Multilayers Of Co Research Articles

Magnetic properties versus interface density in rigid-exchange-coupled amorphous multilayers with induced uniaxial anisotropy

We demonstrate the possibility to tune the saturation magnetization, coercivity, and uniaxial in-plane anisotropy constant in amorphous bilayers and multilayers of Co85(Al70Zr30)15 and Sm11Co82Ti7 through the interface density. From magnetometry and x-ray circular dichroism (XMCD) measurements, we conclude that the easy-axis coercivity μ0Hc increases four times when the number of bilayer repetitions, N, increases from 1 to 10 within a constant total sample thickness of 20 nm. At the same time, the anisotropy constant Ku also increases by a factor four, whereas the saturation magnetization Ms decreases slightly. The Co spin and orbital moments, ms and ml, are found to be approximately constant within the sample series. The average total Co moment is only 0.8–0.9 μB/atom, but the ml/ms ratio is strongly enhanced compared to pure Co. Magnetization curves extracted from XMCD measurements show that the Co and Sm moments are ferromagnetically coupled for all samples.

FORC analysis of nanopatterned vs unpatterned films: Coercivity and switching mechanisms

We have studied the use of self-assembled block copolymers to pattern multilayers of Co and Pd on silicon wafers. Stacks ranging from four to twelve bilayers of Co (0.3 nm)/Pd (0.8 nm) were sputtered onto Ta/Pd seed layers and capped with 3 nm of Ta and were found to have perpendicular magnetic anisotropy as-deposited. The block copolymer polystyrene-block-poly(ferrocenyl dimethylsilane) (PS-b-PFS) was dissolved in toluene and spun onto the wafers. The polymers were phase-separated by heat treatment, leaving self-assembled PFS spheres embedded in PS, which was removed by oxygen-plasma ashing. The PFS spheres were then used as masks to ion-mill the Co/Pd multilayers into nanopillars. To study the effect of etch time and etch angle on the coercivity distribution, we synthesized samples in a Design of Experiments-(DoE)- in these two factors. Scanning electron micrographs showed nanopillars ranging from 15 to 30 nm in diameter, depending primarily on etch time. M-H loops measured on both patterned and unpatterned wafers showed an increase of up to 130% in overall coercivity upon patterning. First Order Reversal Curves (FORC) were measured, and the resulting FORC distributions displayed using a smoothing program (FORCinel) and one that can display the raw data without smoothing (FORC+). We find that FORC+ reveals information about fine-scale structure and switching mechanism that cannot be seen in the smoothed display.

Electrochemical Atomic Layer Deposition of Cobalt Enabled by the Surface-Limited Redox Replacement of Underpotentially Deposited Zinc

Electrochemical atomic layer deposition (e-ALD) process for fabricating cobalt (Co) nano-films is reported. The e-ALD process employs a two-step approach in which underpotential deposition (UPD) is first used to form a sacrificial adlayer of zinc (Zn) on a ruthenium (Ru) substrate. The sacrificial Zn adlayer then undergoes spontaneous surface-limited redox replacement (SLRR) by nobler Co. This provides an atomic layer of Co on the substrate surface. The two-step process is repeated cyclically to build multilayers of Co. The unique feature of the e-ALD approach presented herein is that it utilizes Zn as the sacrificial adlayer instead of Pb or Cu used conventionally in the e-ALD sequence of noble metals. The use of sacrificial Zn uniquely renders its redox replacement by Co to be thermodynamically favorable, thereby enabling Co e-ALD. In the present report, we discuss the electrochemical characteristics of the UPD and SLRR process steps, the e-ALD deposition rate and the deposit surface roughness. The Co deposits formed via e-ALD do not exhibit roughness amplification during the first 10 cycles of e-ALD, which is indicative of atomic-scale layer-by-layer growth of Co on the underlying Ru substrate.

Direct transfer of magnetic sensor devices to elastomeric supports for stretchable electronics.

A novel fabrication method for stretchable magnetoresistive sensors is introduced, which allows the transfer of a complex microsensor systems prepared on common rigid donor substrates to prestretched elastomeric membranes in a single step. This direct transfer printing method boosts the fabrication potential of stretchable magnetoelectronics in terms of miniaturization and level of complexity, and provides strain-invariant sensors up to 30% tensile deformation.

