Courants de spin femtosecondes pour l'électronique de spin ultrarapide – SPINUP
Femtosecond spin currents for ultrafast spintronics
Smaller, faster, lower energy consumption; these are some of the key words of tomorrow’s information and communication technologies (ICTs). Besides, it is nowadays well established that spins can replace charges in terms of information handling. Therefore, mastering the transport and manipulation of spin-information at picosecond and sub-picosecond timescales, ultimate prerequisite to the emergence of ultrafast spintronics devices operating in the terahertz (THz) range.
Enjeux et Etat de l’art
A major breakthrough in nanomagnetism occurred in 1996 when Beaurepaire et al. demonstrated that the magnetization of a nickel thin film could be quenched within a few hundred femtoseconds using intense laser pulses. This was the first evidence that magnetic order can be manipulated on sub-picosecond timescales, opening the possibility of dramatically increasing the speed of spintronic devices. This work gave rise to the field of femtomagnetism. Over the past two decades, significant efforts have been devoted to understanding the mechanisms behind ultrafast magnetic dynamics. A common framework describes electrons, spins, and phonons as three interacting subsystems, known as the “three-temperature model,” which captures the essence of ultrafast demagnetization. The fastest demagnetization occurs when direct laser-induced electronic excitations are involved. The lifetimes and scattering processes of these hot electrons have been extensively studied using time-resolved photoemission. While early models focused on local scattering processes, it is now clear that spin-polarized hot-carrier transport plays a crucial role. Battiato et al. introduced the concept of laser-induced “superdiffusive” spin currents, which carry spin angular momentum away from the excited region. Spin-flip scattering of these carriers leads to loss of spin information and enhances demagnetization, particularly in systems with excitation gradients or multilayer structures containing heavy metals. Experimental evidence of ultrafast spin transport has been obtained through time-resolved second harmonic generation, revealing both ballistic and superdiffusive behavior. More recently, combining ultrafast hot-electron transport with spin-to-charge conversion has enabled the development of efficient terahertz (THz) spintronic emitters. These devices provide a powerful tool for probing sub-picosecond spin dynamics and represent a direct technological application of femtosecond spin currents. Research in this area has grown rapidly in the past five years. Efforts focus on both understanding the underlying physics and improving device performance. On the materials side, metallic systems and their interfaces have been widely studied, with CoFeB/Pt and Fe/Pt bilayers showing the strongest THz emission when layer thicknesses are optimized for spin diffusion and optical absorption. Additionally, tuning laser parameters such as wavelength and pulse duration offers further control over efficiency. Most studies rely on the inverse spin Hall effect (ISHE) as the primary spin-to-charge conversion mechanism. However, alternative mechanisms such as the anomalous Hall effect and the inverse Rashba–Edelstein effect (IREE), demonstrated at Bi/Ag interfaces, are also being explored. Although IREE typically produces weaker THz signals, its sensitivity to external electric fields may enable novel approaches for THz modulation.
The project primarily relied on a combination of advanced experimental techniques to directly probe pure spin currents, including time-resolved second harmonic generation (Tr-SHG), time-resolved magneto-optical Kerr effect (Tr-MOKE), and time-domain terahertz spectroscopy (TDS-THz), applied to a range of representative model systems. Through this approach, the ANR JCJC project aimed to foster the development of a challenging and innovative research activity at the intersection of time-resolved and near-field nonlinear optics. This effort provides valuable insights into the rapidly emerging field of THz spintronics, while more broadly contributing to the advancement of ultrafast nanoscience.
The results obtained within this project provide important advances in the understanding and control of ultrafast spin currents, their conversion mechanisms, and their interaction with complex magnetic systems.
A major achievement is the demonstration of ultrafast spin angular momentum transfer into insulating antiferromagnets. By investigating ferromagnet/antiferromagnet bilayers, it was shown that laser-induced spin dynamics in the ferromagnet can generate coherent terahertz excitations in the antiferromagnet, evidencing the propagation of magnetic information through antiferromagnetic spin waves on picosecond timescales (Science Adv. 11, eeadx1107, 2025). This result establishes insulating antiferromagnets as viable media for ultrafast spin transport.
In parallel, the project developed advanced time-resolved nonlinear optical imaging to probe coupled ferroic orders. Using TR-SHG microscopy, ultrafast antiferromagnetic dynamics were resolved at the sub-picosecond scale within individual domains of BiFeO₃ thin films, demonstrating the possibility to access and manipulate multiferroic order at ultrafast timescales (APL Materials, 2025, under review).
Another key outcome concerns spin-to-charge conversion mechanisms across different timescales. It was shown that, contrary to the inverse spin Hall effect, the inverse Edelstein effect exhibits a strong reduction of efficiency in the terahertz regime, despite high performance in quasi-static conditions (APL 123, 012407, 2023). This behavior was attributed to the non-equilibrium energy distribution of laser-excited hot carriers. This conclusion is reinforced by studies on LaAlO₃/SrTiO₃ interfaces, where a drop of more than four orders of magnitude in conversion efficiency is observed when moving from dc to picosecond regimes (Phys. Rev. B 110, 054412, 2024). These findings highlight critical limitations of Rashba-based systems for ultrafast spintronic applications.
Finally, element-specific ultrafast spectroscopy of CoFeB/Pt bilayers revealed a strong asymmetry between the dynamics of 3d ferromagnetic moments and induced 5d moments in Pt. The induced Pt magnetization exhibits faster and larger demagnetization, consistent with enhanced spin-flip processes driven by strong spin–orbit coupling and magnon-mediated effects (Phys. Rev. Research 5, 013147, 2023).
As a next step, while the JCJC ANR SpinUp project has pioneeringly demonstrated that ultrafast spin currents can be transferred to an insulating antiferromagnet, we now aim to master and control the propagation of these currents. Additionally, we plan to focus on integrating functionalized antiferromagnets into future THz spintronic emitters. Besides, the knowhow acquired on SHG and Tr-SHG imaging allows me to extent my research field toward the physics of new classes of ferroic systems such as antiferroelectrics and ferrotoroidics compounds.
En implémentant une combinaison d'approches optiques ultra-rapides avancées, SPINUP a pour objectif final l’étude des caractéristiques dynamiques ultrarapide de la propagation d'impulsions de courant de spin sub-picosecondes à l'échelle nanométrique. La méthodologie principale reposera sur les mesures directes, résolues spatialement (inférieures à 30 nm) et temporellement (inférieures à 50 fs) des courants de spin purs utilisant des techniques avancées de microscopie et spectroscopie optique non linéaire en champ proche. SPINUP me permettrait finalement de développer un nouveau domaine d’expertise au CEA / SPEC, axé sur l’optique en champ proche résolue dans le temps et permettant d’étudier les nouvelles tendances de la spintronique ultra-rapide. L’impact scientifique attendu de cette ANR JCJC SPINUP sera substantielle pour la prochaine génération de dispositifs de spintronique ultrarapide et, dans un cadre plus général, pour les thématiques de dynamique ultrarapide en nanoscience.
Coordination du projet
Jean-Yves Chauleau (Service de physique de l'état condensé)
L'auteur de ce résumé est le coordinateur du projet, qui est responsable du contenu de ce résumé. L'ANR décline par conséquent toute responsabilité quant à son contenu.
Partenariat
SPEC Service de physique de l'état condensé
Aide de l'ANR 291 753 euros
Début et durée du projet scientifique :
- 42 Mois
