Spin polarization and Gilbert damping of Co2Fe(GaxGe1−x) Heusler alloys

The spin polarization (P) of ferromagnetic Heusler alloys, Co₂Fe(Ga_xGe_1?x) (x = 0, 0.25, 0.5, 0.75, and 1), is investigated by point contact Andreev reflection (PCAR) measurements. While the P values of the ternary Co₂FeGe and Co₂FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co₂Fe(Ge_0.5Ga_0.25) is as high as 0.69. Co₂Fe(Ge_0.5Ga_0.5) alloy shows a strong tendency for L2₁ ordering and a high Curie temperature of 807 °C. Ab initio calculations indicate that the band structures of the Co₂FeGe and Co₂Fe(Ge_0.5Ga_0.5) alloys with L2₁ or B2 structures are half-metallic. Thin films of the quaternary Co₂Fe(Ge_0.5Ga_0.5) alloy grown on MgO(0 0 1) substrates order to the L2₁ structure upon annealing at 500 °C, giving rise to a high P of 0.75. This is the highest P value reported for Heusler alloy thin films using the PCAR technique. Ferromagnetic resonance measurements show that the Gilbert damping constant of the film is ～0.008. All these indicate that the Co₂Fe(Ge_0.5Ga_0.5) alloy is promising as a spin polarized current source for spintronics devices.     

Microstructure and spin polarization of quaternary Co2Cr1−xVxAl,Co2V1−xFexAl and Co2Cr1−xFexAl Heusler alloys

The microstructures, magnetic properties and spin polarization of quaternary Co₂Cr_1−xV_xAl, Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys were investigated. Phase separation into A2 and B2/L2₁ structure occurs in Co₂CrAl and Co₂Cr_0.6Fe_0.4Al, whereas Co₂FeAl exhibits a single-phase B2 structure. The ordered L2₁ structure becomes more stable with increasing vanadium concentration (x ⩾ 0.35). The saturation magnetization measured at 5 K for Co₂Cr_1−xV_xAl alloy changes from 1.4 to 2.0 μ_B when x increases from 0.0 to 0.5 and then becomes 1.4 μ_B for x = 1.0. This behavior can be attributed to the variation in the local magnetic moment of Co atoms. The saturation magnetization of Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys increases with increasing Fe concentration. The spin polarization decreases from 0.62 to 0.56 with increasing x for Co₂Cr_1−xFe_xAl alloy. Also, the spin polarization decreases with increasing x for Co₂Fe_1−xV_xAl and Co₂Cr_1−xV_xAl alloys. Possible reasons for the reduced spin polarization in these alloys are discussed.     

Preparation and thermoelectric properties of p-type (Ga2Te3)x–(Bi0.5Sb1.5Te3)1−x (x = 0–0.2) alloys prepared by spark plasma sintering

p-Type (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x (x = 0–0.2) alloys were prepared by spark plasma sintering technique, and the effect of gallium telluride (Ga₂Te₃) on the thermoelectric properties was experimentally determined. Measurements have shown that the electrical conductivities of (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x are improved and thermal conductivities reduced after the introduction of Ga₂Te₃ in the Bi_0.5Sb_1.5Te₃ alloy without noticeable loss of Seebeck coefficient. The maximum thermoelectric figure of merit ZT of 1.0 is obtained with molar fraction x = 0.1 at 335.5 K, being approximately two times that of the Bi_0.5Sb_1.5Te₃ at the corresponding temperature, thus proving that (Ga₂Te₃)_x–(Bi_0.5Sb_1.5Te₃)_1−x (x = 0–0.2) alloys are to be a promising material for application.     

