Ultrafast Demagnetization through Femtosecond Generation of Non-thermal Magnons

TL;DR

This paper introduces an ab-initio quantum kinetic model to describe ultrafast, non-thermal magnon generation in ferromagnetic metals, revealing that traditional models overestimate demagnetization and highlighting the importance of non-thermal magnons in ultrafast demagnetization.

Contribution

The authors develop a novel, fully ab-initio out-of-equilibrium theory for magnon dynamics that surpasses the limitations of the three-temperature model in ultrafast regimes.

Findings

Magnon distribution is non-thermal during ultrafast excitation.

High-energy magnons are predominantly generated, while low-energy magnons decrease.

The model accurately predicts demagnetization within 200 fs, matching experiments.

Abstract

Ultrafast laser excitation of ferromagnetic metals gives rise to correlated, highly non-equilibrium dynamics of electrons, spins and lattice, which are, however, poorly described by the widely-used three-temperature model (3TM). Here, we develop a fully ab-initio parameterized out-of-equilibrium theory based on a quantum kinetic approach--termed (N+2) temperature model--that describes magnon occupation dynamics due to electron-magnon scattering. We apply this model to perform quantitative simulations on the ultrafast, laser-induced generation of magnons in iron and demonstrate that on these timescales the magnon distribution is non-thermal: predominantly high-energy magnons are created, while the magnon occupation close to the center of the Brillouin zone even decreases, due to a repopulation towards higher energy states via a so-far-overlooked scattering term. We demonstrate that the simple relation between magnetization and temperature computed at equilibrium does not hold in the ultrafast regime and that the 3TM greatly overestimates the demagnetization. The ensuing Gilbert damping becomes strongly magnon wavevector dependent and requires a description beyond the conventional Landau-Lifshitz-Gilbert spin dynamics. Our ab-initio-parameterized calculations show that ultrafast generation of non-thermal magnons provides a sizable demagnetization within 200fs in excellent comparison with experimentally observed laser-induced demagnetizations. Our investigation emphasizes the importance of non-thermal magnon excitations for the ultrafast demagnetization process.