  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.
