Thin Co Films Research Articles

Generation of highly anisotropic physical properties in ferromagnetic thin films controlled by their differently oriented nano-sheets

We fabricated ferromagnetic nano-crystalline thin films of Co, Fe, Co–Fe and Co-rich and Fe-rich, Co–MT and Fe–MT (MT = transition metal), constituted by nano-sheets with a controlled slant. Visualization of these nano-sheets by Scanning Tunneling Microscopy and High-Resolution Transmission Electron Microscopy (HRTEM) showed typically tilt angles ≈56° with respect to the substrate plane, and nano-sheets ≈3.0–4.0 nm thick, ≈30–100 nm wide, and ≈200–300 nm long, with an inter-sheet distance of ≈0.9–1.2 nm, depending on their constitutive elements. Induced by this nano-morphology, these films exhibited large uniaxial magnetic anisotropy in the plane, the easy direction of magnetization being parallel to the longitudinal direction of the nano-sheets. In the as-grown films, typical values of the anisotropy field were between Hk ≈ 48 and 110 kA/m depending on composition. The changes in the nano-morphology caused by thermal treatments, and hence in the anisotropic properties, were also visualized by HRTEM, including chemical analysis at the nano-scale. Some films retained their nano-sheet morphology and increased their anisotropies by up to three times after being heated to at least 500 °C: for example, the thermal treatments produced crystallization processes and the growth of CoV and CoFe magnetic phases, maintaining the nano-sheet morphology. In contrast, other annealed films, Co, Fe, CoZn, CoCu… lost their nano-sheet morphology and hence their anisotropies. This work opens a path of study for these new magnetically anisotropic materials, particularly with respect to the nano-morphological and structural changes related to the increase in magnetic anisotropy.

Ab Initio Characterization of Magnetoelectric Coupling in Fe/BaTiO3, Fe/SrTiO3, Co/BaTiO3 and Co/SrTiO3 Heterostructures

Magneto-electric coupling is a desirable property for a material used in modern electronic devices to possess due to the favorable possibilities of tuning the electronic properties using a magnetic field and vice versa. However, such materials are rare in nature. That is why the so-called superlattice approach to creating such materials is receiving so much attention. In the superlattice approach, the functionality of a combined heterostructure depends on the interacting components and can be adjusted depending on the desired property. In the present paper, we present supercells of ferromagnetic thin films of Fe and Co deposited on ferroelectric and piezoelectric substrates of BaTiO3 and SrTiO3 that exhibit magnetism, ferroelectric polarization and piezoelectric effects. Within the structures under investigation, magnetic moments can be tuned by an external electric field via the ferroelectric dipoles. We investigate the effect of magnetoelectric coupling by means of ab initio spin-polarized and spin–orbit calculations. We study the structural, electronic and magnetic properties of heterostructures, and show that electrostriction can reduce the magnitude of the magnetization vector of a ferromagnet. This approach can become the basis for controlling the properties of one of the ferromagnetic layers of a superconducting spin valve, and thus the superconducting properties of the valve.

Structural dependence of hydrogen evolution reaction on transition metal catalysts sputtered at different temperatures in alkaline media

In the present article, we studied the catalytic activity of magnetron sputtered Mo, V, Ni, and Co thin films for hydrogen evolution reaction (HER) in the alkaline electrolyte. We find that the HER potentials (η10) of the Mo and V thin film catalysts sputtered at 800 °C shift positive with respect to those of the film catalysts sputtered at 25 °C. For Mo metal the observed shift of η10 was 280 mV and for V metal observed shift of η10 was 390 mV. On the other hand, minimal effect of sputtering temperature on both Ni and Co thin film catalyst activity for HER was observed. Structural analysis reveals that Mo and V prepared at 800 °C have uncommon face centered cubic (fcc, 0.74 packing density) structure as opposed to room-temperature sputtered Mo and V thin films which have common body centered cubic (bcc, 0.68 packing density) structure, resulting in significant increases of the packing densities when they are prepared at 800 °C. On the other hand, the high-temperature prepared thin films of Ni and Co retained fcc structures, resulting in no density changes compared to the room-temperature prepared fcc Ni and hexagonal close packed (hcp, 0.74 packing density) Co. Impedance spectroscopy shows that fcc Mo is a better catalyst than fcc Ni, which is considered an industry standard for non-noble pure metal-based catalysts in alkaline media. Stability tests also suggest that fcc Mo thin film catalysts prepared at 800 °C are more durable than fcc Ni thin films. Our study points out that structure phases of catalysts can be a key factor governing the activities of transition metals for HER in alkaline media.

