Kondo behaviour in Ce intermetallic compounds

Houshiar, Mahboubeh (1997) Kondo behaviour in Ce intermetallic compounds. University of Southampton, Doctoral Thesis.

Abstract

A detailed study has been undertaken of the magnetic properties of new Kondo lattice compounds CeTSb₂ (T=Ni, Cu, Ag and Pd). These have been synthesized and investigated by X-ray diffraction, magnetic susceptibility, electrical resistivity, magnetoresistance and inelastic neutron scattering measurements. These compounds crystallise in the tetragonal UCuAs₂-type structure with space group P4/nmm. The magnetic susceptibility of CeAgSb₂ exhibits a sharp rise below 10K indicating ferromagnetic ordering of the Ce moments. CeNiSb₂ orders below 6K with a weak ferromagnetic component, while CeCuSb₂ appears to be paramagnetic down to 5K. The resistivities of these compounds exhibit Kondo-like -1n(T) temperature dependence at high temperatures and maxima in the range 15-25K.

The cross-over between intermediate valence behaviour and localised moment magnetism has been studied in alloys based on CePd₃, namely CePd₃B_x and Ce(Pd_1-xCu_x)₃. In CePd₃B_x alloys, with x up to 0.3 the boron enters the octahedral interstitial site of the AuCu₃ structure. With increasing boron concentration there is a homogeneous cross-over from intermediate valence to Curie-Weiss behaviour. The broad maximum in the resistivity shifts to lower temperatures with increasing x, until for x>0.15 ρ(T) has a negative temperature coefficient at low temperatures. For x=0.3 the resistivity shows normal metallic behaviour between 4K and 300K. The large anomalies in the thermoelectric power of CePd₃ decrease dramatically with increasing boron content.

The Ce(Pd_1-xCu_x)₃ system, where the copper enters the lattice substitutionally, has been claimed to be an example of a cerium system which exhibits gap formation in the electronic density of states. Magnetic and transport measurements have been carried out on Ce(Pd_1-xCu_x)₃ alloys with x in the range 0 - 0.133. With increasing copper content the alloys show a smooth cross-over to Curie-Weiss behaviour; the resistivity maximum shifts to lower temperatures and above 3.3% copper, the resistivity exhibits a negative temperature coefficient down to 10K. The magnetoresistance changes sign from positive to negative with increasing copper substitution. The neutron inelastic scattering data for these alloys show a dramatic change with copper content. For CePd₃ a broad inelastic distribution was observed, characteristic of the intermediate valence state. IN the copper substituted alloys the weight in the high energy response disappears and two relatively well defined crystal field excitations develop. There is no evidence for gap formation in the electronic density of states.

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