Thin Co Films Research Articles

Magnetic easy axis engineering in ultrathin Cu/Co/Cu[110] structures

The magneto-optic Kerr effect in-situ has been used to investigate the effects of exposing ultrathin Co/Cu(llO) films (6ML<d_<Co<30ML) to CO gas and the subsequent effects of depositing submonolayer quantities of Cu. After a given deposition of CO, the adsorption of CO gas (but, not O₂, H₂) is found to cause the magnetic easy axis to switch 90&#x000B0; from the [00l] to the [1-10] direction. The deposition of submonolayer Cu overlayers are observed to reverse the effect of the adsorbed CO thereby switching the easy axis back to [00l]. For thin CO films (d_Co<15ML), Cu overlayers switch the magnetic easy axis back to the [00l] direction abruptly. In thicker CO films (>15ML) the easy axis is found to shift gradually from the [l-10] to the [00l] direction, and it allows us to controllably engineer the direction of the magnetic easy axis at a constant CO thickness.

Effect of annealing and chemical composition on the giant magnetoresistance of electron beam deposited Co xCu (100− x) (11⩽ x⩽45) granular films

A series of granular thin films of compositions Co x Cu 100− x (11⩽ x⩽45) and 150 nm thickness are deposited on mica and glass substrates by the electron beam evaporation technique. Structural and morphological investigations are made by X-ray diffraction, electron diffraction and transmission electron microscopic techniques. Films are nanocrystalline and the crystallite size increases upon annealing at 400°C/0.5 h in vacuum. The magnetization and magnetoresistance of the films are measured. Films with cobalt concentration lower than 13 at% show superparamagnetic and giant magnetoresistance behavior and for Co concentration higher than 30 at% show anisotropic magnetoresistive behavior. Annealed films of compositions (17⩽ x⩽25) show maximum GMR of 5–6% in a magnetic field of 9 kOe.

Modelling of deformation processes by permeable element methods: L.Delaey et al. (Katholieke University, Louvain, Heverlee, Belgium.) Powder Technol., vol 93, no 2, 1997, 127–141

Water adsorption on thin films of Fe, Co, Ni, Pt, Cu and Au obtained in UHV per deposition of the metal vapour on borosilicate glass substrates at 77 K were studied. After the annealing of the films at 323 K the changes in the work function ϕ upon adsorption of submonolayer water doses at 77 K were monitored. In all the studied metals, a steady decrease of ϕ was detected until a saturation value was reached. The mean saturation Δϕ values (in eV) for each metal were: Fe-0.97 ± 0.08; Co-1.20 ± 0.09; Ni-1.10 ± 0.15; Pt-1.02 ± 0.15; Cu-0.73 ± 0.08; and Au-0.60 ± 0.08.

Coincidence correction for coulter particle size analyser: E.J.W.Wynn, M.J.Hounslow. (University of Cambridge, Cambridge, UK.) Powder Technol., vol 93, no 2, 1997, 163–175

Water adsorption on thin films of Fe, Co, Ni, Pt, Cu and Au obtained in UHV per deposition of the metal vapour on borosilicate glass substrates at 77 K were studied. After the annealing of the films at 323 K the changes in the work function ϕ upon adsorption of submonolayer water doses at 77 K were monitored. In all the studied metals, a steady decrease of ϕ was detected until a saturation value was reached. The mean saturation Δϕ values (in eV) for each metal were: Fe-0.97 ± 0.08; Co-1.20 ± 0.09; Ni-1.10 ± 0.15; Pt-1.02 ± 0.15; Cu-0.73 ± 0.08; and Au-0.60 ± 0.08.

Structural characterization of thin film ferromagnetic tunnel junctions

Tunnel junctions comprised of two magnetic metal layers separated by a thin insulating oxide layer have been prepared by reactive sputtering onto thermally oxidized (100) silicon wafers at room temperature. The magnetic layers (thicknesses ∼30–50 nm) consisted of thin films of Co, Fe, and/or CoFe and the oxide barriers (thicknesses in the range 2–10 nm) included CoO, MgO, HfO2, and SiO2. The barriers were prepared by dc reactive sputtering from pure metal sources in mixed oxygen–argon atmospheres or by rf sputtering from oxide targets. Transmission electron microscopy in the cross-sectional geometry was used in this study to characterize the tunnel junction microstructure. Barriers of CoO and MgO were invariably polycrystalline with many crystallites extending across the entire barrier thickness, whereas barriers of SiO2 and HfO2 appeared to be amorphous. Although grain boundary diffusion has been proposed as a possible mechanism for providing shorts or “pinholes” between the magnetic electrodes, it was significant that both HfO2 (amorphous) and MgO (polycrystalline) showed high magnetoresistive response at low temperature.

