CoO Layer Research Articles

Tailoring magnetic properties of Co nanocluster assembled films using hydrogen

Tailoring magnetic properties in nanocluster assembled cobalt (Co) thin films was achieved by admitting a small percentage of H2 gas (∼2%) into the Co gas phase cluster formation chamber prior to deposition. The oxygen content in the films is considerably reduced by the presence of hydrogen during the cluster formation, leading to enhanced magnetic interactions between clusters. Two sets of Co samples were fabricated, one without hydrogen gas and one with hydrogen gas. Magnetic properties of the non-hydrogenated and the hydrogen-treated Co nanocluster assembled films are comparatively studied using magnetic force microscopy and vibrating sample magnetometry. When comparing the two sets of samples the considerably larger coercive field of the H2-treated Co nanocluster film and the extended micrometer-sized magnetic domain structure confirm the enhancement of magnetic interactions between clusters. The thickness of the antiferromagnetic CoO layer is controlled with this procedure and modifies the exchange bias effect in these films. The exchange bias shift is lower for the H2-treated Co nanocluster film, which indicates that a thinner antiferromagnetic CoO reduces the coupling with the ferromagnetic Co. The hydrogen-treatment method can be used to tailor the oxidation levels thus controlling the magnetic properties of ferromagnetic cluster-assembled films.

Магнитные свойства слоистого кобальтита Sr-=SUB=-1-x-=/SUB=-Y-=SUB=-x-=/SUB=-CoO-=SUB=-3-delta-=/SUB=- (x = 0.1)

AbstractThe structure, the magnetic and magnetotransport properties of perovskite Sr_0.9Y_0.1CoO_2.63 have been studied. The sample is shown to have a two-phase structure. The main phase has a tetragonally distorted unit cell and is described by space group I 4/ mmm . The broadening of the reflections with indices corresponding to doubling unit cell parameter c indicates the absence of the rigorous translation symmetry along axis c . The existence of the broadened superstructure reflection observed in the diffraction pattern at small angles at temperature lower than 400 K is explained by the existence of the monoclinic phase whose content is significantly lower than that of the tetragonal phase, but is dominant in the Sr_0.8Y_0.2CoO_3–δ composition. The spontaneous magnetization appears as the monoclinic phase forms. The magnetic structure is mainly G-type antiferromagnetic with magnetic moments 1.5μ_B in the layers of CoO_6 octahedra and 2μ_B in the anion-deficit CoO_4 + γ layers. The conduction of the Sr_0.9Y_0.1CoO_2.63 composition has a semiconducting character. The magnetoresistance is 57% in a field of 14 T at a temperature of 5 K and strongly decreases with the temperature increase.

(Co,Mn)3O4 Spinel Coating for Protecting Metallic Interconnects Thermally Converted from an Electro‐Codeposited Co‐Mn3O4 Composite Coating

Electro‐codeposition was used to prepare the Co and Co‐Mn3O4 precursor coatings on ferritic stainless steel E‐brite. Plated samples were exposed at 800°C in air for different durations (i.e., 10 min and 4 h) for thermal conversion of the deposited layer to a spinel coating. The converted layer was characterized with scanning electron microscopy/energy‐dispersive spectroscopy (SEM/EDS) and X‐ray diffraction (XRD). The results showed that after 10 min heat treatment, the plated Co layer was not fully oxidized and converted into a double‐layer microstructure with an inner CoO layer and outer Co3O4 layer, while for the Co‐Mn3O4 layer, the Mn3O4 particles were not completely dissolved into the oxide layer. After 4 h of exposure, the surface layer was fully converted into the Co3O4 or (Co,Mn)3O4 spinel coating. Though the Mn content in (Co,Mn)3O4 was relatively low, the Mn‐doped spinel coating was advantageous over the pure cobalt spinel coating, as the thermally grown Cr2O3 scale at the coating/substrate interface was more compact and protective. Co diffusion from the deposited layer into the alloy substrate was observed for both coatings.

