Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Microstructure and mechanical behaviour of (Fe88Si12)95Al5 alloy prepared by aluminothermics

Abstract

The (Fe₈₈Si₁₂)₉₅Al₅ alloy was prepared by an aluminothermics. The (Fe₈₈Si₁₂)₉₅Al₅ alloy is composed of spheroidic α-Fe(Si,Al) precipitate with size of 20–50 nm and γ-Fe(Si,Al) matrix. The yield strength and fracture strain in compression of the (Fe₈₈Si₁₂)₉₅Al₅ alloy are 1500 MPa and 23% respectively. The shear bands propagating in the compressive deformation are arrested by the precipitation particles that resulted in large ductility and high strength simultaneously.     

Structural relaxation and embrittlement of Fe74Co10B16 and Fe74Co5Cr5B16 metallic glasses

Structural relaxation and annealing embrittlement behaviour of Fe₇₄Co₁₀B₁₆ and Fe₇₄Co₅Cr₅B₁₆ metallic glasses has been studied by differential scanning calorimetry (DSC) and bend ductility measurements. A pre-anneal technique was employed with DSC to determine activation energies of relaxation at various temperatures. Activation energies of embrittlement were derived from measurements of the embrittlement kinetics. The results obtained for both the alloys are compared to ascertain the effects of chromium addition. A spectrum of activation energies is obtained corresponding to structural relaxation and embrittlement. The activation energies of structural relaxation are found to be slightly higher for the chromium-containing alloy than for the ternary alloy. This observation is consistent with the higher crystallization temperature of the chromium-containing alloy as reported earlier. The ductile-brittle transition temperature of the Fe₇₄Co₁₀B₁₆ glass, however, decreases by 50 K (for 15 min anneal) on addition of 5 at% chromium. Activation energies for embrittlement of the chromium-containing alloy are also considerably smaller than for the ternary alloy. It is concluded that despite increasing the thermal stability, chromium reduces the mechanical stability of Fe₇₄Co₁₀B₁₆ glass.     

Kinetics of crystallization of a Fe74Co10B16 glass

Differential scanning calorimetry (DSC) technique has been employed to study the crystallization kinetics of a Fe₇₄Co₁₀B₁₆ glass. Crystallization of Fe₇₄Co₁₀B₁₆ glass is known to occur in two distinct stages. It was possible to separate out the isothermal kinetics of both the crystallization stages through a thermal treatment scheme in the DSC. The crystallization processes are interpreted in the light of the kinetic data obtained.     

Exchange Spring Effect in RF-Annealed Amorphous Co55Fe25B10Si10 Ribbons

Exchange spring systems are promising for application in high performance data storage, novel permanent magnets and some other technologies. In this work, we report the exchange spring behavior in RF-annealed amorphous Co₅₅Fe₂₅B₁₀Si₁₀ ribbons. The measurements on magnetic and structural properties of irradiated samples revealed that the RF radiation has affected just the outermost surface layers of the ribbons. We have also reported the angular dependence of magnetization reversal and coercivity of a typical annealed sample. Our study shows that the RF annealing can be supposed to be a simple and easy manner to produce bilayer exchange spring systems.     

Magnetic properties of metallic glass Fe39Ni39Mo4Si6B12

Magnetic metallic glass Fe₃₉Ni₃₉Mo₄Si₆B₁₂ has been studied by the⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra consist of broad and overlapping six-line pattern below the Curie temperature. The Curie and crystallization temperatures of this metallic glass have been determined to be 575 ± 3 K and 725 ± 3 K, respectively. The hyperfine magnetic field at room temperature is approximately 225 kOe. The reduced hyperfine fields of this sample decrease much faster than that observed for other iron-rich metallic glasses like Fe₄₀Ni₄₀B₂₀, Fe₈₀B₂₀ etc. This behaviour is attributed to the presence of molybdenum atoms in the sample.     

Synthesis of high-energy-density Pr2Fe14−x Co x B,x⩽3, magnets for practical applications

Stable magnetic powders, of 1–2μm particle size, of partially Co-substituted, Pr₂Fe_14−x Co _x B,x⩽3, alloys together with 2–4 at% excess Pr were prepared by rapidly quenching the associated melts into thin ribbons and then mechanical attriting the ribbons in the refined particle sizes. The saturation magnetizationM _s, remanent magnetizationJ _r, intrinsic coercivityH _ci and Curie temperatureT _c were studied in characterizing the powders for fabricating into sintered or polymer bonded magnets. It is found that the smallx=0·4–0·8 substitution of the Co on Fe sites in this series sensitively leads to an increase in the value ofH _ci, by as much as 40%, with the optimum value of 21 kOe atx ∼ 0·55, together with an improvement in theT _c from 292°C to 325°C, without significantly diluting theM _s∼150 emu/g andJ _r∼8·0 kG values. The Co-substituted Pr₂Fe₁₄B alloy particles are better stable and corrosion resistant in ambient atmosphere. The results are discussed with the microstructure and comparison with the data for Nd₂Fe₁₄B powders processed under the same conditions. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Preparation and tensile properties of amorphous Fe78Si9B13/nano-Ni laminated composite

