Study of the ferromagnetic superconductor URhGe

This thesis deals with the physical properties of the ferromagnetic superconductor URhGe. It is the first study realized on single crystals in extreme experimental conditions i.e. very low temperature, high hydrostatic pressure and intense magnetic field. First of all we describe the crystal growth and the different annealing procedures necessary to obtain samples that exhibit a complete and narrow resistive superconducting transition. Then we present the pressure-temperature phase diagram obtained by calorimetry, resistivity and magnetisation. On contrary to UGe₂ the Curie temperature (T_Curie) increases linearly up to 130 kbar while the ordered moment and the superconducting critical temperature (T_c) decrease monotonically. We also report the influence of the lattice defects and the first try of measuring the critical current. Our results are compatible with an unconventional superconducting ground state. Finally we present the first measurements of the upper critical field (H_c2) on samples with different RRR. A clear jump of H_c2 close to T_c illustrates the presence of the ferromagnetic domains in the superconducting condensate. The value of H_c2 at 0 K and the observed strong anisotropy are coherent with a polar order parameter with spin triplet pairing. (author)