Influence of lattice defects on the Kondo resistance anomaly in diluteZnMn thin film alloys

ThinZnMn films (200–3000 ppmMn) are quench condensed in uhv onto quartz plates held at 5 K. Their resistance is measured down to 0.35 K and then again after stepwise annealing to higher temperatures (maxT _a =350 K). In the lower concentrated films we find an increase by 25% of the slope of the curves in the range where is proportional to logT while increasingT _a to 300 K. This is explained by the decrease in nonmagnetic defect concentration during the annealing process. Further analysis of the data shows that there is also a strengthening of the Rudermann-Kittel interaction between the magnetic impurities because of the increasing mean free path of the conduction electrons during annealing. The effect is more pronounced in the higher concentrated alloy films, where a maximum in the resistance curve is observed similar to bulk material alloy samples.Part of this work was presented at the LT-13 Conference at Boulder, Colorado, in 1972.     

The impurity resistivity ofZnFe alloys

Measurements are presented of the electrical resistivity of several ZnFe alloys with concentrations between 0.04 and 0.05 at % Fe in the temperature interval 0.5–280 K. The low temperature resistivity behavior of even the most dilute alloys is dominated by impurity-impurity interactions leading to fortuitous characteristic temperature ?～90 K. At higher temperatures (T?15 K) the impurity resistivity is dependent mainly on single impurities with much higher ? values. The estimated characteristic temperature of isolated Fe impurities θ₁ ～ 350 K is in better agreement with recent magnetic susceptibility and thermopower measurements.     

The resistivity of Cu and CuMn and CuCr alloys: Kondo effect and deviations from Matthiessen's rule

We have measured the resistivity (1–300 K) of Cu and CuMn and CuCr alloys (10–200 ppm). The data on pure Cu were compared with the Bloch-Gruneisen formula. The impurity contribution (_alloy – _Cu) contains a Kondo term and a deviation from Matthiessen's rule (DMR). The Kondo contribution was analyzed in terms of a modified Hamann-Fischer expression, yielding a good fit to our data with parameters in agreement with expectation. The DMR results in CuCr and CuMn are similar to the DMR reported in nonmagnetic Cu alloys. A phenomenological expression was derived which accounts for the observed concentration and temperature dependences.This work is part of a thesis which was sustained in Grenoble on 7 October 1974 and is registered in the CNRS under A.O.10315.     

Kondo effect in single crystals of dilute Y-Ce alloys

The magnetic susceptibility and electrical resistivity of Ce impurities in single crystals of Y-Ce alloys were measured at temperatures down to 1.5 K in the directions parallel and perpendicular to thec axis of the crystal. The experimental results were interpreted in terms of the Kondo effect and the crystal-field effect. The data show that the crystal field of Y splits the² F _5/2 ground state of the Ce ³⁺ ion into three doublets, the lowest doublet specified byj _z =±1/2. The susceptibility, resistivity, and specific heat results at low temperatures show evidence for the existence of a bound state due to the anisotropic Kondo interaction. Analysis of the low-temperature resistivity data yields a Kondo temperature of 40 K, which is independent of the current direction and the Ce concentration. The susceptibility and the resistivity at 0 K are nearly isotropic.     

Proximity effect tunneling into high-T K Kondo alloys

The proximity effect tunneling into Kondo alloys is studied within the framework of the Matsuura, Ichinose, and Nagaoka (MIN) theory, which is valid for both low-T _K Kondo alloys and high-T _K Kondo alloys. By working with the two-particle propagators, the effects of both the repulsive interaction arising from the virtual polarization of the impurities in the ground state and the pair-breaking interactions on the decrease in the transition temperatures of the proximity effect sandwich can be clearly seen. It is found that the expression for the decrease for the sandwich containing low-T _K Kondo impurities in the normal layer is similar to the expression obtained by Kaiser except for a redefinition of the pair-breaking parameter. For the high-T _K Kondo impurities, the presence of the additional repulsive interaction in the MIN theory leads to an expression for the transition temperature decrease that is different from those previously obtained. The existence of a critical concentration of high-T _K Kondo impurities in the normal layer at which the superconductivity of the sandwich is destroyed is seen to be directly connected to the presence of the contribution due to the repulsive interaction term in the expression for the decrease in the transition temperature.     

