Thin Layers Of Co Research Articles

Kinetic inductance in superconducting CoSi2 coplanar microwave transmission lines

We have looked into cobalt disilicide (CoSi2) as a potential building block for superconducting quantum circuits. In order to achieve this, we annealed a thin layer of Co to create microwave cavities with thickness of d = 10–105 nm from CoSi2 embedded in the silicon substrate. The cavity properties were measured as a function of temperature and power. In the films measuring 10 and 25 nm, we find a significant kinetic inductance LK with a non-BCS power-law variation δLK ∝ T4.3±0.2 at low temperatures. The quality factor of the studied microwave resonances varied from 3 × 103 (d = 10 nm) to ∼5 × 104 (d = 105 nm) and increased as d(A − log d) with thickness, with two-level systems having very little effect. The power dependence of kinetic inductance was analyzed in terms of heat flow due to electron–phonon coupling, which was found to be stronger than estimated for heat relaxation by regular quasiparticles.

Strong effect of crystal structure on the proximity effect between a superconductor and monolayer of cobalt

We present an unexpectedly strong influence of the proximity effect between the bulk Ru(0001) superconductor and atomically thin layers of Co on the crystal structure of the latter. The Co monolayer grows in two different modifications, such as hcp stacking and a reconstructed ε-like phase. While hcp islands show a weak proximity effect on Co and a little suppression of superconductivity in the substrate next to it, the more complex ε-like stacking becomes almost fully superconducting. We explain the weak proximity effect between Ru and hcp Co and the rather abrupt jump of the superconducting order parameter by a low transparency of the interface. In contrast, the strong proximity effect without a jump of the order parameter in the ε-like phase indicates a highly transparent interface. This work highlights that the proximity effect between a superconductor and a normal metal strongly depends on the crystal structure of the interface, which allows to engineer the proximity effect in hybrid structures.

Dependence of shear strength of Sn–3.8Ag–0.7Cu/Co–P solder joints on the P content of Co–P metallization

The mechanical properties of solder joints highly depend on the interfacial reaction between the solders and the metallization on substrates. In this work, we electroplated Co–P films with various compositions on the Cu pads of printed circuit boards and fabricated Sn-3.8 wt% Ag-0.7 wt% Cu/Co–P ball grid array (BGA) solder joints. The BGA solder joints were annealed at 150 °C for 100, 200, 500, and 1000 h and the shear strength of these joints was measured. When the P content of the Co–P metallization was increased from 2.3 to 18.8 at.%, the shear strength after 1000 h annealing initially rose to 107.9 MPa at a P content of 8.5 at.%, then decreased to 84.3 MPa at a P content of 12.5 at.%, and again increased to 96.0 MPa at a P content of 18.8 at.%. The enhancement of the shear strength of the joints with Co-8.5 at.% P, Co-12.5 at.% P, and Co-18.8 at.% P films was 109.5%, 63.7%, and 86.4% in comparison to the joints without Co–P metallization, respectively. The interfacial reaction between the Sn–Ag–Cu (SAC) solder and Co–P films during annealing and the fractured surfaces of the solder joints after the shear test were studied. For the joints with Co-8.5 at.% P and Co-18.8 at.% P films, a thick layer of CoSn3 was formed at the interfaces during annealing, which enhanced the shear strength. For the joints with Co-12.5 at.% P metallization, a thin layer of Co–Sn–P was formed at the interfaces and was peeled off layer by layer with prolongation of the annealing time. The spalled Co–Sn–P was mixed with the solder matrix, increasing the shear strength of the solder joints. The shear strength of the SAC/Co-12.5 at.% P joints was less than that of the joints with Co-8.5 at.% P and Co-18.8 at.% P films because no CoSn3 formed. Therefore, the composition of Co–P metallization played an important role in the interfacial reaction of the SAC/Co–P solder joints, which in turn affected the shear strength of the solder joints. Our experimental results show that the electroplated Co–P film is a promising candidate as the metallization for BGA solder joints.

