Inverted and crossed hysteresis loops in Ag/Ni multilayers

We report on the observation of inverted magnetic hysteresis loops in Ag/Ni multilayers. The samples were prepared by triode dc sputtering, at 100 K, with nickel layer thickness of about 8Å and Ag layer thicknesses ranging from about 5 to 40 Å. The phenomenon was observed in all the samples measured at temperatures larger than 90 K.     

Angular dependence of the in-plane magnetization of (100) Cu/Ni/Cu structures

(100) Cu/Ni/Cu sandwich structures have been deposited on (100) Si using the (100) Cu epitaxially grown as the seed layer. The in-plane epitaxial relation between the metal films and Si allows the study of angular dependence of the magnetization for the field parallel to the film plane. Keeping the Cu layers at 1000 Å each and varying the Ni layers between 50 and 1000 Å, the magnetization along the [110] edge is larger than that along the [100] one. This is observed for both structures with a Ni thickness of 1000 and 500 Å, respectively. For the thinner Ni layers, the angular dependence is interfered by the reversal in magnetic anisotropy reported earlier. For such structures, a squared hysteresis loop is observed for the field perpendicular to the film plane, whereas one with little loop is observed for the in-plane magnetization. The angular dependence observed for the 1000 and 500 Å Ni films is the same as that of single crystal Ni. The (100) Cu/Ni/Cu films thus grown can be used for other magnetic measurements in the exploration of two-dimensional magnetism with controlled orientations.     

Spin depolarization in Ni and Ce

Spin polarized electrons produced by polarized light in a Ge single crystal are depolarized by thin layers of Ni and Ce deposited on the surface of the sample. The mean free path for spin-exchange scattering is found to be (12.5 ± 1.5) Å in Ni and (3.2 ± 1) Å in Ce. The depolarization by Ce is found to be identical at 300 and 4 K. It is concluded that the occupancy of the 4? shell of the Ce atoms is similar at both temperatures.     

197Au Mössbauer study of Au/Ni artificial multilayers

¹⁹⁷Au Mössbauer measurements have been performed at 16 K on the Au/Ni artificial multilayers having three different thickness of the layers those are 10Å Au/10Å Ni, 30Å Au/30Å Ni and 53Å Au/53Å Ni on a 250Å pure Au buffer layer. Mössbauer spectra obtained can be decomposed into mainly two components. One is an unperturbed component having an identical isomer shift value to the bulk Au metal. The other is the component perturbed strongly by the Ni layer indicating a broad contribution at positive velocity side and its intensity depends on the thickness of the Au layer. The spectrum from 10Å Au/10Å Ni multilayer is an entirely perturbed one and its area ratio to the component rising from pure Au buffer layer indicates the large Debye-Waller-factor suggesting the supermodulus effect in this multilayer.     

The effect of particle size on the surface composition of microcrystalline alloys

Thin films a Ni?1at%Pd alloy, ranging in thickness from 10 to 100 Å were deposited onto alumina substrates by RF sputtering. The films were subsequently heat treated to produce small particles of the alloy with average sizes in the range 300 to 1000 Å. The surface composition of aggregates of small particles was determined by Auger electron spectroscopy after equilibration of the samples at 650°C. Average particle sizes were determined by analysis of photomicrographs taken by electron microscopy techniques. The results show a strong dependence of surface composition on particle size. A decrease of a factor of five in surface concentration of palladium is observed over the range of particle sizes studied. The dependence of equilibrium surface composition on particle size has been estimated by means of a mass balance model. While this model accounts qualitatively for the effects of particle size on surface composition, discrepancies between model predictions and the experimental results suggest the intervention of other phenomena, possibly related to capillarity effects.     

Surface structure effects via photoexcited core electron diffraction for Na on Ni

Photoemission from the Na 2p core level has been measured in the range ?ω = 40–120 eV for a c(2×2) Na overlayer on a Ni(001). surface. Weak oscillations in the Na 2p emission intensity vs. wavelength were observed and are interpreted as due to photoelectron scattering. The observed peaks closely match, in energy, those derived in recent calculations which assume the Na occupies a hollow site 2.23Å above the Ni surface layer. The data therefore indicate that observation and calculation of such effects may allow surface structure determinations.     

