Optical manipulation of magnetism holds promise for future ultrafast spintronics, especially with lanthanides and their huge, localized 4f magnetic moments. These moments interact indirectly via the conduction electrons (RKKY exchange), influenced by interatomic orbital overlap, and the conduction electron susceptibility. Here, we study this influence in a series of 4f antiferromagnets, GdT2Si2 (T=Co, Rh, Ir), using ultrafast resonant X-ray diffraction. We observe a twofold increase in ultrafast angular momentum transfer between the materials, originating from modifications in the conduction electron susceptibility, as confirmed by first-principles calculations.

Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light-matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. We study epitaxial Au nanoislands on WSe_2 with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy and momentum of charge-carriers and phonons excited in the heterostructure. We observe a strong non-linear plasmon-exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. Our results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron-phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling.

Raw data for "Exchange scaling of ultrafast angular momentum transfer in 4f antiferromagnets" The h5 files are provided: 1. Windsor_LnRh2Si2_2022_static.h5 This file provides the temperature dependences of the 4f moment, as collected from the (001) magnetic reflection (details described in the supplementary materials, section 1.1. The file is divided by Ln ion, for each LnRh2Si2 material. For each material the sample temperature and the moment (normalized) are given. 2. Windsor_LnRh2Si2_2022_dynamic.h5 This file provides the delay dependences of the 4f moments, as collected from the (001) magnetic reflection (details provided in the methods section). The file is divided by Ln ion, for each LnRh2Si2 material. This is further divided into datasets that correspond to different pump fluences. For each fluence the fields provided are the moment (normalized), the corresponding error (see methods), the delay, and the fluence value.

Ultrafast manipulation of magnetism bears great potential for future information technologies. While demagnetization in ferromagnets is governed by the dissipation of angular momentum 1 – 3 , materials with multiple spin sublattices, for example antiferromagnets, can allow direct angular momentum transfer between opposing spins, promising faster functionality. In lanthanides, 4 f magnetic exchange is mediated indirectly through the conduction electrons 4 (the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction), and the effect of such conditions on direct spin transfer processes is largely unexplored. Here, we investigate ultrafast magnetization dynamics in 4 f antiferromagnets and systematically vary the 4 f occupation, thereby altering the magnitude of the RKKY coupling energy. By combining time-resolved soft X-ray diffraction with ab initio calculations, we find that the rate of direct transfer between opposing moments is directly determined by this coupling. Given the high sensitivity of RKKY to the conduction electrons, our results offer a useful approach for fine tuning the speed of magnetic devices.

The ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh 2 Si 2 . The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time‐ and angle‐resolved photoemission spectroscopy and time‐resolved resonant magnetic soft X‐ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two‐step demagnetization behaviors with two similar timescales (<1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence‐dependent increased Néel temperature. The results indicate that effects beyond a mean‐field description have to be considered for a quantitative description of ultrafast magnetic order dynamics.

By comparing femtosecond laser-pulse-induced spin dynamics in the surface state of the rare earth metals Gd and Tb, we show that the spin polarization of valence states in both materials decays with significantly different time constants of 15 ps and 400 fs, respectively. The distinct spin polarization dynamics in Gd and Tb are opposed by similar exchange splitting dynamics in the two materials. The different time scales observed in our experiment can be attributed to weak and strong 4f spin to lattice coupling in Gd and Tb, suggesting an intimate coupling of spin polarization and 4f magnetic moment. While in Gd the lattice mainly acts as a heat sink, it contributes significantly to ultrafast demagnetization of Tb. This helps explain why all optical switching is observed in FeGd-but rarely in FeTb-based compounds.

Noble metal nanostructures allow to enhance and tune light absorption to efficiently produce plasmonic excitations, which couple strongly to two subsystems of excitations in the semiconductor: hot carriers and phonons. These excitations relax following complex pathways, unlocking numerous nanoplasmonic applications ranging from photocatalysis to photovoltaic. In this work, we distinguish charge carrier and phonon dynamics in a plasmonic metal/semiconductor heterostructure, with the combined use of time- and angle-resolved photoemission spectroscopy (trARPES) and femtosecond electron diffraction (FED). We use trARPES to detect the non-equilibrium charge-carrier population, while with analysis of FED diffraction patterns we single out phonon dynamics. The heterostructure is composed of Au nanoislands, grown epitaxially on single-crystalline, bulk WSe2. The epitaxial relationship between the atoms of the metal and the semiconductor is reflected on the electronic band structure and the diffraction pattern and it allows material-resolved trARPES and FED measurements. Exploiting the surface sensitivity of electron-based techniques, we restrict the probed area to the active interface, and by choosing different pump wavelengths we control the excitation of the semiconductor. Surface decoration of WSe2 with Au is found to cause a significant shortening of the excitons’ lifetime and accelerated lattice heating, which is a strong indication of charge-transfer towards Au. Moreover, Au sensitizes WSe2 to sub-band-gap photons, allowing to observe non-equilibrium phonon populations in WSe2 when the pump wavelength is longer than the semiconductor absorption threshold. The corresponding lattice heating follows a nonlinear relationship with the incident laser fluence, which can be attributed to plasmonically enhanced phonon excitation.