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arXiv · 1704.04228

Low energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu$_2$OSeO$_3$

N. J. Laurita·G. G. Marcus·B. A. Trump·J. Kindervater·M. B. Stone·T. M. McQueen·C. L. Broholm·N. P. Armitage
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Abstract

In this work, 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 not predicted by spin-wave theory with frequency $f$ = 2.03 THz is observed and shown, with accompanying time-of-flight neutron scattering experiments, to be a zone folded magnon from the $\mathrm{R}$ to $\mathrm{\Gamma}$ points of the Brillouin zone. Highly sensitive polarimetry experiments performed in weak magnetic fields, $\mu_0$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 $\beta$ = 0.35 $\pm$ 0.04, in good agreement with the expected value for the 3D Heisenberg universality class of $\beta$ = 0.367. In large magnetic fields, $\mu_0$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 $g_\text{eff}$ = 2.08 $\pm$ 0.03, we observe the uniform mode to decay through a non-Gilbert damping mechanism and to possesses a finite spontaneous decay rate, $\Gamma_0$ $\approx$ 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.

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N. J. Laurita, G. G. Marcus, B. A. Trump, J. Kindervater, M. B. Stone, T. M. McQueen, C. L. Broholm, N. P. Armitage. 2017-04-13. Low energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu$_2$OSeO$_3$. https://doi.org/10.1103/physrevb.95.235155

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