Magneto-plasmonic biosensor with enhanced analytical response and stability

We present novel solutions to surpass current analytic limitations of Magneto-Optical Surface Plasmon Resonance (MOSPR) assays, concerning both the chip structure and the method for data analysis. The structure of the chip is modified to contain a thin layer of Co–Au alloy instead of successive layers of homogeneous metals, as currently used. This alloy presents improved plasmonic and magnetic properties, yet a structural stability similar to Au–SPR chips, allowing for bioaffinity assays in saline solutions. Analyzing the whole reflectivity curve at multiple angles of incidence instead of the reflectivity value at a single incidence angle provides a high signal-to-noise ratio suitable for detection of minute analyte concentrations. Based on assessment of solutions with known refractive indices as well as of a model biomolecular interaction (i.e. IgG–AntiIgG) we demonstrate that the proposed structure of the MOSPR sensing chip and the procedure of data analysis allows for long-time assessment in liquid media with increased sensitivity over standard SPR analyses.

Magnetic field modulation of chirooptical effects in magnetoplasmonic structures

In this work we analyse the magnetic field effects on the chirooptical properties of magnetoplasmonic chiral structures. The structures consist of two-dimensional arrays of Au gammadions in which thin layers of Co have been inserted. Due to the magnetic properties of the Au/Co interface the structures have perpendicular magnetic anisotropy which favours magnetic saturation along the surface normal, allowing magnetic field modulation of the chirooptical response with moderate magnetic fields. These structures have two main resonances. The resonance at 850 nm has a larger chirooptical response than the resonance at 650 nm, which, on the other hand, exhibits a larger magnetic field modulation of its chirooptical response. This dissimilar behaviour is due to the different physical origin of the chirooptical and magneto-optical responses. Whereas the chirooptical effects are due to the geometry of the structures, the magneto-optical response is related to the intensity of the electromagnetic field in the magnetic (Co) layers. We also show that the optical chirality can be modulated by the applied magnetic field, which suggests that magnetoplasmonic chiral structures could be used to develop new strategies for chirooptical sensing.

Characterization of the Surface of LiCoO2 Particles Modified by Al and Si Oxide Using Analytical TEM

LiCoO2 particles about 300 nm in size were synthesized by the Pechini method. LiCoO2 modified using Al oxide and Si oxide were also prepared by the sol-gel method. These samples allowed direct TEM observation without sampling damage, and homogeneous distribution of coating materials on the surface was confirmed. The cycling properties of LiCoO2 coated with Al oxide were improved compared with pristine LiCoO2. Samples modified with Si oxide showed poor electrochemical performance. From an analytical TEM study, there was a thin layer of Co and Al mixture on the LiCoO2 surface in the Al oxide coated LiCoO2. By Si oxide coating, a homogeneous layer about 5 nm in thickness was formed. Using an EELS study, the Li ion concentration in particles was mapped under monochromatized electron beam conditions. Homogeneous Li distribution was confirmed in the cycled particles in LiCoO2 coated by Al oxide. In contrast, an inhomogeneous Li distribution was found in pristine and LiCoO2 with Si oxide coating.

Variable angle magnetometry for exchange-coupled multilayers with in-plane and perpendicular anisotropy

Co/Pd multilayers, including exchange coupled structures with thin layers of Co with different exchange coupling strengths, were investigated using variable angle magnetometry. Hysteresis loops of perpendicular and in-plane components of magnetization were measured using two-axis vibrating sample magnetometer and variable angle magneto-optical Kerr effect system. Co/Pd multilayer structures have high perpendicular remanence and the magnetization reversal mechanism is mainly domain wall motion. For the systems with Co and Co/Pd multilayers, the magnetization reversals of high coercivity (Co/Pd) multilayer and low coercivity Co single layer were found to be independent, if the soft layer (Co) is thicker than 2 nm. In the samples with a high degree of exchange coupling, the magnetization of the multilayer has two components, normal and in-plane. The proportion between them varies with field strength and orientation of the sample. Taking this into account, it was possible to determine the anisotropy energy of the (Co/Pd) multilayer, which is in the range of 400–420 kJ/m3.