Effect of MeV electron irradiation on the free volume of polyimide

The free volume of the microvoids in the polyimide samples, irradiated with 6 MeV electrons, was measured by the positron annihilation technique. The free volume initially decreased the virgin value from ～13.70 to ～10.98 Å³ and then increased to ～18.11 Å³ with increasing the electron fluence, over the range of 5?×?10¹⁴ – 5?×?10¹⁵ e/cm². The evolution of gaseous species from the polyimide during electron irradiation was confirmed by the residual gas analysis technique. The polyimide samples irradiated with 6 MeV electrons in AgNO₃ solution were studied with the Rutherford back scattering technique. The diffusion of silver in these polyimide samples was observed for fluences >2?×?10¹⁵ e/cm², at which microvoids of size ≥3 Å are produced. Silver atoms did not diffuse in the polyimide samples, which were first irradiated with electrons and then immersed in AgNO₃ solution. These results indicate that during electron irradiation, the microvoids with size ≥3 Å were retained in the surface region through which silver atoms of size ～2.88 Å could diffuse into the polyimide. The average depth of diffusion of silver atoms in the polyimide was ～2.5 μm.     

Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

Carbon monoxide adsorption on a nickel iron surface: bonding and electronic structure computational study

CO adsorption on the FeNi(111) surface has been studied by density functional theory calculations. The CO molecule presents its most stable geometry in an intermediate position between the bridge Ni site and the top Fe site. Ni–C (1.94?Å) and Fe–C (2.09?Å) interactions occur after molecular adsorption. The main interactions occur involving C s–metal p and C p–metal d orbitals. The new interactions weaken the metal bonding. As a consequence, the strength of local metal bonds decreases by 15% from the original bulk value.     

Magnetic behaviour of nanocrystalline Ni–Cu ferrite and the effect of irradiation by 100 MeV Ni ions

The effects of 100 MeV Ni ion irradiation on magnetic properties of nanoparticles of Ni_0.8Cu_0.2Fe₂O₄ with average particle sizes of 40 Å and 60 Å, synthesized by chemical co-precipitation method have been studied. The spinel cubic structures were confirmed by XRD. The average particle size estimated by XRD and by Langevin function fitting are in good agreement for both the pristine and irradiated samples. The blocking temperature increases with particle size and does not change after irradiation. On irradiation by 100 MeV Ni ions, significant changes in the hysteresis loop features are observed, which may be attributed to formation of cluster of defects in the nanocrystalline samples due to swift heavy ion (SHI) irradiation. It is also found that SHI irradiation produces more dominant changes in the hysteresis loop of smaller particle size of 40 Å as compared to that of 60 Å.     

Monolayer K-films on Ni(100) — I. Structure and work function

Adsorption of K on Ni(100) at room temperature has been investigated experimentally concerning two-dimensional periodicity and work function change. It is found that the K atoms spread uniformly over the Ni surface and that one well-ordered structure of hexagonal symmetry is formed at 4.9 Å K nearest neighbour separation. In the most densely packed K-layer this separation was observed to be 4.3 Å, i.e. smaller than the bulk separation 4.62 Å. The change in work function at low densities was found to correlate qualitatively with the variation of the effective charge on the adsorbed K atoms.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

Electronic structure of the Au-intercalated graphene/Ni(111) surface

We report the electronic structure of the Au-intercalated graphene/Ni(111) surface using angle-resolved photoemission spectroscopy and low energy electron diffraction. The graphene/Ni(111) shows no Dirac cone near the Fermi level and a relatively broad C 1s core level spectrum probably due to the broken sublattice symmetry in the graphene on the Ni(111) substrate. When Au atoms are intercalated between them, the characteristic Dirac cone is completely recovered near the Fermi level and the C 1s spectrum becomes sharper with the appearance of a 10?×?10 superstructure. The fully Au-intercalated graphene/Ni(111) surface shows a p-type character with a hole pocket of ～0.034?Å^?1 diameter at the Fermi level. When the surface is doped with Na and K, a clear energy gap of ～0.4?eV is visible irrespective of alkali metal.     