Application of heavy-ion RBS to compositional analysis of thin films

A 8 MeV multicharged carbon or oxygen beam was used for Rutherford Backscattering Spectrometry (RBS) compositional analysis. Thin films of Co, Cu, Si, and NiCr of a few nm thickness were deposited on carbon substrates by vacuum evaporation or Ar sputtering. The measured results demonstrate that the mass resolution is much better for the heavy-ion RBS than for the He-RBS analysis. For the compositional analysis, a fitting function formula is proposed based on a simple model in which the thin film consists of many islands. The calculated results agree with the measured spectra, which show asymmetry of the peaks. Numerical analyses using the fitting function give reasonable agreement between the expected and measured ratios of mass elements in the films.

Composition analysis of Co doped FeSi x films by combining standard and heavy-ion RBS

In a UHV deposition system, thin films of Co doped FeSi x were grown on Si (111) substrates by coevaporation of Co, Fe and Si from three separately controlled sources. Heavy-ion RBS (15 MeV 14N 4+) providing a sufficiently high mass resolution was applied to determine the Co Fe ratio. For a Co content of up to [Co] ([Co] + [Fe]) ≈0.3 a linear correlation was found between the relative Co evaporation rate and the concentration of Co in the films. The composition analysis was completed by standard RBS (1.4 MeV 4He +) for more reliable values of the mean Si/metal ratio due to the smaller influence of multiple scattering processes in the spectrum region relevant to Si. IR phonon spectra of the FeSi x :Co films show characteristic changes which correlate with the Co concentration measured by RBS.

Organometallic chemical vapour deposition of cobalt/indium thin films using the single-molecule precursors [(CO) 4Co] aInR 3- a (R = CH 2CH 2CH 2NMe 2; a = 1–3)

Pure cobalt indium alloy thin films of the nominal compositions Co 0.50In 0.50, Co 0.66In 0.33 and Co 0.75In 0.25 were grown by OMCVD using volatile organometallic precursors of the general formula [( CO) 4 Co] a- InR 3-a [ R = CH 2 CH 2 CH 2 N( CH 3) 2; 1 : a = 1 ; 2 : a = 2 ; 3 : a = 3] . These organometallic compounds contain direct CoIn bonds and allow molecular control of the metal ratio of the grown films. The distribution of the metals were uniform throughout the films. The films were examined by AES depth profiling, SEM-EDX and X-ray diffraction. The phase composition of the films follows the phase diagram of the Co/In system, e.g. reflections of the crystalline phase Co, CoIn 2 and CoIn 3 were detected. Due to the multi-phase nature of the films, the coatings were unstable with respect to interfacial reactions with InP, e.g. the formation of CoP was observed by X-ray diffraction.

Electrocatalytic oxidation of H 2O 2 at an oxycobalt film modified glassy carbon electrode for fermentation monitoring

Glassy carbon electrodes coated with thin oxymetallic films of Co(II), Fe(II), Ni(II), Pb(II), Ce(III), Cr(III), Tl(I) and Mn(II) were investigated for the presence of electrocatalytic effect towards H 2O 2 oxidation. Of these, Co(II), Ni(II), Pb(II), Ce(III) and Mn(II) were found to exhibit such an effect in alkaline medium, with Co(II) showing the strongest effect. Further studies with the Co(II) modified glassy carbon electrode as amperometric detector, in the wall-jet configuration, for flow injection analysis (FIA) demonstrated that only a low operating potential of +0.175 V (vs. Ag AgCl , saturated KCl) was needed for good sensitivity. For flow injection analysis, linearity of H 2O 2 calibration was found from 5.00 × 10 −8 to 0.10 M and the detection limit was 5.0 × 10 −9 M ( S N = 3 ), The electrode showed good stability (at least two weeks operation) and can be usefully applied to glucose determination in the fermentation broths without interferences.

Observation of the evolution of anisotropic magnetoresistance in thin magnetic films

The evolutionary process of the anisotropic magnetoresistance from the positive to the negative by changing the field direction was observed in magnetic thin films of Ni-Co, Ni-Fe and Co. Various peculiar magnetoresistance-field curves occur, that can be understood by a model based on the superposition of the positive and negative magnetoresistance with different amplitudes and phases.