Electronic structure and ordering of multilayers of Co and Ag on Cu(111) investigated by photoelectron spectroscopy

The growth and the electronic structure of multilayers of Co and Ag on Cu(111) at room temperature have been studied with photoelectron spectroscopy and low-energy electron diffraction (LEED). The coverage range spans from Co and Ag layers between one monolayer (ML) to stacking of several monolayers. Surface states and ordered structures have been identified at room temperature. A Ag-related surface state with a binding energy of 0.30 eV is identified in normal emission in the ultraviolet photoelectron spectra when silver constitutes the top layer. Core-level binding energy shifts of Ag 3d 5/2 reflect the changing surroundings of Ag. Hexagonal diffraction patterns are observed for sandwiches of consecutive layers of Co and Ag up to 5 layers. Since no interlayer diffusion is observed in the layer-by-layer formation of the films, multilayers of consecutive silver and cobalt on Cu(111) offer preparation of sandwiched magnetic–non-magnetic structures.

Effects of gas-cluster ion beam processing on physical, magnetic, and giant magnetoresistance properties of α-Fe 2O 3 bottom spin-valves

The effects of gas-cluster ion beam (GCIB) processing accelerated by a few thousands of volts on the surface roughness, crystalline orientation texture, magnetic, and giant magnetoresistance (GMR) properties of exchange biased α-Fe 2O 3 bottom spin-valves have been investigated. It was found that all of these properties were strongly influenced by the GCIB processing conditions, such as gas sources including Ar, H 2, O 2, and their mixture combinations, and ion cluster doses. The properties of both GCIB processed and unprocessed areas on 50 and 100-nm thick α-Fe 2O 3 coated Si(1 0 0) wafers with GMR multi-layers of Co(3)/Cu(2.5)/Co(3)/Ta 2O 5(2.5 nm) were compared. The GMR ratio, Δ R/ R 0, was consistently increased by up to 20–30% for the areas that were GCIB processed with condition “P1(10 14 Ar gas)+P4(5×10 14 Ar+H 2 gases) ion cluster ions/cm 2” (Δ R/ R 0=14%) over the unprocessed α-Fe 2O 3 layer (Δ R/ R 0=11.2%), and decreased by up to 80% when using the process with the heaviest ion does, “P3” (Δ R/ R 0=2.4%). In addition, GCIB processing changed the magnetic properties of GMR spin-valve including exchange bias field, pinned, and free-layer coercivities significantly. The free-layer coercivity was decreased dramatically with “P1+P4” GCIB processing from 160 Oe to as low as 27 Oe and the exchange bias field increased from 72 Oe up to ∼125 Oe. The decrease of the GMR ratio, pinned coercivity, and exchange bias field when the “P3” condition process was used are thought to be due to surface damage and composition changes of the α-Fe 2O 3 anti-ferromagnetic layer. X-ray photoelectron spectroscopy (XPS) analysis of the α-Fe 2O 3 thin film surface showed a 4.6% reduction of the oxygen content (unprocessed: 65 kcps (kilo-counter per second), “P3”: 62 kcps) after application of the P3, the heaviest process condition. Atomic force microscopy (AFM), X-ray diffraction (XRD), and cross-sectional transmission electron microscopy (XTEM) analyses suggest that the increase of the GMR ratio and the improved magnetic properties of optimized GCIB-processed α-Fe 2O 3 spin-valve are due to the reduction of surface roughness and improvement of the crystalline orientation texture resulting from a more “defect-free” or denser α-Fe 2O 3 layer.

Pulsed laser deposition of metallic multilayers: the influence of laser power on microstructure

Multilayers of Co and Cu on Si(111) substrates have been produced by pulsed laser deposition at the second harmonic of a Nd:YAG laser (532 nm). The effect of varying the laser power on the film microstructure has been investigated using grazing incidence X-ray reflectivity measurements. Quantitative analysis of the reflectivity curves indicates that higher laser powers are associated with greater intermixing at the Co/Cu interfaces. Offset scans indicate that there is conformal roughness. The deposition process introduces some droplets into the layers, principally of Cu.

Studies of two- and three-dimensional ZnO:Co structures through different synthetic routes

Multilayers of Co and ZnO, with nominal layer thicknesses on the atomic scale with up to 25 bilayers, were deposited by ion beam sputtering on silicon and glass substrates at ambient temperature. Thick epitaxial CoxZn1−xO films on Al2O3(012) substrates were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. All were co-doped with Al. Comparative analysis of magnetization, resistivity, and magnetoresistance measurements, performed in the temperature range 2.5–300 K, is presented. At small thickness of Co layers in the multilayer samples, these structures are diluted magnetic semiconductor (DMS) superlattices, with properties close to the epitaxial films. A crossover from DMS to discontinuous magnetic metal/semiconductor multilayers is observed with increasing metal content in the multilayers. This leads to changes in conduction mechanisms, with increasing contribution of quasithree-dimensional or quasitwo-dimensional intergranular hopping, and superparamagnetism.

