Ultrafast Demagnetization through Rapid Spin-wave Generation

2024-02-02

Excitation of a ferromagnetic material with a femtosecond laser pulse causes an ultrafast magnetization drop, but the deeper mechanism behind this ultrafast drop has been disputed for many years. Uppsala physicists now show that ultrafast generation of non-thermal spin waves can explain a rapid magnetization loss within 200 femtoseconds (1 fs = 10-15 s) consistent with experiments. 

More than 25 years ago, it was discovered that a laser pulse as short as some 50 femtoseconds could alter the magnetization of a ferromagnetic material in less than 200 femtoseconds. Laser excitation of a metal gives generally rise to correlated, out-of-equilibrium dynamics of its fundamental constituents, namely, electrons, lattice vibrations and spin waves. However, it has been a long-standing mystery through what mechanism the ultrafast magnetization loss could be explained. 

Figure 1: The magnon dispersion of iron shown for wave vectors in reciprocal space. The colors show the temperature of the magnon modes during the electron-magnon excitation process. The laser pulse hits the electrons at time t = 0. The consecutive dispersion curves show the time evolution of the wave-vector dependent magnon temperatures. 

To tackle this problem, Uppsala physicists Markus Weißenhofer and Peter Oppeneer developed a fully out-of-equilibrium theory to describe the transfer of energy and spin angular momentum between electrons and quantized spin waves, called magnons. In their theory the laser pulse dumps energy into the electrons that react by transferring their excess energy as well as spin angular momentum into spin waves, exciting thus many magnon modes. The theory then describes the fast dynamics of the magnon modes’ occupations due to electron-magnon scattering. Using only quantities calculated from first principles (i.e., without adjustable parameters) and performing quantitative simulations of the ultrafast laser-induced dynamics in iron, they demonstrated that on femtosecond timescales the magnon distribution is non-thermal: the electrons’ energy is predominantly transferred to high-energy magnons, see Figure 1. The damping of the magnon modes furthermore is found to become strongly magnon wave vector dependent and cannot be described anymore within the standard model of Landau, Lifshitz and Gilbert for spin dynamics. 

Figure 2: Comparison between theory and experiment for the ultrafast loss of magnetization M of an iron film upon laser excitation at t = 0. The full curves present the ultrafast demagnetization computed with the developed theory and the symbols give the experimental data for two different laser intensities. 

Moreover, their parameter-free calculations of the ensuing correlated dynamics show that ultrafast generation of non-thermal magnons provides a sizable demagnetization within 200 fs in excellent comparison with experimental observations, see Figure 2. These investigations thus emphasize the importance of non-thermal magnon excitations for the ultrafast demagnetization process.

Article Reference

Ultrafast Demagnetization Through Femtosecond Generation of Non-Thermal Magnons. Markus Weißenhofer and Peter Oppeneer, Advanced Physics Research 2300103 (2024); published January 10, 2024. DOI: https://doi.org/10.1002/apxr.202300103

Camilla Thulin

Translation: Johan Wall

Last modified: 2023-08-04