This thesis describes neutron diffraction studies of the long-range magnetic ordering of superconducting ErNi2B2C and TmNi2B2C in an applied magnetic field. The magnetic structures in an applied field are especially interesting because the field suppresses the superconducting order parameter and therefore the magnetic properties can be studied while varying the strength of superconductivity. ErNi2B2C: For magnetic fields along all three symmetry directions, the observed magnetic structures have a period corresponding to the Fermi surface nesting structure. The phase diagrams present all the observed magnetic structures, and the spin configuration of the structures are well understood in the context of the mean field model by Jensen et al. . However, two results remain unresolved: 1. When applying the magnetic field along , the minority domain (QBN= (0, Q, 0) with moments perpendicular to the field) shows no signs of hysteresis. I expected it to be a meta stable state which would be gradually suppressed by a magnetic field, and when decreasing the field it would not reappear until some small field comparable to the demagnetization field of 0.1 T. 2. When the field is applied along , the magnetic structure rotates a small angle of 0.5o away from the symmetry direction. TmNi2B2C: A magnetic field applied in the  direction suppresses the zero field magnetic
structure QF = (0.094, 0.094, 0) (TN = 1.6 K), in favor of the Fermi surface nesting structure QN = (0.483, 0, 0). The appearance of the QN phase was initially believed to be caused by the suppression of superconductivity. This suppression should make it energetically favorable to create a magnetic order with a Q-vector determined by the maximum in the magnetic susceptibility at the Fermi surface nesting vector QN. The phase diagram for the magnetic structures is presented, however several properties of the QN magnetic structure cannot be explained within any known models. Quadrupolar ordering is suggested as a possible candidate for explaining several features of the QN structure: The Nèel temperature of QN increases steadily up to the maximum examined field of 6 T, the QN structure appears only parallel to the applied field, not perpendicular to it, and last the QN phase has a low intensity tail extending to temperatures as high as 15 K at 6 T.
|Series||Denmark. Forskningscenter Risoe. Risoe-R|