Electrical resistivity and superconductivity of amorphous La−Ag alloys

We report on measurements of the electrical resistivity and the superconducting transition temperature of amorphous La–Ag alloys (60–74 at % La) obtained by liquid quenching. The temperature coefficient of the resistivity is always slightly negativ, its value cannot be described well by existing theories.T _c depends linearly on the La content and seems to be not very sensitive to the second alloy constituent.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday     

Magnetic properties of amorphous Mn x B100−x alloys

Magnetic properties of amorphous Mn _x B_100–x alloys ranging fromx = 30 to 70 under high magnetic fields and low ac magnetic fields in the temperature range from 4.2 K to room temperature have been investigated. Samples which have Mn concentrations of aboutx = 40–60 show spin-glass-like properties in the low-temperature region. This spin-glass characteristics result from a frustration in the spin system which is caused by the competition of ferromagnetic and antiferromagnetic interactions between randomly distributed Mn atoms. Both magnetization at 4.2 K and paramagnetic momentP _eff as a function of Mn concentration show a peak aroundx 44 which drops rapidly towards both sides of the Mn content.     

Calculation of the temperature dependence of the thermal emf in amorphous alloys CaxAl1−x and AuxNi1−x

The temperature dependence of the thermal emf of Ca_xAl_1–x and Au_xNi_1–x for four different concentrations of the components of the alloys is calculated on the basis of the concept of dynamic concentrated excitations in amorphous metal systems. It is shown that increasing x from 0.15 to 0.50 in Au_xNi_1–x raises the thermal emf, and a further increase in the Au concentration from 0.50 to 0.80 lowers S(T). For Ca_xAl_1–x the dependence S(T) is calculated in the interval of Ca concentrations from 0.55 to 0.75. In this concentration interval the thermal emf decreases as x is increased. It is shown that for both types of alloys the S(T) curve bends abruptly at a temperature near 10T₀ (where T₀ is the concentration-dependent characteristic temperature of amorphous alloys separating the ranges of strong and weak scattering of electrons by dynamic concentration excitations). The so-called S(T) knee shifts toward lower temperatures when the thermal emf increases with increasing x and toward higher temperatures when S(T) decreases with increasing x. The results agree with experimental data.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–48, August, 1994.     

Magnetostriction of amorphous GdxAg1−x alloys

Magnetostriction measurements above 4.2 K and up to 2.2 T have been performed in the amorphous alloys Gd_xAg_1−x (0.30 ⩽ x ⩽ 0.40). Magnetostriction is purely of volume character. It shows with composition a maximum at x = 0.35, and this behaviour can be explained if forced volume magnetostriction derives form the strain dependence of the two-ion longitudinal spin correlation. The isotropic spin exchange correlation seems irrelevant in these series, as also put forward by thermal expansion measurements.     

Pressure dependence of the irradiation-induced ferromagnetism in Fe–Ni invar alloys

Pressure dependence of the irradiation-induced ferromagnetism recently found in Fe–Ni invar alloys was investigated under hydrostatic pressures up to 7.5 GPa. A rectangular sheet of Fe–30.2 at% Ni invar alloy was irradiated with 80 MeV Xe ions. The range was much smaller than the thickness of the sample. The Curie temperature of the irradiated part increased by 63 K, and the absolute value of the pressure coefficient, dT_C/dp   was smaller than that of non-irradiated part. The relation p≈(_TC0-_TC)ⁿ$p\approx (T_{{\mathrm{C}}_0}-T_{\mathrm{C}}{)}^n$ holds with n=2$n=2$ for both non-irradiated and irradiated part. The itinerant character was not so much modified by irradiation.     

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.     

On the random partial coordination numbers of amorphous ZrxCo100−x alloys

An approach based on the limitation of the solid-angle-sum interval enables the calculation of possible random partial coordination numbers. The results and recent experiments are discussed for ZrCo amorphous alloys.     

Eu valence mixture in amorphous EuxPd1−x alloys

The electronic configuration of Eu in amorphous Eu_x Pd_1−x alloys as determined by Mössbauer spectroscopy is found to change from a trivalent to a divalent state over the concentration range 0.18 ≦ x ≦ 0.35. The intermediate valence state of Eu in these alloys is characterized by a mixture of Eu occuring in two different integral valence states.     

Magnetic properties of amorphous MnxPd82−xGe18 alloys

Magnetoresistivity, magnetic, susceptibility and high field magnetization measurements were performed on the amorphous Mn _x Pd_82–x Ge₁₈ alloys for 1x7. These amorphous alloys were prepared by radio frequency (R. F.) sputtering using argon, depositing onto fused quartz substrates to a thickness of about 20 microns. The negative magnetoresistivity is approximately proportional to the square of the magnetization and the susceptibility obeying the Curie-Weiss law between 18 and 293 K. These results lead to the conclusion that a Kondo-type s-d exchange interaction exists in the amorphous Mn-Pd-Ge alloys and the d-d, spin correlation between magnetic atoms is, in general, weaker in the amorphous alloys than in the corresponding crystalline alloys.     