Magnetic nanoarrays on flexible substrates.

In this work, we used nanosphere lithography to fabricate large area 2-D magnetic nanoparticle (MNP) arrays on a flexible polyimide substrate (Kapton). Samples were fabricated by assembling polystyrene (PS) spheres on thin films of Co capped with Au. Etched PS spheres were used to mask Co–Au particle arrays. The MNP arrays were subjected to superconducting quantum interference device measurements; flat samples (10 nm Co coated with 10 nm Au) exhibited an Ms of 117.3 emu g−1, which was lower than the reported literature value for bulk Co (162.7 emu g−1). When compared to the flat film, coercivity, Hc, increased in a linear fashion with respect to particle size. These preliminary results reveal that future investigations of the magnetic properties on flexible substrates should account for residual Co remaining in the polymeric material, the unique MNP shape, the effect of order (or lack or order) of the 2D array, and positioning with respect to the direction of the magnetic field.Graphical Supplementary InformationThe online version contains supplementary material available at 10.1557/s43580-021-00193-z.

Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains

Abstract It has been implicitly assumed that ices on grains in molecular clouds and protoplanetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (Ic) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano-crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a protoplanetary disk is shown. These results have important implications for the chemical evolution of molecules, nonthermal desorption, collision of icy grains, and sintering.

Studying structural, magnetic and morphological features of electrochemically fabricated thin films of Co–Ni–Fe with different Fe compositions

In this work, a detailed study on the influence of the Fe composition on the crystallite size, lattice parameter, crystallographic texture, coercive field (Hc), magnetic squareness ratio (SQR), domain structure, surface roughness and grain size in electroplated Co–Ni–Fe deposits was carried out. For this aim, Co–Ni67, Co–Ni61–Fe6 and Co–Ni54–Fe13 thin films with different Fe compositions were fabricated using electrochemical deposition technique. The results showed that the composition of Fe in the deposit depends on the concentration of Fe ions in the electroplating solution. An increase in the Fe composition caused a noticeable reduction in the size of crystallites and induced a considerable enhancement in the strength of the preferred orientation being in the [111] direction. It was also observed that an increase in the Fe composition makes the lattice parameter larger. Compared to other deposits, the Co–Ni54–Fe13 with the highest Fe composition exhibited a lower surface roughness and a more compact nodular surface topography consisting of relatively finer grains. All deposits displayed a stripe domain (SD) structure, an in–plane magnetic hysteresis loop with a vertical magnetization component and a semi–hard magnetic property. However, the Hc value, SQR parameter and SD width varied in accordance with the Fe composition.

Broadband Terahertz Probes of Anisotropic Magnetoresistance Disentangle Extrinsic and Intrinsic Contributions

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e. spin-dependent electron scattering), whereas intrinsic (i.e. scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, Ni81Fe19 and Ni50Fe50 over the frequency range from DC to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, Ni81Fe19 and Ni50Fe50 samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizeable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than 2/3 of the DC AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magneto-transport phenomena of standard magnetic thin films on ultrafast time scales.

Ultrasmooth cobalt films on SiO2 by chemical vapor deposition using a nucleation promoter and a growth inhibitor

Very smooth thin films of Co are deposited on SiO2 by chemical vapor deposition from the precursor dicobalt octacarbonyl Co2(CO)8 by augmenting the process conditions in two ways: (1) pretreating the surface to promote nucleation and (2) adding a coflow of a growth inhibitor. The surface pretreatment involves brief exposure of the SiO2 surface at 70 °C to a tetrakis(dimethylamido) metal complex M(NMe2)4, where M = V, Ti, or Hf. This pretreatment affords a self-limiting, submonolayer coverage of a metal-containing intermediate. When the pretreated surface is then exposed to 0.025 mTorr of the Co2(CO)8 precursor, nucleation occurs to afford a high density (1 × 1012 cm2) of nanoscale islands; the rms surface roughness for a sample with an areal density of 100 Co atoms/nm2 is 1.0 nm. Comparisons show that the M(NMe2)4 pretreatment has three benefits: it accelerates the nucleation process, it increases the nucleation density by about a factor of 3, and it reduces the surface roughness by about a factor of 2. The rms surface roughness of a sample with ∼100 Co atoms/nm2 can be further reduced to only 0.3 nm by adding up to 4 mTorr of ammonia NH3 to the feed gas along with the Co2(CO)8 precursor. Ammonia serves as a growth inhibitor that reduces the steady-state growth rate of Co by a factor of 4. This slower growth allows additional nucleation to occur on bare areas, and the growth of these nuclei affords coalescence with a smaller roughness.

