UC San Diego

Magnetic materials have played an enormous role in the advancement of technology over the past two centuries. Much of the current understanding of magnetism, however, is limited by the complexity of the interactions between particles from which collective behavior and interesting phenomena emerge. In the regime where the miniaturization of technologies combined with ultrafast responses are desired, these phenomena can become even more obscure. Probing these systems and interactions to gain insight into the origin of these phenomena is required for both fundamental understanding and for the engineering of novel materials and devices for application. In this dissertation, we study three magnetic systems exhibiting complex phenomena in confined geometries, mostly confining in one dimension as thin films. Elemental chromium is an antiferromagnetic metal with coupled spin density wave, charge density wave, and periodic lattice distortion. We track this ordering primarily through x-ray diffraction measurements statically, on ultrafast time scales following photoexcitation, and over longer time scales to follow the evolution and recovery to the static state. Certain antiperovskite manganese nitride compounds exhibit magnetovolume effects including negative thermal expansion tied to the transition in magnetic ordering between ferrimagnetic, antiferromagnetic, and paramagnetic phases. These magnetovolume effects are demonstrated for thin films through static x-ray diffraction measurements as well as dynamically following photoexcitation. And iron rhodium is an alloy with a room temperature transition between an antiferromagnetic phase and a ferromagnetic phase that also exhibits large magnetoresistance and magnetostructural effects. We conduct a study of its magnetic phase diagram through resistivity measurements and x-ray magnetic circular dichroism measurements at high field and with theoretical calculations. We also examine the effects of confinement in a film and in wires through polarized neutron reflectometry measurements. These experiments provide insight into the collective interactions in these materials and into the strong coupling of the charge, spin, and lattice degrees of freedom seen in each of them.