Superior Inorganic Ion Cofactors of Tetraborate Species Attaining Highly Efficient Heterogeneous Electrocatalysis for Water Oxidation on Cobalt Oxyhydroxide Nanoparticles.

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a Na2B4O7 solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K2SO4 solution at the same pH. HB4O7- and B4O72- ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The Na2B4O7 concentration dependence of the catalytic current was analyzed based on a Michaelis-Menten-type kinetics to provide an affinity constant of cofactors to the active sites, Km = 28 ± 3.6 mM, and the maximum catalytic current density, Imax = 2.3 ± 0.13 mA cm-2. The Imax value of HB4O7- and B4O72- ions was 1.4 times higher than that (1.3 mA cm-2) for the previously reported case of CO32- ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HB4O7- and B4O72- ions compared with that of CO32- ions.

An efficient catalyst film fabricated by electrophoretic deposition of cobalt hydroxide for electrochemical water oxidation

The hexagonal-shaped β-Co(OH)2 discs were electrophoretically deposited on an electrode to yield a stably adherent and uniform β-Co(OH)2 layer. The electrocatalytic water oxidation at the β-Co(OH)2 layer-deposited electrode was investigated to seek efficient water oxidation catalyst for artificial photosynthesis as promising energy-providing systems for the future. The β-Co(OH)2 layer did not work as a water oxidation catalyst in a K2SO4 solution at pH 7.0 when applying 1.70V vs. reversible hydrogen electrode (RHE), although the layer was oxidized to the CoO(OH) layer. In a phosphate buffer solution (pH 7.0), the β-Co(OH)2 layer was dissolved and oxidatively re-deposited as the Co-Pi catalyst (Science, 321 (2008) 1072–1075.) when applying the same potential. While in a Na2B4O7 solution at pH 9.4, the β-Co(OH)2 layer was oxidized to the CoO(OH) layer with the hexagonal-disc-shape remained when applying 1.7V vs. RHE, and the formed CoO(OH) layer works efficiently for electrocatalytic water oxidation in contrast to no catalytic activity in a K2SO4 solution at pH 9.4. The catalytic performance of the formed CoO(OH) layer was evaluated; the overpotential (η) of 0.42V for current generation of 100μAcm−2 and the Tafel slope of 0.21∼0.29V dec−1 were provided for water oxidation at pH 9.4, which are comparable with efficient cobalt-based catalyst films reported so far.

Kerr microscopy study of thermal and athermal training effects in a Co/CoO exchange bias system

The present work reports the magnetic domain evolution during the magnetization reversal and the training effect in a polycrystalline Co/CoO exchange bias system. Co/CoO bilayers with different cobalt (Co) layer thicknesses are being studied. The measurements are carried out using the Kerr microscopy at different temperatures (≥ 80 K) after the field-cooling across the Néel temperature of the antiferromagnetic (AFM) CoO layer. It is observed that with the increasing ferromagnetic (FM) Co layer thickness, the exchange bias reduces and the training effect becomes weaker. Analysis of the temperature variation of the exchange bias field indicates an increasing disorder at the FM-AFM interface with the decreasing FM thickness. Two different training effects, viz., athermal and thermal mechanisms, are observed depending on the thickness of the FM layer. For the lower FM thickness an athermal training effect is observed, whereas for the thicker FM layer thermal training effect is dominated. The domain structure of FM layers drastically changes during the athermal training effect with the cycling and domain size significantly decreasing during the magnetization reversal process below blocking temperature (TB), whereas in the thermal training mechanism no significant changes are found in the magnetic domain evolution throughout the temperature range with cycling. These results are expected to provide key inputs to various theoretical models that are being used to study the exchange bias phenomena in the recent literature.

