On the Engineering of Interfaces in Chiral Ferromagnetic Heterostructures

Abstract

Chiral magnetic thin film heterostructures have been of great interest in recent years for their potential for fast domain wall motion stabilized by an antisymmetric exchange called the Dzyaloshinskii-Moriya interaction (DMI). Additionally, these materials can stabilize a topological spin texture called the magnetic skyrmion, which is a bubble-like magnetic domain that requires low current densities for motion and shows promise for use in racetrack magnetic memory. At the fundamental level, these unique chiral textures arise from an interplay between interaction energies including DMI, magnetic anisotropy, and magnetostatic energy. In this thesis, we seek to answer questions about the relation between DMI and these tuning parameters in Pt-Co based magnetic heterostructure systems. We find that the inclusion of a Cu spacer layer in a Pt-Co system induces DMI via symmetry breaking. In contrast, Cu decreases anisotropy in Pt-CoFeB-MgO films by reducing Co-O bonding at the upper interface. Both of these material changes result in decreased domain widths. Additionally, we find that skyrmion lattice textures can be stabilized by decreasing the nonmagnetic spacer layer thickness due to changes in dipolar coupling between the magnetic layers. Further, we examine the temperature dependence of DMI in a Pt-Co-Cu heterostructure, finding that it decreases more rapidly with temperature than the magnetic anisotropy or the saturation magnetization. This finding indicates that the two-dimensional nature of DMI makes its temperature dependence unique compared to bulk magnetic properties, perhaps due to interfacial strains or the temperature dependence of the locally induced Pt magnetic moments. And finally, we examine how nitrogen-vacancy (NV) center magnetometry can be used to image the spin texture of a Neel skyrmion, wherein we find that multilayer coupled skyrmions exhibit Neel caps rather than a uniform chirality throughout the stack. We also take advantage of the flexible and sensitive platform that NV center magnetometry provides to examine the behavior of single layer magnetic thin films that are engineered to exhibit multidomain zero field states. We find that they indeed show stripe domains which can be transformed into skyrmion lattices via applied magnetic fields. These results imply the potential for usage of single layer chiral magnetic films in practical data storage devices.