Field dependence of orbital magnetic moments in the Heusler compounds Co<Subscript>2</Subscript>FeAl and Co<Subscript>2</Subscript>Cr<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>Al

Orbital and spin magnetic moments of the Heusler compounds Co₂FeAl and Co₂Cr_0.6Fe_0.4Al were measured by magnetic circular dichroism in X-ray absorption (XMCD). The orbital magnetic moments per spin are quite large (0.1–0.2) compared to bulk values of Fe and Co metals, indicating a considerable spin–orbit coupling in these Heusler compounds. A strong localization of the 3d electron states might be responsible for this observation. The Co and Fe orbital to spin moment ratio shows a distinct decrease of r(Fe)=0.04±0.02 and r(Co)=0.06±0.02 with increasing external field for the ternary compound Co₂FeAl, while the ratio is within error limits independent of the field for Co₂Cr_0.6Fe_0.4Al. This is discussed in terms of a relation to magnetocrystalline anisotropies. PACS 75.50.Cc; 71.20.Lp; 78.40.Kc     

Crystal structure and magnetic anisotropy of ludwigite Co<Subscript>2</Subscript>FeO<Subscript>2</Subscript>BO<Subscript>3</Subscript>

Co₃O₂BO₃ and Co₂FeO₂BO₃ single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co₂FeO₂BO₃ undergoes two-stage magnetic ordering at the temperatures characteristic of Fe₃O₂BO₃ (T _N1 ≈ 110 K, T _N2 ≈ 70 K) rather than Co₃O₂BO₃ (T _N = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.     

Mechanochemical reactions in Co<Subscript>80</Subscript>B<Subscript>20</Subscript> and Co<Subscript>60</Subscript>B<Subscript>40</Subscript> mixtures during ball milling and postannealing

The solid-phase interaction of cobalt with boron during high-energy ball milling of Co₈₀B₂₀ and Co₆₀B₄₀ mixtures was studied. It was found that Co₃B boride forms during the mechanical alloying of components in the presence of both modifications of Co with a reduction in the size of blocks of up to 7–15 nm. The transformation of the metastable Co₃B phase into stable Co₂B and CoB borides is observed in the temperature range of 590–700°C.     

Transport properties of Co<Subscript>2</Subscript>CrAl Heusler alloy films

The effect of atomic disorder on the electron transport and the magnetoresistance (MR) of Co₂CrAl Heusler alloy (HA) films has been investigated. We show that Co₂CrAl films with L2₁ order exhibit a negative value for the temperature coefficient of resistivity (TCR) in a temperature range of 10 < T < 290 K, and the temperature dependence of electric conductivity varies as T ^3/2 similarly to that of the zero-gap semiconductors. The atomic or the site disorder on the way of L2₁ → B2 → A2 → amorphous state in Co₂CrAl HA films causes the deviation from this dependence: reduction in the absolute value of TCR as well as decrease in the resistivity down to ?(T = 293 K) ～ 200 μΩ cm in comparison to ?(T = 293 K) ～ 230 μΩ cm typical for the Co₂CrAl films with L2₁ order. The magnetic-field dependence of MR of the Co₂CrAl films with L2₁ order is determined by two competing contributions: a positive Lorentz scattering and a negative s-d scattering. The atomic disorder in Co₂CrAl films drastically changes MR behavior due to its strong influence on the magnetic properties.     

Surface-modified Li[Li<Subscript>0.2</Subscript>Mn<Subscript>0.54</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>]O<Subscript>2</Subscript> nanoparticles with LaF<Subscript>3</Subscript> as cathode for Li-ion battery

The LaF₃-coated Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ nanoparticles were synthesized via co-precipitation method followed by simple chemical deposition process. The crystal structure, particle morphology, and electrochemical properties of the bare and coated materials were studied by XRD, SEM, TEM, charge–discharge tests. The results showed that the surface coating on Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ nanoparticles were amorphous LaF₃ layer with a thickness of about 10–30 nm. After the surface modification with LaF₃ films, the coating layer served as a protective layer to suppress the side reaction between the positive electrode and electrolyte, and the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ oxide demonstrated the improved electrochemical properties. The LaF₃-coated Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ electrode delivered the capacities of 270.5, 247.9, 197.1, 170.0, 142.7, and 109.5 mAh g^?1 at current rates of 0.1, 0.2, 0.5, 1, 2, and 5 C rate, respectively. Besides, the capacity retention was increased from 85.1 to 94.8 % after 100 cycles at 0.5 C rate. It implied surface modification with LaF₃ played an important role to improve the cyclic stability and rate capacity of the Li-rich nickel manganese oxides.     

