M. Sc. / The substitution series CeCu5−xAlx where x 2 {1.0, 1.5, 2.0, 2.1, 2.2, 2.3, 2.4} was synthesized and characterized via x-ray diffraction and scanning electron microscope analysis. All the compounds formed single phase in the P6/mmm hexagonal crystal structure, which pushed the solubility limit of Al within this structure from CeCu3Al2 to CeCu2.6Al2.4. The ascast samples were annealed, but the annealing process had no visible effect on the crystalline nature nor on the physical properties of the samples. The electric, magnetic and thermal properties were measured on Quantum Design’s Physical Properties Measurement System and Magnetic Properties Measurement System. The electrical resistivity showed single-ion Kondo scattering at intermediate temperatures with no signs of coherence for x 2 {1.0, 1.5, 2.0, 2.1, 2.2, 2.3} CeCu2.6Al2.4 on the other hand showed a weak maximum at 2 K, which was the onset of antiferromagnetic ordering confirmed by other measurements. The magnetoresistance was fitted by the Beth´e ansatz spin-1/2 model and the obtained Kondo temperatures had a maximum at CeCu3Al2, with linear dependencies on Al concentration on either side of this point. The Kondo moments followed an exponential dependence on the Al concentration, but these effective moments were far lower than the free-ion value, due to the Kondo interaction. The thermal conductivity in this series was phonon dominated with the magnitude of the electronic component being inversely proportional to the Kondo temperature, showing the effect that the Kondo scattering mechanism has on the availability of the conduction electrons to participate in heat transport. The magnitude of the peak in the thermoelectric power S(T) was strongly dependent on the Al concentration, having a maximum at CeCu2.8Al2.2. Below the peak there were two temperature dependencies: S(T) / T for x 2 {1.0, 1.5, 2.4} which is metallic and for x 2 {2.0, 2.1, 2.2, 2.3} S(T)/T /−ln(T) which is characteristic of a quantum critical point. The data were also fitted with a phenomenological model and the obtained Kondo temperatures were quantitatively similar to those obtained from the Beth´e ansatz. The heat capacity Cp(T) data confirmed these compounds to be heavy fermions and also showed Cp(T)/T /−ln(T) below 10 K, which is the hallmark for a magnetically tuned quantum critical point. The magnetic susceptibility showed a Curie-Weiss temperature dependence at temperatures above 100 K, with magnetic moments close to the free-ion value, indicating the presence of localized magnetic moments. The low temperature data found (T) /−ln(T) for Al concentrations near CeCu3Al2 and the ordering present in CeCu2.6Al2.4 was suppressed by 0.3 K in going from 0.01 T to 2 T, suggesting that the ground state is antiferromagnetic in this compound.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:8656 |
| Date | 07 June 2012 |
| Creators | Britz, Douglas |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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