Thesis presented October 29, 2004

Abstract:
In the present work, inelastic neutron scattering has been performed to study the magnetic excitations of the heavy fermions systems Ce_1-xLa_xRu₂Si₂ and CeIn_3-xSn_x. Those systems are characterized by a quantum phase transition (at T = 0) between a magnetically ordered ground state and a Fermi liquid paramagnetic ground state. The first experimental part is devoted to the system Ce_1-xLa_xRu₂Si₂. A detailed study of the temperature dependence of the magnetic fluctuations is carried out for the compound of critical concentration x_c and for the paramagnetic compound of concentration x = 0. We find i) that the magnetic fluctuations saturate at low temperatures, and ii) that the dynamical susceptibility, which corresponds to those fluctuations, follows a high temperature E/T^ß scaling law with ß < 1. Such results do not enter the framework of quantum phase transitions theories, since i) a divergence of the spin fluctuations is expected, and ii) a E/T^ß scaling law can only occur with ß > 1 in those theories. A study of the ordered compounds x > x_c permits to conclude that the antiferromagnetic fluctuations are critical and that they govern the transition, even if they do not diverge at the magnetic instability. The second part concerns the system CeIn_3-xSn_x. A study of the excitations spectra of CeIn₃ illustrates the duality of heavy fermions systems: well-defined spin waves are a manifestation of the localized character of the f electrons, while the broadening the crystal field excitations is due to their itinerant character. A study of the magnetic fluctuations of the critical compound of concentration x_c=0.6 seems also to indicate their saturation below 10 K.

Keywords:
Heavy Fermions, Kondo Lattice, Magnetic Fluctuations, Inelastic Neutron Scattering, Quantum Phase Transition, Magnetic Instability, Criticality, Scaling Laws

On-line thesis.