Low-energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu ₂ OSeO ₃

In this paper, we present a comprehensive study of the low-energy optical magnetic response of the skyrmionic Mott insulator Cu ₂ OSeO ₃ via high resolution time-domain THz spectroscopy. In zero field, a new magnetic excitation (f ₀ = 2.03 THz) which has not been predicted by spin-wave theory is observed and shown, with accompanying time-of-flight neutron scattering experiments, to be a zone folded magnon from the R to Γ points of the Brillouin zone. Highly sensitive polarimetry experiments performed in weak magnetic fields, μ ₀ H < 200 mT, observe Faraday and Kerr rotations which are proportional to the sample magnetization, allowing for optical detection of the skyrmion phase and construction of a magnetic phase diagram. From these measurements, we extract a critical exponent of β = 0.35 ± 0.04, in good agreement with the expected value for the 3D Heisenberg universality class of β = 0.367. In large magnetic fields, μ ₀ H > 5 T, we observe the magnetically active uniform mode of the ferrimagnetic field polarized phase whose dynamics as a function of field and temperature are studied. In addition to extracting a g _eff = 2.08 ± 0.03, we observe the uniform mode to decay through a non-Gilbert damping mechanism and to possess a finite spontaneous decay rate, Γ ₀ ≈ 25 GHz, in the zero temperature limit. Our observations are attributed to Dzyaloshinkii-Moriya interactions, which have been proposed to be exceptionally strong in Cu ₂ OSeO ₃ and are expected to impact the low-energy magnetic response of such chiral magnets.