Explaining the redox imbalance between the H and O escape fluxes at Mars by the oxidation of methane

From a comparison between the different observations of Martian methane existing today, including the new TES methane maps ( Fonti and Marzo, 2010), we show that all sets of data are globally consistent with each other, and that a well definite seasonal cycle of methane has been at work for at least 10 yr. With a simple model of the balance between the loss fluxes of H and O, using up-to-date values of the escape fluxes, we show that the long-standing enigma of the imbalance between H and O escape fluxes may be solved by assuming that the missing sink of oxygen is the oxidation of methane. If no H 2 is released together with CH 4, a good agreement is found between the present CH 4 flux and the value imposed by the balance between H and O escape fluxes, an average over the last ≈10 3 yr. If H 2 is released together with CH 4, as expected if CH 4 originates in serpentinization, the average level of CH 4 during the last 10 3 yr should have been at least ten times lower than the present one. The lack of present H 2 release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H 2 has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO 2 ice covering the permanent southern polar cap could result from the release of methane since the end of the last obliquity transition (time scale: 1 Myr), at an average rate of 0.1 Mt yr −1, consistent with the values derived from: (i) the present observations of methane (time scale: 10 yr), (ii) the estimate from the observed imbalance between the H and O escape fluxes (time scale: 1 kyr). If so, the present release of methane from subsurface clathrates would have acted at a similar rate since at least 3 Myr.

Formation of Co-capping during sintering of straight WC–10wt% Co

Abstract Co-capping is a term that refers to a thin layer of Co occasionally observed on the surface of sintered WC–Co parts. It is usually considered undesirable and needs to be removed. The mechanism of its formation has been a myth in the industry and not fully understood to date. In this study, the effects of several factors, including cooling rate, atmosphere and carbon content in specimens, on the formation of Co-capping during sintering of straight WC–Co materials were experimentally examined. The mechanism of Co-capping formation was discussed based on the principles of liquid-phase migration.

Lower Temperature Si Thin Films by Metal Induced Growth for Lower Cost Solar Cells

AbstractMetal induced growth (MIG) was used to form epitaxial thin films of microcrystalline Si (μc-Si). By substituting Al as the catalyst metal in place of the usual Ni, x-ray diffraction (XRD) confirmed that μc-Si was successfully grown at temperatures between 350-525°C. At 525°C, a preferred orientation of (220) Si was observed with additional (111) and (311) Si orientations, while a temperature of 350°C resulted in a shift in preferred orientation to (111) Si. The lower limit for Al thickness was found to be between 10-20 nm with little crystallization and a smooth surface observed at 10 nm with XRD and SEM, respectively. Electrical measurements on Schottky diodes revealed space-charge limited conduction (SCLC) with an exponential distribution of trap levels due to diffusion of Al atoms into the Si, which was supported by analysis with energy dispersive x-ray spectroscopy (EDX) near the film surface. By depositing a thin layer of Co on top of Al prior to sputtering, the films exhibited increased crystallinity and a more uniform surface likely due to increased confinement of Al atoms. Electrical measurements demonstrated a shift from SCLC to thermionic emission in resulting Schottky diodes.

Band-filling effects in the magnetic anisotropy of atomic thin layers of Co

Ultrathin Co layers grown on Ru(0001) form a unique structure, which shows a variety of spin reorientation transitions linked to the completion of atomic layers. The close relation between atomic layer completion and magnetic anisotropy is maintained when Co is covered by thin noble metal films. Here we explore the effect of different metal cappings with unfilled d shell based on fully relativistic calculations within the Screened Korringa-Kohn-Rostoker (SKKR) method. The complex interplay of electronic and surface contributions is determined by means of a layer-resolved analysis, which allows us to predict the conditions leading to perpendicular magnetization for ultrathin Co films.

Soft magnetic properties of FeCo films with high saturation magnetization

The magnetic properties of multilayer composed of Fe65Co35 and thin layers of Co (less than 5nm) have been investigated. It is found that the microstructure, crystallographic texture, and magnetic softness of the multilayers depended on the initial layer. Multilayers with saturation magnetic induction higher than 23kG and hard axis coercivity smaller than 2Oe have been successfully prepared by using Co as the initial layer. Furthermore, the structure provided an effective process to tailor the in-plane magnetic anisotropy field without deteriorating the magnetic softness of the multilayers.