The ductility and shape-memory properties of Ni–Mn–Co–Ga high-temperature shape-memory alloys

Ni–Mn–Co–Ga alloys with Ni/Mn or Ni and Mn substituted by Co were investigated as candidates for high-temperature shape-memory alloys. Ni_56?xCo_xMn₂₅Ga₁₉ alloys with x < 8 consist of single phase martensite, whereas Ni_56?xCo_xMn₂₅Ga₁₉ (x ? 8), Ni₅₆Mn_25?yCo_yGa₁₉ (y = 4, 8) and Ni_56?z/2Mn_25?z/2Co_zGa₁₉ (z = 4, 6) alloys consist of a two-phase mixture of martensite and γ phase. The mechanical and shape-memory properties of Ni₅₆Mn_25?yCo_yGa₁₉ and Ni_56?z/2Mn_25?z/2Co_zGa₁₉ alloys, which were hot-rolled into 0.5 mm thin plates by conventional hot rolling process, were investigated. The ductility and hot-workability of Ni–Mn–Co–Ga alloys were greatly improved by increasing the amount of ductile γ phase. Dynamic tensile tests and scanning electron microscopy observations of fracture surfaces confirm that the existence of γ phase plays a key role in improving the ductility of Ni–Mn–Co–Ga alloys.     

The effect of molybdenum on the corrosion behaviour of the high-entropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments

The purpose of this study is to investigate the electrochemical properties of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x high-entropy alloys in three aqueous environments which simulate acidic, marine, and basic environments at ambient temperature (∼25 °C). The potentiodynamic polarisation curves of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x alloys, obtained in aqueous solutions of H₂SO₄ and NaOH, clearly revealed that the corrosion resistance of the Mo-free alloy was superior to that of the Mo-containing alloys. On the other hand, the lack of hysteresis in cyclic polarisation tests and SEM micrographs confirmed that the Mo-containing alloys are not susceptible to pitting corrosion in NaCl solution.     

Preparation of CuIn1−xGaxS2 (x = 0.5) flowers consisting of nanoflakes via a solvothermal method

CuIn_1−xGa_xS₂ (x = 0.5) flowers consisting of nanoflakes were successfully prepared by a biomolecule-assisted solvothermal route at 220 °C for 10 h, employing copper chloride, gallium chloride, indium chloride and l-cysteine as precursors. The biomolecule l-cysteine acting as sulfur source was found to play a very important role in the formation of the final product. The diameter of the CuIn_0.5Ga_0.5S₂ flowers was 1-2 μm, and the thickness of the flakes was about 15 nm. The obtained products were characterized by X-ray diffraction (XRD), energy dispersion spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction spectroscopy (SAED), and UV-vis absorption spectroscopy. The influences of the reaction temperature, reaction time, sulfur source and the molar ratio of Cu-to-l-cysteine (reactants) on the formation of the target compound were investigated. The formation mechanism of the CuIn_0.5Ga_0.5S₂ flowers consisting of flakes was discussed.     

Martensitic transformation and magnetic properties in Ni–Fe–Ga–Co magnetic shape memory alloys

Ni–Fe–Ga–Co is a promising system for magnetic shape memory alloy applications, due to its good ductility, mobile twin boundaries and high transformation temperatures. Unlike previous studies which focused on compositions with a Ga content of 27 at.%, here the martensitic transformation and magnetic properties over a large composition range of Ni_54−xFe₂₀Ga₂₆Co_x, Ni_54−xFe₁₉Ga₂₇Co_x, Ni_56−xFe₁₇Ga₂₇Co_x and Ni_54−xFe₁₈Ga₂₈Co_x (x = 0, 2, 4) are investigated. The martensitic transformation temperature T_m and the Curie temperature T_c can be tailored in a wide range by changing composition and heat treatment. A coupling of martensitic and magnetic transformations at ∼90 °C is found for Ni₅₂Fe₁₇Ga₂₇Co₄. Additionally, the effect of thermal cycling on the martensitic transformation of single- and two-phase Ni–Fe–Ga–Co alloys is discussed. Furthermore, an intermediate face-centered cubic phase induced by powderization and transformed into a body-centered cubic phase by aging is reported. The saturation magnetization is significantly decreased by powderization, while recovered by the subsequent aging.     

