Quasi-One-Dimensional van der Waals Lattices with Diverse Magnetism: New Platforms Towards Ultrathin Magnetic Nanowires

Abstract

One-dimensional (1D) or quasi-1D van der Waals (vdW) magnets, which feature covalently bonded spin chains or ladders separated by weak vdW interactions, could potentially offer twofold benefits for the current field of 1D magnets. On the one hand, the bulk crystals of these phases are more ideal 1D magnets because large vdW gaps effectively prevent any inter-chain or inter-ladder exchange couplings. This allows for the study of unique 1D magnetic fluctuations coupled with 1D characteristic transport behaviors. On the other hand, the vdW gaps open up possibilities to exfoliate these 1D vdW magnets for magnetic nanowire production. These nanowires would then be used to investigate 1D confinement effects and for densely packed spintronics. In this thesis, efforts to synthesize new quasi-1D vdW magnets, control their bulk magnetism, and efficiently exfoliate them into high-quality nanowires are detailed. Chapter 1 reviews the definitions and fundamental physics of 1D magnets, discusses limitations with the current routes to access 1D magnets, and introduces lessons from the breakthroughs in two-dimensional (2D) magnets, which serve as the starting point of this thesis work. Chapter 2 demonstrates the first exfoliation strategy developed for quasi-1D vdW magnet CrSbSe₃ and presents the properties of the resulting nanowires. The exfoliated CrSbSe₃ nanowires have high aspect ratio, well-defined crystallinity, smooth surfaces, and high stability, and exhibit stronger coercivity compared to bulk CrSbSe₃ due to the stronger shape anisotropy therein. Chapters 3 and 4 are concerned with expanding the library of quasi-1D vdW magnets and efficiently controlling their bulk magnetic properties. Chapter 3 demonstrates the substitution of Se in CrSbSe₃ with S switches the overall magnetic ordering from ferromagnetic (FM) to antiferromagnetic (AFM) and discusses the metamagnetic transition and strong spin-phonon coupling in the resulting AFM spin-ladder phase CrSbS₃. Chapter 4 demonstrates that Bi alloying into the Sb sites in CrSbSe₃ and CrSbS₃ is an efficient strategy to enhance the magnetic anisotropy without altering the original 1D structural features or magnetic ground state. This offers an independent dimension to finely tune the magnetic behaviors of these quasi-1D vdW magnets.