Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2024)

This thesis describes the synthesis and fabrication of various two-dimensional (2D) crystals and heterostructures that exhibit long-range charge and spin ordering. By combining intercalation with the precise design of 2D interfaces, we tune and engender targeted magnetic and charge density wave (CDW) phases in 2D crystals. Furthermore, by marrying transmission electron microscopy (TEM) imaging, optical probes, and electron transport measurements, we establish robust structure-property relationships that may guide the development of 2D crystals for miniaturized electronic devices.

Chapter 1 presents a broad overview of the field of 2D magnets and CDWs. We discuss the fundamental hindrances in realizing magnetism in the atomically thin limit and build upon these insights to outline the design principles for 2D magnets. Further, intercalated transition metal dichalcogenides (TMDs) are introduced as promising material platforms for realizing 2D crystals with long-range spin order. This chapter also provides insights into the mechanisms underlying CDW formation through an orbital-based approach. Lastly, we give a brief introduction to the CDW properties of 1T-TaS2, a layered material that is extensively studied in this thesis.

Chapter 2 describes the first synthesis and magnetic characterization of iron-intercalated tantalum disulfide (FexTaS2) in the two-dimensional (2D) limit. The synthesized 2D FexTaS2 crystals exhibit hard ferromagnetic behavior down to the thinnest limit of this intercalation compound (Fe-intercalated bilayer TaS2). Magnetic properties of this system are highly tunable and depend on the interplay between magnetocrystalline anisotropy, intercalation amount, homogeneity, and dimensionality.

Chapter 3 describes a synthetic framework for realizing heterostructures comprising intercalation compounds of TMDs through directed topotactic intercalation of van der Waals (vdW) assembled layers. The developed topochemistry method enables the creation of previously inaccessible magnetic TMD heterostructures that demonstrate strong interfacial magnetic exchange effects only possible with atomically clean heterointerfaces.

Chapter 4 describes the topotactic transformation of nanothick 1T-TaS2 crystals into verti-lateral heterostructures of 1T-TaS2 and H-TaS2. These polytype heterostructures exhibit reliable multi-step resistance and chirality switching behavior encoded by the number of H-TaS2 layers. The potential of strain engineering for realizing multifunctional phase change materials is also highlighted.

Chapter 5 describes the synthesis of Fe-intercalated polytype heterostructures comprising 1T-TaS2 and H-TaS2. In these materials, modulating the intercalant amounts leads to simultaneous changes in long-range spin and charge ordering, which coexist in these materials. This chapter explores the competition and coexistence of structural and electronic phases, offering insights into designing multifunctional materials.

Chapter 6 describes the electronic and optical properties of moiré heterostructures comprising 2D 1T-TaS2 and monolayer 1H-WSe2. Interfacial charge transfer and moiré strain encode the global CDW ordering in 1T-TaS2 while simultaneously modulating the optoelectronic behavior of 1H-WSe2. This study outlines how local perturbations can be engineered to design ensemble electronic behavior.

Chapter 7 provides a brief summary of the tuning knobs used in this thesis and outlines possible avenues for future research.

    • Article
    • Peer Reviewed
    LBL Publications (2023)
    Transition metal intercalated transition metal dichalcogenides (TMDs) are promising platforms for next-generation spintronic devices based on their wide range of electronic and magnetic phases, which can be tuned by varying the host lattice or intercalants identity, stoichiometry, or spatial order. Some of these compounds host a chiral magnetic phase in which the helical winding of magnetic moments propagates along a high-symmetry crystalline axis. Previous studies have demonstrated that variation in intercalant concentrations can have a dramatic effect on the formation of chiral domains and ensemble magnetic properties. However, a systematic and comprehensive study of how atomic-scale order and disorder impact these chiral magnetic textures is so far lacking. Here, we leverage a combination of imaging modes in the (scanning) transmission electron microscope (S/TEM) to directly probe (dis)order across multiple length scales and show how subtle changes in the atomic lattice can tune the mesoscale spin textures and bulk magnetic response in Cr1/3NbS2, with direct implications for the fundamental understanding and technological implementation of such compounds.
      Cover page: Consequences and Control of Multiscale Order/Disorder in Chiral Magnetic Textures.
      • Article
      • Peer Reviewed
      LBL Publications (2023)
      Some magnetic systems display a shift in the center of their magnetic hysteresis loop away from zero field, a phenomenon termed exchange bias. Despite the extensive use of the exchange bias effect, particularly in magnetic multilayers, for the design of spin-based memory/electronics devices, a comprehensive mechanistic understanding of this effect remains a longstanding problem. Recent work has shown that disorder-induced spin frustration might play a key role in exchange bias, suggesting new materials design approaches for spin-based electronic devices that harness this effect. Here, we design a spin glass with strong spin frustration induced by magnetic disorder by exploiting the distinctive structure of Fe intercalated ZrSe2, where Fe(II) centers are shown to occupy both octahedral and tetrahedral interstitial sites and to distribute between ZrSe2 layers without long-range structural order. Notably, we observe behavior consistent with a magnetically frustrated and multidegenerate ground state in these Fe0.17ZrSe2 single crystals, which persists above room temperature. Moreover, this magnetic frustration leads to a robust and tunable exchange bias up to 250 K. These results not only offer important insights into the effects of magnetic disorder and frustration in magnetic materials generally, but also highlight as design strategy the idea that a large exchange bias can arise from an inhomogeneous microscopic environment without discernible long-range magnetic order. In addition, these results show that intercalated TMDs like Fe0.17ZrSe2 hold potential for spintronic technologies that can achieve room temperature applications.
        Cover page: Near Room-Temperature Intrinsic Exchange Bias in an Fe Intercalated ZrSe2 Spin Glass.
        Top