Steering the magnetic properties of Ni/NiO/CoO core-shell nanoparticle films: The role of core-shell interface versus interparticle interactions

Supported core-shell Ni/NiO/CoO nanoparticle (NP) films were obtained by deposition of preformed and mass-selected Ni NPs on a buffer layer of CoO, followed by a top CoO layer. The resulting NPs have core/shell morphology, with a McKay icosahedral Ni core and a partially crystalline CoO shell. X-ray photoelectron spectroscopy evidenced the presence of a thin NiO layer, which was shown to be between the Ni core and the CoO shell by elemental TEM mapping. CoO and NiO shells with different thickness values were obtained, allowing us to investigate the evolution of the magnetic properties of the NP assemblies as a function of the oxide shell thickness. Both exchange-coupling and magnetostatic interactions significantly contribute to the magnetic behavior of Ni/NiO/CoO NP films. After the Ni/NiO/CoO NPs are cooled in a weak magnetic field, they have blocking temperature higher than room temperature because of strong magnetostatic interactions, which support the formation of a spin-glass-like state below $\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Exchange coupling dominates the magnetic behavior after the NPs are cooled in a strong magnetic field. The exchange bias (EB) is in the 0.17--2.35 kOe range and strongly depends on the CoO thickness (0.4--2.7 nm), showing the onset of the EB at the few-nanometer scale. The switching field distribution showed that the EB opposes the magnetization reversal from the direction along the cooling field but it does not significantly ease the opposite process. The EB depends on ${t}_{\mathrm{CoO}}$ only for ${t}_{\mathrm{NiO}}\ensuremath{\le}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, but when NiO is 0.7 nm thick it strongly interacts with CoO and a large increase of the EB and coercivity is observed.

Field dependence of antiferromagnetic domain switching in epitaxial Fe/CoO/MgO(001) systems

Utilizing the magneto-optic Kerr effect and Kerr microscopy measurements, we investigated the antiferromagnetic (AFM) domain switching process at different magnetic fields in a single-crystalline Fe/CoO bilayer grown on MgO(001) substrate. In spite of the zero-net magnetic moment in the CoO layer, we find that the activation energy barrier of CoO AFM domain switching decreased at larger magnetic field. To separate the different behaviors of domain nucleation and domain wall motion during the CoO spin switching process, a new analytical method was developed. Using this method, we found that the CoO domain nucleation energy barrier exhibited a jump at a critical magnetic field while the CoO domain wall motion experienced only a tiny energy barrier variation. The field-dependent behaviors of the energy barriers were attributed to the formation of a spiral domain wall in the Fe layer during its magnetization reversal and this was supported by micromagnetic simulations.

The synthesis of novel LiNiPO4 core and Co3O4/CoO shell materials by combining them with hard-template and solvothermal routes

Pure CoO and Co3O4 surface-modified LiNiPO4 nanoscale powders are successfully synthesized for the first time by newly elaborated hard-template and solvothermal methods. Using this extraordinary technique, LiNiPO4 particles are coated with a thin CoO or Co3O4 layer with a perfect core-shell morphology. The technique’s positive contribution to electrochemistry is investigated in detail. The HR-TEM analysis proves that it possible to achieve a continuous surface coverage of about 5nm, a result that contributes towards solving the chronic electrochemical problems of high-voltage cathode material. Our data reveal that cobalt cannot be diffused into the LiNiPO4 crystal system during the calcination process and that the LiNiPO4@CoO cathode demonstrates superior cycle stability and specific capacity at relatively low rates. The LiNiPO4@CoO cathode exhibits the best electrochemical properties, achieving a discharge capacity of 149mAhg−1 at 0.1C rate, and shows almost 82% capacity retention after 80 charge-discharge cycles. It therefore achieves partial success in terms of improving the electrochemical properties of the LiNiPO4 cathode material. This is especially crucial for energy storage with regard to electric vehicles and plug-in hybrid electric applications.

Exchange-biased SiO2/Co/CoO granular multilayers deposited by sequential sputtering

We investigate the magnetic properties in a system engineered to present Co grains laterally surrounded by a CoO matrix. By considering ferromagnetic SiO2/Co/CoO granular multilayers produced by sequential sputtering deposition, we verify that the Co grains present strong interfacial exchange coupling with the CoO matrix, assigned by the exchange bias effect as well as the raise of the Co blocking temperature. Morever, through magnetization and Hall resistivity experiments, we observe that the antiferromagnetic ordering temperature and exchange bias are dependent on the thickness of the antiferromagnetic material, attributed to finite size effects, as well as we find that the Co blocking temperature is raised as the CoO layer becomes thicker. We discuss the experimental results in terms of the growth mode of the layers, morphological properties of the multilayers, and temperature dependence of the magnetic behavior. The results place the sequential sputtering deposition as a promising alternative technique to produce Co/CoO granular multilayers.

