Observation of coherent helimagnons and gilbert damping in an itinerant magnet

We study the magnetic excitations of itinerant helimagnets by applying time-resolved optical spectroscopy to Fe_0.8Co_0.2Si. Optically excited oscillations of the magnetization in the helical state are found to disperse to lower frequency as the applied magnetic field is increased; the fingerprint of collective modes unique to helimagnets, known as helimagnons. The use of time-resolved spectroscopy allows us "format"to address the fundamental magnetic relaxation processes by directly measuring the Gilbert damping, revealing the versatility of spin dynamics in chiral magnets.