Magnetic field dependent cycloidal rotation in pristine and Ge-doped CoCr2O4

We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of Co$_{0.975}$Ge$_{0.025}$Cr$_2$O$_4$ (Ge-CCO) and CoCr$_2$O$_4$ (CCO), under low- and high-magnetic fields, from 0.2 T up to 6.5 T. A characterization of Ge-CCO at a low magnetic field is performed and the results are compared to those of pure CCO. The ferrimagnetic phase transition temperature $T_C \approx 95$ K and the multiferroic transition temperature $T_S \approx 27$ K in Ge-CCO are comparable to those observed in CCO. In Ge-CCO, the ordering wave vector $\textit{(qq0)}$ observed below $T_S$ is slightly larger compared to that of CCO, and, unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity and therefore the electric polarization was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.

Magnetic and electrical transport properties of Ru doped cobalt ferrite thin films with perpendicular magnetic anisotropy

Epitaxial ferrimagnetic thin films of (Co, Ru) Fe2O4 were grown on MgO (001) substrate using pulsed laser deposition technique. Ruthenium substitution in cobalt ferrite has increased the conductivity by orders of magnitude, but it has a minimal effect on magnetic properties. The film has a high coercivity and perpendicular magnetic anisotropy (PMA), where the magnetic easy axis points perpendicular to the film surface. We report the magnetic and electrical transport properties here. The temperature variation of resistivity showed different conduction mechanisms at high and low-temperature regimes. Room temperature Hall measurement reveals “n” type carrier with a carrier concentration of 4×1020/cm3. The film showed negative MR and a linear decrement with the magnetic field without any saturation.

Electrosynthesis and characterization of Layered Double Hydroxides on different supports

Thin films of Al, and Co or Ni based Layered Double Hydroxides (LDH) have been electrochemically deposited on four different supports (Grafoil, Toray Carbon Paper, Carbon Cloth and Nickel Foam) through a potentiostatic or a potentiodynamic approach. The obtained films have been fully characterized to compare their properties in dependence on the different deposition techniques and substrates. Finally, the Grafoil support modified with a Ni /Al-LDH has been employed as cathode in combination with a Grafoil anode modified with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) also electrosynthesized, in order to develop an all-binder-free hybrid supercapacitor as a proof of concept to demonstrate the applicability in the field of energy storage.

Hydrothermally derived co, Ni co-doped ZnO nanorods; structural, optical, and morphological study

Thin films of ZnO and Co, Ni co-doped ZnO nanorods were successfully synthesized on the silicon substrate (100) by the hydrothermal method. The grown thin films were annealed at 105 °C for 4.5 h and characterized by employing different analytical techniques to investigate the effects of co-doping on structural, morphological, and optical properties of ZnO nanorods. X-ray diffraction (XRD) analysis confirmed the successful substitution of dopants (Co and Ni) without altering the hexagonal wurtzite structure of ZnO. The crystallite size and micro-strain of grown films were calculated by employing Scherer, W–H, and SSP methods. The unit cell volume and lattice parameters were increased for Co doping, moreover by increasing Ni concentration as a co-dopant these parameters were decreased. Crystallite size of ZnO, ZnCo, ZnCoNi3, ZnCoNi6 and ZnCoNi9 grown thin films were 73 nm, 42 nm, 71 nm, 60 nm, and 52 nm respectively. The Fourier transform infrared spectroscopy (FTIR) has shown the existence of characteristic vibrational bonds of Zn–O in all co-doped samples at 499.35 cm −1 , 561.25 cm −1 , 570.89 cm −1 , 565.10 cm −1 , and 553.53 cm −1 , respectively. SEM images show the surface morphology of grown thin films, which indicates that the diameter of nanorods is increased by increasing Ni contents. The UV–visible spectroscopy was employed to measure the energy bandgap ( E g ) using Tauc's plot. The optical energy band gap was decreased (3.37–3.16) eV for all co-doped samples. The redshift in energy bandgap by Co and Ni co-doping is mainly due to the strong orbital coupling and generation of new energy states between the VB and CB . The other optical parameters including optical absorption coefficient ( α ) , transmittance (T) , skin depth, optical density (OD) , extinction coefficient ( k ), refractive index ( n ), optical conductivity ( σ opt ), and dielectric constants ( ε r , ε i ) of the grown thin films were also discussed. • Hydrothermally derived Co, Ni co-doped ZnO nanorods; Structural, optical, and morphological study. • Variation in microstructural parameters calculated by Scherrer, W–H, and SSP methods. • Nanorods are successfully grown on thin film of Si (100) substrate. • The optical parameters i.e. energy band gap, optical absorption coefficient, transmittance, skin depth, etc. were studied.