Structure and magnetic properties of multilayer Co/CN granular films

The magnetic and structural properties of a series of granular Co/CN films prepared by annealing multilayers of Co and CN have been studied. The films contained both hcp and fcc phases, and grain sizes were in the range 18–27 nm. The coercivity varied with cobalt fraction and with bilayer thickness, and a maximum room temperature coercivity of 1230 Oe was obtained for 50% cobalt and 2.7 nm bilayer thickness. Measurements of the time and temperature dependence of the highest coercivity films suggest that their coercivity is due primarily to the hcp phase and to the grain size. All films had relatively low values of magnetic viscosity (2%–3%/decade).

Ultra-thin cobalt films on Mo(110): overlayer growth and chemisorption of CO and D 2

The overlayer growth of cobalt on Mo(110) and the adsorption of CO and deuterium on Co/Mo(110) have been studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD). Co grows layer by layer on Mo(110) at a sample temperature of 115 K. Annealing multilayers of Co on Mo(110) to > 400 K results in the formation of three-dimensional (3D) Co clusters. The TPD spectra of Co from Mo(110) reveal two desorption peaks, β 1 and β 2, with activation energies of 82 and 85 kcal/mol, corresponding to desorption from 3D Co clusters and the first uniform Co overlayer, respectively. Submonolayer Co( < 0.75 ML, where 1 ML corresponds to the atomic density of the Co(0001) surface) is believed to form a pseudomorphic layer on Mo(110) upon annealing to 400 K. Co coverages from 0.75 to multilayers, annealed to 500 K, exhibit the following LEED patterns: a 9 × 2 structure, a hexagonal structure, and a 2 × 2 structure. The 9 × 2 and hexagonal structures are interpreted to correspond to a distorted lattice of Co(0001) and to 3D Co clusters on Mo(110), respectively. The TPD spectra of CO or D 2 on Co overlayers show desorption features with lower activation energies than the analogous desorption features from bulk Co(0001).

Square-loop cobalt/gold multilayers

Multilayers of Co and Au with perpendicular hysteresis loop squareness ratios of ∼1 have been prepared by e-beam evaporation. These films have perpendicular anisotropy in the as-deposited condition in contrast to other work in which Co/Au multilayers, prepared by ion beam sputtering, showed perpendicular anisotropy only after annealing at 300 °C. The Faraday rotation of these square-loop multilayers is about 9×105 deg/cm of Co or 1×105 deg/cm of total thickness at a wavelength of 633 nm. These values indicate an enhancement of the Faraday rotation of Co at this wavelength by about a factor of 2. This may be a plasma-edge enhancement effect similar to that reported by Katayama et al. [Phys. Rev. Lett. 60, 1426 (1988)] in the Kerr effect of Fe/Au multilayers.

Physisorption of N 2 and CO on Al(111): A combined HREELS-UPS investigation

Mono- and multilayers of CO and N 2 physisorbed on Al(111) at 20 K were investigated by means of high-resolution electron energy-loss spectroscopy (HREELS) and UV photoelectron spectroscopy (UPS). From the desorption temperature an upper limit for the bonding energy to the Al(111) surface of 0.2 eV is estimated for both CO and N 2. Essentially, UPS and HREELS reproduce the gas phase data. The UP spectra are considerably broadened and shifted by 1 to 2 eV to smaller electronic binding energies. The energies of the molecular stretching vibrations are equal to the gas phase value within ± 1 meV. At the CO monolayer the electrons are inelastically scattered in the dipole mode. From the strong scattering cross section a complete in-plane orientation of CO can be excluded. The strong cross section for N 2 is due to resonance scattering via a known temporary negative-ion state.

Amorphization of crystalline Co and Sn multilayers by solid state reaction

Co x Sn 1− x amorphous alloys have been formed by solid state reaction at room temperature, from polycrystalline multilayers of Co and Sn deposited at 77 K. The forming ability range, temperature variation of the resistance, crystallization temperature of the amorphous alloys obtained by this new technique are very similar to those of amorphous Co x Sn 1− x alloys formed by vapour quenching.

Epitaxial Growth of a New Crystal Structure of Solid CO

The adsorption of multilayers of CO on a Cu(100) surface at 10 K has been studied by LEED and infrared spectroscopy. By starting from an ordered chemisorbed CO monolayer, it was possible to grow a three-dimensional crystal with hexagonal planes parallel to the surface and with a lattice constant much larger than that of the ordinary $\ensuremath{\alpha}$-phase CO solid.