Real space and reciprocal space investigations of the spin dynamics in skyrmion-hosting materials

Magnetic skyrmions are whirl-like spin configurations with particle-like properties protected by non-trivial topology. Due to their unique spin structures and dynamical properties, they have attracted tremendous interests over the past decade, from fundamental science to technical applications. However, experimental methods to study skyrmion dynamics in real space are limited until present. The goal of this thesis is to both apply and develop novel methodologies to firstly discover new skyrmion lattice hosting systems through neutron scattering experiments, and then directly observe the dynamics in these materials in real space by time-resolved imaging. The main content of this thesis is divided into two parts: firstly, I will demonstrate by means of inelastic neutron scattering how the magnon spectrum was measured in VOSe2O5, one of the few known systems hosting a Néel-type skyrmion lattice. By comparing the measured spectra with SpinW simulations, the exchange interactions and magnetic anisotropy were extracted so that the spin Hamiltonian in this system could be constructed. The results provide positive feedback to the recently developed quantum chemistry calculations. Further, the reasonable consistency among experiments, simulations and calculations may enable the community to use quantum chemistry as a novel tool to predict skyrmion hosting materials and understand their formation. The second part of this thesis will introduce my work in integrating a microwave system into a time-resolved transmission electron microscope (TEM). In this project, a pump-probe technique using microwave to coherently excite spin wave dynamics in skyrmion lattice hosting materials in the GHz frequency range was developed and implemented. The time-resolved TEM will enable the direct observation of excited spin waves and skyrmion dynamics in real space. This instrumental development will also allow the study of the spatial homogeneity of resonant modes, and their behavior around individual impurities or grain boundaries. The insights gained from such experiments would help the community to understand microscopic aspects of skyrmion dynamics and tailor spintronic devices using skyrmions in the future. The chapter layout is presented as follows: in Chapter One, I introduce fundamental concepts about skyrmions, and then discuss their general magnon spectrum and how they can be measured. Chapter Two describes methodologies adopted during this thesis work, including experimental techniques and modelling methods. The scattering experiment details and results obtained at various beamlines are presented in Chapter Three. The progress concerning the development of the new technique in time-resolved real space imaging starts from Chapter Four. Here, the implementation of a microwave system in ultrafast TEM is demonstrated systematically. The last chapter contains the conclusion and outlook.