High-temperature ordering-phase separation transition in the Fe<Subscript>50</Subscript>Co<Subscript>50</Subscript> alloy

High-temperature (above 1200°C) B2 ordering has been detected in the Fe₅₀Co₅₀ alloy by selected area electron diffraction. X-ray photoelectron spectroscopy data indicate that the transition from phase separation to ordering is accompanied by changes in both the d-electron valence band spectrum (ordering increases 3d-electron localization at the Fe atoms) and the 3s core level spectrum (phase separation increases the exchange interaction between the electron spins of the partially filled 3d shell and ionized 3s shell of the Fe and Co atoms). We conclude that an ordering-phase separation transition occurs not only at 730°C but also at a temperature slightly above 1200°C.     

Irreversibility Line Measurement and Vortex Dynamics in High Magnetic Fields in Ni- and Co-Doped Iron Pnictide Bulk Superconductors

The de-pinning or irreversibility lines were determined by ac susceptibility, magnetization, radio-frequency proximity detector oscillator (PDO), and resistivity methods in Ba(Fe_0.92Co_0.08)₂As₂ ( T _c = 23.2 K), Ba(Fe_0.95Ni_0.05)₂As₂ ( T _c = 20.4 K), and Ba(Fe_0.94Ni_0.06)₂As₂ ( T _c = 18.5 K) bulk superconductors in ac, dc, and pulsed magnetic fields up to 65 T. A new method of extracting the irreversibility fields from the radio-frequency proximity detector oscillator induction technique is described. Wide temperature broadening of the irreversibility lines, for any given combination of ac and dc fields, is dependent on the time frame of measurement. Increasing the magnetic field sweep rate (dH/dt) shifts the irreversibility lines to higher temperatures up to about dH/d t = 40,000 Oe/s; for higher dH/dt, there is little impact on the irreversibility line. There is an excellent data match between the irreversibility fields obtained from magnetization hysteresis loops, PDO, and ac susceptibility measurements, but not from resistivity measurements in these materials. Lower critical field vs. temperature phase diagrams are measured. Their very low values near 0 T indicate that these materials are in mixed state in nonzero magnetic fields, and yet the strength of the vortex pinning enables very high irreversibility fields, as high as 51 T at 1.5 K for the Ba(Fe_0.92Co_0.08)₂As₂ polycrystalline sample, showing a promise for liquid helium temperature applications.     

Angular-Dependent Vortex Pinning Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>/Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Quasi-Multilayers

A series of quasi-multilayers of YBa₂Cu₃O_7?δ (YBCO)/Y₂O₃ specifically 70 × (m YBCO/n Y₂O₃) were prepared on SrTiO₃ single crystal using pulsed-laser deposition (PLD) with a controlled deposition pulses of m = 40 and n = 2, 5, and 10 for YBCO and Y₂O₃, respectively. The x-ray diffraction patterns indicate that all the present quasi-multilayers exhibit good c-axis orientation. The angular dependence of critical current density (J _c ) on applied magnetic field directions are systemically measured to study the anisotropic vortex pinning performances for those quasi-multilayers. It is revealed that compared with the pure YBCO films, the quasi-multilayers with n = 2, i.e., a proper constituent pulse of Y₂O₃, exhibits the enhanced vortex pinning abilities in all angles between c-axis orientation and the applied magnetic field direction. As well, such a quasi-multilayer film (n = 2) shows the higher lift factor J _c (Θ)/ J _c (90°) and much better vortex pinning properties at high fields and high temperatures, showing promising potential for coated conductor application.     

