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This thesis describes inelastic neutron scattering (INS) experiments on several iron-based materials. The experiments were primarily designed to investigate the link between magnetic dynamics and superconductivity. The work contributes to evidence that magnetic fluctuations influence or are influenced by superconductivity. It is demonstrated that the INS response of a material, in conjunction with theoretical models, can provide valuable information about both superconductivity and magnetism. I measured the magnetically ordered parent-compound SrFe2As2 to investigate the nature of magnetism in iron-based systems. Comparison of the data to models based on both itinerant and localised magnetism showed that an itinerant model offers the best description of the data. LiFeAs is a superconductor that shows no magnetic order, however I was able to distinguish a magnetic signal in its INS spectrum. The signal is consistent with the magnetic resonance observed in several other iron-based superconductors. This indicates that LiFeAs likely hosts an s± gap symmetry. I investigated two iron-phosphide systems, LaFePO and Sr2ScO3FeP, and in this case I was unable to identify any magnetic scattering. Comparison to LiFeAs showed that any signal in LaFePO is at least 7 times weaker. These results suggest that magnetic fluctuations are not as influential to the electronic properties of iron-phosphide systems as they are in other iron-based superconductors. In CsxFe2−ySe2 I found two independent signals that appear to be related to phase-separated magnetic and superconducting regions of the sample. I showed that fluctuations associated with the magnetically ordered phase are consistent with localised magnetism, and do not respond to superconductivity. The second signal, however, increases in intensity below the superconducting transition temperature Tc = 27K, consistent with a magnetic resonance. This could be indicative of a pairing symmetry in CsxFe2−ySe2 that is distinct from most other iron-based superconductors. Finally, the molecular intercalated FeSe compound Li0.6(ND2)0.2(ND3)0.8Fe2Se2 revealed strong magnetic fluctuations. Again the signal was consistent with a magnetic resonance responding to Tc = 43 K. The results suggest that Lix(ND2)y(ND3)1−yFe2Se2 is similar to the superconducting phase of CsxFe2−ySe2, placing constraints on theoretical models to describe the molecular intercalated FeSe compounds.

Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO₂. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb₂O₆, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb₂O₆. It has a square lattice of Cu²⁺ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.

The R2PdSi3 (R = heavy rare earth) have been synthesized first in 1990 in the search for materials with unusual electronic properties. The availability of single crystals was the starting point for several investigations of the magneto-crystalline anisotropy, also in applied magnetic fields. The results of the observed properties in resistivity, magnetization and susceptibility lead to the summary that these compounds range from interesting to exotic and that their magnetic properties are low dimensional, spin-glass like and altogether “novel”. The focus of this thesis is the careful analysis of the magnetic properties and magnetic structures of single crystalline R2PdSi3 (R = Gd, Tb, Dy, Ho, Er, Tm). The investigation of macroscopic properties uses magnetization and ac-susceptibility measurements. Resulting from these investigations are magnetic phase diagrams. Neutron and resonant X-ray diffraction measurements elucidate the magnetic structure for the investigated compounds. The phase diagram of Tb2PdSi3 is the starting point of a detailed neutron diffraction study in applied magnetic fields up to 6.5 T and in the temperature range from 0.05 K to 100 K on this compound. Key to the understanding of the R2PdSi3 is the strong coupling of crystallographic structure to the magnetic properties. Thus the established framework of exchange interaction and magneto-crystalline anisotropy allows a collective description instead of a “novel” behavior. / Die R2PdSi3 (R = schwere seltene Erde) sind erstmals 1990, im Rahmen der Suche nach Materialien mit ungewöhnlichen elektronischen Eigenschaften, synthetisiert worden. Die Verfügbarkeit von Einkristallen war der Startpunkt für eine Vielzahl von Untersuchungen, auch in angelegten Magnetfeldern, der magneto-kristallinen Anisotropie. Das Ergebnis der untersuchten Eigenschaften Widerstand, Magnetisierung und Suszeptibilität führte zu dem Schluss, dass diese Verbindungen interessant bis exotisch und das ihre magnetischen Eigenschaften niedrig dimensional, spin-glas ähnlich und insgesamt “neuartig“ sind. Der Schwerpunkt dieser Dissertation ist die genaue Analyse der magnetischen Eigenschaften und Magnetischen Strukturen von einkristallinen R2PdSi3 (R = Gd, Tb, Dy, Ho, Er, Tm). Magnetisierungs- und Suszeptibilitäts-Messungen werden zur Untersuchung der makroskopischen Eigenschaften benutzt. Resultat dieser Untersuchungen sind magnetische Phasendiagramme. Neutronen und resonante Röntgendiffraktrometrie klären die magnetische Struktur der untersuchten Verbindungen auf. Das Phasendiagramm von Tb2PdSi3 ist der Startpunkt einer detaillierten Neutronendiffraktionsuntersuchung dieser Verbindung in Magnetfeldern bis 6.5 T und im Temperaturbereich von 0.05 K und 100 K. Der Schlüssel zum Verständnis der R2PdSi3 ist die starke Kopplung der kristallografischen Struktur und der magnetischen Eigenschaften. Dadurch erlaubt das etablierte System aus Austauschwechselwirkung und magneto-kristalliner Anisotropie eine gemeinsame Beschreibung anstatt „neuartigem“ Verhalten.