FeCoNiCrCu0.5Alx高熵合金的结构和性能（英文）

研究Al含量和热处理对FeCoNiCrCu0.5Alx多主元高熵合金的相结构、硬度和电化学性能的影响规律。随着Al含量的增加,铸态合金的相结构由FCC相向BCC相转变。当x从0.5增加到1.5时,FeCoNiCrCu0.5Alx高熵合金的稳定结构由FCC结构向FCC+BCC双相结构转变。BCC相的硬度高于FCC相的,在氯离子及酸性介质中BCC相的耐腐蚀性均优于FCC相的。FeCoNiCrCu0.5Al1.0铸态合金具有高硬度和良好的抗腐蚀性能。     

An investigation on hydrogen storage kinetics of nanocrystalline and amorphous Mg2Ni1−xCox (x = 0-0.4) alloy prepared by melt spinning

In order to improve the hydrogen storage kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂Ni_1−xCo_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The structures of the as-cast and spun alloys are characterized by XRD, SEM and TEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the as-spun alloys is tested by an automatic galvanostatic system. The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The electrochemical impedance spectrums (EIS) and the Tafel polarization curves are plotted by an electrochemical workstation. The results show that the substitution of Co for Ni notably enhances the glass forming ability of the Mg₂Ni-type alloy. Furthermore, the substitution of Co for Ni, instead of changing major phase Mg₂Ni, leads to forming secondary phases MgCo₂ and Mg. Both the melt spinning treatment and Co substitution significantly improve the hydrogen absorption and desorption kinetics. The high rate discharge ability, the hydrogen diffusion coefficient and the limiting current density of the alloys significantly increase with raising both the spinning rate and the amount of Co substitution.     

Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments

High-entropy alloys (HEAs) are a newly developed family of multi-component alloys. The potentiodynamic polarization and electrochemical impedance spectroscopy of the Al_xCrFe_1.5MnNi_0.5 alloys, obtained in H₂SO₄ and NaCl solutions, clearly revealed that the corrosion resistance increases as the concentration of aluminium decreases. The Al_xCrFe_1.5MnNi_0.5 alloys exhibited a wide passive region, which extended >1000 mV in acidic environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. SEM micrographs revealed that the general and pitting corrosion susceptibility of the HEAs increased as the amount of aluminium in the alloy increased.     

Phase evolution, crystal structure and dielectric behavior of (1 − x)Nd(Zn0.5Ti0.5)O3 + xBi(Zn0.5Ti0.5)O3 compound ceramics

The phase evolution, crystal structure and dielectric properties of (1 − x)Nd(Zn_0.5Ti_0.5)O₃ + xBi(Zn_0.5Ti_0.5)O₃ compound ceramics (0 ≤ x ≤ 1.0, abbreviated as (1 − x)NZT-xBZT hereafter) were investigated. A pure perovskite phase was formed in the composition range of 0 ≤ x ≤ 0.05. The B-site Zn²⁺/Ti⁴⁺ 1:1 long range ordering (LRO) structure was detected by both XRD and Raman spectra in x ≤ 0.05 samples. However, this LRO structure became gradually degraded with an increase in x. The dielectric behaviors of the compound ceramic at various frequencies were investigated and correlated to its chemical composition and crystal structure. A gradually compensated τ_f value was obtained in (1 − x)NZT-xBZT microwave dielectrics at x = 0.03, which was mainly due to the dilution of dielectric constant in terms of Claussius-Mossotti differential equation.     

Effect of gallium on the crystal structure and magnetic properties of PrFe11 − x Ga x C y compounds

A homogeneity range of the PrFe_{11 ? x} Ga _x C _y phase was determined. As the gallium content increases, the lattice parameters were shown to increase, whereas the Curie temperature decreases; the type of magnetic anisotropy at room temperature is unchanged. In the alloys with x > 3, the transformation of the tetragonal lattice of the compound into an orthorhombic lattice with the axial ratio a/b ≤ 1.006 is observed. This is likely to be caused by the ability of gallium to form covalent bonds. Gallium atoms were found to occupy preferentially 16j ₂ and 4e ₂ sites in the orthorhombic lattice of the FePr_6.5Ga_4.5C compound. The atoms located in the 16j ₂ sites form wavy chains along the [010] direction of the orthorhombic lattice, whereas the iron atoms located in the 16j ₁ sites form analogous chains along the [100] direction. Owing to this fact, one of these directions is likely to become an easy axis. The alloy with x= 4.5 has a low coercive force at room temperature; at 114 K, its coercive force is H _c = 42.4 kA/m.     

