Observation of Coherent Gapless Magnons in an Antiferromagnet

Antiferromagnetic magnons possess high speed and are immune to external disturbance, making them promising for future magnonic circuits. In this Letter, we report the observation of gapless magnons in an easy-axis antiferromagnet α-Fe_{2}O_{3} at low temperatures. These antiferromagnetic magnons are detected at nearly zero frequency by all-electrical spin-wave spectroscopy and propagate along antiferromagnetic domain walls as revealed by our theoretical model and simulations. Moreover, we demonstrate high coherency of these gapless magnons by showing their strong coupling with microwave photons. Our results open the pathway for antiferromagnetic texture based magnonic devices operating at microwave frequencies.