Die Wirkung von Seltenerd-Stoerelementen in Seltenerd-Seltenerd-Nickel-Borkarbid-Verbindungen, die im gleichen Temperaturbereich Supraleitung und magnetische Ordnung zeigen, wird in Hinblick auf die Unterdrueckung der Supraleitung untersucht. Sie aeussert sich in magnetischer Paarbrechung, ferngeordneten Magnetstrukturen und Unordnung. Die Verbindungen wurden entsprechend der an das Material gestellten Anforderungen mittels Lichtbogenschmelzen, Rascherstarrung oder durch Einkristallzucht hergestellt. Die Unterdrueckung der supraleitenden Uebergangstemperatur in diesen Mischreihen mit magnetischen und nichtmagnetischen Seltenerdionen wird durch den Einfluss des effektiven de Gennes-Faktors, der der Konzentration der magnetischen Seltenerdionen Rechnung traegt, des Kristallfeldes und der durch die unterschiedlichen Radien der Seltenerdionen verursachten Unordnung erklaert. Die Unordnung wird durch die Varianz der Ionenradien quantifiziert und ihre Auswirkung auf die supraleitende Uebergangstemperatur und das obere kritische Feld sowie dessen Kruemmung in der Naehe der Uebergangstemperatur beobachtet. Die Uebergagstemperatur von Yttrium-Seltenerd-Nickel-Borkabid-Verbindungen wird im Bereich schwacher Konzentration von magnetischer Seltenerdionen fuer alle magnetischen Seltenerdionen qualitativ erfasst. In antiferromagnetisch geordneten Seltenerd-Seltenerd-Nickel-Borkabid-Verbindungen wirken unmagnetische Verunreinigungen stark paarbrechend, wobei dieser Mechanismus durch die Unordnung verstaerkt wird. Durch Neutronenbeugungsexperimente wurden die magnetischen Strukturen der Seltenerd-Seltenerd-Nickel-Borkabid-Verbindungen nachgewiesen. Bei vollstaendig ausgebildeter antiferromagnetischer Ordnung verzerrt das Kristallgitter aufgrund magnetoelastischer Effekte. In Holmium-Seltenerd-Nickel-Borkabid-Verbindungen konnten die drei bekannten magnetischen Strukturen gefunden werden, wodurch die Korrelation zwischen dem reentrant-Verhalten und magnetischer Fernordnung nachgewiesen wurde. Im aeusseren Magnetfeld werden die Strukturen anhand eines bekannten Feld-Temperatur-Phasendiagramms erklaert, in dem die magnetischen Ueberstrukturen als Funktion von Feld und Temperatur aufgestellt wurden.