Microstructure and properties of Al0.3CrFe1.5MnNi0.5Ti x and Al0.3CrFe1.5MnNi0.5Si x high-entropy alloys

In this article, the microstructure, hardness, and corrosion resistance of the Al0.3CrFe1.5MnNi0.5Tixand Al0.3CrFe1.5MnNi0.5Six(x = 0, 0.2, 0.5, 1.0) high-entropy alloys were investigated via X-ray diffraction(XRD)scanning electron microscopy(SEM), digital display Vickers hardness tester, and electrochemical technique These alloys are mainly composed of BCC solid-solution structure. When adding high content of Ti or Si elemen(x C 0.5), some intermetallic compounds are found in the microstructure, which makes the alloys have a high hardness, high brittleness, and easy cracking. While the alloys with low content of Ti or Si(x = 0.2) have a hardness of HV 420–HV 430, and its hardness increases about 14 %compared with that of Al0.3CrFe1.5MnNi0.5. Electrochemical results in 3.5 % NaCl solution show that the alloying elements Ti and Si have a negative influence on the corrosion resistance of the Al0.3CrFe1.5MnNi0.5alloys.     

Thermal expansion behavior,microhardness and electrochemical corrosion resistance properties of Au52Cu27Ag17–x(NiZn0.5)x alloys

The thermal expansion behavior, microhardness and electrochemical corrosion resistance of Au₅₂Cu₂₇Ag_17–x(NiZn_0.5)_x (x=0, 6 and 12) alloys were investigated by dilatometer (DIL), microhardness tester, electrochemical workstation, X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). With increasing x, the relative length expansion and DIL maximum temperature T_{l m} (i.e., thermal stability) of the alloys increase in thermal expansion measurements, which can be explained by the change of the atomic binding energy, mismatch entropy together with phase transformation. With the increase of x, the microhardness can be improved, but the corrosion resistance decreases; in addition, the anodic peak current densities of polarization curves decrease, which are related closely with the solid solution degree and dissolution of Ag, Ni and Zn alloying elements in Cl⁻-containing solution.     

Crystal structures of the compounds formed in DyCo2–DyGa2 pseudo-binary system at 773 K

DyCo_2?xGa_x alloys with x from 0 to 2 were prepared by arc melting and investigated by X-ray powder diffraction. Six ternary compounds with different Ga contents x, DyCo₂, DyCo_1.33Ga_0.67, DyCoGa, DyCo_0.67–0.35Ga_1.33–1.65, DyCo_0.25–0.15Ga_1.75–1.85, and DyGa₂, were formed in the system. The crystal structures of these compounds were determined by Rietveld refinement method with TOPAS V3.0 software. The Rietveld refinements reveal that DyCo_1.33Ga_0.67 crystallized in the MgZn₂-type structure (P6₃/mmc) with a = 5.414(1) and c = 8.452(1) Å, DyCoGa in the Co₂Si-type structure (Pnma) with a = 7.052(1), b = 4.393(1) and c = 6.958(1) Å, DyCo_0.5Ga_1.5 in the GdPdSi-type structure (Imma) with a = 4.341(1), b = 7.007(1) and c = 7.551(1) Å, and DyCo_0.2Ga_1.8 in the AlB₂-type structure (P6/mmm) with a = 4.339(1) and c = 3.634(1) Å. The relationships between these structures were discussed.     

Electronic structures of Heusler alloy Co2FeAl1-xSix surface

We have calculated the electronic structures of Co2FeAl1-xSix(101) surface using first-principles method based on the density functional theory. Because of the surface effect, the minority spin band gap at the Fermi level disappears at the surface of bulk Co2FeAl1-xSix. However, beneath the surface, the minority spin gap opens at the Fermi level, which indicates that the electronic structures of Co2FeAl1-xSix(101) be-come close to that of bulk phase. Accordingly, the Co2FeAl1-xSix(101) surface is a composite tri-layer structure that corresponds to the weakening of half-metallic property in Co2FeAl1-xSix films. Even though, the spin polarization of Co2FeAl1-xSix(101) surface is still larger than that of Co2FeAl or Co2FeSi materials, making Co2FeAl1-xSix a promising spintronics material.     

