Pressure dependence of superconductivity in amorphous La100−xAlx alloys

The pressure dependence of the superconducting transition temperatureT _c of amorphous La_100–x Al _x alloys (x=16.5, 20, 25, 30, 34) has been studied under hydrostatic pressure to 3.5 GPa and, additionally, forx=16.5 under quasihydrostatic pressure to 12 GPa. For amorphous samples having Al-concentrationx>20%, T _c /P is positive in sign and decreases linearly with increasingx, extrapolating atx=0 to the same value (+1.1 K/Gpa) determined for pure crystalline La. For thex=16.5 sample, however, T _c /P is observed to be anomalously large, more than twice the value expected. The present results are compared with previous studies on amorphous and crystalline La-systems.This paper is dedicated to Prof. Methfessel of this institution on his 60th birthdayThis work is supported by the Minister für Wissenschaft und Forschung des Landes Nordthein-Westfalen     

Effect of Fe on the glass-forming ability,structure and devitrification behavior of Zr–Cu–Al bulk glass-forming alloys

We report on the glass-forming ability and devitrification behavior of Zr₆₀Cu₃₀Al₁₀, Zr₆₀Cu₂₅Al₁₀Fe₅ and Zr_62.5Cu_22.5Al₁₀Fe₅ bulk glass-forming alloys on heating. The effect of Fe addition on the structure of Zr–Al–Cu alloys is also discussed. Crystallization kinetics and structural changes in the glassy alloys were studied using X-ray diffraction, transmission electron microscopy, differential scanning and isothermal calorimetry methods. The results indicate that good glass-formers, such as Zr_62.5Cu_22.5Al₁₀Fe₅, are located somewhat beyond the equilibrium eutectic point. Possible phase separation in the supercooled liquid on heating and electron beam-induced in situ crystallization are observed and discussed.     

Phase transformation behaviour in continuously cooled Zr65Al7.5Ni10Cu17.5− x Pd x (x = 0 − 17.5) glass-forming alloys and consequences for structure and property control

Continuous cooling transformation (CCT) diagrams were successfully constructed for Zr₆₅Al_7.5Ni₁₀Cu_{17.5? x} Pd _x (x?=?0???17.5) glass-forming alloys, comparing phase-transformation features in the alloy system to composition. While a low-Pd alloy (x?=?5) showed a single transformation curve, corresponding to the formation of a crystalline phase on the high-temperature side of the undercooled-liquid region, for a given time-scale, a high-Pd alloy (x?=?17.5) revealed an additional curve, corresponding to quasicrystalline phase formation on the lower temperature side. The result provides a clue to the structural and property control on the alloy system. Glassy specimens of the same size but with different intrinsic structure, evaluated by structural relaxation during continuous heating, could be fabricated for the low-Pd alloy (x?=?5). Plasticity was found to increase proportionally with the relaxation enthalpy. On the other hand, the critical size for glass formation could be improved considerably from 5 to 7 mm in diameter for the high-Pd alloy (x?=?17.5).     

Electrical resistivity of dilute (Cr1−xAlx)95Mo5 alloys

The temperature dependence of the electrical resistivity of (Cr_1−xAl_x)₉₅Mo₅ alloys with x = 0.11, 0.44, 0.77, 1.49, 2.07, 3.11, 4.84, 7.31 and 10.63%, was measured. The non-magnetic behaviour of the antiferromagnetic alloys (x = 0.11, 0.44, 0.77, 1.49, 7.31 and 10.63%) was determined by using the temperature dependence of the alloys with x = 2.07, 3.11 and 4.84% that remain paramagnetic at all temperatures, as a model. This allowed an analysis of the data in terms of existing theories.     

Peculiarities of α → β transformation in the Pd-In-Ru-H system

A precise X-ray diffractometry method is employed to investigate a Pd-In-Ru alloy foil subjected to electrolytic hydrogenation. It is revealed that an increase in the scattering volume of the sample and its multiphase disintegration are caused by the α → β transformation. The basic characteristics of the given transformation are established.     

