Spin-current-mediated rapid magnon localisation and coalescence after ultrafast optical pumping of ferrimagnetic alloys

Ezio Iacocca ,Hendrik Ohldag ,A V Kimel ,Stefano Bonetti ,Giacomo Coslovich ,Patrick W Granitzka ,Konstantin Hirsch ,Roopali Kukreja ,W F Schlotter ,Andrei Kirilyuk ,Richard F L Evans ,T M Liu ,R.w Chantrell ,T Chase ,Z Chen ,A X Gray ,Sergiu Ruta ,Loïc Le Guyader ,Emmanuelle Jal ,J Stöhr ,Markus Hoffmann ,Georgi L Dakovski ,Catherine E Graves ,T J Silva ,Arata Tsukamoto ,Zhiqiang Fu ,Thomas Ostler ,Mark Hoefer ,Daniel J Higley ,S Carron ,Alexander H Reid ,Th Rasing ,H A Dürr

doi:10.1038/s41467-019-09577-0

Abstract

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 107 A cm−2. Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.

Highlights

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996
To further investigate the fundamental physics involved in the evolution of spatially varying magnetisation after ultrafast optical pumping, and to elucidate which physical mechanisms are most important for the recovery of local magnetic order at picosecond timescales, we study the space- and time-dependent magnetisation dynamics in ferrimagnetic Gd0.24Fe0.665Co0.095 alloys with time-resolved resonant X-ray scattering
The substantial, highly turbulent flux of angular momentum in the vicinity of magnon drops during the coalescence stage can be estimated by use of numerical simulations accompanied by a hydrodynamic formulation of magnetisation dynamics that show the presence of strong exchange flow spin currents (EFSCs)[23,24], which are equivalent to a 100% spin polarised charge current density on the order of 107 A cm−2

Summary

Introduction

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. These processes describe the nucleation and subsequent dynamics of localised textures that arise from attractive nonlinear interactions between thermalized magnons[19] This is in contrast to theories that predict the order parameter recovery of the spatially averaged magnetisation, as described by the damping of a heated spin-wave distribution[20]. The substantial, highly turbulent flux of angular momentum in the vicinity of magnon drops during the coalescence stage can be estimated by use of numerical simulations accompanied by a hydrodynamic formulation of magnetisation dynamics that show the presence of strong exchange flow spin currents (EFSCs)[23,24], which are equivalent to a 100% spin polarised charge current density on the order of 107 A cm−2. The magnon processes identified here shed light upon the physical mechanisms that are important in the initial stages of unstable growth and pattern formation triggered by ultrafast optical pumping

Methods

Results

Conclusion