Magnetic Phenomena in Interface-Engineered Multilayer Thin films

Abstract

The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and non-magnetic material are of particular interest and importance for current and future spintronic applications. Consequently, the effect of proximity induced magnetization (PIM), the interfacial Dzyaloshinskii-Moriya interaction (DMI) and the spin-orbit-torque (SOT) in Pt are studied in this thesis with a detailed investigation of the mechanism and the implications on engineered interfacial multilayers thin films. In this work, Co2FeAl (CFA) and Co are the ferromagnet material investigated.
PIM is investigated in CFA-based multilayer structures under the influence of annealing for different heavy metals (HMs). In some cases the PIM is presented along with the interfacial DMI. However, interfacial quality plays a crucial role in the strength of PIM and the influence of interfacial quality and thermal processes on DMI and PIM are not understood. Here, it was found that the thermal process increased the PIM in Pt samples. On the other hand, the magnetism was cancelled by the annealing process for the Ir samples.
The DMI determines the type and symmetry of non-uniform magnetic structures and arises at interfaces between a ferromagnet and a nonmagnetic material with strong spin-orbit coupling. As that DMI depends of the symmetry of the system, interfacial roughness further breaks the symmetry at the interface in a heterostructure.
A description of how controlled induced interfacial roughness can be used to tailor the DMI in model Pt/Co/Ta(Pt) thin-film multilayer structures. Magneto-optic Kerr effect microscopy is used to quantify the strength of the DMI indirectly via the asymmetric growth of bubble-like domains. Brillouin Light Scattering (BLS) is also used to directly quantify the DMI from the asymmetry induced in the spin-wave dispersion. The influence of engineered interface roughness on DMI as measured by BLS is presented. Fluctuations were found in the DMI when the roughness increases from 0 to 35 A by both methods of analysis, which suggests that changes in the interfacial characteristics in the multilayer modify the interfacial DMI.
Furthermore, micromagnetic simulations are used to show how a current-driven domain wall motion using SOT is employed to create a bubble-skyrmion on the top of a square pillar using the difference in perpendicular magnetic anisotropy in the different regions on a strip. High bubble-skyrmion velocity was found once the structure was depinned.
Future spintronic applications rely on the understanding of interfacial engineering devices. Towards this goal, this thesis provide contributions to the advantageously use of thermal processes, interfacial quality in terms of roughness and intermixing in multilayer thin-film stacks and the creation of skyrmion-bubbles to be used as potential information carriers.