Conductivity of nanostructured India oxide films containing Co<Subscript>3</Subscript>O<Subscript>4</Subscript> or ZrO<Subscript>2</Subscript>

The effect of additives of cobalt and zirconium oxides on the conductivity of nanostructured composites based on indium oxide is studied. It is shown that addition of up to 20 wt % ZrO₂ to In₂O₃ leads to a sharp decrease in the conductivity of the composite. For the Co₃O₄?In₂O₃ system, the conductivity decreases up to a Co₃O₄ content of 60 wt %, after which it increases. At a Co₃O₄ content in the Co₃O₄?In₂O₃ system of up to 60 wt %, n-type conduction takes place, changing to p-type at 80 to 100 wt % Co₃O₄. Zirconium oxide exhibits practically no n-type conduction, so electric current in the ZrO₂?In₂O₃ system flows through In₂O₃ nanocrystals, i.e., n-type conduction takes place. Possible causes of the observed effects are considered.     

Sensor Properties of Nanostructured Systems Based on Indium Oxide with Co<Subscript>3</Subscript>O<Subscript>4</Subscript> or ZrO<Subscript>2</Subscript> Additives

The effect of Co₃O₄ and ZrO₂ additives on the sensory response of In₂O₃-based nanostructured composites to H₂ and CO is studied. It is shown that the addition of small amounts of Co₃O₄ or ZrO₂ to In₂O₃ leads to a sharp increase in the sensory response to hydrogen. The maximum sensory response of the ZrO₂?In₂O₃ composite to 1100 ppm of hydrogen increases from 80 to 270 as the ZrO₂ content changes 0 to 20 wt %. The response to CO varies only slightly. For Co₃O₄?In₂O₃ composites, the maximum response to H₂ and CO increases with the Co₃O₄ content within 0?10 wt %. A further increase in the Co₃O₄ content leads to a significant decrease in the response, with composites containing ～60 wt % Co₃O₄ being characterized by a very low efficiency. In the Co₃O₄?In₂O₃ system with a content of up to 60 wt % Co₃O₄, electronic conduction is realized, which changes to hole conduction at Co₃O₄ within 80?100 wt %. In the ZrO₂?In₂O₃ system, electric current flows through In₂O₃ nanocrystals, i.e., n-type conduction takes place. Possible reasons for the observed effects are discussed.     

Observation of multiple superconducting gaps in the infrared reflectivity spectra of Ba(Fe<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe_0.9Co_0.1)₂As₂ are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/h (2Δ is the superconducting gap and h is Planck’s constant). This is evidence for the s ^+/− or s ^+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe_0.9Co_0.1)₂As₂ manifest opening of several superconducting gaps at temperatures lower than critical T _c .     

Structural and magnetic properties of layered Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> thin films

Layered cobalt oxides Ca₃Co₄O₉ thin films have been grown directly on c-cut sapphire substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy characterizations show that the deposited films present the expected monoclinic structure and a texture along the direction perpendicular to the Al₂O₃(001) plane. The Ca₃Co₄O₉ structure presents six variants in the film plane. Rutherford backscattering spectroscopy shows that the films are stoichiometric and that the film thickness agrees with the nominal value. The susceptibility χ of the films, recorded along the c-axis of the substrate, after field cooling and zero field cooling in an applied field of 1 kOe shows two magnetic transitions at 19 and 370 K which agree well with previous findings on single crystal samples. In turn, at low temperature (5 K), the magnetization curve along the c-axis exhibits coercive field and remanent magnetization much smaller than those reported for bulk samples, which can be related to the influence of structural variants and structural defects.     

