Graduation Year

2024

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Hariharan Srikanth, Ph.D.

Committee Member

Manh-Huong Phan, Ph.D.

Committee Member

Dario Arena, Ph.D.

Committee Member

Manfred Albrecht, Ph.D.

Keywords

Anisotropy, Dirac, Magnetism, Semimetal, Weyl

Abstract

Semimetals have gained intense interest recently due to their exotic magnetic and electronic properties. One of the most widely studied semimetals is graphene, a Dirac semimetal. The utilization of graphene in devices and sensors requires interfacing it with other materials, which may induce potentially strong interfacial effects. Furthermore, graphene alone does not possess magnetic order. Studying the interfacial effects between graphene and magnetic materials is therefore of great importance in the application of graphene to meet modern technological needs. Furthermore, by understanding the fundamental interfacial physics between graphene and magnetic materials, new properties can be unlocked, broadening the possible applications of graphene. In this work we study the interfacial magnetism and anisotropy in Dirac (graphene) and Weyl (Co2MnGa) systems. It has been found that not only can the type of magnetic substrate affect the properties of graphene, but also that graphene can impact the properties of the magnetic material, and therefore it is important that the bulk magnetic behavior of the magnetic substrate is well understood. This is manifested in the changes in the bulk magnetic properties in the ferrimagnetic insulator nickel ferrite due to the inclusion of graphene. Furthermore, it is shown that there exists surface-termination dependent magnetism at the interface, applicable to a broader class of magnetic substrates/graphene. The changes in the surface properties due to the inclusion of graphene in variable-phase iron oxide heterostructures was also studied. It was found that graphene can induce an inversion of the sign of the magnetoresistance at finite temperature as compared to heterostructures without graphene. As potential magnetic substrates that can be interfaced with graphene to form novel heterostructures, the properties of emerging garnet Tm3Fe5O¬12 and Weyl semimetal Co2MnGa were studied. The temperature-dependent magnetic and spin transport properties of Tm3Fe5O¬12 films grown on different substrates were characterized. The magnon propagation length, one of the key factors in determining a system’s viability for spintronic applications, was quantified through a thickness dependent study. Finally, experiments and first principles calculations demonstrate the impact of modulating the crystal structure on the Weyl points in Co2MnGa. The changes in the microscopic properties of the Weyl points are shown to give rise to the nonlinear changes in the magnetic properties as a function of temperature. This opens a new degree of freedom for the control of the magnetic and electronic properties in Weyl semimetal systems through structural changes and paves the way for future work studying the interfaces between Dirac and Weyl semimetals.

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