Angle-dependent X-ray magnetic circular dichroism study of enhanced perpendicular magnetic anisotropy in hybrid [CoO/Pd]2[Co/Pd]7 multilayers

We have investigated the angle-dependent orbital and spin magnetic moments of the [Co/Pd] multilayer (ML) films with intervening CoO layers under annealing by employing angle-dependent soft X-ray magnetic circular dichroism (XMCD). After annealing, the orbital magnetic moments of Co ions are found to be enhanced, with the maximum values along the perpendicular direction of [Co/Pd] ML, providing evidence for the origin of the improved perpendicular magnetic anisotropy (PMA) being the interface spin–orbit coupling. The induced Pd polarization was observed after annealing, demonstrating the strong Co-Pd hybridization arising from the Co-Pd alloy formation near the interface. The angle-dependent coercivity follows the 1/cosθ behavior, suggesting that the magnetization reversal in the hysteresis curve in [Co/Pd] ML occurs mainly through the pinning mechanism.

Enhanced exchange bias fields for CoO/Co bilayers: influence of antiferromagnetic grains and mechanisms

The emergence and optimization of devices that can be applied to spintronics have attracted considerable interest, and both experimental and theoretical approaches have been used in studies of exchange bias phenomena. A survey of the literature indicates that great efforts have been devoted to improving exchange bias fields, while only limited attempts have been made to controll the temperature dependence of exchange bias. In this study, the influence of antiferromagnetic grains on exchange bias phenomena in CoO/Co bilayers on a semiconductor surface was investigated. Based on an antiferromagnetic grain model, a correlation between grain size, grain density, blocking temperature, and the exchange bias field was established. For crystallites with a smaller median diameter, the dependence of the thickness of the CoO layer on blocking temperature showed a less pronounced variation. This is due to the larger thermal agitation of the atomic spin moments in the grain, which causes a weaker exchange coupling between atomic spin moments. The enhanced density of antiferromagnetic/ferromagnetic pinning sites resulting from an increased grain density is responsible for the enhancement in the exchange bias fields. The results reported herein provide insights into our knowledge related to controlling the temperature dependence of exchange bias and related mechanisms.

Investigation of local structural and magnetic properties of discontinuous to continuous layer of Co at Co/MgO interface in MgO/Co/MgO trilayer structure

We investigate influence of oxygen migration in MgO/Co/MgO trilayer structure by varying Co and MgO layer thickness. To resolve Co/MgO interface structural and magnetic properties, we used near edge X-ray absorption spectra (XANES), X-ray absorption fine structure (XAFS) spectroscopy and Magneto-optical Kerr effect (MOKE) on MgO/Co/MgO trilayer structure. O K-edge XANES spectra show the variation in hybridization of Co 3d orbitals with neighboring O 2p orbitals as a function of Co layer thickness. Co L3 -edge XANES and Co K-edge XAFS results revealed the quantity of CoO formation as a function of Co layer thickness from discontinuous layer to continuous layer. Formation of cobalt oxide is mainly due to island formation of Co ≤ 10 nm layer and oxygen migration from MgO layer at interface via redox reaction to form CoO bonds. The oxygen migration is driven by imperfect stoichiometry of MgO at the Co/MgO interface. The formation of CoO layer influences the CoO orbital hybridization modifying in-plane magnetic anisotropy as a function of Co and MgO layer thickness. Higher coercivity (Hc) for lower thickness of Co layer confirms the formation of domain wall pinning. A model has been drawn to describe the interaction of Co layer with MgO layer at Co/MgO interface from discontinues to continuous region of Co thin film of magnetic tunnel junctions (MTJ)s, which may help to design high-performance spintronic devices.

