Abstract
Dissipation-free manipulation of magnetic order remains a long-standing goal for future spintronic 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 4ƒ antiferromagnets experimentally.
First, we demonstrate that the rate of such direct angular momentum transfer between antiparallel localized 4ƒ moments scales with the strength of the RKKY interaction by singling out the role of the localized 4f moments, the source, and of the conduction electrons, the mediator of the RKKY interaction. Our study focuses on ultrafast magnetic order dynamics in a series of lanthanide-based antiferromagnets LnT2Si2. Our key observation is that the 4ƒ angular momentum transfer rate scales with the de Gennes factor, a fundamental quantity derived from the L and S atomic numbers of localized 4ƒ electrons (Chapter 4) and with the spin polarization of 5d conduction electrons around the Fermi level (Chapter 5). Supported by ab-initio calculations, we reveal a direct correspondence between angular momentum transfer rates to the strength of antiferromagnetic RKKY coupling.
Next, we study the on-site RKKY coupling between the localized 4ƒ moments and itinerant conduction electrons by measuring ultrafast transient temperature dynamics along with surface and bulk magnetic order dynamics. Our key observations are that (i) the conduction electrons and localized moments exhibit very similar demagnetization timescale, which suggests a strong on-site RKKY coupling and (ii) there are robust magnetic orderings of both conduction electrons and localized moments even when the transient temperature is well above the phase transition temperature for hundreds of picoseconds. A microscopic three temperature model based on the Landau-Lifshitz-Bloch equation phenomenologically explain this with effective increase of phase transition temperature during ultrafast spin dynamics (Chapter 6). We also demonstrate that, in extreme case, the localized 4f moments can stay non-thermalized upon optical excitation for hundreds of picoseconds exhibiting 200-ps-long disparate dynamics of exchange couplings between 4ƒ moments and long-range antiferromagnetic 4ƒ ordering (Chapter 7).
Our results are of fundamental importance for ultrafast magnetic order dynamics in that they demonstrate a systematic relation between microscopic magnetic coupling and inter- and intraatomic flow of angular momentum and energy. From a practical perspective, they also present new opportunities for controlling or even engineering magnetic order dynamics, because the RKKY magnitude can be sensitively tuned, such as by modifying either the 4ƒ moments or the conduction electrons. This is particularly relevant due to the recent rise in prominence of antiferromagnetic spintronics. With these considerations, we systematically explored the role of the RKKY interaction in ultrafast spin dynamics of 4ƒ antiferromagnets in this thesis.