Spin dynamics in the spin glass phase of Y(Mn1−xAlx)2

Zero field SR has been used to probe spin dynamics in the concentrated spin glass Y(Mn_0.9Al_0.1)₂. The spectra follow a stretched exponential form, G_z(t)=exp(–(t)), with reaching 1/3 as the freezing temperature, T_g=60K, is approached. The evolution of G_z(t) with temperature is suggestive of the behaviour of the spin correlation function found by Ogielski in Monte Carlo simulations of the dynamics of the Ising spin glass model. The SR results are correlated with results previously obtained by inelastic neutron scattering.     

The influence of microstructure on the martensitic transformation in Cu–Zn–Al melt-spun ribbons

The martensitic transformation was investigated in a set of twin roller melt-spun Cu–Zn–Al shape memory alloys, solidified at tangential wheel speeds between 20 and 40 m/s. The resulting microstructures were analyzed using X-ray diffraction, optical and transmission electron microscopy techniques. The characteristic martensitic transformation temperature, M _S, was determined for each condition by conventional resistometric methods. The ribbons are homogeneous in shape and for each quenching rate they exhibit a quite uniform M _S temperature. By proper thermal treatments, the different factors affecting M _S could be separately examined and from temperature measurements, the contribution of L2₁ antiphase boundaries evaluated. A calculation of this contribution using pair interchange energies is in good agreement with the experimental results.     

Influence of C on Ge incorporation in the growth of Ge-rich Ge1−x−ySixCy alloys on Si (100)

Amorphous carbon nitride powders are successfully produced by ammonothermal synthetic routes. The existence of the chemical bonding of C≡N in the powders is confirmed by Raman and infrared spectra. Raman spectra of various samples show that the G-band frequency shifts to higher energies, and the intensity ratio I_D/I_G decreases as the N content increases. The Knoop hardness of the sintering sample is up to 1200 kgf/mm². The hardness of the sample increases with increasing of the N content. Received: 24 May 2000 / Accepted: 26 May 2000 / Published online: 2 August 2000     

Mössbauer study of spin states in amorphous Fe100−x Zr x alloys

Spin states in amorphous Fe_100–x Zr_x (7x12) are investigated using hyperfine field distributions (HFDs) as derived from⁵⁷Fe transmission Mössbauer experiments. The existence of low-, medium- and high-spin states is demonstrated by three-dimensional projections of HFDs. Temperature dependences of the corresponding average values revealed anomalous behaviour in the vicinity ofT _fc. An increase in the hyperfine magnetic fields observed towards low temperatures is discussed in terms of a ferromagnet-to-spin-glass transition. Utilizing also theT _c values, which were obtained from a thermal scanning Mössbauer technique, we propose a diagram of spin states in amorphous Fe-Zr.     

Effect of K-doping to the weak link behaviour of (Cu0.5Tl0.5−xKx)Ba2Ca2Cu3O10−δ superconductors

A systematic study of the weak link behaviour for (Cu_0.5Tl_0.5?xK_x)Ba₂Ca₂Cu₃O_10?δ (x = 0, 0.25) superconductors samples has been carried out using electrical resistivity and AC-susceptibility techniques. The K-doped (Cu_0.5Tl_0.5?xK_x)Ba₂Ca₂Cu₃O_10?δ samples were synthesized by solid-state reaction method. In magnetic susceptibility measurements, the real (χ′) and imaginary (χ″) parts of the magnetic susceptibility of (Cu_0.5Tl_0.5?xK_x)Ba₂Ca₂Cu₃O_10?δ (x = 0, 0.25) samples were measured as a function of temperature under various DC-magnetic fields up to 172 Oe. It is observed from these studies that the magnitude of the diamagnetism is substantially enhanced by K-doping. The possible reasons for the enhanced magnitude of diamagnetism have been investigated. It is observed from in-field magnetic measurements that the inter-grain coupling is improved with the K-doping. It is concluded from these studies that potassium atoms appearing at the crystal boundaries enhance inter-grain coupling and pinning mechanism in K-doped (Cu_0.5Tl_0.25K_0.25)Ba₂Ca₂Cu₃O_10?δ superconductors.     

