Einfluß der magnetischen Ordnung auf Supraleitung und Kristallstruktur in Seltenerd-Nickel-Borkarbid-Verbindungen

Rare-earth nickel borocarbids RNi2B2C are particularly suitable for investigations on one of the most interesting problems in modern solid-state physics: these compounds display competition and coexistence of superconductivity and magnetism. Depending on the R3+ ion, the transition temperatures are in an experimentally easy accessible range of 1 K to 25 K. This thesis presents experimental studies on the interplay of both ordering phenomena. Neutron diffraction is used to determine the magnetic order and the resulting changes of the crystal structure. Experiments are performed on polycrystalline and single crystal samples in dependence on temperature and external magnetic fields. The Ni-B stoichiometry of the tetragonal RNi2B2C compounds is systematically varied and the magnetic R3+ ions are partially substituted by other magnetic or nonmagnetic R?3+ ions. The experimental results are compared with macroscopic magnetic and electrical properties. For HoNi2B2C three different magnetic structures are found in a narrow temperature range. While for two magnetic structures the Ho3+ moments are modulated along the c axis, a third magnetic structure with a modulation in a direction is observed. Both, partial substitution of Ho3+ ions and variation of the Ni-B stoichiometry, strongly modify the formation of these different types of magnetic order. The comparison with the concomitant changes of the superconducting properties yields the following scenario for HoNi2B2C-based compounds: superconductivity coexists with both magnetic structures with modulations in c direction. However, the onset of magnetic order weakens the superconductivity. For the magnetic structure with modulation along the a axis, components of the magnetic moments arise in c direction. The resulting local magnetic fields on Ni sites yield a strong suppression of the superconductivity. The observed competition between superconductivity and the magnetic structure with modulation along the a axis strongly suggests that the modification of the electronic structure due to the superconducting state influences the magnetic ordering. As a further impact of the magnetism in RNi2B2C compounds with R = Ho, Dy, Tb and Er changes of the crystal structure are investigated. Using high-resolution neutron diffraction, tetragonal-to-orthorhombic lattice distortions are found. They are induced by those magnetic structures with either parallel or anti-parallel alignement of R3+ magnetic moments. The direction of the lattice distortions, the dependence of their size on the square of the effective ordered magnetic moment and on the type of the R3+ ions indicate that the magneto-elastic interactions are determined by crystal-field effects. This fact also facilitates the elucidation of the magnetic phase diagrams by neutron diffraction experiments in external magnetic fields. For a given phase, absence or presence of magneto-elastic lattice distortions restrict the set of possible magnetic structures. For HoNi2B2C the magnetic phases reported in literature are confirmed. The experimental results for DyNi2B2C are interpreted using a simple model to determine the magnetic structures. Based on mean field calculations, the differences in the magnetic structures for increasing and decreasing magnetic fields can be understood as very strong hysteresis effects in connection with first-order phase transitions.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Technische Universität Dresden

Year

2001

Author dc:creator

• Kreyßig, Andreas

Contributors dc:contributor

• Loewenhaupt, Michael
• Schultz, Ludwig
• Winzer, Klaus

• Borkarbide (PACS=74.70.Dd)
• Korrelation Magnetismus-Supraleitung (PACS=74.25.Ha)
• Magnetische Phasenübergänge (PACS=75.30.Kz)
• Magnetische Strukturen (PACS=75.25.+z)
• Magneto-elastische Effekte (PACS=75.80.+q)
• Neutronendiffraktion (PACS=61.12.-q)
• Supral
• Borocarbides (PACS=74.70.Dd)
• Correlation magnetism-superconductivity (PACS=74.25.Ha)
• Magnetic phase transitions (PACS=75.30.Kz)
• Magnetic structures (PACS=75.25.+z)
• Magneto-elastic effects (PACS=75.80.+q)
• Neutron diffraction (PACS=61.12.-q)
• Supercond