Magnetic extended x-ray absorption fine structure of transition metal films and alloys (abstract)

A very large helicity dependence of the extended x-ray absorption fine structure (EXAFS) above the L2,3 edges of the ferromagnetic transition metals Fe, Co, and Ni are observed. Transmission measurements of circularly polarized soft x rays through thin films of bcc Fe, hcp Co, fcc Ni, and FexNi1−x alloys deposited on ultrathin, semitransparent substrates show L-edge magnetic EXAFS (MEXAFS) oscillations nearly two orders of magnitude larger than the respective K-edge MEXAFS oscillations. Surprisingly, comparing the EXAFS and MEXAFS oscillations, after separation of the combined L2 and L3 spectra by an iterative deconvolution, show a linear dependence of the MEXAFS intensity on the spin moment of the scattering atom. Furthermore, the resulting fourier transforms show that, for the MEXAFS, the scattering paths traditionally associated with multiple scattering paths are enhanced compared to the L-edge EXAFS, suggesting the increased importance of multiple scattering in MEXAFS.

Measurements of mechanical properties of the liquid CO 2-water-CO 2-hydrate system

Microscopic observations of the liquid CO 2-water-CO 2-hydrate system were conducted to measure both the interfacial tensions in these phases and the strength of the CO 2-hydrate film. Measurements on hanging drops revealed that the interfacial tension between liquid CO 2 and water decreased slightly as the temperature was raised from 266.3 to 284.9 K, and that the average of the interfacial tension was about 29 dyn-cm −1. Contact angles between a water droplet and a teflon plate were measured before and after the thin film of CO 2 hydrate was formed on the droplet. Comparisons of these contact angles indicated that the sums of the interfacial tensions between liquid CO 2 and CO 2-hydrate film and between CO 2-hydrate film and water were about 26% less than between liquid CO 2 and water. The destructive strength of CO 2-hydrate film formed on the pendant drop was examined by using a process for breaking of the film. Measurements indicated that the strength of the CO 2-hydrate film depended on the film thickness. The film thickness of CO 2 hydrate was then estimated from the film strength.

Exchange coupling in single-crystalline spinel-structure (Mn,Zn)Fe2O4/CoFe2O4 bilayers.

We have observed strong exchange coupling between single-crystalline spinel-structure ferrite thin films of Co${\mathrm{Fe}}_{2}$${\mathrm{O}}_{4}$ and magnetically soft (Mn,Zn)${\mathrm{Fe}}_{2}$${\mathrm{O}}_{4}$ at room temperature. The exchange coupling constant of the interface, ${A}_{\mathrm{int}}$, which is estimated from exchange-biased (Mn,Zn)${\mathrm{Fe}}_{2}$${\mathrm{O}}_{4}$ loops is comparable to estimated exchange coupling constants within each individual layer, ${A}_{\mathrm{MZF}}$ and ${A}_{\mathrm{CF}}$. This result contrasts with exchange coupling in polycrystalline ${\mathrm{Ni}}_{0.8}$${\mathrm{Fe}}_{0.2}$/${\mathrm{Mn}}_{0.5}$${\mathrm{Fe}}_{0.5}$ and ${\mathrm{Ni}}_{0.81}$${\mathrm{Fe}}_{0.19}$/${\mathrm{Ni}}_{0.53}$${\mathrm{Mn}}_{0.47}$ bilayers where ${A}_{\mathrm{int}}$ is at best two orders of magnitude smaller than estimates of ${A}_{\mathrm{NiFe}}$.

Determination of the energy dependence of the off-diagonal terms of the dielectric tensor by means of M2,3 reflection MCD measurements (abstract)

We present the first magnetic circular dichroism (MCD) measurements around the M edges of 3d metals utilizing SRCs recently developed quadruple reflection polarizer system. The reflection polarizer converts linearly polarized light by means of the phase shift between s- and p-reflections into either left or right circular polarized radiation. The concept of the phase shifter has been tested at a bending magnet beam line at the Aladdin storage ring using the M2,3 absorptions of magnetically ordered thin films of Fe, Co and Ni. The normalized MCD signal e.g., (IR−IL)/0.5*(IR+IL) is on the order of several % from peak to peak and scales linearly with the magnetic moments. The shape of the reflection MCD signals has a very pronounced angular dependence with a maximum around a reflection angle of θ=60°–65°. The main trends in the angular dependence of the reflection MCD signal can be verified by model calculations. The model also allows us to extract the energy dependence of the off-diagonal terms in the dielectric tensor from the experimental data. The potential use and high sensitivity to variations in the local magnetic environment will be demonstrated with high resolution measurements of several ferrimagnetic compounds.

