Experimental and Theoretical Studies of Novel Magnetic Behaviors in Amorphous Ferrimagnets

Author: ORCID icon orcid.org/0000-0002-6802-7205
Ma, Chung, Physics - Graduate School of Arts and Sciences, University of Virginia
Poon, Joseph, AS-Physics, University of Virginia

Amorphous rare-earth (RE) transition-metal (TM) alloys are ferrimagnetic (FiM) materials. In RE-TM, magnetic moments in RE and TM couples antiferromagnetically and form two magnetic sublattices. With the two sublattices, the magnetic properties of RE-TM thin films can vary significantly with temperature, composition, and thickness. For example, the magnetization of RE-TM alloys vanishes at the compensation temperature (Tcomp). Such flexibility has both advantages and disadvantages. On the one hand, their properties can be tuned for specific applications, such as magneto-optical recording devices and skyrmion based devices. On the other hand, with so many tunable parameters, it could be very time consuming to optimize their properties experimentally. Guidance from simulations can provide crucial knowledge to reduce the time and the cost of experiments.
This thesis focuses on the tuning of magnetic properties in RE-TM thin films through simulations and experiments. Based on experimental results, computational models are developed. Then, simulations results can be used to guide experiments. In this study, RE- TM thin films were deposited on various substrates by radio frequency (RF) magnetron sputtering at room temperature. In TbFeCo on thermally oxidized Si substrates, the exchange bias effect was revealed near Tcomp. Magnetic modeling was needed to investigate the origin of the exchange bias effect. To study this effect, the micromagnetic model is adopted into the two-sublattice model, where each sublattice evolves under its own Landau-Lifshitz-Gilbert (LLG) equation. Using the two-sublattice model, exchange interactions between two nanoscale amorphous phases were found to be the origin of the exchange bias. Using this model, one can explore different FiM heterostructure to develop desirable exchange bias properties for applications at room temperature.
In addition to exchange bias, the ability to control magnetic anisotropy can provide great flexibility in writing and storing information. Using TbFeCo deposited on Kapton, the magnetic properties of TbFeCo are varied through the bending of Kapton. Furthermore, in VO2/TbFeCo heterostructure, a decrease in magnetic anisotropy in TbFeCo was found to correspond to the tensile strain from the structural transition of the underlying VO2 films. These results provide guidance on tuning magnetic anisotropy through interfacial strain.
Amorphous RE-TM thin films are also promising materials for hosting magnetic skyrmions, which are proposed for magnetic memory devices to improve density and efficiency. In RE-TM thin films, skyrmions are stabilized through the interfacial Dzyaloshinskii-Moriya Interaction (DMI), originates from an adjacent heavy metal layer. In this study, computational models are employed to investigate room-temperature skyrmions in GdCo thin films. Using atomistic simulations, various parameters, including thickness, composition, anisotropy, and interfacial DMI, are varied to explore their effects on skyrmions. Results show that increasing thickness and reducing interfacial DMI in GdCo can further reduce the size of skyrmion at room temperature, which is crucial for applications.

PHD (Doctor of Philosophy)
Ferrimagnetism, Perpendicular magnetic anisotropy, Exchange bias, Magnetic skyrmions, Magnetic thin films
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