Oral: Theory of magnetic interactions and spin textures in amorphous FeGe

Magnetic properties and topological spin textures such as skyrmions are both fundamentally intriguing and relevant to novel information storage and advanced logic technologies. Topological spin textures have long been studied and considered mostly to form due to Dzyaloshinskii-Moriya interaction (DMI) caused by global symmetry-breaking or stabilized by spin frustration induced by alternating higher-order Heisenberg exchange interactions. Recently, experimental observation of chiral spin textures, such as helical spins and skyrmions, was reported for amorphous FeGe thin films [1]. However, there is yet to be a theoretical explanation or model to explain the magnetic properties and topological spin textures found in amorphous systems. How are topological spin textures formed in an amorphous system? Is DMI or spin frustration important? How does magnetic property differ in amorphous systems with respect to their crystalline counterparts? Here, we provide answers to these questions by performing first-principles calculations, ab-initio-based molecular dynamics, and Monte Carlo simulations on amorphous FeGe. Our results show that the magnetic exchange coupling parameters can be much stronger in amorphous FeGe compared to the crystalline value and we find nano-metric-sized topological spin-texture called anti-Skyrmion. Furthermore, anti-Skyrmions were found to form by the magnetic exchange coupling parameters rather than DMI or single-ion-anisotropy. Our results give first hints toward finding topological spin textures in any amorphous magnetic system without a heavy element.

[1] Streubel, et al., Chiral Spin Textures in Amorphous Iron–Germanium Thick Films. Adv. Mater. 2021, 33, 2004830.