Preparation of different thin film catalysts by direct current magnetron sputtering for hydrogen generation.

In this study, thin films of Co, Ni, Pd, and Pt were prepared on Co 3 O 4 support material in pellet form using the direct current (DC) magnetron sputtering method for use as catalysts for hydrogen generation from NaBH 4 .Characterization of the catalysts was carried out using X-ray diffraction (XRD), scanning electronic microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). According to cross-sectional SEM images, catalyst thicknesses were observed in the range of approximately 115.3–495.8 nm. The particle sizes were approximately 25.0, 21.4, 33.9, and 9.5 nm for Ni-Co 3 O 4 , Co-Co 3 O 4 , Pd-Co 3 O 4 , and Pt-Co 3 O 4 catalysts, respectively. The increase in NaOH initial concentration provides an increase in the rate of hydrogen generation for Co, Ni, and Pd catalysts. A maximum hydrogen generation rate of 1653 mL/g cat .min was obtained for the Pt-Co 3 O 4 catalyst.

A novel viewpoint of an imidazole derivative ionic liquid as an additive for cobalt and nickel electrodeposition.

Thin films of Co and Ni electroplated onto a copper electrode from acidic sulfate and Watts baths, respectively, were investigated. The use of an ionic liquid additive in the electrolyte is widespread for producing thin films by electrodeposition. In the present work, the influence of a new ionic liquid, namely, 1-methyl-3-((2-oxo-2-(2,4,5-trifluorophenyl)amino)ethyl)-1H-imidazol-3-ium iodide (Im-IL), in the electrodeposition of two metals was investigated using cathodic polarization (CP), cyclic voltammetry (CV), and anodic linear stripping voltammetry (ALSV) measurements and cathodic current efficiency (CCE%). The surface morphology of the Co- and Ni-coated samples was examined using Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and atomic force microscopy (AFM). The corrosion protection of the Co and Ni samples in a marine environment (3.5% NaCl solution) was studied by the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results show that the addition of Im-IL inhibits Co2+ and Ni2+ deposition, which leads to more fine-grained deposits, especially at low Im-IL concentrations. The inhibition of Co2+ and Ni2+ reduction in the presence of Im-IL ions occurs via adsorption, which obeys the Langmuir adsorption isotherm. The CCE% is higher in the presence of Im-IL. SEM images show smoother deposits of Co and Ni in 1 × 10−5 M and 1 × 10−4 M Im-IL solution respectively. The results prove that Im-IL acts as an efficient additive for electroplating soft Co and Ni films.

Reversible Control of Magnetic Anisotropy and Magnetization in Amorphous Co40Fe40B20 Thin Films via All-Solid-State Li -ion Redox Capacitor

Modulation of magnetism by electric-field-driven ion migration has huge potential for tuning of magnetic properties and inducing magnetic functionalities. This article reports the use of a solid-electrolyte Li${}^{+}$ redox capacitor for reversible modulation of magnetic anisotropy and magnetization in thin films of Co${}_{40}$Fe${}_{40}$B${}_{20}$, an alloy much studied for spintronics. The technique is a means to control the magnetic properties of metallic thin films via solid-state ionics, by marrying the physics of lithium-ion batteries and magnetic tunnel junctions. Strain engineering via the lithiation-delithiation cycle is also a possibility.

