Ultrafast optical excitation of coherent magnons in antiferromagnetic NiO

Abstract

Antiferromagnets are rapidly gaining importance as crucial ingredients in many applications. They are abundant in nature and they are robust against externally applied fields. Rather usefully, their spin resonances often lie within the THz regime, which makes them ideal candidates for optical studies. In this paper, the authors present a thorough experimental and theoretical exploration of the optical spin excitation in antiferromagnetic NiO. Based on a phenomenological theory, they derive expressions for the optically induced magnetization via the inverse Faraday effect and the inverse Cotton-Mouton effect. Light polarization is conserved by pumping and probing along the optical axis of the material, facilitating the comparison between theory and experiment. Those agree amazingly well, making possible the identification of the driving mechanism behind the ultrafast magnon excitations. Moreover, the authors succeed in obtaining information about the otherwise elusive spin-domain distribution in NiO and in showing that the energy transfer into the magnon mode is about three orders of magnitude more efficient via the inverse Cotton-Mouton effect than via the inverse Faraday effect.