Dissipation-free manipulation of magnetic order remains a long-standing goal for future spin- tronic devices, and is of particular focus in the field of ultrafast magnetism. Ferromagnets, which have long been the primary focus of this field, suffer from inherent angular momentum dissipation, which sets fundamental limits on achievable time scales and energy efficiency. In contrast, antiferromagnets can overcome these limits, and achieve dissipation-free spin dynamics by direct angular momentum transfer between opposing magnetic sublattices. While presenting appealing prospects for devices, a fundamental understanding of how such transfer is mediated remains largely unexplored, in particular when indirect magnetic couplings are at play. A prime example is the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange coupling, in which conduction electrons mediate between localized moments. There are two important aspects of RKKY coupling: (i) the inter-atomic coupling between localized moments and (ii) the intra-atomic coupling between localized moments and itinerant conduction electrons. This thesis explores these two different aspects of the RKKY coupling by studying ultrafast spin dynamics of 4f antiferromagnets experimentally.