Effect of annealing conditions on high temperature superconductivity in La2−xSrxCuO4

Abstract

Besides investigating a number of oxides of the series La_2?xM_xCuO₄ (M=Ba, Pb, Sr), we have, in this investigation emphasised on the studies of the effect of annealing conditions on superconducting properties of composition La_1.8Sr_0.2CuO₄. The effect is remarkable and is di'scuss'ed in the light of X-ray diffraction results. These samples, studied after a gap of two months, showed interesting changes which has been discussed in the present paper.     

Low-temperature specific heat of icosahedral and amorphous Pd−U−Si alloys

Icosahedral (I) Pd_0.588U_0.206Si_0.206 can be obtained from melt-spun amorphous (A) ribbons by annealing. The specific heatC (measured betweenT=0.1 K and 20 K) shows very similar behavior for both phases. The main features ofC are as follows. (i) The vibrational heat capacityC _ph dominatesC at highT.C _ph is almost identical in both phases, in agreement with recent inelastic neutron scattering data. (ii) Shallow maxima in (C–C _ph)/T vs.T are found at 5.4 and 4.3 K forI andA phases, respectively, associated with magnetic order. These maxima are suppressed by 20% in an applied magnetic field of 6 T. (iii) A large quasi-linear contribution is observed with a low-T coefficient =165 mJ/mole U K² for theI phase and =120 mJ/mole U K² for theA phase. In the low-T region,C is hardly affected by a field of 6 T. This hints at the formation of a narrow 5f band with a comparable density of states for bothI andA phases.     

Temperature dependence of the upper critical field as an indicator of boson effects in superconductivity in Nd2−x CexCuO4−y

The temperature dependence of the upper critical field B _c 2 was determined from the shift of the resistive transition ΔT(B) in nearly optimally doped Nd_2?x Ce_xCuO_4?y single crystals. Within the experimental accuracy, the weak-field data are described by the power function B _c 2∝(ΔT)^3/2. This result is compared with the data on heat capacity and analyzed in the context of possible manifestations of boson effects in superconductivity. The T dependence of B _c 2 persists down to the lowest temperatures, but the numerical values of B _c 2 below 1 K are different for different samples.     

Kinetics and products of crystallization of Fe84−xVxB16 amorphous alloys

Isothermic crystallization of a series of Fe_84–xV_xB₁₆ (x = 0 to 8 at. % V) amorphous alloys was studied by the electrical resistivity, coercivity and⁵⁷Fe Mössbauer spectroscopy methods. Two stages of the process were observed corresponding to the primary -Fe(-V) formation by the growth-of-nuclei mechanism and the eutectic mixed borides — metal crystallization. Concentration dependences of kinetics parameters (Avrami exponent, activation energy) and the changes of electrical and magnetic properties were found and the influence of vanadium on the crystallization process is discussed.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.Thanks are due to Prof. L. Potocký, afárik University, Koice, for his kind providing the amorphous ribbons, and to Dr. J. Krejí from the author's Institute for the EDAX analyses.     

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.     

Phenomenological model of superconductivity in U1−xThxBe13

A phenomenological model taking into account the interaction between superconductivity and the coherence of Kondo screening is introduced. This model describes the main experimental data on UBe₁₃, including the behaviour of T_c in U_1−xTh_xBe₁₃ under ambient and elevated pressures.     

Pressure dependence of the bowing parameter of direct band gap of CdSexTe1−x

The variation of the lowest direct band gap of compounds CdSe_xTe_1-x with hydrostatic pressure has been measured by photoluminescence at room temperature. This allowed determination of pressure dependence of the bowing parameter. Our data compared to theoretical calculations based on the dielectric model indicates that the average ionic potential is constant with pressure. This result has been previously assumed in the calculation of energy-band pressure coefficients and proved fundamental for the agreement with experimental values for a wide variety of semiconductors.     

Pressure Effects in the Raman Spectra of La 2−x Sr x CuO 4

Micro-Raman measurements under hydrostatic pressures up to 6 GPa have been carried out on high-quality La 2 m x Sr x CuO 4 polycrystalline compounds with Sr concentration up to x =0.45. The zz scattering polarization has been investigated, where two strong modes due to La/Sr and the apex oxygen, and (in the low Sr concentrations) the soft mode at ～100 cm m 1 are observed. The frequency of the strong modes increases almost linearly with pressure for the Sr concentrations studied. Modifications in the increment rate d y /d p and the phonon width have been detected depending on the amount of doping. For x =0.45 a considerable increase in the width of both strong phonons with pressure was found, which must reflect a separation into phases, since this concentration is close to the solubility limit. The relative intensity of the strong phonons was investigated in connection with its correlation to the transition temperature.