Neutron diffraction stud- ies of magnetic ordering in superconducting ErNi2B2C and TmNi2B2C in an ap- plied magnetic field

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. [1]. However, two results remain unresolved: 1. When applying the magnetic field along [010], the minority domain (QN= (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 [110], the magnetic structure rotates a small angle of 0.5 away from the symmetry direction. TmNi2B2C: A magnetic field applied in the [100] 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. It should be noted that part of the work for this thesis has been carried out in collaboration with fellow researchers. In general, I have initiated all the neutron diffraction and scattering experiments, and performed most of them myself. Some of them have been performed while I was either pregnant or on maternity leave, by A. B. Abrahamsen, A. Jensen and N. H. Andersen. The results for ErNi2B2C with the field along [001] are presented in section 5.5, note that the experiment and data analysis for this field direction is done by A. Jensen. The remaining data analysis presented in chapters 5 and 6 was done by myself. The model work on the magnetic structures, described in sections 2.4, 2.5, and 2.6 was done by J. Jensen [1, 2]. Many of the results presented in this thesis are also included in the following publications: K. Nørgaard, M.R. Eskildsen, N.H. Andersen, J. Jensen, P. Hedeg̊ard, S.N. Klausen, and P.C. Canfield, Interdependence of Magnetism and Superconductivity in the Borocarbide TmNi2B2C, Phys. Rev. Lett. 84, 4982-4985, 2000. M.R. Eskildsen, K. Nørgaard, A. B. Abrahamsen, N. H. Andersen, K. Mortensen, P. L. Gammel, D. Lopez, D. J. Bishop, P. Vorderwisch, M. Meissner, and P. C. Canfield, Neutron scattering studies of the flux line lattice and magnetic ordering in TmNi2B2C, pp. 333-340 in Rare Earth Transition Metal Borocarbides (Nitrides), Kluwer Academic Publishers, 2001. K. Nørgaard Toft, A. B. Abrahamsen, N. H. Andersen, D. F. McMorrow, A. Jensen, J. Jensen, P. Hedeg̊ard, J. Klenke, K. Prokes, P. Smeibidl, S. Danilkin, V. Sikolenko, M. R. Eskildsen, and P. C. Canfield, The Magnetic state of ErNi2B2C in an in-plane field, pp. 14-15 in Superconductivity and Magnetism: Materials Properties and Developments, 24th Risø International Symposium on Materials Science 2003. A. Jensen, K. Nørgaard Toft, A. B. Abrahamsen, D. F. McMorrow, M. R. Eskildsen, N. H. Andersen, J. Jensen, P. Hedeg̊ard, J. Klenke, S. Danilkin, K. Prokes, V. Sikolenko, P. Smeibidl, S. L. Budko, and P. C. Canfield, Field-induced magnetic phases in the normal and superconducting states of ErNi2B2C. Accepted for publication in Phys. Rev. B. Present version attached to this thesis. K. Nørgaard Toft, A. B. Abrahamsen, M. R. Eskildsen, K. Lefmann, N. H. Andersen, P. Vorderwisch, P. Smeibidl, M. Meissner, and P. C. Canfield, A Neutron Diffraction Study of the Anomalous High Field Magnetic Phases in TmNi2B2C, Submitted to Phys. Rev. B. Present version attached to this thesis. This thesis is submitted in partial fulfilment of the requirements for a ph.d. degree at the University of Copenhagen. The work has been carried out at Risø National Laboratory in the Condensed Matter Physics and Chemistry Department and in the Materials Research Department. The supervisors were Niels Hessel Andersen, Risø and Jens Jensen, University of Copenhagen. ISBN 87–550–3286–9; 87–550–3287–7 (Internet) ISSN 0106–2840 Print: Pitney Bowes Management Services Denmark A/S · 2004