Observation of magnetic excitations of skyrmion crystal in a helimagnetic insulator

TL;DR

This study reports the observation of two distinct low-energy microwave excitations in the skyrmion crystal phase of a helimagnetic insulator, revealing insights into skyrmion dynamics and potential manipulation methods.

Contribution

First direct observation of microwave excitations of skyrmion crystal in an insulator, identifying two modes with different polarization characteristics.

Findings

Detected counter-clockwise mode at 1 GHz

Observed breathing mode at 1.5 GHz

Revealed potential for skyrmion manipulation via these excitations

Abstract

The skyrmion is a topologically stable spin texture, in which the spin direction wraps a sphere. The topological nature gives rise to emergent electromagnetic phenomena such as topological Hall effect. Recently, the crystallization of nanoscale skyrmions was observed in chiral helimagnets with use of the neutron diffraction and Lorentz transmission electron microscopy. The skyrmions are quite mobile under electric current density as low as $\sim$ 10$^6$ A/m$^2$, and, in some cases, stable up to near room temperature. These features suggest that the skyrmions may work as a magnetic stable-variable similar to the bubble memory but equipped with topological functionalities. Here, we investigate the low-energy excitations of the skyrmion crystal in a helimagnetic insulator Cu$_2$OSeO$_3$ in terms of microwave response. We have observed two distinct excitations of the skyrmion with different polarization characteristics; the counter-clockwise circulating mode at 1 GHz and the breathing mode at 1.5 GHz. These modes may play a crucial role in the low energy dynamics of skyrmions and hence in their manipulation via external stimuli.