Exploring Magnetism in van der Waals 2D Materials and Heterostructures
Open Access
- Author:
- Chuang, Ya Wen
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 09, 2020
- Committee Members:
- Jun Zhu, Dissertation Advisor/Co-Advisor
Jun Zhu, Committee Chair/Co-Chair
Vincent Henry Crespi, Committee Member
Mauricio Terrones, Committee Member
Roman Engel-Herbert, Outside Member
Nitin Samarth, Program Head/Chair - Keywords:
- magnetism
2D Material
Van Der Waals Heterostructure
bilayer graphene
Landau level
WSe2
GaSe
MnSb1.8Bi0.2Te4
PZT
piezoelectric
strain - Abstract:
- Since the successful isolation of graphene in 2004, 2D van der Waals materials have drawn a lot of interests of the community because of their potential in advancing fundamental physics understandings and next-generation device applications. Van der Waals materials with diverse properties have been synthesized and studied and many fundamental phenomena of condensed matter physics including the quantum Hall effect, superconductivity, and magnetism have been observed. Robust room-temperature 2D magnetism remains a goal of the field. We have explored three van der Waals 2D materials to realize magnetism. This includes bilayer graphene, gallium selenide (GaSe), and manganese bismuth telluride (MnSb1.8Bi0.2Te4). A strain-tuning technique based on a piezoelectric substrate is developed and will offer convenient tuning of magnetic properties. This dissertation begins with an introduction, which is followed by four chapters, each of which discusses a material platform or technique. Chapter 1 gives an overview of the properties of 2D materials, which forms the basis of our investigations in the following chapters. We discuss different ways to manipulate the properties of 2D materials. Chapter 1 also gives a brief introduction on the current state of magnetism in 2D. Chapter 2 presents findings in WSe2/bilayer graphene heterostructures. Our original goal was to introduce spin-orbit coupling into bilayer graphene through proximity coupling. The effect was found to be very small. Instead we observed large modifications to the Landau level energies of bilayer graphene, which suggests a possible way to engineer the quantum Hall effect. A systematic study of Landau level gaps at filling factors ν = 1, 2, 3 is presented and offers insight to the rich competing many-body interactions in bilayer graphene. Chapter 3 describes our effort in making electrical contacts to GaSe using various materials and procedures. We also discuss different fabrication ideas to enhance the quality of GaSe devices. Our original goal was to study the predicted gate-tunable ferromagnetism in few-layer GaSe. We were not successful in the end however this work provides valuable information to the challenging problem of contacting thin GaSe flakes. Chapter 4 reports our observations of a ferromagnetic ground state in MnSb1.8Bi0.2Te4, a van der Waals magnetic compound. This work was done with collaborators in synthesis and structural characterizations. The development of ferromagnetism in MnSb1.8Bi0.2Te4, in contrast to commonly observed anti-ferromagnetism in this family of materials, is supported by magneto-transport measurements, bulk magnetometry and neutron scattering. Our finding contributes to the understanding of a newly discovered family of ferromagnetic topological insulators. Chapter 5 describes our design and fabrication of a strain-tuning substrate that is based on piezoelectric lead zirconate titanate (PZT) and suitable for devices made of van der Waals materials. Strain is a powerful tool for manipulating 2D material properties, including magnetism. Our substrate integrates strain tuning, Al2O3 gating, ionic liquid gating, and electrical measurement capabilities. We aim for a platform compatible with cryogenic measurements and available for convenient and reliable strain tuning.