Spin scattering in ferromagnetic thin films.

By combining parallel and transverse magnetoresistance measurements on thin films of Co and Ni, the contribution of spin scattering at the domain walls is separated from the anisotropic magnetoresistance (AMR). A model, based on the Larmor-precession-induced deviation of the conduction electron spin direction during domain-wall traversal is developed. By using a scattering probability which varies with the cosine of the angle between the carrier spin and the local exchange field (as used for giant magnetoresistance systems) it is possible to account for the amplitude of the measured magnetoresistive effect. \textcopyright{} 1996 The American Physical Society.

Kinetics of the formation of CO 2 hydrate on the surface of liquid CO 2 droplet in water

Kinetic studies on the formation of CO 2 hydrate on the surface of a liquid CO 2 droplet in water were conducted. Dimensionless equations for the CO 2 hydration on the surface of a liquid CO 2 droplet were obtained. The thickness of the CO 2 hydrate was investigated in terms of the diffusion coefficient, reaction rate constant, and radius of droplet by the use of dimensionless equations. Some conclusions were obtained from the viewpoint of ocean CO 2 disposal for the reduction of CO 2 emission. When a liquid CO 2 droplet is injected into deep ocean, it will be immediately covered with a very thin film of CO 2 hydrate. It will be very difficult to produce a droplet mostly composed of CO 2 hydrate by liquid CO 2 injection into the ocean.

The structure and magnetic properties of thin films of Ni and Co deposited in Penning discharge

The results of investigation of the structure and magnetic properties of thin Ni and Co films obtained in Penning discharge with simultaneous bombardment of the growing films by low energy Ar + and Kr + ions are presented.

MODIFICATION OF THE ELECTRICAL RESISTANCE OF THIN COBALT FILMS UPON THE ADSORPTION OF CARBON MONOXIDE

Thin films of pure Co with thicknesses ranging between 10 and 50 nm were produced in UHV conditions. The films were deposited onto mica substrates with dimensions of 1 cm ×1 cm ×0.1 cm . Thin gold wires were attached to the ends of the films in order to monitor their electrical resistivity. The films were introduced into a UHV chamber which consists of a quadrupole mass spectrometer and an Arsputtering gun for cleaning. The films were exposed to 10−6 torr of high-purity CO and the resistivity of the films was monitored during exposure. The resistivity changed in a “sawtooth” fashion similar to the changes observed by Pick in the Nb/H 2 system. The aim of these studies is to correlate changes in resistivity of the film with well-characterized adsorption states of CO on the Co surface. Carbon monoxide desorption from the cobalt films was studied (using a mass spectrometric method in an ultrahigh vacuum system) and two carbon monoxide desorption peaks were observed identical to prior work using Co foils. These two states correspond to molecular CO and presumably dissociated CO. Changes in resistivity of the thin Co films appear to be related only to the molecular CO adsorbed on the Co surface.

Chemistry, microstructure, and electrical properties at interfaces between thin films of cobalt and alpha (6H) silicon carbide (0001)

Thin films (4–1000 Å) of Co were deposited onto n-type 6H-SiC(0001) wafers by UHV electron beam evaporation. The chemistry, microstructure, and electrical properties were determined using x-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and I-V and C-V measurements, respectively. The as-deposited contacts exhibited excellent rectifying behavior with low ideality factors and leakage currents of n &lt; 1.06 and 2.0 × 10−8 A/cm2 at −10 V, respectively. During annealing at 1000 °C for 2 min, significant reaction occurred resulting in the formation of CoSi and graphite. These annealed contacts exhibited ohmic-like character, which is believed to be due to defects created in the interface region.

Structures and properties of mechanically alloyed aluminium with transition metal oxides: J. Kaneko et al (Nihon University, Narashino, Japan), J. Japan Inst. of Metals, (Vol 57, No 11, 1993, 1325–1332. (In Japanese)

Water adsorption on thin films of Fe, Co, Ni, Pt, Cu and Au obtained in UHV per deposition of the metal vapour on borosilicate glass substrates at 77 K were studied. After the annealing of the films at 323 K the changes in the work function ϕ upon adsorption of submonolayer water doses at 77 K were monitored. In all the studied metals, a steady decrease of ϕ was detected until a saturation value was reached. The mean saturation Δϕ values (in eV) for each metal were: Fe-0.97 ± 0.08; Co-1.20 ± 0.09; Ni-1.10 ± 0.15; Pt-1.02 ± 0.15; Cu-0.73 ± 0.08; and Au-0.60 ± 0.08.