Mode locking phenomena of the current-induced skyrmion-lattice motion in microfabricated MnSi

Using voltage fluctuation spectroscopy, we observe the mode locking phenomena of current-induced skyrmion-lattice motion in microfabricated MnSi. When only a dc electric current is applied, the frequency of the emergent narrow-band noise (NBN) monotonously increases with increasing dc current. By contrast, if an ac electric current is further added, the NBN frequency becomes much less dependent on the dc current when it satisfies a simple integer ratio to the ac current frequency, a hallmark of mode locking. In the mode locked state, the linewidth of the NBN is sharpened, indicating the enhanced coherence of the skyrmion-lattice motion. The overall features of the mode locking phenomena are qualitatively reproduced within the framework of the classical rigid-sphere model.