Superconducting properties of amorphous transition metal alloys

We have co-sputtered amorphous films of several Mo and W-based superconducting alloys. Measurements of T_c, ($\text{d}\text{H}{}_{\mathrm{c2}}\text{d}\text{T}$)_Tc and the normal state resistivity near T_c were made on a number of these alloys. Our results and other data from the literature are studied to examine the correlation between T_c and the dressed density of states at the Fermi level.     

Magnetostriction of transition metal substituted Co-based amorphous alloys

The effects of the substitutions of 3d-, 4d- and 5d-transition metals for silicon on magnetostriction of Co-based amorphous alloys have been investigated by measuring the tension dependence of anisotropy field in magnetic hysteresis loop. Fractional changes of the magnetostriction with transition metal solutes in Co-based amorphous alloys are qualitatively discussed in terms of the magnetic moment and coordination number, and classified into 3 types, i.e., positive, negative and complicated types, from a viewpoint of the contribution to magnetostriction.     

Electronic structure and glass forming ability in early and late transition metal alloys

A correlation between the change in magnetic susceptibility (Δχ_exp) upon crystallisation of Cu–Zr and Hf metallic glasses (MG) with their glass forming ability (GFA) observed recently, is found to apply to Cu–Ti and Zr–Ni alloys, too. In particular, small Δχ_exp, which reflects similar electronic structures, ES, of glassy and corresponding crystalline alloys, corresponds to high GFA. Here, we studied Δχ_exp for five Cu–Ti and four Cu–Zr and Ni–Zr MGs. The fully crystalline final state of all alloys was verified from X-ray diffraction patterns. The variation of GFA with composition in Cu–Ti, Cu–Zr and Cu–Hf MGs was established from the variation of the corresponding critical casting thickness, d_c. Due to the absence of data for d_c in Ni–Zr MGs their GFA was described using empirical criteria, such as the reduced glass transition temperature. A very good correlation between Δχ_exp and d_c (and/or other criteria for GFA) was observed for all alloys studied. The correlation between the ES and GFA showed up best for Cu–Zr and NiZr₂ alloys where direct data for the change in ES (ΔES) upon crystallisation are available. The applicability of the Δχ_exp (ΔES) criterion for high GFA (which provides a simple way to select the compositions with high GFA) to other metal-metal MGs (including ternary and multicomponent bulk MGs) is briefly discussed.     

The electronic structure of some transition metal alloys

Thermal neutron scattering experiments have provided detailed information on the distributions of magnetic moment in a number of disordered ferromagnet binary alloys. The general features of these distributions together with saturation magnetization data are discussed and compared with various simple theories. Attention is focused on dilute alloy systems. After an introduction the paper is divided into four sections, the first of which deals with alloys which tend to follow the Slater-Pauling curve. Here a simple Thomas-Fermi treatment due to Friedel suggests that magnetic moment changes, largely confined to the minor constituent (solute) sites, should occur with a sign dependent on the nature of the density of states at the Fermi level in the pure major constituent (solvent). Comparison with experiment shows qualitative agreement except in the case of Fe-based alloys containing transition element solutes from the right of Fe in the periodic table. This discrepancy is examined and an explanation put forward. The next section outlines a discussion of the electronic structure of alloys of transition elements with non-transition metal solutes. The view is taken that the electronic configuration of a solute atom is roughly similar to the configuration found in the pure non-transition metal: it follows that no partially filled d orbitals are expected at solute sites. Use of a simple Thomas-Fermi model based on this assumption indicates that some of the electric screening associated with a non-transition metal solute takes place in the surrounding transition metal slovent. Additional electrons introduced in this way into the solvent occupy mainly d states and cause a reduction in magnetic properties. This reduction together with the total loss of d-state effects from the solute sites themselves can account qualitatively for the changes observed in Ni, Pd and Fe-based alloys with non-transition elements. The fourth section deals with the transition metal alloys which show marked departures from Slater-Pauling behaviour, e.g. NiCr. An explanation for these alloys has been provided by Friedel's bound impurity state model and the mechanism suggested by Comly, Holden and Low to account for the similarity in shape of the magnetic disturbances observed in different systems. The final section discusses ferromagnetic alloys of PdFe and PdCo. The giant moments associated with the Fe and Co solutes result from a widespread polarization of the Pd solvent contiguous to the solute atoms. This polarization can be interpreted with the use of a non-local exchange-enhanced susceptibility function for the Pd host. With increasing solute content this function becomes modified to an extent dependent on the shift of d holes from one spin direction to the other, i.e. on the mean polarization of the Pd.     

