Nonreciprocal Magnons in Noncentrosymmetric Magnets

In a noncentrosymmetric crystalline material, the propagation of particles or quasiparticles can be nonreciprocal, i.e., the left-moving and right-moving (quasi)particles become inequivalent. In a noncentrosymmetric magnet, such nonreciprocity is expected for magnons, the quantized collective spin fluctuations that propagate as a wave in a magnetically ordered phase. Even though the nonreciprocal propagation of the magnons was theoretically proposed decades ago, experimentally, little attention has been given to the phenomenon, partly because of its putative subtleness originating from the weak relativistic spin-orbit coupling. The situation has markedly changed recently, as the possibility of measuring and controlling a magnon spin current in noncentrosymmetric magnets begins to gain wider recognition. In this article, we will review recent progress in the detection of the nonreciprocal magnons in noncentrosymmetric magnets. Particular emphasis will be placed on the neutron scattering studies where the magnon dispersion is directly measured in a microscopic length scale.