Magnon-skyrmion scattering in chiral magnets (1405.1568v3)

Abstract: Chiral magnets support topological skyrmion textures due to the Dzyaloshinskii-Moriya spin-orbit interaction. In the presence of a sufficiently large applied magnetic field, such skyrmions are large amplitude excitations of the field-polarized magnetic state. We investigate analytically the interaction between such a skyrmion excitation and its small amplitude fluctuations, i.e., the magnons in a clean two-dimensional chiral magnet. The magnon spectrum is found to include two magnon-skyrmion bound states corresponding to a breathing mode and, for intermediate fields, a quadrupolar mode, which will give rise to subgap magnetic and electric resonances. Due to the skyrmion topology, the magnons scatter from a Aharonov-Bohm flux density that leads to skew and rainbow scattering, characterized by an asymmetric differential cross section with, in general, multiple peaks. As a consequence of the skew scattering, a finite density of skyrmions will generate a topological magnon Hall effect. Using the conservation law for the energy-momentum tensor, we demonstrate that the magnons also transfer momentum to the skyrmion. As a consequence, a magnon current leads to magnon pressure reflected in a momentum-transfer force in the Thiele equation of motion for the skyrmion. This force is reactive and governed by the scattering cross sections of the skyrmion; it causes not only a finite skyrmion velocity but also a large skyrmion Hall effect. Our results provide, in particular, the basis for a theory of skyrmion caloritronics for a dilute skyrmion gas in clean insulating chiral magnets.

Analysis of Magnon-Skyrmion Scattering in Chiral Magnets

The paper of magnon-skyrmion interactions in chiral magnets, as presented in the paper by Sch\"utte and Garst, explores the fundamental dynamics and effects arising from the complex interplay between magnons and skyrmions. This research contributes to the theoretical foundation necessary for understanding skyrmion phenomena in magnetic systems, particularly focusing on insulating chiral magnets where magnon currents play a significant role.

Skyrmion Dynamics and Magnon Interactions

Chiral magnets are known for their unique magnetic textures, such as skyrmions, which owe their stability and properties to the Dzyaloshinskii-Moriya interaction. In the context of a sufficiently large external magnetic field, the paper examines the conditions under which skyrmions emerge as stable excitations against a field-polarized magnetic backdrop. This investigation is situated within the two-dimensional non-linear sigma model framework — a common approach for exploring magnetization dynamics in theoretical physics.

The interaction of magnons with skyrmions is shown to lead to the formation of bound states within the magnon spectrum. Two distinct types of bound states are identified: the breathing mode, which spans the entire examined magnetic field range, and the quadrupolar mode, which only exists for certain intermediate field strengths. These modes are characterized by specific energy levels within the subgap range, predicting resonances that are pertinent to magnetic resonance and spintronic studies.

Scattering Characteristics and Topological Hall Effects

A critical aspect addressed in the paper is the scattering behavior of magnons in the presence of skyrmions, revealing distinctive skew and rainbow scattering patterns. The skew scattering results in a topological magnon Hall effect, analogous to the electron Hall effect observed in skyrmion lattices. From the scattering analysis, the differential cross-section is found to be asymmetric, indicating complex interactions influenced by the topology inherent to skyrmions.

Another notable implication of the magnon-skyrmion interaction is the transfer of momentum from magnons to skyrmions. The paper quantitatively describes how magnon currents exert pressure on skyrmions, which is interpreted as a reactive force leading to a finite skyrmion velocity and a pronounced skyrmion Hall angle. These effects underscore important potential for manipulating and utilizing skyrmion motion in spintronic applications and contribute to the emergent field of skyrmion caloritronics.

Future Directions and Practical Implications

The insights provided in this paper have significant implications for both theoretical and practical developments in the field of magnetism and spintronics. By expanding our understanding of magnon-skyrmion interactions, this research opens avenues for further exploration into manipulating magnetic skyrmions via magnon currents. The predictions regarding the subgap resonances can be experimentally explored to fine-tune magnetization dynamics in chiral magnets and innovate in low-power spintronic devices.

Understanding and controlling skyrmion dynamics is crucial for advancing nanoscale magnetic technologies and could revolutionize approaches to data storage and energy-efficient computational systems. The comprehensive analysis of magnon-skyrmion interactions here sets a strong foundation for future studies to explore complex interactions in multi-skyrmion systems and to investigate thermal effects on skyrmion propagation.

Overall, the paper provides a robust theoretical exploration of key phenomena in chiral magnets, contributing to the ongoing development of skyrmion-based technologies and the broader understanding of topological excitations in condensed matter physics.