Exchange biasing and interlayer coupling in Co/Pt/Co/CoO multilayers with perpendicular anisotropy

Investigations have been performed on exchange biasing and interlayer coupling in the Co/Pt/Co/CoO multilayers with perpendicular anisotropy. We have observed the asymmetry of the magnetization reversal of the top Co layer in contact with CoO layer and the shifting EHE loops of the bottom Co layer, which could be ascribed to the competition between the interfacial exchange biasing of the top Co layer and interlayer coupling between Co layers across the Pt layer. Due to other oxide phases observed in CoO layer, non-stoichiometric structure of CoO lowers the blocking temperature for the exchange biasing systems. The blocking temperature also depends on the CoO thickness and we observed lower blocking temperature for the thinner CoO layer.

High-temperature oxidation behaviour of novel Co-Al-W-Ta-B-(Mo, Hf, Nb) alloys with a coherent γ/γ'–dominant microstructure

In this work, 2at% Mo, 2at% Nb and 2at% Hf were substituted for the same amount of W into a Co-9Al-9W-2Ta-0.02B alloy (hereafter referred as to 2Mo, 2Nb and 2Hf alloys, respectively, while the original alloy is denoted as 0Me alloy). The effect of the Mo, Hf and Nb additions on the isothermal oxidation resistance, oxide scale evolution and failure mechanism, of the Co-9Al-9W-2Ta-0.02B alloy when exposed at 800°C and 900°C for 100h was investigated. It was found the Mo, Hf and Nb additions degraded the oxidation resistance of the Co-9Al-9W-2Ta-0.02B alloy, while the 2Mo alloy always displayed the poorest oxidation resistance, resulted from heavy spallation of the oxide scale. An oxide scale composed of an outer Co3O4+CoO layer, a middle complex oxide layer enriched with Al, W and Ta, and a γ/needle-like Co3W zone adhering to the γ/γ' substrate was gradually formed; moreover, a continuous or discontinuous Al2O3 layer and dispersive Al2O3 dots or slices were observed within the γ/needle-like Co3W zone, depending on the oxidation temperature and added elements (Mo, Hf and Nb). The formation of volatile MoO3 in the oxide scale of the 2Mo alloy enhance the exfoliation of the oxide products, resulting in severe spallation and poor oxidation resistance.

Ultrathin Co-O oxide layer-driven perpendicular magnetic anisotropy in a CoO/[Co/Pd]m multilayer matrix upon annealing.

Ferromagnetic/noble metal multilayer (ML) frames are expected to serve as reliable building blocks in a variety of perpendicular magnetic anisotropy (PMA) based spintronic devices. However, ultrathin ML matrices are highly susceptible to unintended reduction of electron spin polarization in the as-grown or annealed states and often require a large repeat number. Here, we introduce a simple approach to achieve thermally stable PMA in ultrathin [Co/Pd]3 MLs involving the incorporation of an ultrathin CoO capping layer. The thickness and oxygen content of the CoO layer are critical parameters to achieve enhanced PMA in ultrathin [Co/Pd]3/CoO MLs post-annealed up to 400 °C. An extensive analysis of structural features identified that robust PMA characteristics in [Co/Pd]3/CoO MLs are linked with thermally activated oxygen atom diffusion leading to structural reconfiguration upon annealing. The possible origin of the enhanced PMA in our [Co/Pd]3/CoO ML samples after high-temperature annealing is discussed, thereby enabling their use in future spintronic-related devices.

Exchange bias of nanostructured films assembled with Co/CoO core–shell clusters

Nanostructured films assembled with Co/CoO core–shell clusters were prepared by low energy cluster beam deposition. The structure and magnetism of the films were investigated. It is shown that core–shell clusters are detected with the average diameter of 6.3nm. And films assembled with clusters present exchange bias effect, which is attributed to the exchange interaction at the interface of Co and CoO layers. The present nanostructured films assembled with Co/CoO core–shell clusters promote its applications in ultrahigh-density magnetic recording, giant magnetoresistance, and spin-electronic devices.