MOC observations of four Mars year variations in the south polar residual cap of Mars

The residual south polar cap of Mars (RSPC) is distinct from the residual north polar cap both in composition and in morphology. CO 2 frost in the RSPC is stabilized by its high albedo during southern spring and summer despite the relatively large insolation during that period. The morphology of the RSPC in summer displays a bewildering variety of depressions that are formed in relatively thin layers of CO 2. The increase of the size of these depressions between each of the first three years of Mars Global Surveyor (MGS) observations may possibly signal some sort of climate change on the planet. For example, the erosion of the bright plateaus might reduce the RSPC albedo and affect the energy balance. The Mars Orbiter Cameras (MOC) on MGS observed Mars for four consecutive martian years before contact with the spacecraft was lost in late 2006. During this period coverage of the polar regions was particularly dense because MGS flew over them on every orbit. In this paper we report on the four-year behavior of the morphological features in the RSPC and on the large-scale variability in RSPC albedo over the period. The changes in the size of the surface features in the RSPC due to backwasting that were first observed between Mars years (MY) 24 and 25 and subsequently between MY25 and M26 was observed to continue at the same rate through MY 27. The results indicate that on average thicker layers in the RSPC retreat faster than thinner ones, roughly in proportion to their thickness. We argue that a simple difference in porosity between the A and B layers can explain this difference although other factors could be involved. The large-scale albedo of the RSPC decreases as the depressions are uncovered by sublimation of seasonal CO 2. However, any interannual differences in albedo due to the backwasting process are masked by interannual differences in the summer dust opacity in the RSPC region.

Photoelectrodeposition of thin Co films on p-Si from sulfate solutions

The electrodeposition of thin layers of Co on hydrogen terminated p-Si substrates from sulphate solutions was analyzed by means of cyclovoltammetry, impedance and atomic force microscopy (AFM). The reduction of Co2+ on p-Si involves the discharge of photogenerated electrons at the conduction band edge and/or surface state levels at potentials more negative than the flat band condition. The diffusion controlled growth of Co films is characterized by the advance of a smooth front with a constant root mean square (rms) roughness of 6 to 7 nm. The deposition mechanism was analyzed by means of the Power Spectral Density of the AFM images. Then, the quality of the interface Si/Co was determined by its quasi-ideal rectifying behaviour with a Schottky barrier Φ=0.57 eV.

A photoemission study of CoO-films on Au(1 1 1)

Angle-resolved ultraviolet photoemission spectroscopy has been used to study thin layers of Co and CoO on an Au(1 1 1) substrate. A bulk-like CoO layer was obtained for a nominal coverage of 7 ML Co, with Co evaporated in an oxygen atmosphere of 10 −4 Pa and the Au(1 1 1) substrate at 350 K. Low-energy electron diffraction of this layer exhibits a (1 × 1) structure, indicating an unreconstructed CoO(1 1 1) surface. Considering the instability of a bulk-like terminated CoO surface, we assume stabilization by OH termination. At low coverage, the CoO layer is oxygen deficient and exhibits metallic character. For the bulk-like CoO layers we found band-like oxygen states and no detectable dispersion for the states near the Fermi edge, which is in agreement with previous investigations of single-crystalline CoO.

Positive giant magnetoresistance in ferrimagnetic/Cu/ferrimagnetic films

Spin valves composed of ferrimagnetic/Cu/ferrimagnetic layers were fabricated with the magnetization perpendicular to the film planes. By changing the composition of ferrimagnetic layers, both negative and positive giant magnetoresistance (GMR) can be observed in ferrimagnetic spin valves. For samples consisting of both transition-metal (TM-) rich TbFeCo and GdFeCo, negative GMR values were obtained. Due to the high resistivity of amorphous ferrimagnetic films, the shunting effect of Cu led to relatively small MR ratio. The negative MR effect was 1% for 1.7 nm Cu. For spin valves consisting of rare-earth (RE-)rich TbFeCo and TM-rich GdFeCo, positive GMR values were observed. A thin layer of Co was inserted between RE-rich TbFeCo and Cu to manipulate the sign of GMR.