Study of roughness effect in Fe and Co thin films prepared by plasma magnetron sputtering

We report the experimental and theoretical study of a substrate roughness surface induced magnetic anisotropy in thin films of Fe and Co. The experimental results confirm previously reported data on NiFe thin films and indicate that rough substrates increase the magnetic coercive fields in magnetic thin films. This effect is most prominent in films of thickness comparable to the surface roughness values and materials of small volume magnetic anisotropy. We determined the coercive field of 15 nm Fe thin film sample deposited onto rough PVDF to be 256 Oe, which is more than doubled the value of the coercive field of 15 nm Fe films coated under identical conditions, onto smooth Si substrates. The effect is visible for Co films but weaken by its increased volume magneto-crystalline anisotropy. These results are important for applications based on magnetic thin film where the magnetic properties could be adjusted via substrate roughness engineering.

Development of quasicrystal morphology in gas-atomized icosahedral-phase-strengthened aluminum alloy powders

The microstructures in gas-atomized powders and in blind-die-compacted material from three Al-Cr-Mn-Co-Zr alloys have been studied using X-ray diffraction and electron microscopy. The materials consist of a mixture of FCC Al and icosahedral quasicrystalline phases, but three different phase distributions are observed which depend on the powder particle size, and hence on the cooling rate. The finest particles contain a nano-composite mixture of equiaxed Al grains and fine (<200 nm) quasicrystalline dispersoids. Coarser particles exhibit large cellular dendritic Al grains with thin films of Co- and Cr/Mn-rich phases at the cell boundaries and coarser (up to 2 µm) quasicrystals at the particle surfaces. The largest particles contain coarse (up to 5 μm) radial quasicrystalline growths within the particle. The same microstructures were observed for each alloy, but the volume fractions of the microstructural types varied with the alloy content. These effects are explained on the basis of competing nucleation and growth phenomena during solidification of the atomized droplets. Since these microstructures are retained in the bulk material following blind-die-compaction, they could have a profound influence on the mechanical and other properties of materials made from such powders. The importance of such effects for alloy and process design are discussed.

Tunability of Domain Structure and Magnonic Spectra in Antidot Arrays of Heusler Alloy

Understanding the magnetization dynamics in lattices of antidots (holes, rather than bumps) is interesting for applications in spintronics and data-storage technology. Magnetic antidot lattices have potential as magnetophotonic crystals, due to their higher spin-wave velocities (steeper dispersion), compared to dot arrays. This study shows that, by tuning the shapes of the holes (and hence the anisotropy) in the arrays, one can control the spin-wave spectra and engineer the domain structure in a thin film of Co${}_{2}$Fe${}_{0.4}$Mn${}_{0.6}$Si. This work should have significant impact on magnetic filters, splitters, and other magnonic devices.

Microwave magnetoelectric effect in structures based on ferromagnetic metals

The results of a theoretical calculation of the resonant magnetizing field values for ferromagnetic resonance in thin films of nickel, iron and cobalt for frequencies of 3, 10, 30 GHz for various orientations of this field are presented. Analytical dependences of the ferromagnetic line shifts on the electric field strength are obtained for two- layer magnetoelectric composites in which PZT, PMN-PT or PZN-PT disks are used as piezoelectrics on which thin films of Ni, Fe or Co are deposited.

Anisotropic magnetothermoelectric power of ferromagnetic thin films

In this article, we report the measurements of the magnetothermoelectric power (MTEP) in metallic ferromagnetic thin films of Ni80Fe20 (Permalloy; Py), Co and CrO2 at temperatures in the range of 100K to 400K. In 25nm thick Py films and 50nm thick Co films both the anisotropic magnetoresistance (AMR) and MTEP show a relative change in resistance and thermoelectric power (TEP) of the order of 0.2% when the magnetic field is reversed, and in both cases there is no significant change in AMR or MTEP after the saturation field has been reached. Surprisingly, both Py and Co films have opposite MTEP behaviour although both have the same sign for AMR and TEP. The data on half metallic ferromagnet CrO2 films show a different picture. Films of thickness of 100nm were grown on TiO2 and on sapphire. The MTEP behavior at low fields shows peaks similar to the AMR in these films, with variations up to 1%. With increasing field both the MR and the MTEP variations keep growing, with MTEP showing relative changes of 1.5% with the thermal gradient along the b-axis and even 20% with the gradient along the c-axis, with an intermediate value of 3% for the film on sapphire. It appears that the low-field effects are due to the magnetic domain state, and the high-field effects are intrinsic to the electronic structure of CrO2 and intergarian tunnelling magnetoresistance that contributes to MTEP as tunnelling-MTEP. Our results will stimulate the research work in the field of spin dependent thermal transport in ferromagnetic materials to further develop spin-Caloritronics.