Local Moments and Itinerant Electrons: Gaining New Insights through Investigating Electronic and Dynamical Properties

Magnetic materials are often categorized in terms of either a purely local or a purely itinerant picture despite the fact that the vast majority actually fall within a spectrum that ranges between these two extremes. It is from such a starting point that this thesis aims at developing an understanding of how the complex interplay between local moments and itinerant electrons ultimately affects the electronic and dynamical properties. Such ideas are explored in greater detail using two materials as case studies: the chiral helimagnet Cr_1/3NbS₂ [Cr intercalated Niobium Disulfide] and YFe₂Ge₂ [Yttrium Iron Germanide] a Pauli paramagnet believed to be near a magnetic quantum critical point. From this investigation, new microscopic insights into macroscopic phenomena such as the intriguing magneto-transport properties of Cr_1/3NbS₂ and the presence large longitudinal fluctuations in the moment of YFe₂Ge₂ were gained. Focusing on Cr_1/3NbS₂ it was found that both Cr- and Nb-derived states participate in the formation of the Fermi surface. Based on this finding, it is evident that the same states responsible for magnetism are likewise those responsible for transport. Consequently, a clear separation between the magnetic and itinerant degrees of freedom is not evident; a finding that is most clearly illustrated by the increase in spectral weight observed at EF as the temperature is lowered below the magnetic transition temperature. Extending this work to include real-time measurements of the electron, lattice and spin dynamics, an anonymously long demagnetization dynamic was observed in the magnetic phase of Cr_1/3NbS₂. However, rather implying the spins to be thermally insulated from the electronic degrees of freedom, such a finding may simply imply that spin flips occurring via Elliot-Yafet mechanism are inefficient due to an imbalance in the spin polarized density-of-state in the ferromagnetic state. Finally, after expressing the limitations of single color optical pump-optical probe experiments, the design, construction and commissioning of a time-resolved ARPES system is presented with particular interest in laying out future plans enabling this system to resonantly excite collective modes in strongly correlated systems.