Kondo effect in superconducting diluteMoCo,MoFe, andMoMn alloys

The superconducting critical fieldsH _c and the critical temperaturesT _c have been measured down to 0.05 K in a series of dilute alloys ofMo Co,Mo Fe, andMo Mn. These systems haveT _K /T _c0 of 50, 0.3, and 15, respectively, whereT _K is the Kondo temperature of the alloy andT _c0 is the critical temperature of pure Mo. The critical fields atT=0 and the specific heat jump atT _c have been estimated from the observedH _c and compared with the theories of the magnetic impurity effect in superconductors. The behavior of these quantities and the density of states are found to depend strongly uponT _K /T _c0 in agreement with theory. From the results it is inferred that in the limit of largeT _K the superconducting properties of a Kondo alloy would tend to those of the Abrikosov-Gor'kov theory.     

The dc Josephson effect for superconducting proximity Kondo alloys

We calculate the maximum dc Josephson current I _Mof S-I-NX/S tunnel junctions, where N is a normal metal containing a low concentration c of magnetic impurities X in proximity with a superconducting film S. Our calculation shows a reentrant behavior of I _Mvs temperature T. A three-dimensional representation of f(I_M; T; c) = 0 is given.Laboratoire associé au CNRS.Research fellow of the Belgian IIKW.     

The impurity dependence of the electrical resistivity in the intermediate state

Measurements have been made of the electrical resistivity in the intermediate state of a superconducting cylinder of tin showing the resistivity to have the expected temperature and field dependence, but indicating that the dependence on the electron mean free path appears to be inadequately characterized by present theory.     

Magnetic impurity ordering in dilute alloys

The interaction of conduction electrons with the ordered system of magnetic impurities randomly distributed in a normal metal is investigated in terms of the s-d exchange model. In the calculation of the electronic self-energy part the most divergent terms are summed. The internal exchange field arising due to the impurity ordering is determined in a self-consistent manner. It is shown that in the system under investigation the electron spectrum is greatly renormalized; in particular, the effective mass on the Fermi surface and also the low-temperature electronic heat capacity become several times larger. The addition to the electronic heat capacity has a broad temperature maximum and depends on the impurity concentration. The concentration and temperature dependences are qualitatively in good agreement with the experimental results. The mechanism responsible for the appearance of the long-range ferromagnetic interaction is also discussed.     

On properties of superconducting alloys containing Kondo impurities

The phase transition temperature T _c and the specific heat discontinuity C of a superconductor containing Kondo impurities are expressed self-consistently in terms of the t-matrix for Kondo scattering, which has been calculated within the Tomonaga approximation, thereby taking care of the infrared singularities. The approximation contains the Abrikosov-Gor'kov theory and the exact result for the classical spin as limiting cases. The initial slopes of T _c and C as a function of antiferromagnetic exchange coupling have a maximum, and the zero-temperature transition concentration has a minimum, showing that the impurity undergoes a magnetic-nonmagnetic transition when the coupling is increased. This agrees with previous results for the impurity spin susceptibility. The condition for a reentrant phase boundary is discussed in connection with its implications for the specific heat jump. The results are compared quantitatively with experimental data for LaAl₂ containing Ce and Gd impurities and qualitatively with recent data for the (La _1-x Th _x )Ce system.     

The low-temperature resistivity of platinum-rhodium alloys

Resistors of high stability at liquid helium temperatures have been made with platinum-rhodium alloys. The resistivity and temperature coefficient of resistance in the range 2 to 4.2 K are reported for alloys containing 10%, 13% and 30% rhodium. Resistivity minima occurred within the above temperature range.     

