The magnetic phase diagrams of (calcium,strontium) (ruthenium,titanium) oxide and iron pnictide systems revealed by muon spin relaxation

This thesis comprises two projects exploring the relationships between superconductivity (SC) and magnetism: (1) a comprehensive muon spin relaxation and susceptibility study of the ruthenate systems (Ca,Sr)2RuO 4 and Sr2(Ru,Ti)O4, and (2) a muon study of the iron pnictide systems R(O,F)FePn, (R = La or a lanthanide, Pn = P or As).;Interest in Sr2RuO4 was spurred by the discovery of its SC in 1994, soon revealed to be unconventional, highly sensitive to both magnetic and non-magnetic impurities and exhibiting superfluid 3He-like spin-triplet pairing. Additionally, being isostructural to La2CuO4 evoked comparison with the cuprates. Sr 2RuO4 and Ca2RuO4 are connected via a Mott transition system (Ca,Sr)2RuO4, and SC is suppressed and incommensurate magnetism induced by doping non-magnetic Ti in Sr2(Ru,Ti)O4. A comprehensive and synoptic study of both systems has been undertaken, revealing static magnetism across nearly the entire doping range, indicating proximity of the SC state to magnetic order, contrasting earlier pictures in which only the Ca-rich range exhibited magnetic order.;Much research in the iron pnictides was stimulated by the discoveries of SC in LaOFeP in 2006 and La(O,F)FeAs in 2008; SC arises in the FePn layers, analogous to the cuprates and Sr2RuO4. We studied both magnetic parent compounds and doped SC compounds, probing both magnetism and the SC vortex state in the latter. While our results were limited by the available samples, comparison to our subsequent studies on higher-quality pnictide samples reveals that the linear relationship between Tc and superfluid density observed in the cuprates holds in the pnictides, though considerable questions remain.;While a comprehensive theory of unconventional SC remains elusive, enumerating the similarities and differences between unconventional SC families leads us toward that goal.;While "conventional" SCs are well understood via the BCS theory of electron pairing via phonon interactions, in "unconventional" SCs this model is inadequate and a comprehensive theory remains elusive. It is widely believed, however, that magnetism, particularly conduction electron interactions with magnetic fluctuations and proximity to a static magnetic state, is crucial to unconventional superconductivity.