Collective spin excitations of helices and magnetic skyrmions: review and perspectives of magnonics in non-centrosymmetric magnets (1702.03668v1)

Abstract: Magnetic materials hosting correlated electrons play an important role for information technology and signal processing. The currently used ferro-, ferri- and antiferromagnetic materials provide microscopic moments (spins) that are mainly collinear. Recently more complex spin structures such as spin helices and cycloids have regained a lot of interest. The interest has been initiated by the discovery of the skyrmion lattice phase in non-centrosymmetric helical magnets. In this review we address how spin helices and skyrmion lattices enrich the microwave characteristics of magnetic materials. When discussing perspectives for microwave electronics and magnonics we focus particularly on insulating materials as they avoid eddy current losses, offer low spin-wave damping, and might allow for electric field control of collective spin excitations. Thereby, they further fuel the vision of magnonics operated at low energy consumption.

Collective Spin Excitations of Helices and Magnetic Skyrmions

The paper of magnetic materials with complex spin structures, particularly spin helices and skyrmion lattices, presents significant opportunities for advancing the field of magnonics, which exploits spin waves (magnons) for information processing. This paper, authored by Markus Garst, Johannes Waizner, and Dirk Grundler, provides a comprehensive review of the collective spin excitations associated with helices and skyrmions in non-centrosymmetric magnets, focusing on their implications for magnonics in high-frequency applications.

Spin Structures and Their Characteristics

Magnetic materials such as ferromagnets, ferrimagnets, and antiferromagnets have traditionally dominated applications in data storage and signal processing. In these materials, the arrangement of magnetic spins is typically collinear. However, non-collinear spin structures such as spin helices and skyrmions have garnered interest due to their unique properties. Helical spin structures are stabilized by the Dzyaloshinskii-Moriya interaction (DMI), which results from relativistic spin-orbit coupling. This interaction is prevalent in certain crystal structures that lack inversion symmetry, leading to non-collinear magnetic order, such as in the skyrmion phase observed in chiral magnets like MnSi and Cu$_2$OSeO$_3$.

Skyrmions, in particular, are of great interest due to their topological protection, nanometer size, and particle-like properties. They can form lattice structures known as skyrmion crystals. The paper describes key spin wave modes in skyrmion crystals, including the clockwise (CW), counterclockwise (CCW), and breathing modes, and how these modes can be excited by microwave fields.

Experimental Insights and Numerical Models

The paper reviews spectroscopic studies of GHz excitations in materials hosting skyrmion lattices, delivering insights into their dynamic properties. It compares experimental findings in compounds such as MnSi, Cu$_2$OSeO$_3$, and Fe$_{0.8}$Co$_{0.2}$Si, among others. These studies highlight that skyrmion-hosting materials display a uniform set of excitation modes across different compositions and phases, yielding a distinct signature that can be utilized in magnonic applications.

From a theoretical perspective, the authors develop models to simulate the band structures of magnons in the helical and skyrmion phases. They calculate the Chern numbers for skyrmion crystals, thus identifying the presence of topologically protected magnon edge states due to the non-trivial topology of these structures.

Potential Applications in Magnonics

The authors suggest manifold applications for non-collinear spin structures in magnonics:

• Magnonic Crystals: By exploiting the inherent periodicity of helical magnets, one can create reconfigurable magnonic crystals without the need for nanopatterning. This can lead to advanced functionalities like nonreciprocal spin wave propagation and spin-wave channeling.
• Microwave Signal Control: Skyrmion crystals can be used as tunable grating couplers for short-wavelength spin waves. The ability to control spin wave directionality and frequency through electric fields opens up avenues for spin-based logic and communication devices.
• Low-energy Technologies: Insulating materials are highlighted as potential candidates for energy-efficient magnonics, minimizing losses associated with eddy currents and enabling electric-field control over spin excitations.

Future Directions

Looking forward, the paper emphasizes the need for further exploration of chiral magnetic materials, particularly those with high Curie temperatures and insulating properties, to achieve practical magnonic devices operable at room temperature. Furthermore, integrating these materials into thin-film technologies can dramatically extend their applicability in nanoscale devices. The authors also suggest that the coupling of skyrmions with spin waves might fuel the development of novel computational and storage technologies.

In conclusion, this paper provides a detailed theoretical and experimental analysis of spin excitations in helical and skyrmionic phases, emphasizing their potential to drive the next generation of magnonics-based technologies. It sets a foundation for future research to explore and harness the complex dynamics of these captivating magnetic structures.