Exchange-biased Py/CoO vortex structures: Magnetization reversal, cooling-field dependence, and training

In this paper, the magnetization reversal of exchange-biased Py/CoO vortex structures was investigated. The vortex structures were prepared by film deposition onto closely packed arrays of silica particles. By zero field cooling below the N\'eel temperature of the antiferromagnetic CoO, a so called circular exchange bias can be imprinted. By completing the same cooling process in an applied magnetic field, either a displaced vortex or even a $c$ state can be stabilized, which depends strongly on the CoO layer thickness. A strong athermal training effect mainly on the descending branch after the second loop is observed, which is accompanied by a strong reduction in loop shift, indicating a rearrangement of the CoO spin configuration during reversal. By analyzing the nucleation and annihilation field of the exchange-biased loops, a strong asymmetry is observed, which hints to a magnetization reversal via formation of a distorted viscous vortex. This asymmetry gets more pronounced with increasing CoO layer thickness. Furthermore, we investigated the blocking temperature distribution of the Py/CoO sample series. Below the mean blocking temperature, an increased coercive field is observed when the vortex is cooled in a field larger than the annihilation field compared to zero field cooling, revealing the competition between the vortex state formation and the alignment of spins along the cooling-field direction.

Interface mixing and its impact on exchange coupling in exchange biased systems

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni80Fe20/CoO(tCoO)/Co trilayers as a model system, and identified non-stoichiometric Ni-ferrite and Co-ferrite at the surface and interface, respectively. These ferrites, being ferrimagnets typically, should influence the exchange coupling. However, in our trilayers the interface ferrites were found not to be ferro- or ferri-magnetic; thus having no observable influence on the exchange coupling. Our analysis also revealed that (i) interlayer exchange coupling was present between Ni80Fe20 and Co even though the interlayer thickness was significantly larger than expected for this phenomenon to happen, and (ii) the majority of the CoO layer (except some portion near the interface) did not contribute to the observed exchange bias. We also identified that the interlayer exchange coupling and the exchange bias properties were not interdependent.

Effect of Co Substitution on the Crystal and Magnetic Structure of SrFeO2.75-δ: Stabilization of the "314-Type" Oxygen Vacancy Ordered Structure without A-Site Ordering.

A study of the structure-composition-properties correlation is reported for the oxygen-deficient SrFe1-xCoxO2.75-δ (x = 0.1-0.85) materials. The introduction of Co in the parent SrFeO2.75 (Sr4Fe4O11) structure revealed an interesting structural transformation. At room temperature (RT), an orthorhombic (space group Cmmm, 2√2ap × 2ap × √2ap type, ap = lattice parameter of the cubic perovskite) → tetragonal (space group P4/mmm, ap × ap × 2ap type) → tetragonal (space group I4/mmm, 2ap × 2ap × 4ap type) structural transformation is observed in parallel with increasing Co content and decreasing oxygen content in the structure. At the same time, a rich variation in the magnetic properties is explored. The samples with x = 0.25, 0.3 show temperature-induced magnetization reversal. With increasing Co content in the structure, magnetic interactions start to weaken due to the random distribution of Fe and Co in the structure; the x = 0.5 sample shows frustration in the magnetic behavior with much smaller magnetization value. With a further increase in the Co content in the structure, RT ferrimagnetic-type behavior is observed for the sample with x = 0.85. The nuclear and magnetic structure refinements using RT and low-temperature neutron powder diffraction (NPD, 10 K) patterns confirm the formation of a "314-type" novel oxygen vacancy ordered phase for the sample with x = 0.85, which is the first case of "314-type" novel oxygen vacancy ordering without A-site (ABO3-δ type perovskite) ordering. The magnetic structure is G-type antiferromagnetic starting at room temperature. Further, the stabilization of the "314-type" complex superstructure is related to the ordering of oxygen vacancies in the oxygen-deficient Co-O layers, and the same assists in building a network of Co ions with different coordination environments, each with different spin states, and forms the spin-state ordering.