UC Santa Barbara

Intermetallic materials, which are compounds of two or more metals and metalloids, host a diversity of magnetic phenomena that promise to enable next-generation technologies. For example, several families of intermetallics can undergo large changes in temperature when placed into a magnetic field. This phenomenon, known as the magnetocaloric effect, can be used to build heat pumps that can replace traditional vapor compression refrigerators and air conditioners. Such “magnetic refrigeration” can operate with high efficiencies and without the harmful chlorofluorocarbon or hydrofluorocarbon refrigerants that traditional vapor compression devices rely on. As another example, magnetic intermetallics with non-centrosymmetric crystal structures can host skyrmions: nanoscale vortices of the magnetic moments that behave like stable particles. Skyrmions can be manipulated using small electrical currents or magnetic fields, potentially allowing for ultra-high-density magnetic memory that is more energy-efficient than present alternatives.

The interesting properties of intermetallics can be attributed to their ability to simultaneously host several different types of structural and magnetic interactions, including a mix of covalent, metallic and ionic bonding and magnetic interactions that are a combination of itinerant and local-moment-like. Furthermore, these structural and magnetic interactions often couple. This flexibility presents tremendous opportunities for the realization of desirable functionality in intermetallics; however, it also means that the behavior of these systems is often difficult to understand. In order to harness the potential of magnetic intermetallics, an improved understanding of the interplay between crystal structure and magnetism is required. In this dissertation, I investigate several key magnetocalorics and skyrmion hosts in order to uncover the mechanisms by which magnetism and crystal structure couple in highly functional intermetallics.

The first portion of the dissertation aims to establish a general understanding of the physical origins of large magnetocaloric effects. Chapters 2 and 3 present strategies for rapid computational screening of potential magnetocalorics using high-throughput density functional theory calculations. These studies provide evidence that magnetostructural coupling, generically, is the primary driver of strong magnetocaloric effects across a very broad range of ferromagnetic materials. This connection is made concrete in Chapters 4 and 5, which provide detailed computational and experimental studies of two specific magnetocalorics: MnAs and MnB. In each case, it is found that competition between magnetism and chemical bonding leads to coupling between magnetic and structural degrees of freedom. This competition-driven coupling results in systems that are delicately balanced between magnetic and structural stability and can easily be “tipped’ in one direction by the application of a small field, resulting in a large magnetic entropy changes. The results and ideas included in this section of the dissertation are pulled together in a perspective review on magnetostructural coupling in magnetocalorics, which is included in the introductory chapter of this dissertation (Chapter 1).

The latter portion of the dissertation focuses on skyrmionic magnetic phases in non-centrosymmetric intermetallics. Chapter 6 presents an experimental method to study subtle magnetic phase diagrams using magnetic entropy measurements and applies this method to understand the magnetic behavior of the near-room-temperature skyrmion host FeGe. Chapter 7 uses this technique, along with synchrotron and neutron scattering and electronic structure calculations, in an in-depth study of the CoxZnyMnz family of high-temperature skyrmion hosts. The experimentally observed crystal structures are found to arise from competing magnetic and bonding effects, and these structures are found to host frustrated magnetic interactions. This results in an interesting magnetic state involving the coexistence of ordered and disordered magnetism on the Co and Mn atoms, respectively. This unique phenomenon is proposed to enable the formation of remarkable disorder-driven skyrmionic phases that have been observed in CoxZnyMnz.