Synthesis,characterization and magnetic properties of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanotubes

Well-aligned Co₃O₄ nanotubes were synthesized within the nanochannels of porous anodic alumina membranes using a single-source chemical vapor deposition method. Scanning electron microscopy and transmission electron microscopy showed that the Co₃O₄ nanotubes are highly ordered with uniform diameter in the range of 100–300 nm and length up to tens of microns. X-ray diffraction, the Raman spectrum, energy-dispersive spectroscopy and selected-area electron diffraction demonstrated that the nanotubes are composed of pure cubic phase polycrystalline Co₃O₄. Magnetic measurements using a SQUID magnetometer suggested the presence of a strong antiferromagnetic interaction with Weiss constant θ= -248 K. The real and imaginary parts of the ac susceptibility at f= 10 Hz had a maximum at 4.0 K, and the field dependence of the magnetization at 1.8 K showed a small hysteresis loop with a coercivity of ∼ 98 Oe. PACS 81.07.De; 81.15.Gh; 78.30.-j; 75.75.+a; 61.46.Np     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Synthesis and electrochemical properties of Li<Subscript>1.2</Subscript>Mn<Subscript>0.54</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>O<Subscript>2</Subscript> cathode material for lithium-ion battery

The layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ lithium-rich manganese-based solid solution cathode material has been synthesized by a simple solid-state method. The as-prepared material has a typical layered structure with R-3m and C2/m space group. The synthesized Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has an irregular shape with the size range from 200 to 500 nm, and the primary particle of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has regular sphere morphology with a diameter of 320 nm. Electrochemical performances also have been investigated. The results show that the cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ prepared at 900 °C for 12 h has a good electrochemical performance, which can deliver a high initial discharge capacity of 233.5, 214.2, 199.3, and 168.1 mAh g^?1 at 0.1, 0.2, 0.5, and 1 C, respectively. After 50 cycles, the capacity retains 178.0, 166.3, 162.1, and 155.9 mAh g^?1 at 0.1, 0.2, 0.5, and 1 C, respectively. The results indicate that the simple method has a great potential in synthesizing manganese-based cathode materials for Li-ion batteries.     

Synthesis and electrocatalytic activities of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocubes

In this article, a hydrothermal method was developed to synthesize Co₃O₄ nanocubes using hydrogen peroxide (H₂O₂) as oxidant, Co(NO₃)₂·6H₂O as a cobalt source. The products are characterized in detail by multiform techniques including X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the obtained products are Co₃O₄ nanocubes with size ranging between 20 and 40 nm. The effects of the hydrogen peroxide concentration on the size of the products have been studied. The electrocatalytic activities of H₂O₂ reduction on Co₃O₄ nanocubes in phosphate buffer were also evaluated.     

Fast electrochromic response of porous-structured cobalt oxide (Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) thin films by novel nebulizer spray pyrolysis technique

Electrochromic effect of cobalt oxide thin films was studied as a function of substrate temperature (573–673 K). Tricobalt tetraoxide (Co₃O₄) thin films were deposited on glass and fluorine-doped tin oxide (FTO) substrates by nebulized spray technique using cobalt nitrate as precursor material. The XRD patterns indicated (311) plane was dominant for all the films irrespective of the deposition temperature. Williamson-Hall (W-H) analysis was made to understand the strain variation in the prepared Co₃O₄ films under different deposition temperature by employing uniform deformation model (UDM). Scanning electron microscopy images revealed porous morphology for the film prepared at 623 K. The optical parameters such as refractive index (n), extinction coefficient (k), and band gap were derived from UV-visible spectra of Co₃O₄ films. The electrochromic performance of the deposited Co₃O₄ films was analyzed through cyclic voltammetry, chronocoulometry, chronoamperometry, and iono-optical studies.     

Rectifying and photovoltage characteristics of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript>/Nb-doped SrTiO<Subscript>3</Subscript> heterojunction

A heterojunction was fabricated by growing a layer of Bi₂Sr₂Co₂O _y thin film on the 0.7 wt% Nb-doped SrTiO₃ substrate. Such heterojunction showed good rectifying characteristics over a wide temperature range, and its transport mechanism under the forward bias can be attributed to a space charge limited conduction mechanism via defects near the interface of the heterojunction. Photovoltaic properties of the heterojunction were studied by using both continuous-wave and pulsed irradiations and the results can be well explained by the photovoltaic effect of a p–n junction.     