Anomalous Hall effect in Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> granular films deposited by magnetron co-sputtering

In this research, Co _x Si_1?x granular films with Co volume fraction (x) varying from 0.36 to 1 were prepared by using a magnetron co-sputtering method. The influence of x on the temperature-dependent longitudinal resistivity (\({\rho _{xx}}\)) and anomalous Hall effect (AHE) in Co _x Si_1?x granular films were systematically studied. The dependence of \({\rho _{xx}}\) on measuring temperature could be explained by the superposition of fluctuation-induced tunneling (FIT) process and temperature-dependent scattering. Interestingly, the contribution from scattering and FIT can be distinguished. The saturated Hall resistivity (\(\rho _{{xy}}^{A}\)) increased monotonically with decreasing x. In addition, the \(\rho _{{xy}}^{A}\) amplified nearly 26-fold as x decreased from 1 to 0.4. For the pure Co film (x?=?1), the intrinsic and side jump mechanisms dominated the Hall transport. However, the deviation of classical scaling relation was observed in other samples, which was mainly ascribed to the influence of interfacial scattering.     

Effects of <Emphasis Type="Italic">β</Emphasis>-Sn grain <Emphasis Type="Italic">c</Emphasis>-axis on electromigration behavior in BGA Sn3.0Ag0.5Cu solder interconnects

The anisotropy of β-Sn grain can significantly affect the electromigration (EM) behavior in Sn3.0Ag0.5Cu (SAC305) solder interconnects. A real ball grid array (BGA) specimen with a cross sectioned edge row suffered electromigration for 600 h to investigate the effects of β-Sn c-axis on the behavior of electromigration in SAC305 solder interconnects. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were used to obtain the microstructure and orientation of β-Sn grains in as-reflowed and low current density conditions. Besides, the orientation of c-axis had a great effect on the growth direction of IMCs in solder matrix. The solder interconnect with the Sn grain c-axis pointing the positive direction of ND would emerge serious electromigration phenomena. The density of Cu₆Sn₅ IMCs distributing at the surface of solder matrix increased obviously. However, when Sn grain c-axis was in the same direction with the opposite direction of ND, the original Cu₆Sn₅ IMCs in as-reflowed solder interconnect disappeared. Therefore, the results show that the solder interconnects will performance a different electromigration behavior due to the direction of c-axis in Sn grain: the growth direction of Cu₆Sn₅ IMCs in solder matrix will along the c-axis accompanied growing into solder matrix or gathering at the surface of the cross section.     

The H-T and P-T Phase Diagram of the Superconducting Phase in Pd:Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

We study the magnetic field vs. temperature (H – T) and pressure vs. temperature (P – T) phase diagrams of the T _c ≈ 5.5 K superconducting phase in Pd _x Bi₂Te₃ (x ≈ 1) using electrical resistivity versus temperature measurements at various applied magnetic fields (H) and magnetic susceptibility versus temperature measurements at various applied magnetic fields (H) and pressure (P). The H – T phase diagram has an initial upward curvature as observed in some unconventional superconductors. The critical field extrapolated to T = 0 K is H _c (0) ≈ 6–10 kOe. The T _c is suppressed approximately linearly with pressure at a rate d T _c /d P ≈ ?0.28 K/GPa.     

Hybrid Josephson Junctions with Iron-based and Conventional Superconductor Electrodes

We investigate the iron-based superconductor Ba(Fe _1?x Co _x ) ₂As ₂ (Ba-122) regarding its superconducting properties and possible applications. Therefore, Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and a b-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I– V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I– V characteristics and I _c R _n– T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I _c R _n product of the interface-engineered barrier junction was 12 μV and the planar junction with Au barrier showed 18 μV, we could increase the I _c R _n to 90 μV for planar TiO _x barrier junctions.     

Structural and Magnetic Properties of Transition Metal-Adsorbed MoS<Subscript>2</Subscript> Monolayer

The electronic and magnetic properties of transition metal (TM) atoms (TM = Co, Cu, Mn Fe, and Ni) adsorbed on a MoS₂ monolayer are investigated by density functional theory (DFT). Magnetism appears in the case of Co, Mn, and Fe. Among the three magnetic cases, the Co-adsorbed system has the most stable structure. Therefore, we further study the interaction in the two-Co-adsorbed system. Our results show that the interaction between the two Co atoms is always ferromagnetic (FM) and the p–d hybridization mechanism results in such ferromagnetic states. However, the FM interaction is obviously suppressed by increasing the Co–Co distance, which could be well explained by the Zener–Ruderman–Kittel–Kasuya–Yosida (RKKY) theory. Moreover, similar magnetic behavior is observed in the two-Mn-adsorbed system and a longrange FM state is shown. Such interesting phenomena suggest promising applications of TM-adsorbed MoS₂ monolayer in the future.     