X-Ray Techniques for Silicides

ABSTRACTPreviously we have found that the addition of a thin layer of Co between Ni and the Si substrate dramatically improves the thermal stability of the resulting silicide. Here we describe the use of four different x-ray techniques to obtain complementary information on film structure, providing a possible explanation for the observed thermal stability.

Magnetic and transport properties of NiFe/Ag and Co/NiFe/Co/Ag multilayers.

We present a comprehensive study of the structural, magnetic, and transport properties of NiFe/Ag multilayers grown by magnetron sputtering onto substrates maintained at liquid-nitrogen temperature. The influence of the Ag thickness (${\mathit{t}}_{\mathrm{Ag}}$), NiFe thickness (${\mathit{t}}_{\mathrm{NiFe}}$), and number of bilayers (n) has been studied. These multilayers exhibit antiferromagnetic and biquadratic coupling for 9 \AA{}${\mathrm{t}}_{\mathrm{Ag}}$12.5 \AA{}. For a given Ag thickness in this interval, the saturation field is inversely proportional to ${\mathit{t}}_{\mathrm{NiFe}}$. Furthermore, giant magnetoresistance (GMR) is observed with a maximum amplitude for ${\mathit{t}}_{\mathrm{NiFe}}$\ensuremath{\approxeq}25 \AA{}. However, the maximum GMR sensitivity \ensuremath{\Delta}R/R/H is obtained at larger NiFe thicknesses. The dependence of the resistivity and magnetoresistance on the thickness of the NiFe layers has been analyzed within the Camley and Barnas semiclassical theory. The effect of adding thin layers of Co at the NiFe/Ag interfaces has also been investigated. A doubling of the GMR amplitude at room temperature and a significant increase in the saturation field are observed with the introduction of only one atomic plane of Co at each NiFe/Ag interface. A quantitative analysis of the data shows that the increase of the GMR is due to a better transmission of the spin-up electrons through the NiFe/Co/Ag interface than through the NiFe/Ag interface. The results are interpreted in terms of a reinforcement of the magnetic ordering at the NiFe/Co/Ag interfaces, and correlatively a reduction of the interfacial magnetic scattering caused by the presence of the Co layer. \textcopyright{} 1996 The American Physical Society.

Temperature and thickness dependence of coercivity and magnetization of Co/Cu and Co/Si multilayers

Co/Cu and Co/Si multilayers of total thickness ∼3000 Å were prepared by rf and dc magnetron sputtering. The nominal thicknesses of the individual layers were in the range of 4 to 100 Å. A large coercivity (Hc) at 10 K was observed for very thin layers of Co in Co/Cu samples, and it decreased with increase of the Co layer thickness. For very thin layers of Co in Co/Cu samples, the layer behaved superparamagnetically. Similar behavior was not to be observed in Co/Si samples. With increased substrate temperature (Ts) during deposition, Hc was also observed to increase (decrease) for Co/Cu (Co/Si) samples. Magnetization data were modeled to determine the diffusion layer thicknesses.

Epitaxial strain, metastable structure, and magnetic anisotropy in Co-based superlattices (invited)

We explore the relationship between interface structure and magnetic anisotropy in three types of Co-based superlattices: Cohcp-Au; Cofcc-Cu; and Cohcp-Cr, grown epitaxially on GaAs(110). For very thin layers of Co, Co-Au, and Co-Cu superlattices exhibit a perpendicular easy axis due to magnetoelastic contributions to the anisotropy energy. The magnetic anisotropy in Co-Cr is found to be strongly dependent on growth conditions. At slow deposition rates of Co the interface between Co and Cr becomes diffuse as is evidenced by a low saturation moment and a shift toward perpendicular anisotropy whereas samples with abrupt interfaces show predominantly parallel anisotropy. The Cr layers grow in a metastable hcp phase which appears to be paramagnetic. The results illustrate the influence of the heterointerface on magnetic properties.