J. W. Loram

Low temperature electrical resistivities of some dilute Pd Fe alloys

Abstract The electrical resistivity of several dilute Pd Fe alloys has been measured in the temperature range 0.5°–77°K. Immediately below the Curie temperature (Tc) a linear decrease of the incremental esistivity (Δρ) with temperature is observed, while at lower temperatures (T ⪡ Tc), Δρ is proportional to T 3 2 . The experimental results are compared with the theoretical predictions of a model based on s electron-magnon scattering. Estimates of the effective exchange integral J are made.

The resistivity of dilute Au Co alloys

Abstract Measurements are presented of the resistivity of Au containing between 0.005 and 3 per cent Co between 0.5° and 300°K. The results are consistent with a Kondo Temperature of around 400°K for the most concentrated Au Co alloys. At low temperatures the resistances vary approximately linearly with temperature, with a slope proportional to (concentration)1.5. Possible explanations for this unusual concentration dependence are discussed, and it is concluded that this results from a Kondo temperature which decreases with increasing concentration.

The magnetic specific heat of spin-glass and ferromagnetic AuFe alloys between 2 and 300K

The magnetic specific heat Delta C of Au1-xFex has been determined between 2 and 300K from measurements of the difference in specific heat of pairs of Au1-xFex and Au1-xNx alloys. The concentration range investigated, 0.02 or=0.15). It is shown that the results provide evidence against substantial spatial short-range order but can be explained assuming a random distribution of Fe atoms. No discontinuity in Delta C is observed at the freezing temperature TF in the spin-glass and cluster-glass alloys, at which a susceptibility cusp has previously been observed, but TF is found to correlate well with a pronounced maximum in Delta C/T where the entropy is increasing most rapidly with temperature. The magnetic entropy is shown to be consistent with a spin between 1.2 and 1.5 for the Fe atoms.

Spin fluctuation contributions to the specific heat of CuNi alloys

The electronic and magnetic specific heat of CuNi alloys has been determined in the temperature range 1.5-240K for Ni concentrations between 25 and 70 at.%. For concentrations up to 58 at.% Ni the specific heat at 240K is close to the unenhanced electronic value deduced from a recent CPA band-structure calculation for paramagnetic CuNi. At lower temperatures the electronic specific heat is strongly enhanced in the critical concentration region. Arguments are presented against a superparamagnetic cluster explanation, and it is concluded that the results may best be understood in terms of an electron effective mass enhancement due to their coupling to spin fluctuations localised in regions of high Ni density.

Dilute PdNi-a homogeneous magnetic system of fluctuating moments

It is shown that published magnetic susceptibility and magnetisation data for PdNi alloys are consistent with a homogeneous model in which the Ni atoms have a spin fluctuation temperature Ts approximately 55K and are coupled by ferromagnetic Ni-Ni interactions. Using a molecular-field model the equivalent classical RMS moment per Ni atom square root ( mu 2) is shown to be concentration independent for concentrations between 1.3 and 16.7 at.% Ni and for temperatures between the ordering temperature Tc and 700K. square root ( mu 2) exhibits a temperature-dependent exchange enhancement at high temperatures, and at low temperatures falls to zero due to intrinsic spin fluctuations. The exchange enhancement and the ferromagnetic coupling between Ni moments are shown to be related in accordance with the indirect-coupling model of Takahashi and Shimizu (1965). The critical concentration for the onset of ferromagnetism and the concentration dependence of the ordering temperature Tc are determined from the curve for square root ( mu 2) and are shown to be in excellent agreement with experiment. A simple model is presented which qualitatively reproduces the magnetisation curves for paramagnetic and ferromagnetic alloys.

Magnetoresistance of a dilute PtFe alloy

The electrical resistivity of a Pt 0.05 at% Fe sample has been measured between 1.5 K and 10 K and in magnetic fields up to 60 kOe. The magnetoresistance is found to be negative, and is well described using an s-d model in which the s electrons scatter off the giant moments known to exist in this system. Values of the spin and g-factor for the giant moment are found, and the latter is compared with a theoretically predicted value.

Evidence on the nature of superconductivity in La1.85Ba0.15CuO4 from specific heat measurements

Information on the nature of the superconducting state in La1.85Ba0.15CuO4 has been obtained from high-precision differential specific heat measurements on superconducting La1.85Ba0.15CuO4, and non-superconducting La2CuO4 and La1.85Ba0.15Cu0.9Ni0.1O4 samples between 1.5 and 300 K. From the very small jump of 0.07 mJ mol-1 K-2 in C/T at Tc approximately 29 K the authors obtain (dHc/dTc)=33 Oe K-1 and Hc(0)=475 Oe, where Hc is the thermodynamic critical field. A condensation of real-space Bose pairs (e.g. bipolarons) is excluded by the authors results, which indicate that a fraction of around 0.06 of the electrons within kTc of the Fermi surface are in Cooper pairs. The estimated electron mass enhancement (1+ lambda ) with lambda approximately 0.82 is lower than expected for conventional phonon-mediated superconductivity with Tc approximately 29 K and indicates that an alternative, possibly excitonic, pair interaction may be involved. Phonon spectra are determined that are in agreement with theoretical predictions, and evidence is obtained for strong high-temperature anharmonicity. A striking anomaly at 58 K in the specific heat of the superconducting compound may indicate a soft phonon mode.

Electrical resistivity of the AuV system

Measurements are presented of the resistivity of two AuV alloys in the temperature range 0.5-273K. The impurity resistivity, uncorrected for deviations from Mathiessens rule, decreases as T2, T and then log T, in general agreement with recent calculations (Whall et al. 1972). Estimates are made of the deviation from Matthiessens rule. This is found to be large in the temperature range 15-300K, and considerably affects the details of the temperature dependence of the impurity resistivity.

Magnetization and magnetoresistance of the localized spin fluctuating system PtCo

The susceptibility, magnetization, resistivity and magnetoresistance of a Pt‐0.061 at.% Co sample have been measured between 1.5 and 15 K in applied magnetic fields varying up to 59 kOe. The susceptibility data have been fitted to two phenomenological expressions, both of which yield a characteristic temperature of a few degrees. The magnetization data are complex, but they can be understood qualitatively by using the idea of an effective temperature. Finally, except at the highest field an lowest temperature, the magnetoresistance data is well represented by the predictions of localised spin fluctuation (lsf) theory, from which an lsf temperature of 0.7±0.1 K is estimated for this system.

PtNi-a system of interacting Kondo moments

It is demonstrated that published magnetic susceptibility and magnetisation data on Pt1-cNic alloys for 0<c<1 are consistent with a localised model of ferromagnetically interacting Kondo moments in which the Ni atoms are magnetic at all concentrations and have a Kondo temperature of around 300 K. The Ni effective moment at low concentrations is around 2.4 mu B with a giant moment enhancement factor of approximately 1.46, the latter value being close to that for dilute PtFe. This interpretation is also shown to provide a better explanation for the concentration dependence of the electronic specific heat coefficient than current models of weak itinerant ferromagnetism.