Amorphous Fe₇₈Si₉B₁₃/nano-Ni laminated composite was prepared by electrodeposition method. The tensile properties of laminated composite at room temperature were examined. The laminated composite exhibits a very high tensile strength and reasonable tensile elongation. This is attributed to a good bonding between amorphous Fe₇₈Si₉B₁₃ layer and nano-Ni layers. The amorphous layer can deform in conformity with Ni layers and be significantly stretched without fracture. The apparent surface energy γ_f of amorphous Fe₇₈Si₉B₁₃ ribbon in the laminated composite is 7 times larger than γ_f of amorphous ribbon in monolithic form. The isostrain model may be insufficient to explain the tensile behavior of the laminated composite.     

The effect of a precursor on the magnetostatic and dynamic characteristics of Co<Subscript>68</Subscript>Fe<Subscript>4</Subscript>Cr<Subscript>4</Subscript>Si<Subscript>12</Subscript>B<Subscript>12</Subscript> amorphous ribbons

We have studied the influence of a precursor on the magnetostatic and dynamic characteristics of Co₆₈Fe₄Cr₄Si₁₂B₁₂ amorphous alloy ribbons. It is established that a rational choice of the precursor provides for a significant increase in the properties of soft magnetic ribbons and noticeably increases the magnetoimpedance.     

Electronic and Magnetic Properties of Superconducting Sr4V2Fe2As2O6 Versus Sr4Sc2Fe2As2O6

First principles of FLAPW-GGA calculations have been performed with the purpose to understand the peculiarities of band structure and Fermi surface topology for recently discovered 37 K superconductor: Sr₄V₂Fe₂As₂O₆—in comparison with isostructural phase Sr₄Sc₂Fe₂As₂O₆. Our main finding is that the replacement of Sc with V leads to drastic transformation of electronic, magnetic and conductive properties of these materials: as against non-magnetic Sr₄Sc₂Fe₂As₂O₆ which is formed from non-magnetic conducting [Fe₂As₂] and insulating [Sr₄Sc₂O₆] blocks, Sr₄V₂Fe₂As₂O₆ consists of non-magnetic conducting [Fe₂As₂] blocks and [Sr₄V₂O₆] blocks which exhibit magnetic half-metallic properties.     

Formation mechanism of Fe80Si8B12 metallic glass

In order to gain an understanding of the glass-forming mechanism during the rapid quenching of a metallic alloy, the nucleation and growth process of the crystalline phase which competes with the metallic glass must be investigated. The microstructures of melt-spun Fe₈₀Si₈B₁₂ alloy ribbons with different thicknesses were examined using optical and electron microscopy. The phase competing with the metallic glass is-(Fe, Si) ferrite, nucleated by the homogeneous nucleation. The growth process of-(Fe, Si) dendrites was explained well by Liptonet al.'s theory of dendritic growth in an undercooled alloy melt. It was concluded that the easy glass-forming ability during rapid quenching of the Fe₈₀Si₈B₁₂ alloy is due to (i) the slow growth rate of-(Fe, Si) dendrite, and (ii) the wide gap between the temperatures of the maximum nucleation rate and the maximum growth velocity.     

Effect of composition on the crystallization and magnetic properties of Fe–Co–Si–B–Nb amorphous alloys

The crystallization behavior and magnetic properties of Fe₆₂Co₁₀Si_15−x B_18−y Nb_{(x + y)−5} amorphous alloys with x = 0–5 and y = 0–5 were examined. Primary crystallization temperature of Fe₆₂Co₁₀Si_15−x B₁₃Nb _x and Fe₆₂Co₁₀Si₁₀B_18−y Nb _y alloys increased with the addition of Nb. The primary and secondary crystallization temperatures were well separated when the Nb content is above 4 at%. The alloys with 15–18 at% B showed a distinct supercooled region. The Nb addition decreased the Curie temperature as well as room temperature saturation magnetization. The glassy-type Fe₆₂Co₁₀Si₁₀B₁₈ alloy exhibited good soft magnetic properties as well as a supercooled liquid region of 39 K. The finding of the glassy-type Fe-based alloy without Nb element exhibiting high Bs above 1.4 T is promising for future use as a soft magnetic glass material.     

Relaxation processes and pure shear stress creep in a metallic glass ribbon of composition (Fe0.05Co0.95)75Si15B10

The effect of annealing time and temperature and of pre-annealing on the creep properties of metallic (Fe_0.05Co_0.95)₇₅Si₁₅B₁₀ glass ribbons has been studied. The creep strains, measured in a torsional creep mode, can be well interpreted using the concept of an activation energy spectrum. Observed Int kinetics as well as deviations from this are comprehensible by considering the form of the characteristic annealing function.     