First-principle study of the electronic,magnetic and structural characteristics of the Mn2CoAs1−xAlx (x = 0,0.25,0.50,0.75) Heusler alloys

First-principles density functional theory approach is adopted to determine the electronic, magnetic and structural characteristics of the Mn₂CoAs_1−xAl_x (x = 0,0.25,0.50,0.75) Heusler alloys. The computations are carried out by WIEN2k code based on full-potential linearized augmented plane wave method (FP-LAPW). Moreover, the exchange-correlation energy functional is treated at the level of the generalized gradient approximation (GGA). Analysis of our computed results of the electronic band structure, as well as the density of states of the Mn₂CoAs compound, show it a stable and half-metallic material with an energy band gap value of 0.48 eV. The calculated spin gap values are: 0.627 eV, 0.22 eV and 0.188 eV for Mn₂CoAs_0.75Al_0.25, Mn₂CoAs_0.50Al_0.50 and Mn₂CoAs_0.25Al_0.75 respectively. Furthermore, the calculated total magnetic moment of the Mn₂CoAs (4 µB) is found to be in agreement with the Slater–Pauling rule. Thus, our calculations show the Mn₂CoAs_1−xAl_x (x = 0, 0.25, 0.50, 0.75) Heusler alloys potential materials for near future applications in spintronic because of their half-metallic ferromagnetism property.     

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni₅₀Mn₃₅In_{15 ? x} Si _x (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220?C280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150?C200 ??? cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.     

Magnetic behaviour of alloys in the series (Fe1−xCox)2P

Magnetization and Mössbauer spectroscopic measurements are reported on five alloys in the series (Fe_1−xCo_x)₂P (corresponding to x=0.03, 0.10, 0.30, 0.50 and 0.70). All the five alloys are ferromagnetically ordered. With increasing cobalt, the Curie temperature initially increases becoming maximum for x=0.30 at 480 K and then drops. In going from x=0.10 to 0.30, when the crystalline symmetry changes from Fe₂P-like hexagonal one to Co₂P-like orthorhombic one, the magnetic moment on the pyramidal metallic site tilts off the V_zz-axis. In all the compositions, substitution of cobalt reduces the internal magnetic field at the tetrahedral site whereas that at the pyramidal site shows little change. The system with x=0.03 exhibits temporal relaxation of magnetization over a long period of time. Mössbauer spectra of this alloy exhibit a reduction in linewidth with increasing temperature from 80 to 125 to 160 K. These observations are indicative of the existence of magnetic clusters.     

Origin of the strong influence of rhodium on the Curie temperature of Pd–Fe alloys: the spin reorientation in (Pd100−xRhx)90Fe10 alloys

The hyperfine field distributions and the local spin configurations for Fe atoms in the (Pd_100−xRh_x)₉₀Fe₁₀ alloys for x=0, 10 and 20 are investigated by the Mössbauer spectroscopy technique. It was found that the anomalous behavior of T_C in these alloys is attributable to the spin reorientation in some part of Fe atoms with the formation of local antiferromagnetic spin configurations.     

Hall effect of noncrystalline Zr−Cu films

Quench condensed Zr _x Cu_1–x films offer a wide concentration range (0.1x0.9) for measurements of the Hall coefficientR _H of amorphous Zr–Cu alloys.R _H changes sign as a function of composition, from negative to positive, as the Zr concentration is increased. The sign change is observed in a narrow concentration range at aboutx=0.22, without any peculiarity of the conductivity at this crossover concentration. The Hall coefficient of the unannealed Zr-rich films is nearly independent of temperature. However, both, the values ofR _H and of the temperature dependence ofR _H change with a heat treatment well below the crystallization temperature. This is most obvious for the films with a composition close to the cross-over concentration.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Itinerant electron metamagnetism in Y(Co1−xAlx)2

Magnetization measurements on the itinerant electron magnet Y(Co_1−xAl_x)₂ up to 350 kOe have clearly revealed metamagnetic transitions in the paramagnetic region (0.07⩽x⩽0.11). The transition is sharp for low x, suggesting itinerant electron metamagnetism to exist in YCo₂ at 1 MOe, while it is broadened and disappears upon the onset of ferromagnetism at x∼0.13.