Magnetism of 3d Transition Metals Doped 2H, 4H and 6H-GaN Polytypes

We present spin-polarized density-functional theory study of substitutional 3d transition metal (TM) atoms (Sc → Ni) in various host polytypes of GaN. For the structural parameters, we found that a(c) decreases (increases) from Sc to Ni. Additionally, calculations reveal decreasing (increasing) of the lattice constants a (c/a ratio) with increasing hexagonality h. In spite of the small cell used, TM atoms doped GaN polytypes with Ti, Cr, Mn, Fe, Co, and Ni show ferromagnetic character; whereas Sc and V dopant systems show, respectively, semiconducting and metallic behavior. The origin behind magnetism in 3d transition metal atoms doped 2H, 4H, and 6H-GaN polytypes is clarified through the ordering of spin-up (t+, e+) and spin-down (t-, e-) states.     

Studies of the Local Structure and the g Factors of the Cu<Superscript>2+</Superscript> Site in Y<Subscript>2</Subscript>BaCuO<Subscript>5</Subscript>

The local structure and the g factors (g _x , g _y , and g _z ) of the Cu²⁺ site in Y₂BaCuO₅ are theoretically studied using the perturbation formulas of the g factors for a 3d⁹ ion in orthorhombically elongated octahedra. The orthorhombic field parameters in these formulas are determined from the superposition model and the local geometry of the system. From the calculations, the oxygen octahedron is found to undergo the local elongation ΔZ (≈0.05 Å) along c-axis and the relative bond length variation ΔX (≈0.1 Å) along a- and b-axes, respectively. The calculated g factors based on the above local structure are in good agreement with the experimental data. The relationships between the anisotropies of the g factors and the low symmetrical (orthorhombic) distortions of the Cu²⁺ site in Y₂BaCuO₅ are discussed.     

Effect of Co partial substitution on the valence state of Ru in the Gd<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Polycrystalline samples of Gd_2?xCo _x Ru₂O₇ with x = 0.0, 0.1 and 0.4 were synthesized by the molten salt method. The samples were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrical resistivity measurements. Rietveld refinements of the XRD patterns and XPS measurements showed that the Co²⁺ ion replaces Gd³⁺ sites. As a result, the lattice parameter a and the Ru–O bond length decrease; then, the Ru–O–Ru bond angle increases. Those changes induce a charge compensation which was detected by XPS measurements. The analysis of these results shows that the Ru 3d_5/2 core level could be fitted assuming the contribution of two different chemical states of the Ru. The Ru 3d_5/2 core level is localized at 280.7 and 281.6 eV, which corresponds to Ru⁴⁺ and Ru⁵⁺. The valence band XPS spectra show an increase in Co 3d states at the Fermi level as the Co content increases, which contribute to the decrease in the electrical resistivity.     

Crystal Structure of Nb<Subscript>5</Subscript>Si<Subscript>3 − <Emphasis Type="Italic">x</Emphasis></Subscript>P<Subscript>0.5 + <Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0–0.5)

The structure of the ternary compound Nb₅Si_{3 ? x}P_{0.5 + x} (x = 0–0.5) is determined by single-crystal x-ray diffraction: sp. gr. P6₃mc, a = 1.32350(5) nm, c = 0.52954(1) nm; R _F = 0.028, R_w = 0.081 for 547 reflections with F _hkl > 4.0σ(F _hkl ). Relations between the structure of Nb₅Si_{3 ? x}P_{5 + x}, the Mn₅Si₃ structure type, and its derivatives are discussed.     