Properties of Co-alloyed Ni-Fe-Ga Ferromagnetic Shape Memory Alloys

It is experimentally found that in the Ni₅₄Fe_20-x Co _x Ga₂₆ ferromagnetic shape memory alloys, Co variation from 0 to 9 at.% leads to: (i) almost linear change of martensitic transformation temperatures from ?70 °C to 120 °C; (ii) a non-monotonous change of the Curie temperature, and (iii) a linear decrease of saturation magnetization from 60 to 43 Am²/kg. The selected alloys grown as single crystals have been (magneto-) mechanically tested. The superelastic effect has been measured to be about 4%. The magneto-strain shows a training effect which is an evidence of the effect of magnetic-field-induced twin-boundary motion.     

Magnetic properties and phase diagram of Ni50Mn50−xGax ferromagnetic shape memory alloys

We present the magnetic properties and the magnetic phase diagram of Ni₅₀Mn_50?xGa_x ferromagnetic shape memory alloys across a wide concentration range. Martensitic transformation, intermediate transformation, B2–L2₁ order–disorder transformation, Néel and Curie temperatures are determined for the prepared samples. The martensitic transformation temperature decreases with increasing Ga concentration and bends two times when crossing the Curie temperature and the intermediate-phase transformation temperature. Spontaneous magnetization and its composition dependence were also investigated. Composition dependence of the transformation temperatures and the spontaneous magnetization in the martensite phase of Ni₅₀Mn_50?xGa_x are compared with those of Ni₅₀Mn_50?xIn_x and Ni₅₀Mn_50?xSn_x, revealing a similarity in the NiMn-based alloy systems.     

Influence of Ti additions on martensitic transformation and magnetic properties of cast Ni51Fe22−xGa27Tix shape memory alloys

The effect of Ti addition on the microstructure, martensitic transformation, magnetic and mechanical properties of polycrystalline Ni₅₁Fe_22?x Ga₂₇Ti _x (x=0, 2 and 4) ferromagnetic shape memory alloy was investigated by scanning electron microscope, differential scanning calorimetry and X-ray diffraction. The results showed that the martensitic transformation temperature increases monotonously with the increase of fraction of Ti substitution for Fe. The increase in the martensite transformation temperatures should be related to the change of the electron concentration after the addition of Ti to Ni₅₁Fe_22?x Ga₂₇Ti _x alloys. According to the results of X-ray diffraction and magnetic properties, Ti has significant effect the structure of Ni₅₁Fe_22-x Ga₂₇Ti _x . Adding of 4 at% Ti altered the structure of the matrix from five-layered tetragonal martensite of Ni₅₁Fe₂₂Ga₂₇ and Ni₅₁Fe₂₀Ga₂₇Ti₂ alloys to non-modulated tetragonal martensite. Magnetic properties proved that the alloy transits from ferromagnetic, five-layered tetragonal martensite, to paramagnetic, non-modulated martensite structure, with increasing Ti content to 4 at.%. Saturation magnetization, remnant magnetization and coercivity of the alloy were significantly influenced by Ti additions. Hardness values of Ni₅₁Fe₂₂Ga₂₇ increased by the addition of Ti.     

Coexistence of room temperature ferroelectricity and ferrimagnetism in multiferroic BiFeO3-Bi0.5Na0.5TiO3 solid solution

The structure, ferroelectric and magnetic properties of (1 − x)BiFeO₃-xBi_0.5Na_0.5TiO₃ (x = 0.37) solid solution fabricated by a sol-gel method have been investigated. X-ray diffraction and Raman spectroscopy measurements show a single-phase perovskite structure with no impurities identified. Compared with pure BiFeO₃, the coexistence of ferroelectricity and ferrimagnetism have been observed at room temperature for the solution with remnant polarization P_r = 1.41 μC/cm² and remnant magnetization M_r = 0.054 emu/g. Importantly, a magnetic transition from ferrimagnetic (FM) ordering to paramagnetic (PM) state is observed, with Curie temperature T_C ∼ 330 K, being explained in terms of the suppression of cycloid spin configuration by the structural distortion.