Ultrafast coherent nonlinear magnonics in antiferromagnets*

The wildly growing field of antiferromagnetic spintronics is currently addressing several fundamental questions. A major topic of investigations concerns the possibility to drive and manipulate coherent magnons on the ultrafast timescale. The proposition that driving such magnon modes, in a strongly nonlinear regime, could even result in switching coherently the order parameter on the femtosecond timescale needs to be considered. In my talk, I will discuss two approaches to nonlinear regime of coherent magnon dynamics. 

Domains are usually perceived as a nuisance, occurring in the ground state of antiferromagnets, to be avoided for an efficient control of spins. First, I will discuss recent results, which experimentally disprove this commonly accepted wisdom. Relying on a spectroscopic opto-magnetic investigation of the femtosecond spin dynamics in the archetypal antiferromagnet NiO in a multidomain state, I will demonstrate: i) the excitation and a novel mechanism to arbitrary amplify a THz magnon mode via the exciton-magnon transition; ii) nonlinear femtosecond spin dynamics, in the form of coupling between the different magnon modes, typically orthogonal in a single-domain state; iii) the microscopic nature of the coupling between modes, which is due to the presence of domain walls. This last point was supported by a phenomenological model and, most importantly, by means of a control experiment performed in a single domain of the material. 

Second, I will outline the unprecedented femtosecond coherent spin dynamics obtained by resonantly drive high-energy magnons, near the edges of the Brillouin zone. Although non-resonant excitation of such modes had been previously explored, the technical difficulties demanded by a resonant drive have been hitherto forbidden to unlock this scenario. The spin dynamics induced following this approach discloses coherent coupling of several magnon modes throughout the Brillouin zone.