保护气氛对Fe80Si9B11非晶带材表面特征及磁性能的影响

非晶合金带材的表面特征对磁性能有着重要的影响。为研究CO保护气氛对Fe₈₀Si₉B₁₁非晶带材表面鱼鳞纹的形成及非晶合金带材磁性能的影响规律,本文采用平面流铸法制备了Fe₈₀Si₉B₁₁非晶合金带材,利用金相显微镜、X射线衍射仪和磁性能测试仪研究了熔潭保护气体CO流量对非晶合金带材表面特征、微观结构及磁性能的影响。研究表明,不同CO流量条件下制备的合金带材均为非晶态,且CO流量对带材厚度几乎无影响;随CO流量增大,熔潭周围气体密度和气压降低,熔潭稳定性增强,裹入熔潭的气体变少,带材贴辊面气泡尺寸变小且数量减少,带材表面的鱼鳞纹间距增大。当CO流量为0时,带材表面比较粗糙,观察不到鱼鳞纹;当CO流量为0.25和0.5 m³/h时,带材表面鱼鳞纹间距分别为1.5、2.3 mm。随CO流量增大,由于气泡对畴壁的钉扎和鱼鳞纹细化磁畴的协同作用,使带材矫顽力和铁损降低,振幅磁导率增大;磁损耗分离结果表明,随CO流量增大,磁滞损耗降低,涡流损耗增大,且铁损降低主要由磁滞损耗降低引起的。     

Fabrication of Co3O4 cubic nanoframes: Facet-preferential chemical etching of Fe ions to Co3O4 nanocubes

The novel Co₃O₄ cubic nanoframes, sized in ca. 30 nm, were firstly fabricated via a facile solvothermal route. Based on the transmission electron microscopy and the powder X-ray diffraction analyses of the time-dependent products, a mechanism of facet-preferential chemical etching of Fe³⁺ ions to the pre-synthesized Co₃O₄ nanocubes is proposed for the formation of Co₃O₄ cubic nanoframes. This synthetic strategy can probably be extended to fabricate nanoframes of some other binary metal oxides, by designing similar chemical etching process.     

Effect of Crystallization on the Electrical Resistance and Structure of the Co67Cr7Fe4Si8B14 Amorphous Alloy

The variations in the electrical resistance and structure of amorphous Co₆₇Cr₇Fe₄Si₈B₁₄ during constant-rate heating were studied by four-probe ac measurements and x-ray diffraction. The crystallization process was shown to follow the eutectic mechanism. The phase composition of the crystallization products was determined. Between 870 and 920 K, the alloy is in an amorphous–nanocrystalline state. The resistance of the as-prepared, amorphous alloy shows anomalies at about 497 and 843 K, which are interpreted in terms of structural changes.     

The magnetic properties of the glassy ferromagnets Fe81.5B14.5Si4 and Fe40Ni40B20

The galssy ferromagnets Fe_81.5B_14.5Si₄ and Fe₄₀Ni₄₀B₂₀ have been studied by Mössbauer spectroucopy from 77.5 up to 650 and 680K, respectively, In the glassy state, the average magnetic hyperfine field decreases with increasing temperature due to a distribution of exchange inter-actions. At low temperatureH(T)/H(0) has a temperature dependence (1 -BT ^/32 -CT ^/52) whereB andC are constants, indicative of spin-wave excitations in glassy ferromagnets. The value of B_/32) = 0.40 = 0.05 is almost four times larger than those of crystalline iron and nickel. On the other hand, fluctuations of the exchange interaction constant are found to decrease with increasing temperature. The Curie temperaturesT _c - 608 K for the glassy Fe_81.5B_14.5Si₄ andT _c = 583K for the glassy Fe₄₀Ni₄₀B₂₀ are found to be well defined. AtT close toT _c H(T) behaves as a power law with critical exponent = 0.3. The crystallizationT _cr glassy Fe_81.5B_14.5Si₄ was found to be 645K. The crystalline material obtained contains three different phases, namely -Fe, Fe₂B and Fe-8% Si.     

Hydrogen trapping in metallic glasses investigated by positron annihilation and differential scanning calorimetry

Positron annihilation experiments have been performed on the metallic glasses Co₅₈Ni₁₀Fe₅ B₁₆Si₁₁, Fe₇₈B₁₃Si₉ and Fe₈₁B_13.5Si_3.5C₂ produced by melt-spinning. For the metallic glass Co₅₈Ni₁₀Fe₅B₁₆Si₁₁ it has been found that the positron lifetime spectrum is either single-component or resolvable in two exponential terms, depending on the production parameters of the ribbons. The commercial metallic glass Fe₇₈B₁₃Si₉ (2605S2) shows a two-component spectrum while the alloy Fe₈₁ B_13.5Si_3.5C₂ (2605SC) exhibits a single-component spectrum. After hydrogen charging no significant changes are observed in the positron lifetime of those metallic glasses which exhibit a single-component spectrum. For ribbons showing a two-term spectrum, the intensity of the long-lived component_{l 2} decreases with the cumulative charging time. This decrease is interpreted in terms of hydrogen trapping in the structural defects responsible for the long-lived component. Differential scanning calorimetry of ribbons of Co₅₈Ni₁₀Fe₅B₁₆Si₁₁ suggest that hydrogen in extended structural defects should induce some local structural change.