Structural,magnetic, and magnetothermal properties of the Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>Co<Subscript>2</Subscript> compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb_0.3Dy_0.7Co₂ compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb_0.3Dy_0.7Co₂ compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb_0.3Dy_0.7Co₂ have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.     

Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@MnO<Subscript>2</Subscript> core shell arrays on nickel foam with excellent electrochemical performance for aqueous asymmetric supercapacitor

At present, a lot of attention has been paid to the reasonable design and synthesis of materials with core shell structure for high-performance supercapacitors. Herein, the Co₃O₄@MnO₂ core shell arrays on nickel foam are successfully synthesized via a facile and effective hydrothermal method followed with annealing process. The sample was characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Electrochemical performance of the Co₃O₄@MnO₂ material was studied using cyclic voltammetry, charge/discharge cycling, and electrochemical impedance measurements in 6 mol L^?1 KOH aqueous electrolyte. The results indicated that the Co₃O₄@MnO₂ material presented excellent electrochemical performance in terms of specific capacitance, cyclic stability, and charge/discharge stability.     

Effects of Sn doping on electrochemical characterizations of Li[Ni<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>]O<Subscript>2</Subscript> cathode material

Li[Ni_1/3Co_1/3Mn_1/3]O₂ and Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials for lithium battery are synthesized by a solid-state method. The samples are characterized by X-ray diffraction, scanning electron microscope, electrochemical impedance spectroscopy (EIS), and charge–discharge test. The results show that the Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ has a typical hexagonal α-NaFeO₂ structure and strawberry-like shape with uniform particle size. It has also been found that the Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ reveals better electrochemical performances than that without Sn doping. The EIS results suggest that Sn presence decreases the total resistance of Li[Ni_1/3Co_1/3Mn_1/3]O₂, which should be related to the improvement on the electrochemical properties.     

Causes of the Metamagnetism in a Disordered EuMn<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>O<Subscript>3</Subscript> Perovskite

The magnetic properties of the EuMn_0.5Co_0.5O₃ perovskite synthesized under various conditions are studied in fields up to 140 kOe. The sample synthesized at T = 1500°C is shown to exhibit a metamagnetic phase transition, which is irreversible below T = 40 K, and the sample synthesized at T = 1200°C demonstrates the field dependence of magnetization that is typical of a ferromagnet. Both samples have T_C = 123 K and approximately the same magnetization in high magnetic fields. The metamagnetism is assumed to be related to a transition from a noncollinear ferromagnetic phase to a collinear phase, and the presence of clusters with ordered Co²⁺ and Mn⁴⁺ ions leads to ferromagnetism. The noncollinear phase is formed due to the competition between positive Co²⁺–Mn⁴⁺ and negative Mn⁴⁺–Mn⁴⁺ and Co²⁺–Co²⁺ interactions, which make almost the same contributions, and to the existence of a high magnetic anisotropy.     

Electrospinning preparation of one-dimensional Co<Superscript>2+</Superscript>-doped Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> nanofibers for high-performance lithium ion battery

One-dimensional Co²⁺-doped Li₄Ti₅O₁₂ nanofibers with a diameter of approximately 500 nm have been synthesized via a one-step controllable electrospinning method. The Co²⁺-doped Li₄Ti₅O₁₂ nanofibers were systematically characterized by XRD, ICP, TEM, SEM, BET, EDS mapping, and XPS. Based on the cubic spinel structure and one-dimensional effect of Li₄Ti₅O₁₂, Co²⁺-doped Li₄Ti₅O₁₂ nanofibers exhibit the enlarged lattice volume, reduced particle size and enhanced electrical conductivity. More importantly, Co²⁺-doped Li₄Ti₅O₁₂ nanofibers as a lithium ion battery anode electrode performs superior electrochemical performance than undoped Li₄Ti₅O₁₂ electrode in terms of electrochemical measurements. Particularly, the reversible capacity of Co²⁺-doped Li₄Ti₅O₁₂ electrode reaches up to 140.1 mAh g^?1 and still maintains 136.5 mAh g^?1 after 200 cycles at a current rate of 5 C. Therefore, one-dimensional Co²⁺-doped Li₄Ti₅O₁₂ nanofiber electrodes, showing high reversible capacity and remarkable recycling property, could be a potential candidate as an anode material.