each of the intermediate phase layers at a particular temperature. The lifetime of the coating with an outer Ni2Al3 phase layer of any specified initial thickness and a sufficiently thick inner Ni layer can then be estimated using the lifetime design model delineated in this study.

Effect of Ni Layer Thickness on Cu-Based {100} Textured Substrate for Coated Conductor

We prepared substrates with the structure electroplated Ni/textured Cu/stainless steel (SS) and clarified the effect of the electroplated Ni layer thickness on the substrate performance by investigating the dependences of the degree of crystal orientation and the surface roughness on the Ni layer thickness. It was confirmed that the orientation of the Ni layer did not significantly change up to a thickness of 10 µm; however, the surface roughness of the Ni layer decreased when the layer thickness exceeded 3 µm. From these results, in order to achieve a high critical current density (Jc) on a Cu substrate, it is suggested that the preferred maximum thickness of the Ni layer on the textured Cu should be approximately 3 µm. In addition, the possibility of broad availability for the coating process was investigated by estimating the Cu concentration on the surface of a 3-µm-thick Ni layer.

Effect of Ni Layer Thickness on Cu-Based {100}&lt;001 &gt; Textured Substrate for Coated Conductor

Effect of Ni Layer Thickness on Cu-Based {100}&lt;001 &gt; Textured Substrate for Coated Conductor

Structural and magnetic properties of Ru/Ni multilayers

Ru/Ni multilayers of different Ni thicknesses have been fabricated using magnetron sputtering. The structure of the multilayers has been determined by grazing incidence x-ray diffraction and x-ray reflectivity and their magnetic properties by magnetization and polarized neutron reflectivity measurements. The presence of Ru leads to the formation of a hexagonal Ni structure within an interfacial layer ∼1 nm above each Ru layer, while the rest of the Ni layer relaxes to the equilibrium fcc structure. The hcp Ni interfacial layer has a substantially increased cell volume, and is ferromagnetic with an atomic magnetic moment that increases with Ni layer thickness but remains lower than the value predicted from ab initio calculations.

Magnetic Studies in Sputtered Ni/Ti Multilayers

Magnetic properties of Ni/Ti multilayers, prepared by the DC triode sputtering method, have been studied by magnetic measurements. Both metal layers are crystalline with a (111) fiber structure when they are thicker than 20 A. The magnetization decreases with a decrease in Ni layer thickness tNi and the analysis of the results at 5 K indicates the presence of a dead Ni layer about 13 A thick. The effective anisotropy Keff of Ni/Ti multilayers is obtained using a torque magnetometer. Spin-wave theory has been used to explain the temperature dependence of the magnetization. Approximate values for bulk exchange interaction Jb, surface exchange interaction JS and interlayer coupling strength JI for various Ni layer thicknesses have been obtained.