Doctoral Dissertations
Date of Award
8-2023
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Physics
Major Professor
Dustin A. Gilbert
Committee Members
Christian Batista, Joon Sue Lee, Haidong Zhou
Abstract
Magnetic skyrmions are topologically protected chiral spin textures with great potential for next-generation consumer technologies. These magnetic structures can be described as spins continuously wrapping into a closed coplanar loop, featuring a core and fencing perimeter with opposite out-of-plane orientations. While conventional depictions of magnetic skyrmions use a two-dimensional projection, recent research underscores the importance of their three-dimensional structure in determining their topology and stability. Magnetic skyrmions typically emerge just below the curie temperature of a magnetic material, creating what is known as a skyrmion pocket. In most materials the stability pocket is at low temperatures and finite fields, however recent studies have stabilized skyrmions in Fe/Gd ML systems under ambient conditions. The key to achieving enhanced stability in this system was the unique 3D structure of the skyrmion, consisting of both Néel-like and Bloch-like components along the length of the skyrmion tube. My first work focuses on experimentally determining the structure of these hybrid skyrmions using neutron scattering. To interpret the neutron data, micromagnetic simulations were conducted and the simulated diffraction pattern calculated using a Born approximation. The resulting magnetic profile offers insights into the structure of the skyrmions, including the depth profile, thickness of the Bloch and Néel segments, and the core diameter. Beyond their primary structure as a topologically protected soliton, skyrmions often form ordered hexagonal lattices. While the nucleation and annihilation of single skyrmions is expected to be very fast, on the order of nanoseconds, recent works have suggested that the ordering of the skyrmion lattice is much slower. To investigate these slow dynamics, the formation and destruction time scales of skyrmion lattice were investigated using neutron scattering, performed on three different prototypical skyrmion materials: MnSi, Fe0.85Co0.15Si, and Cu2OSeO3. Using a step-like magnetic field to move the skyrmions into and out-of the stability pocket, the lattice ordering is captured using time resolved small-angle neutron scattering. Fitting the time-dependent diffraction data, the slow timeframes were confirmed, with ordering occurring in 10-100 ms. The formation and destruction times were shown to depend on the magnitude of the field strep, the temperature, the magnetic anisotropy, and the saturation magnetization.
Recommended Citation
Liyanage, W-L-Namila Chandula, "Exploring Skyrmions Dynamics and Structure using neutron scattering. " PhD diss., University of Tennessee, 2023.
https://trace.tennessee.edu/utk_graddiss/8634