The borocarbide intermetallics RNi2B2C (R = Y, Ce-Lu) were discovered in 1994 and attracted a large attention because superconductivity and magnetic ordering are coexisting in R = Dy, Ho, Er and Tm. Early reports concluded that these compounds should be considered as anisotropic BCS superconductors with a phonon-mediated moderately strong coupling between the conduction electrons causing the creation of Cooper pairs. The magnetism is of the indirect Ruderman-Kittel-Kasuya-Yosida(RKKY) type where the conduction electrons are polarized by the local magnetic moment of the rare-earth ions and thereby mediates a coupling between the ions, which orders into a spin density wave at sufficiently low temperatures. Thus an intricate interplay between the magnetism and the superconducting state is expected and has indeed been observed. The compound TmNi2B2C has previously been studied by Small Angle Neutron Scattering(SANS) with the applied field along the crystalline c-axis and a very rich phase diagram in terms of flux line lattices(FLL) with different symmetries have been found. One of the FLL transitions is coincident with a magnetic phase transition between two spin density waves. In this thesis additional SANS studies of the FLL phases in TmNi2B2C are reported and an interpretation of the phase diagram in the paramagnetic region is presented. It is suggested that the observed square FLL is stable in between two transition lines determined by two different length scales. The lower transition field is reached when the distance between the flux lines becomes comparable to the non-locality radius resulting from non-local electrodynamics, whereas the upper transition field is determined from the crossover from intermediate to high flux line density where the vortex cores start to overlap and the superconducting order parameter is suppressed in between the flux lines. A detailed examination of the intensity of the neutron diffraction spots caused by scattering on the flux line lattice in TmNi2B2C is presented and analyzed on the basis of the form factor of an isolated flux line. This analysis can not provide a good explanation for the observed scattering and it is suggested that the scattering from the Tm ions must be considered. One can argue that the moments of the Tm ions are modulated by the flux line lattice, because the RKKY interaction between the Tm ions might be different inside the vortex cores than outside in the superconducting phase. A calculation of the neutron scattering cross section of such a magnetic flux line lattice has been performed and compared to the SANS data. This offers a qualitative explanation of some of the observations, but future work is needed to make a more quantitative comparison.
|Place of Publication||Roskilde|
|Publisher||Risø National Laboratory|
|Number of pages||118|
|Publication status||Published - 2003|
|Series||Denmark. Forskningscenter Risoe. Risoe-R|