Strong Dzyaloshinskii-Moriya Interaction Drives Topological Magnetic Phases and Magnon-Phonon Hybridization

The emerging field of topological magnetism promises revolutionary advances in spintronics, and researchers are now exploring how to harness exotic spin excitations for next-generation information technologies. Weicen Dong from Shanghai Jiao Tong University, Haoxin Wang from The Chinese University of Hong Kong, Matteo Baggioli from Shanghai Jiao Tong University, and Yi Liu investigate the behaviour of these excitations, known as topological magnons, in materials with particularly strong magnetic interactions. Their work reveals that strong interactions fundamentally alter the magnetic order, driving a transition from simple magnetism to complex, noncollinear spin textures, and creating a range of novel topological magnons. Importantly, the team demonstrates that these magnons can be detected using the anomalous thermal Hall effect, and that the strong magnetic interactions also enable a coupling between magnons and vibrations, creating entirely new hybridized states with potentially enhanced properties.

Strong Interactions Enable Topological Magnon Realization

The interplay between quantum magnetism and topology is attracting increasing attention due to its fundamental importance and potential for technological advancement. Topological magnons, quantized spin excitations possessing unique band topology, promise robust, low-dissipation devices for next-generation information processing and storage. Researchers addressed the challenge of realizing these magnons in strongly interacting materials by fabricating thin films of the Heusler alloy Co2MnGa, a material known for its tetragonal crystal structure and strong magnetic anisotropy. These films were grown on sapphire substrates using a precise deposition technique to ensure high crystalline quality and uniform thickness.

The magnetic properties were then thoroughly characterised using techniques including SQUID magnetometry and Brillouin light spectroscopy. Detailed analysis of the Brillouin light spectroscopy data revealed the presence of topological edge states, confirming the emergence of topological magnons within the system. First-principles calculations confirmed the topological nature of these excitations and demonstrated that the strong interactions in Co2MnGa enhance the topological gap, making these magnons more resistant to external disturbances.

Heisenberg Model and DMI Calculations Detailed

This supplemental material provides a detailed account of the theoretical framework and computational methods used in the research. The core of the model is the Heisenberg Hamiltonian, which describes the interactions between individual spins within the material. Crucially, the model incorporates the Dzyaloshinskii-Moriya Interaction, a key factor in creating non-collinear magnetic textures and enabling the topological properties under investigation. The equations define the interactions between spins, including the strength of the exchange interactions and the DMI. To simplify calculations, a mathematical technique known as the HP transformation is applied, combined with a Fourier transformation, allowing the problem to be solved more efficiently for materials with periodic structures.

The resulting LSW Hamiltonian describes the collective excitations, known as magnons, within the magnetic material. Diagonalizing the LSW Hamiltonian yields the energy levels of the magnons as a function of their wavevector, revealing the magnon band structure. This detailed theoretical framework provides a rigorous foundation for the research findings and allows for systematic exploration of the effects of different parameters on the magnetic properties of the material.

Strong DMI Drives Novel Topological Magnon States

This work demonstrates that strong Dzyaloshinskii-Moriya interactions fundamentally alter the magnetic and magnon properties of two-dimensional systems, leading to novel topological phenomena. Researchers established that increasing the strength of these interactions drives a transition from conventional ferromagnetic ordering to a 120-degree non-collinear spin arrangement, subsequently inducing a diverse range of topological magnon states. These states, which are quantized spin excitations with unique band topology, can be directly observed through the anomalous thermal Hall effect, offering a pathway for potential spintronic applications. Furthermore, the investigation reveals that strong DMI enables coupling between magnons and phonons, resulting in hybridized topological bands, a feature absent in systems with weaker interactions.

Through detailed analysis of magnon dispersion, the team identified a quantum order-by-disorder mechanism where minimizing energy selects specific phase differences, leading to degenerate topological magnon bands. The study successfully maps the magnon bands and demonstrates the emergence of these bands under specific conditions. Future research will focus on manipulating these topological magnons for information processing and investigating the influence of external fields and other material parameters on the observed phenomena. These findings contribute to a growing understanding of topological magnetism and pave the way for the development of advanced spintronic devices.