Kagome Magnets Achieve Multiple Topological Phases Via Dzyaloshinskii-Moriya and Pseudo-Dipolar Interactions

Kagome magnets, materials possessing a unique geometric structure, present exciting opportunities for exploring novel magnetic phenomena, and a team led by Jin-Yu Ni and Xia-Ming Zheng, from their respective institutions, alongside Peng-Tao Wei and colleagues, now reveals a wealth of previously unseen magnon phases within these materials. Their research investigates how competing magnetic interactions, specifically the Dzyaloshinskii-Moriya interaction and a pseudo-dipolar interaction, influence the behaviour of magnons, quantum excitations carrying spin, in two-dimensional Kagome ferromagnets. The team demonstrates that these interactions generate distinct magnetic phases with high Chern numbers, offering unprecedented control over magnon behaviour and potentially paving the way for advanced magnonic devices. Significantly, they predict a reversal in thermal Hall and Nernst conductivities at certain temperatures, offering a possible explanation for puzzling experimental observations and highlighting the potential for manipulating heat flow with these materials.

Topological Spin Waves and Magnonic Materials

Scientists are actively investigating magnonics, the study of spin waves, and topological magnonics, a field focused on manipulating these waves using materials with unique topological properties. Research encompasses fundamental magnonic concepts, such as the behavior of magnons, quantized spin waves, and the influence of interactions like the Holstein-Primakoff transformation and dipolar interactions. A key area of focus is understanding how topology, described by concepts like Chern numbers and topological invariants, affects the propagation of magnons, particularly in materials designed to host edge states protected from backscattering. Investigations extend to the thermal Hall effect, where magnons mediate heat transport, and the relationship between this effect and the Berry curvature, a measure of quantum state changes.

Materials like Kagome lattice compounds, possessing a unique geometric arrangement of atoms, and Kitaev materials, predicted to exhibit exotic magnetic states, are central to these studies. Theoretical frameworks, including the Heisenberg, Kitaev, and Dzyaloshinskii-Moriya models, provide the foundation for understanding these complex magnetic interactions. A major theme is how different magnetic interactions give rise to topological magnonic states, requiring careful materials selection to achieve desired properties. Understanding quantum spin liquids and their fractionalized excitations is a key goal, alongside exploring how magnons interact with polarons and exhibit nonlinear behavior. Combining spin and thermal transport opens possibilities for energy-efficient spintronic devices, while understanding the role of disorder and imperfections is crucial for realizing robust topological properties. Engineering magnonic edge states and translating these discoveries into practical devices, such as thermal diodes and spin transistors, represent the ultimate aims of this research.

Kagome Magnonics and Competing Magnetic Interactions

Scientists are investigating the magnetic properties of Kagome lattices, two-dimensional structures with unique geometric characteristics, to explore novel magnonic phenomena. They developed a comprehensive mathematical model, a Hamiltonian, to describe the complex interplay of magnetic forces within this lattice, incorporating interactions like the Heisenberg exchange, Dzyaloshinskii-Moriya interaction, pseudo-dipolar interaction, and single-ion anisotropy. This model accounts for the influence of external magnetic fields on the system. To analyze the system, researchers employed the Holstein-Primakoff transformation, a standard technique to map spin operators onto bosonic creation and annihilation operators.

This conversion simplifies the complex many-body problem, allowing for analysis in momentum space. By diagonalizing the resulting Hamiltonian, scientists determined the energy spectrum of magnons, revealing how energy levels change with momentum and predicting their behavior within the Kagome lattice under various conditions. Scientists further characterized the topological properties of magnons by calculating the Chern number, a topological invariant describing the non-trivial characteristics of magnon bands. This calculation involved determining the Berry curvature, a measure of how the quantum state of a magnon changes as it moves through momentum space. An efficient numerical method was implemented to ensure the accuracy of these calculations, allowing the team to identify and characterize topological magnon insulators, materials exhibiting protected surface states due to their topological properties, and paving the way for designing materials with tailored magnonic properties.

Topological Magnons Controlled by Magnetic Interactions

Recent work details a breakthrough in understanding topological magnons, quantum excitations of spin waves, within Kagome lattice magnets. Scientists investigated how multiple magnetic interactions, specifically the Dzyaloshinskii-Moriya interaction and pseudo-dipolar interaction, influence the behavior of these magnons and create novel topological phases. The research demonstrates that distinct combinations of these interactions lead to completely different phase diagrams and magnon states, revealing a high degree of control over magnon behavior. Experiments reveal the emergence of multiple magnon phases characterized by high Chern numbers, a measure of their topological properties.

The interplay between the Dzyaloshinskii-Moriya interaction, pseudo-dipolar interaction, and the inherent Dirac and flat bands within the Kagome lattice controls a variety of phase transitions, suggesting significant potential for manipulating magnons. Notably, the team observed a sign reversal in thermal Hall and Nernst conductivities, properties related to heat transport, induced by temperature changes in specific topological regions. This reversal originates from the energy gap and Berry curvature near magnetic phase transitions, offering a possible explanation for previously puzzling experimental observations. The study confirms that the combination of anisotropic interactions, the Dzyaloshinskii-Moriya interaction and pseudo-dipolar interaction, creates a platform for realizing novel magnonic devices and exploring possibilities in quantum computing. Researchers found that the pseudo-dipolar interaction, arising from the combined effects of spin-orbit coupling and orbital quenching, is more ubiquitous than the Kitaev interaction, and its interplay with the Dzyaloshinskii-Moriya interaction is crucial for controlling magnon behavior.

Tunable Magnon Phases and Topological States

This research demonstrates how multiple magnetic interactions within a two-dimensional Kagome lattice give rise to a variety of novel magnon phases and tunable topological states. By investigating the interplay between Heisenberg exchange interactions, the Dzyaloshinskii-Moriya interaction, and pseudo-dipolar interaction, scientists have revealed that distinct combinations of these interactions produce dramatically different magnetic phase diagrams. The emergence of magnon phases with high Chern numbers, linked to these interactions and the unique Dirac and flat bands present in the Kagome lattice, highlights the potential for controlling magnetic excitations. Importantly, the team observed a sign reversal in thermal Hall and Nernst conductivities induced by temperature changes, which they attribute to energy gaps and Berry curvature near magnetic phase transitions.

This finding offers a possible explanation for previously puzzling experimental observations and underscores the complex interplay of factors governing thermal transport in these materials. The research confirms that these materials can host multiple tunable topological states, achievable in real transition-metal materials, suggesting a pathway towards advanced magnonic devices. The authors acknowledge that their model simplifies certain aspects of real materials, such as the assumption of uniform interactions and the neglect of long-range effects. Future work could explore the impact of these factors and investigate the potential for realizing these exotic magnonic properties in specific material systems, potentially leading to the development of novel information storage and processing technologies.