Composition dependence of the electrical transport properties of amorphous transition metal alloys

Summary A quadratic composition dependence of the electrical resistivity of amorphous transition metal alloys has been investigated at room temperature. A very good agreement between the theoretical and the observed values has been obtained in the case of Ni_xZr_1−x and Cu_xZr_1−x for all compositions. The thermoelectric power was then correlated with the electrical resistivity satisfactorily for the Cu_xZr_1−x and Ni_xZr_1−x To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Electronic theory for the metal-non-metal transition in simple metal alloys

The metal-non-metal transition in liquid alloys such as Cs_1–x Au _x and Li_1–x Pb _x is explained as resulting from concentration dependent electron charge transfer causing short range atomic order and a change from metallic to ionic bonds. Numerical results for the electronic density of states, the electron charge transfer, the free energy of mixing and the volume change are given.     

On the concentration dependence of transport coefficients in amorphous transition metal alloys

We discuss the concentration dependence of resistivity of CuTM (TM  Ti, Zr, Hf) amorphous alloys within a framework of a two-band model in which the s-states dominate conduction at the Cu end while the conduction of the d-states becomes increasingly important at TM rich end. We present a simple empirical relation that well describes the concentration dependence of the Hall coefficient and its sign reversal.     

Electronic structure and stability of binary transition metal compounds

It is shown that the ordered structures occurring in binary substitutional transition metal alloys on simple lattices (f.c.c., b.c.c.) can systematically be obtained from the electronic structure of the disordered alloys. Generalized structural maps are introduced; their physical meaning and their relation to the phenomenological structural maps used previously by other authors are discussed.     

Electronic structure of transition metal monoantimonides: Analogy to the transition metals

A simple metallic band model is proposed for the transition metal monoantimonides, by analogy to the transition metals. It is assumed that the metal 3d electrons are itinerant in narrow bands which energetically overlap broader antimony 5p-derived bands, and the Fermi level lies in the overlap region. Consistency of the model with available data is assessed and found to be quite good.     

Electronic properties and superconductivity of amorphous Si1-xAux alloys near the metal-insulator transition

We report on specific-heat and resistivity measurements on quench-condensed Si_1-xAu_x films for 0.11 ⩽ x 0.36 in the temperature range 0.35 K ⩽ T ⩽ 6 K. A distinct increase of the specificheat derived electronic density of states at the Fermi level is observed at x_b ≈ 0.2, i.e., in the vicinity of the metal-insulator transition occurring for our samples at x_c = 0.16. This suggests a different type of bonding between Au and Si for x < x_b and x > x_b. While resistive transitions to superconductivity are observed for x⩾0.21, the absence of a specific-heat anomaly at the transition points to filamentary superconductivity except for × = 0.35 where a sizable anomaly is seen. The difference in various electronic properties between differently prepared samples of these metastable alloys, in particular the influence of different preparation and annealing temperatures is emphasized. It is suggested that these differences are caused by incipient phase separation in the room-temperature prepared samples.     

Magnetic clusters in amorphous metal alloys

The effect of thermal treatment in combination with an external magnetic field and/or elastic stress on the magnetic characteristics of amorphous metal alloys of the 2NSR type is studied. The complex behavior of the magnetization and coercivity values in dependence on the length of annealing at temperatures below crystallization is described. It is assumed that the observed changes in the macroscopic magnetic characteristics are associated with the formation of clusters with different degrees of exchange interaction.     

Magnetic phases of amorphous transition metal-metalloid alloys

Hysteresis loop and ac susceptibility measurements were performed on three series of amorphous alloys: (A_wB_1-w)₇₅P₁₆B₆Al₃, where (A, B) are (Fe, Ni), (Co, Ni) and (Fe, Mn). Upon cooling, low w alloys undergo paramagne t to spin glass transitions. Alloys with higher w first experience a Curie transition to a ferromagnetic state, and then a spin freezing transition to a spin glass state. the T dependence of the width of the ac hysteresis loop is used to determine the spin freezing transition temperature. A magnetic phase diagram is presented for each alloy series and the value of w required for ferromagnetism, w_C, is determined. When measured in the presence of small constant fields, the ac susceptibility of alloys with w just above w_C has maxima near both transition temperatures. The field and temperature dependences of the peaks are explained by scaling arguments, used to determine the critical exponent δ for the Curie transition, and suggest that a similar scaling law holds for the ferromagnet to spin glass transition.     

