H. v. Löhneysen

Delocalization of the Ce 4f shell in amorphous Ce75.5Co24.5

The amorphous alloy Ce75.5Co24.5 prepared by melt spinning has been studied through measurements of the magnetic susceptibility, magnetization, electrical resistivity, thermoelectric power and specific heat. The results are interpreted in terms of a homogeneous intermediate valence state of the Ce ions. This is inferred from a temperature-independent magnetic susceptibility at low temperature and the absence of magnetic ordering, a large linear term in the specific heat, and aT2 dependence of the electrical resistivity at low temperature followed by a steep increase with temperature up to 50 K. At this temperature, the thermoelectric power displays a maximum. The intrinsic properties are partially obscured at low temperatures by a contribution from roughly a few percent of magnetic ‘impurities’, presumably Ce3+ ions. They manifest themselves by an increase of the susceptibility towards low temperatures and by a broad Schottky-like contribution to the specific heat resulting from the excitation of magnetic clusters.

Low temperature thermal properties of amorphous Mg0.7Zn0.3

Measurements of the specific heat (0.09 K ⩽ T ⩽ 1.7 K) and the thermal conductivity (0.5 K ⩽ T ⩽ 20 K) on the metallic glass Mg0.7Zn0.3 are reported. The electronic specific heat coefficient ψ can be described by the free electron density of states with an electron-phonon mass enhancement factor 1 + λ = 1.24. Bulk superconductivity is observed below Tc= 0.11 K. From Tc, λ is calculated to be 0.3 in good agreement with the value deduced from ψ. The small λ value suggests that amorphous Mg0.7Zn0.3 is a weak-coupling superconductor. This is supported by the size of the specific-heat jump at Tc which is close to the BCS value. The thermal conductivity is mostly due to conduction electrons. The phonon heat transport is limited by scattering from conduction electrons which can be accounted for in the free electron model, and by scattering from low energy excitations (two-level systems) with roughly the same coupling strength to phonons as found in other amorphous metals.

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureTf(ν) is determined for the dilute spin glass systems (La, Gd)B6 and (Y, Gd)Al2 in the frequency range 10–1,000 Hz. While for (La, Gd)B6,Tf(ν) is found to be weak, for (Y, Gd)Al2Tf(ν) is even stronger than for the previously studied system (La, Gd)Al2. Both, measurements of the temperature dependence of the susceptibility nearTf and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B6 and ferromagnetic for (Y, Gd)Al2 as well as (La, Gd)Al2.

Effect of structural relaxation on low energy excitations in amorphous Zr x Cu1−x

We report on an investigation of the liquid-quenched metallic glass ZrxCu1−x (0.6≦x≦0.74) subjected to heat treatments below the glass transition temperatureTg. Annealing temperatures up to 200°C (<0.8Tg) were chosen as to achieve topological relaxation only. The superconducting transition temperaturesTc are lowered, as already observed for other metallic glasses. Low temperature measurements of the thermal conductivity (0.5 K≦T≦15 K) and of the specific heat (0.1 K≦T≦3 K) were carried out in order to determine the effect of structural relaxation on the low energy configurational excitations characteristic of the amorphous state. The annealed samples show no detectable (<20%) change in the specific heat forT≪Tc, but an increase of the thermal conductivity by a factor of 2 forT≪Tc is observed. Within the tunneling model of two level systems (TLS) for the low energy excitations, this behavior can be qualitatively understood in terms of a change of the TLS relaxation time distribution upon annealing. This distribution differs from that of the commonly used standard tunneling model. The change of the phonon scattering by TLS directly observed forT≪Tc is largely responsible for the enhancement of the thermal conductivity found also aboveTc.

Competition between spin-glass and antiferromagnetic order in EuxSr1−xAs3

Abstract Measurements of the magnetic a.c. susceptibility χ of EuxSr1−xAs3 single crystals (0.10 ⩽ x ⩽ 0.78) are reported. For the lower concentrations, spin-glass like maxima with a.c. frequency dependent positions and heights are observed. Samples with x = 0.54 and 0.65 exhibit a complex magnetic behavior (with directional dependence of χ observed for x = 0.54) which gives way to antiferromagnetic ordering when x is further increased towards EuAs3.

Magnetic field dependence of the thermal conductivity in EuxSr1-xS

Abstract The thermal conductivity κ of two Eu x Sr 1- x single crystals ( x = 0.25 and 0.54) was measured between 1.5 and 25 K. In magnetic fields of ≈ 7 T κ is enhanced for temperatures up to 20 K with respect to κ( B = 0). At 1.5 K, where the relative increase κ( B )/κ(0) is largest, this ratio is 1.5 for x = 0.54 and ≈ 4 for x = 0.25. Two possible mechanisms for this effect, i.e. freezing of phonon scattering by magnetic excitations in a magnetic field, and heat transport by field-induced magnons, are discussed.

(Eu, Sr)As3: Competition between spin glass and antiferromagnetic order☆

Abstract The ac susceptibility of Eu x Sr 1- x As 3 single crystals with 0.1 ⩽ x ⩽ 0.8 has been measured. The magnetic phase diagram based on these results shows a region of reentrant antiferromagnetism, where the system goes upon cooling first through an antiferromagnetic state before forming a spin glass.

Specific heat of EuSSrS multilayers

Specific-heat measurements on epitaxially grown magnetic EuSSrS multilayers are reported. The thickness of the individual EuS and SrS layers varied between dM = dN = 1.25 and 40 nm, with a total thickness of 160 nm for each sample. With decreasing thickness, a smooth transition from the specific heat of pure ferromagnetic EuS for large-dM samples to that of EuxSr1−xS spin-glasses with x ≈ 0.5 for dM = 1.25 is found. This can be attributed to the existence of interfacial EuxSr1−xS layers of thickness d ≈ 3 nm produced by diffusion. Possible magnon quantization effects are probably masked by the large specific-heat contribution of the interfacial layers.

Low-temperature properties of amorphous (Mo1−xRu x )0.8P0.2 alloys

Amorphous (Mo1−xRux)0.8P0.2 alloys (0.3≤x≤0.7) have been investigated with measurements of the specific heatC and thermal conductivity κ. Also the superconducting properties (critical temperatureTc and upper critical field) have been determined. Well belowTc, all alloys show the familiar behavior known for glasses, i.e.C∼Tn and κ∼Tm withn≈1 andm≈2 which is attributed to tunneling states (TLS). The largeTc allows an unambiguous determination of the coefficients ofC and κ. Compairing our data with literature data, we find no correlation between the TLS density of states and the glass temperature or crystallization temperature, as opposed to insulating glasses where such a correlation appears to exist. The unusual annealing behavior found previously in amorphous Zr−Ni and Zr−Cu, which was attributed to a change in the TLS relaxation-time spectrum, is confirmed in the present work.

Thermal properties of EuxSr1-xS in high magnetic fields☆

Abstract We report measurements on EuxSr1-xS of the thermal conductivity ϰ between 1.5 and 40 K and of the specific heat C between 0.07 and 30 K in field B up to 7 T. In high fields, ϰ is enhanced by up to a factor of 5 in the spin-glass concentration range. The specific heat (varying linearly with T in zero field) decays increasingly faster for T → 0 with increasing B. For B ⩾ 3 T, C decreases exponentially, suggesting the opening of a gap in the spectrum of magnetic excitations.