Thesis presented December 16, 2010

Abstract:
In this PhD thesis, three low dimensional spin systems are studied by means of elastic and inelastic neutron scattering. Macroscopic measurements in the DMA-CuCl₃ compound indicate the coexistence of two kinds of dimers : antiferromagnetic and ferromagnetic. The magnetic structure determined by our neutron diffraction survey at H = 0 shows irrevocably the existence of these two kinds of dimers. It has been shown that the Ising-like compound BaCo₂V₂O₈ should be the first realization of a system in which a longitudinal spin density wave (LSDW) magnetic order occurs when a magnetic field is applied. In a first time, we have determined the magnetic structure in zero magnetic field. Then, we focused on the effect of a magnetic field on the propagation vector, showing an entrance in the LSDW phase at H_c = 3.9 T. The magnetic structure refined above this critical field confirms that BaCo₂V₂O₈ is the first compound in which occurs a LSDW phase. In the organic compound D-F₅PNN, it has been shown that this compound is well described at low temperature by spin chains with alternating couplings. However, the crystallographic structure determined at room temperature implies that the interactions are uniform. By means of neutron diffraction, we characterized a structural transition at low temperature (H_c = 450 mK) making the system evolve from C2/c space group to Pc. This transition explains the alternating behavior of the interactions. We have also evidenced a field-induced structural transition (H_c = 1.1 T). Above this field, the system is back to the C2/c space group, implying that the interactions are back to uniform. We have confirmed this by studying the magnetic excitations.

Keywords:
Low dimensional magnetism, Spin chain, Neutron scattering, Magnetic structure, Magnetic excitationss

On-line thesis.