Sagkovits, Dimitrios; (2024) Spin Dynamics and Magneto-Transport in Van der Waals Magnetic Systems and Photon-Magnon Hybrids. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The present thesis investigates the static and dynamic properties of van der Waals (vdW) magnetic systems. These systems can be reduced in ultra-thin counterparts termed two-dimensional (2D) materials, with thickness ranging from a few nanometers down to the size of a single atom. 2D materials, which can be thought of as the ultimate extension of the thin film technology underlying much of the modern technological revolution, can exhibit completely different properties compared to those of their bulk counterparts. They have been the subject of extensive research, and the addition of magnetic systems in this material class has led to a surge in the research of the field of 2D spintronics. In this thesis, the properties of bulk vdW systems are characterized and also of exfoliated flakes while developing novel platforms to electrically probe these ultra-thin systems by fabricating electronic devices. In the first chapter, one of the earliest members of the magnetic vdW family, Fe3GeTe2 (F3GT), is thoroughly characterized in their bulk form, probing its static and dynamic responses. Based on the electrical contacting scheme (pre-patterned bottom electrodes) that has been developed as part of this thesis, an electrical device is fabricated made of an exfoliated flake of F3GT, and its magneto-transport properties are investigated as a function of temperature, revealing the existence of a planar Hall effect, with a maximum of 0.45 μΩ cm at 100K, when the external magnetic field is precisely directed along the in-plane direction. The origin of this phenomenon is attributed to an out-of-plane magnetic field component due to the presence of topological spin structures. Another vdW material that was subsequently developed with bulk Curie temperature (TC) exceeding room temperature (RT) is Fe5GeTe2 (F5GT). The spin dynamics of this system is explored from 300 down to 5 K. The gyromagnetic ratio, effective magnetization and Gilbert damping are extracted as a function of temperature for the in- and out-of-plane orientations. A perpendicular anisotropy field up to 0.8 T is measured at 110 K, where a phase transition possible takes place. A deviation of g-factor from the value of 2 suggests that orbital moment is present in this system. The evolution of damping in this system is positively correlated with its resistance for the entire measured temperature range. The nature of this correlation is surprising according to the established phenomenology that considers the electronic scattering mechanisms in magnetic systems as a source of magnetic relaxation. Finally, superconducting resonators are fabricated from high-temperature superconducting (HTS) material YBa2Cu3O7 (YBCO), in a meander shape. This type of resonator can confine the magnetic field to a very small spatial region, proving beneficial in the future study of spin dynamics in micrometer-sized exfoliated magnetic vdW systems. This prototype system is coupled to the high spin density ferrimagnetic system Y3Fe5O12 (Yttrium Iron Garnet - YIG) and an avoided crossing heralds the onset of strong coupling between the photon and magnon systems, with a maximum coupling strength of 410 MHz and a cooperativity exceeding 10.

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