Perchlorosilanes and perchlorocarbosilanes as precursors for SiC synthesis

Homologues with the general stoichiometry a(SiCl₄): bSi: cC: d(SiC) are shown to be potential precursors for the low-temperature gas-phase synthesis of silicon carbide. Thermal decomposition of these precursors yields the chemically stable gaseous species SiCl₄ and condensed Si, C, SiC, SiC + Si, or SiC + C. We use thermodynamic modeling of the thermal decomposition of octachlorotrisilane, Si₃Cl₈, to analyze the key features of the thermolysis of perchlorosilanes with the general stoichiometry a(SiCl₄): bSi. The equilibrium compositions of reaction products in the Si₃Cl₈-CO system are determined. This reaction system enables low-temperature (400–1200 K) formation of silicon carbide.     

Carrier mobility in Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> crystals

We have studied the 300-K carrier mobility in Czochralski-grown single crystals of undoped and heavily boron doped Si_{1 ? x} Ge _x (0 < x < 0.02) alloys. Comparison of our experimental data with theoretical predictions indicates that the Ge atoms in dilute Si_{1 ? x} Ge _x alloys do not act as neutral scattering centers. The carrier mobility related to the scattering by Ge atoms is governed by disorder (alloy) scattering. The carrier mobility in the boron-doped Si_{1 ? x} Ge _x alloys is much lower than that in the undoped alloys, depends very little on Ge content, and is governed by ionized impurity scattering.     

AC Potential-Dependent Concentration Variation and Domain Wall Pinning in Co1−x Zn x (x=0.4−0.5) Nanorods

The Co_1?x Zn _x (x=0.4?0.5) nanorods were synthesized via an AC electrochemical deposition method into anodized aluminum oxide (AAO) templates at different voltages ranging from 10 to 18 V, and nanorods of varying concentrations of Co and Zn were obtained. The characterization tools were used to examine different aspects of nanorods, e.g., shape, size, morphology, chemical composition, and magnetic behavior. Scanning electron microscope (SEM) images show that CoZn nanorods have length L=1μm and diameter d=50 nm. The grain size was calculated to be 25.4 nm using an X-ray diffraction (XRD) technique. The XRD also shows some other phases of ZnCoO. The M?H loops measured by a vibrating sample magnetometer (VSM) at room temperature show pure ferromagnetic behavior at all AC potentials. The nanorods show magnetic isotropic behavior due to strong magnetic interactions and presence of random nanorods. The potential-dependent coercivity H _c and saturation magnetization M _s show a non-linear curve which is explained on the basis of magnetic islands and domain wall pinning. This study is useful to tune the magnetic properties of nanorods by a simple and low-cost technique.     

Effects of LiNbO<Subscript>3</Subscript> doping on the microstructures and electrical properties of BiScO<Subscript>3</Subscript>–PbTiO<Subscript>3</Subscript> piezoelectric system

Piezoelectric ceramics xLiNbO₃–yBiScO₃–(1?x?y)PbTiO₃ (LN–BS–PT, 0.00?≤?x?≤?0.10, 0.30?≤?y?≤?0.36) were synthesized and their phase diagram and morphotropic phase boundary between rhombohedral and tetragonal phases have been confirmed. The optimal properties were found at the composition of 0.03LN–0.36BS–0.61PT with piezoelectric coefficient d₃₃^* value of 702 pm/V, d₃₃ of 551 pC/N, planar electromechanical coupling factor k_p of 0.51, remnant polarization P_r of 46.5 µC/cm², Curie temperature T_c of 337 °C, and a large strain of 0.351% at an electric field of 50 kV/cm and frequency of 2 Hz with a low strain hysteresis of 5.9%. The Curie temperature of the ternary system presents a linear relationship with LiNbO₃ and BiScO₃ contents. The optimization of these electric properties was probably ascribed to the enhancement in domain walls and the improving mobility of domain switching due to LiNbO₃ doping.     

Specific Heat Jump at T<Emphasis Type="Italic">c</Emphasis> of High T<Emphasis Type="Italic">c</Emphasis> Superconductors: Effect of Van Hove Singularity

In the BCS framework, exact expressions for the ratio between the jump in the specific heat at T _c and the normal phase specific heat are derived within the Van Hove singularity scenario. Analytical results are obtained for an isotropic s-wave and anisotropic d-wave pairing symmetries. Graphical solutions of the ratio as functions of ω _D /T _c and E _F /T _c , where ω _D is the cutoff energy and E _F is the Fermi energy, show significant deviations from the BCS value of 1.43.     

