Over the past decade, the field of terahertz driven magnetic phenomena has witnessed a remarkable surge of interest, particularly in the study of antiferromagnetic materials, which has emerged as a captivating sub-field within the area of ultrafast pump-probe spectroscopy studies.
This thesis is dedicated to theoretical and experimental studies of a specific class of magnetic oxides, namely rare-earth orthoferrites (REOs) and the magnetic crystal known as Terbium Gallium Garnet (TGG). These materials are characterised by a plethora of fascinating physical phenomena, including the occurrence of spin reorientation phase transitions (SRTP), which can be effectively manipulated and controlled through the use of THz driving/excitation.
The first two chapters of this thesis explain the motivations behind this research and provide an overview of the theoretical and experimental methods necessary for understanding the later chapters.
Chapter 3 develops the theoretical formalism used to describe THz-driven magnetic switching phenomena in rare-earth orthoferrites with non-Kramers ions. It provides insights into the dynamics induced by THz radiation on iron spins and analyses the mechanisms that facilitate the iron spin-switching process during the spin-reorientation phase transition in Thulium orthoferrite (TmFeO3). Based on the available experimental data this chapter analyses static and dynamic properties of TmFeO3 in the course of SRPT, explains the effects responsible for the spin switching behaviour and presents theoretical results with the realistic values of threshold fields necessary for achieving effective and minimally dissipative iron spin switching showing a good match with experimental findings.
Chapter 4 delves into our experimental results on the signatures of the magnetic analogue of the Jahn-Teller effect during the spin-reorientation phase transition in Terbium orthoferrite, (TbFeO3), supported by a developed theoretical analysis. It also explores the features of the strong coupling regime between Fe and Tb ions, comparing TbFeO3 with other strongly coupled systems as reported in previous research.
Chapter 5 describes the experimental comparison of THz- and optically-induced spin dynamics of Tb ions in Terbium Gallium Garnet (Tb3Ga5O12). This crystal is well-known for its magneto-optical properties but lacks the specific magnetic order found in orthoferrites, making it an ideal candidate for investigating the features of low-temperature magnetism of pure Tb3+ ions unaffected by interactions with other magnetic ions.
The final chapter summarises our theoretical and experimental results obtained throughout this research. It provides an outlook of how these results correlate with each other, and discusses future experimental and theoretical steps that emerged in the course of this work.