An atomic scale study of strain, chirality, topology and domain wall networks in antiferromagnetic multilayers of Mn on Ir(111)

Abstract

Antiferromagnetic spin textures are rapidly emerging as key building blocks for next-generation spintronic devices. Their inherent lack of stray fields, robustness against external perturbations, and ultra-fast spin dynamics in the terahertz regime make them especially attractive for high-density, low-power memory technologies such as racetrack memory. Beyond their potential for device applications, antiferromagnetic systems also host a rich landscape of topological phenomena, including emergent orbital magnetism and exotic Hall effects, offering a fertile ground for fundamental research at the intersection of topology, spintronics, and materials design.

In this thesis, a model type system of a few atomic layers of Manganese (Mn) deposited on a hexagonal lattice of Ir(111) has been investigated with spin-polarized scanning tunneling microscopy (SP-STM) for antiferromagnetic spin textures and domain walls. By introducing localized strain via embedded argon bubbles, magnetic imaging of domain wall networks has been shown. The elements of the domain wall networks, i.e. triple magnetic domain wall junctions and hexa-junctions have been studied down to the atomic scale.

A systematic study of the double, triple, and quadruple Manganese layers on Ir(111), reveals a strong correlation between structural relaxation and magnetic behavior. In particular, stress-relief-induced reconstruction patterns in the triple and quadruple layers significantly influence the orientation of surrounding magnetic domain walls. The double layer has been characterized to have the row-wise antiferromagnetic (1Q) state. Moreover, multi-Q states such as the 2Q and 3Q have also been observed in the different layers, suggesting the presence of higher-order exchange interactions.

Across all the layers, structurally chiral domain wall junctions have been observed, which reflect the broken symmetries inherent to the system. Notably, a lateral shift of the double layer relative to the monolayer has been observed. This has been attributed to an enhanced inter-layer antiferromagnetic coupling and symmetry reduction arising from the onset of the row-wise antiferromagnetic (RW-AFM) order.

Furthermore, the thesis reports the presence of the non-trivial topological 3Q state, appearing as localized spin textures in the double layer and as extended magnetic domains and domain walls in the thicker layers. A collaborative density functional theory (DFT) analysis reveals a topological orbital moment associated with the 3Q state in the double layer of Manganese on Ir(111). These findings open new pathways for incorporating antiferromagnetic textures into future device architectures. The ability to induce domain walls in an antiferromagnet through localized strain highlights a viable route for strain-engineered control of magnetic states, with potential implications for ultra-fast domain wall dynamics. Moreover, the emergence of the topological 3Q state and its associated orbital moment lays the groundwork for transport experiments and computational studies exploring exotic Hall effects—including the recently proposed topological orbital Hall effect.

Together, this work offers a foundation for both experimental and theoretical investigations into topologically driven spin phenomena in antiferromagnetic systems.