Thesis presented December 06, 2012

Abstract:
In this thesis, three Yb-based heavy fermion compounds, YbCu₂Si₂, YbCo₂Zn₂0 and YbRh₂Si₂, are studied under extreme conditions, i.e, high pressure, low temperatures and high magnetic fields. An important part of the work has been the set up of an in situ tuning pressure device to measure diamond anvil cells at dilution fridge temperatures. This has enable most of the experimental results presented here. In YbCu₂Si₂ the nature of the pressure-induced magnetic order that arises for P_c> 8 GPa has been clarified to be ferromagnetic by ac-susceptibility and magnetization measurements under pressure. The interplay of magnetism and valence change has been investigated by measuring the valence of the title compound at high pressures and low temperatures using resonant inelastic x-ray scattering (RIXS). As expected, pressure favors the trivalent state but the Yb ion valence remains below 3 even at the highest pressure and at low temperatures very close to the onset of the magnetic order. We have also performed a detailed search for signatures of metamagnetism. The second part of my thesis focuses on YbCo₂Zn₂0. We have extended the (P-T) phase diagram up to 14 GPa by ac-calorimetry and ac-susceptibility measurements. Our results shed some light on the nature of the magnetic order that arises for P>1 GPa which is antiferromagnetic. The strong field effects observed in the physical properties in YbCo₂Zn₂0 are probably related to the interaction between the magnetic field and the spin of the 4ƒ electrons rather than to their charge as the valence of the Yb ion is insensitive to the application of a magnetic field of 10 T. The final part of this thesis is devoted to the detailed study of the (H-T) phase diagram under pressure in YbRh₂Si₂ to determine the evolution of the magnetic ordering temperatures as a function of H and P to better understand the interplay of the two control parameters.

Keywords:
Heavy-fermion compounds, Intermediate-valence compounds, High pressure, YbCu₂Si₂, YbCo₂Zn₂0, YbRh₂Si₂

On-line thesis.