From equilibrium to ultrafast antiferromagnetism.

Although ferromagnets are known from ancient times, antiferromagnets were only discovered in the twentieth century and represent a much larger, less explored class of magnets. In this Review, we present the current understanding of the ultrafast response of spins in antiferromagnets to ultrashort laser stimuli. We progressively transition from equilibrium to ultrafast out-of-equilibrium dynamics, highlighting fundamental pillars that make the ultrafast magnetic response in antiferromagnets qualitatively distinct, compared with that in ferromagnets. Throughout this Review, we demonstrate how ultrashort laser excitation serves as a powerful tool for investigating and controlling antiferromagnets, providing access to coherent terahertz spin dynamics, dissipation mechanisms, nonlinear interactions and magnetic switching on ultrafast timescales. This unique combination of rich fundamental physics and advanced control capabilities positions antiferromagnets as both a fascinating subject of basic research and a promising platform for next-generation ultrafast and low-dissipation spintronic technologies.