Thesis presented September 28, 2011

Abstract:
In this thesis, the study on the superconducting ferromagnet UGe₂ is presented. Crys- tal growth of UGe₂ single crystals was realized in a tetra-arc furnace using the Czochralski technique. This technique was also used to obtain high quality single crystals of other uranium compounds, notably UCoGe and URu₂Si₂. The Curie temperature of UGe₂ (T_Curie = 53 K) decreases with pressure and is suppressed at p_c = 1.5 GPa. Before being suppressed, the ferromagnetic transition changes from second to first order at a tricritical point. Precise resistivity and Hall resistivity measurements under pressure and magnetic field revealed the position of the tricritical point as well as its evolution under magnetic field which draw a wing structure phase diagram. Despite the theoretical prediction that this diagram is general for a ferromagnet, here we present the first experimental observation. Other measurements focus on the superconductivity (T_sc = 0.75 K) which coexists with ferromagnetism under pressure. The bulk nature of the superconductivity is investigated by AC calorimetry measurements under pressure. The attention is turned to the interesting phenomenon of field enhanced superconductivity.

Keywords:
Superconducting ferromagnet, Tricritical point, UCoGe, URhGe, URu₂Si₂

On-line thesis.