Theoretical studies of the geometries of O and S overlayers on the (100) surface of nickel

Geometries for O and S overlayers on the (100) surface of Ni have been calculated using $\text{a b initio}$ wavefunctions for O and S bonded to small clusters of Ni atoms (1 to 5 Ni atoms). The calculated distance of the adatom from the surface is 0.96 Å and 1.33 Å for O and S, respectively, in excellent agreement with the results of dynamic LEED intensity calculations, 0.9 ± 0.1 Å and 1.3 ± 0.1 Å, respectively. This indicates that accurate geometries of chemisorbed atoms may be obtained from calculations using clusters.     

Full-scale atomistic simulations of dislocations in Ni crystal by embedded-atom method

Full-scale atomistic simulations by the nudged elastic band method are performed to determine the energetics and core structures of dislocations in a Ni lattice using an embedded-atom method potential. We find that for an edge dislocation, the potential yields very weak coupling between the partials which move almost individually. For a screw dislocation, the coupling between the partials is somewhat stronger and the partials move with some dependence. As expected, the results indicate that stacking fault energy has a controlling influence on the coupling behaviour of the partials. The effective Peierls energies and stresses are 1.30?×?10^?6?eV/Å and 2.79?×?10^?6?μ for the edge dislocation, and 1.62?×?10^?4?eV/Å and 2.02?×?10^?4?μ for the screw dislocation.     

Fe/Dy artificial multilayered films studied by57Fe Mössbauer spectroscopy

Magnetic properties of multilayered Fe/Dy films with artificial superstructures have been investigated by⁵⁷Fe Mössbauer spectroscopy. Doublet peaks are observed at room temperature when the Fe layer is thinner than 20Å. Mössbauer spectra for thicker Fe layers correspond to α-Fe spectra. In certain samples, i.e. [Fe(44Å)/Dy(6Å)], a gradual spin reorientation takes place, which is evidenced from the change of relative intensities of Δm=0 lines with decreasing temperature.     

Localized excitonic states in ZnS–ZnSe single quantum wells

We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well.     

Positron annihilation in Ni fine particles

It is found that almost all positrons thermalized in Ni fine particles of mean diameters of 100 Å and 300 Å escape from the particles without annihilating with electrons inside the particles. In Ni fine particles of mean diameter 1000 Å about one-half of the positrons annihilate with electrons inside the particles and the other half annihilate from trapped surface states.     

Structure determination of Ni(110)-(2 × 3)-N by use of SEXAFS measurements: on-surface and sub-surface sites on a pseudo-(100) reconstructed surface

Surface-extended X-ray-absorption fine-structure measurements have been performed on the Ni(110)-(2 × 3)-N system. The data are consistent with a model in which nitrogen chemisorbs on a pseudo-(100) reconstructed and largely corrugated surface with a nearest-neighbour N-Ni bond length of 1.86 ± 0.03 Å. The corrugation results in two raised and two lowered [11&#x0304;0] Ni rows which are not uniformly distributed in the [001] direction. Nitrogen chemisorbs in four-fold hollow sites slightly (0.19Å) above the lowered Ni rows and in pseudo-four-fold hollow sites slightly (0.38 Å) below the plane defined by a raised and a lowered Ni row. In both sites N is equidistantly bonded to Ni atoms in the second layer. Structural models with long-bridge adsorption sites can be safely excluded.     

Magnetic properties of ultrathin Ni films

The magnetic properties of Au/Ni/Si(100) films with Ni thicknesses of 8–200 Å are studied at T=77 K using a scanning magnetic microscope with a thin-film high-temperature dc SQUID. It is found that the Ni films, with an area of 0.6×0.6 mm, which are thicker than 26 Å have a single-domain structure with the magnetic moment oriented in the plane of the film and a saturation magnetization close to 0.17 MA/m. For films less than 26 Å thick, the magnetization of the film is found to drop sharply.