On the theory of the Kondo effect

The Kondo effect is investigated by methods developed in the theory of fields with strong coupling. Renormalizability relations analogous to the scaling laws in the theory of second-order phase transitions are found. The behavior of the scattering amplitude of an electron by an impurity is analyzed as a function of the electron energy. Formulas are derived for the resistance as a function of the temperature and for the magnetic moment as a function of the magnetic field and temperature. For the antiferromagnetic interaction sign the resistance tends to a constant and the magnetic moment to zero asT 0 according to power laws with universal exponents that depend only on the spin of the impurity. For sufficiently large spin the susceptibility tends to infinity asT 0.     

Spin relaxation of a spin-1/2 Kondo impurity in a superconductor

The behavior of a spin-1/2 Kondo impurity coupled to a superconducting metal by thes-d exchange interaction is analyzed. The impurity spin correlation function is expressed by the spin relaxation spectrum, which was calculated within the Tomonaga approximation, thereby taking care of the infrared singularities. The static spin correlation function ₀ is self-consistently obtained from the relaxation spectrum. When the characteristic Kondo temperature is of the order of or larger than the superconducting gap, ₀ is finite at zero temperature, showing that the ground state is a nonmagnetic complex with total spin 0. This result is related to previous calculations on bulk properties of the superconductor, such as the reduction of the phase transition temperature and of the specific heat jump due to magnetic impurities. The impurity relaxation rate is discussed in connection with retardation effects, and analogies to the relaxation of a spin in a normal metal with an applied magnetic field are pointed out.     

The effect of gas purity on the reflectance and resistivity of magnetron sputtered aluminium and its alloys

The relative optical reflectance and electrical resistivity have been measured of thick films (～1μm) deposited by sputtering Al in a planar magnetron using Ar and Ar + O₂ mixtures. The results obtained have a practical value in indicating the power input and related deposition rate required for given discharge conditions to obtain Al-films with a high reflectance and bulk resistivity.     

The resistivity of β-manganese and nickel-stabilized alloys

Resistivity-temperature measurements have been carried out on pure manganese metal in which the phase had been retained by quenching, and on two alloys in which the phase was found to be stabilized at room temperature. The resistivity of -manganese is seen to decrease monotonically with temperature and no anomaly is revealed in the temperature range 4.2–7 K. A low-temperature resistance minimum, however, appears in the nickel-stabilized alloys.The work reported here formed part of a Ph.D. thesis submitted at the University of Oxford by one of us, the late K. C. Whittaker, Professor of Physics, University of Science and Technology, Kumasi, Ghana.     

Temperature dependence of the high-frequency resistivity in dilute magnetic alloys

The temperature dependence of the microwave resistivity at 70 GHz in diluteCuCr,CuMn, andCuFe alloys was measured between 1.6 and 4.2 K by the calorimetric method. The observed resistivity decreases slightly with decreasing temperature inCuFe. These results are in qualitative agreement with the recent theory of Moriya and Inoue on the high-frequency conductivity of Kondo alloys. InCuCr the resistivity increases remarkably with decreasing temperature, whereas, according to the theory, it should decrease. This fact seems to suggest that our present knowledge of thes-d scatteringt matrix is not sufficient to describe the high-frequency resistivity of dilute magnetic alloys.     

Behaviour of hydrogen impurity in aluminium alloys during anodizing

The present study examines the behaviour of hydrogen impurity in an Al-6.5 at.% W alloy during anodizing, using elastic recoil detection and nuclear reaction analyses. Increased concentrations of hydrogen are found near the alloy/anodic film interface, amounting to ∼2×10¹⁵ H atoms cm⁻² for the particular alloy, containing 0.1-0.3 at.% hydrogen in the bulk regions, and conditions of anodizing. The enrichment arises from hydrogen in the alloy (i) diffusing to the interface, which acts as a trap, or (ii) accumulating at the interface, due to the growth of the anodic film, or a combination of both processes. Diffusion is consistent with the known mobility of hydrogen in aluminium near ambient temperature. Further, accumulation, and subsequent oxidation, of hydrogen are expected based on the general behaviour of alloying elements in anodized aluminium. The anodic films contained ∼0.1-0.3 at.% hydrogen, originating from either the electrolyte or the alloy.