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

TL;DR

This paper investigates ultrafast optical pumping in ferrimagnetic alloys, revealing rapid magnon localization and coalescence driven by spin currents, which facilitate quick magnetic order recovery and potential stabilization of meta-stable states.

Contribution

It uncovers the spatial dynamics of magnetization after ultrafast optical excitation, highlighting the roles of magnon processes and spin currents in magnetic order recovery.

Findings

Identification of localization and coalescence regimes in magnetization dynamics

Estimation of exchange-mediated spin currents reaching 10^8 A/cm^2

Demonstration of ultrafast magnetic order recovery mechanisms

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 rapid magnetic order recovery in materials with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify both localisation and coalescence regimes, whereby localised magnetic textures nucleate and subsequently evolve in accordance with a power law formalism. Coalescence is observed for optical excitations both above and below the switching threshold. Simulations indicate that the ultrafast generation of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of $10^8$ A/cm$^2$. Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. These processes suggest an ultrafast optical route for the stabilization of desired meta-stable states, e.g., isolated skyrmions.