Examining how effectively circularly polarized THz waves drive antiferromagnetic resonance within crystalline antiferromagnets

In this work, we use frequency-domain THz spectroscopy to characterize the antiferromagnetic resonance of three different orientations of monocrystalline nickel oxide (NiO). Our spectroscopy system generates a circularly polarized THz beam, with a frequency that can be adjusted between 100 - 1800 GHz. We experimentally find that the (100) and (110) orientations of NiO more strongly absorb the THz radiation compared with the (111) sample at the expected resonance frequency near 1000 GHz. To explain our experimental results, we use a macrospin model of an easy-plane antiferromagnet that is driven by a circularly polarized driving field. The model considers both the misalignment of the easy plane with the optical axis of the THz beam as well as the orientation of the Néel vector within the plane. Our model predicts that the (111) orientation of NiO most weakly couples to the circularly polarized driving field, in qualitative agreement with our experimental results. In general, our model predicts that the misalignment of the easy plane with the optical axis strongly influences the absorption of circularly polarized THz radiation and that, to a lesser effect, there is a minute dependence on the orientation of the Néel vector within the plane.