Synthesis and Superconductivity of Layered Compounds with Orthogonal [Zr<Subscript>2</Subscript>N<Subscript>2</Subscript>] Network

Layered α-form ZrNX (X: Cl and Br) compounds with high quality were prepared by chemical vapor transport. The intercalation of alkali metal A (A: Li, Na, K, Rb) was carried out to realize electron doping into the orthogonal [Zr₂N₂] layers. The Rietveld refinement analysis reveals that the [Zr₂N₂] crystalline layers in the intercalation compounds shift mutually in the ab plane when compared with the hosts. Magnetic measurements show that the intercalation compounds A _x ZrNX are changed into superconductors with transition temperature T _c of up to 12 K. Upon the cointercalation of solvent molecules such as THF, T _c decreases to as low as 6.1 K with increasing the interlayer spacing d up to 14 Å, which is similar to the d dependence of T _c recently found in electron-doped α-form TiNX series. We also succeeded in synthesizing another new polymorph of α-Zr₂N₂S by the topochemical reaction between α-form ZrNX and Na₂S. α-Zr₂N₂S (space group: Immm, a = 4.1375(1) Å, b = 3.5422(1) Å, and c = 11.5204(3) Å) has the same α-[Zr₂N₂] layers, whereas the interlayer spacing between two adjacent [Zr₂N₂] layers is effectively decreased by 1/3 when compared with the parent compounds of ZrNX.     

First-principles Investigation of Half-metallicity and Ferrimagnet Properties of Co2ScZ (Z = As,Sb, and Bi)

Based on the first-principles methods, the half-metallic ferrimagnet properties of Co₂ScZ (Z = As, Sb, and Bi) full-Heusler alloys have been predicted by employing the full-potential linearized plane wave plus local orbital method (FP-LAPW + lo) within the framework of density functional theory (DFT) and generalized gradient approximation plus Coulomb repulsion (GGA + U) parameterization. The GGA scheme is served in this work to obtain only the equilibrium structural parameters in both paramagnetic and ferromagnetic phases, whereas the GGA + U scheme is exploited to treat the “d” electrons through electronic and magnetic properties of these Heusler alloys. The electronic structure investigation shows that all Co₂ScAs, Co₂ScSb, and Co₂ScBi Heusler alloys are stable half metals with half-metallic gap energies (E _HM) of 0.351, 1.080, and 0.836 eV, respectively. The total magnetic moment follows the generalized Slater-Pauling rule, where its value is around 2 μ _B for all these compounds. We have also found opposite signs between atomic magnetic moments of Co and Sc transition elements, demonstrating the ferrimagnetic behavior.     

Structural Formation and Improved Performances of Chemically Synthesized Composition-Controlled Micron-Sized Fe100−x Co x Particles

Micron-sized composition-controlled Fe_100?x Co _x (20 < x < 75) alloy particles with high purity have been prepared by an optimized reduction reaction. The influence of Co content on the alloying process, structures, and magnetic properties of the products has been studied. The as-synthesized Fe_{100 ?x} Co _x with x < 65 exhibit a single bcc crystal structure. A bcc-FeCo/fcc-Co composite structure can be formed in the Fe_100?x Co _x products with x > 65. Very slight surface oxidation is observed in all the products. The high purity and single bcc-FeCo phase for the well-alloyed Fe_{100 ?x} Co _x particles with x < 65 lead to their high saturation magnetization of 182–220 A m² kg^?1. All the well-alloyed Fe_{100 ?x} Co _x show nearly spherical morphologies with an average particle size of 2–8 μm, which results in their good compactibility with a high compacted density of about 7.4–7.6 g cm^?3. The simple preparation and improved performances for these chemically synthesized composition-controlled FeCo particles show their great potential for applications in near-net-shaped and complex-shaped FeCo-based soft magnetic composite devices.