Theory of electronic structure of non-dilute transition metal alloys : Ni-Cu

The density of states for the Ni-Cu alloy system has been calculated as a function of concentration by the supercell method. This method takes into account inter-site interactions in contrast with current effective medium or single site theories. We show that the method is practical for the study of disordered systems. Comparison is made with previous work as well as with photoemission data.     

Electronic transport properties of amorphous NiTi alloys

Measurements of electrical resistivity, magnetoresistance and Hall effect on amorphous Ni_100–x Ti _x alloys withx=39, 43, 60, 65, 70, and 76 are presented. The resistivity at 4.2 K is in the range 210 cm to 325 cm. The Hall coefficient turns out to be negative for low Ti contents and positive for high Ti contents with values around –6×10^–11 m³/As and +7×10^–11 m³/As, respectively. At low temperatures, the temperature and magnetic field dependences of the quantities measured are analysed and discussed in terms of weak localisation and electron-electron interaction effects.     

Effect of transition metal additions on the resistivity of Fe83B17 amorphous alloys

Alloying effects of transition metals on Fe₈₃B₁₇ amorphous alloy were investigated. The resistivity versus temperature curves of Fe₈₃T₃B₁₇ between 4.2–300 K depend on the magnetic behaviour of the transition elements, while below 4.2 K show universal behaviour as it was suggested recently.     

Electronic structure of nin-isocoric transition metal alloys: CuxRh1-x

The electronic structure of the copper—rhodium alloys is determined in the frame of recently developed LCAO-based theory which allows to treat also non-isocoric alloys with strong off-diagonal disorder exactly within the single-site approximation. The total and component densities of states are calculated and compared with available experimental data.     

Magnetovolume effect in transition metal-metalloid amorphous alloys

The thermal expansion, spontaneous volume magnetostriction ω_s, forced volume magnetostriction $\text{(}\text{?ω}\text{?H}\text{)}$ and Young's modulus of amorphous Fe-B, Fe-P, Co-B and (Fe-M)₇₇Si₁₀B₁₃ (M = Cr, Mn, Co, Ni) alloys have been measured to make clear the magnetovolume effect in transition metal-metalloid amorphous alloys. The thermal expansion coefficient α, ω_s and $\text{(}\text{?ω}\text{?H}\text{)}$ are dependent on the number of d-electrons per transition metal atom n_eff calculated based on the charge transfer model. The n_eff vs. α, ω_s and $\text{(}\text{?ω}\text{?H}\text{)}$ curves are quite similar to the corresponding curves in fcc alloys. The maxima in those curves are, however, found at n_eff ≈ 8.2 for the amorphous alloys in contrast with n_eff ≈ 8.7 for the fcc Fe-Ni alloys. On the other hand, Young's modulus measured under the saturation of magnetization is governed by the molar volume, irrespective of n_eff. The magnetovolume effect in transition metal-metalloid amorphous alloys is discussed in connection with the instability of ferromagnetism of amorphous Fe.     

Further investigations of cocondensation as a preparation method for amorphous states in transition metal alloys

Thin films of Pt–Ir, Ag–Ir, Ag–Ni and Ag–Y were cocondensed onto sapphire substrates at room temperature. Their atomic structures were investigated by X-ray diffraction. Although the first three systems have endothermic heats of fusion we did not find any sign of decomposition in the Ir containing cocondensed films. Instead, homogeneous solid solutions with fcc structure were formed. For the Ag–Ni system we found partial decomposition. But even here, the solubilities of one component in the other were strongly enhanced with regard to thermodynamical equilibrium. The X-ray diffraction patterns indicate transitions of the two crystalline fcc phases into amorphous states for Ni-contents of about 70 at %. Homogeneous amorphous solid states can be prepared in the Ag–Y system, which has an exothermic heat of fusion. At higher substrate temperatures amorphous and crystalline phases tend to be in metastable equilibrium. The experimental results we present are very suitable to discuss the importance of special material parameters for the formation of amorphous states in cocondensation experiments.