Role of polarized phonons during ultrafast demagnetization*

The transfer and control of angular momentum is a key aspect for spintronic applications. When a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times. This phenomenon offers opportunities for rapid information processing or ultrafast spintronics. Consequently, ultrafast demagnetization is central to modern material research, but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum on such short time scales?

 

We use molecular dynamics simulations to investigate the role of phonons during ultrafast demagnetization in nickel. For this purpose, we transfer angular momentum corresponding to the observed amount of demagnetization into the lattice and calculate the resulting changes in the diffraction pattern. Our results are in line with ultrafast electron diffraction measurements which show an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear as quickly as the magnetic order is lost. Theory and experiment indicate a rotational lattice motion on atomic dimensions after the excitation with the laser pulse that takes up the missing angular momentum [1] before the onset of a macroscopic Einstein-de Haas rotation [2].

 

In the second part of the talk we report on a new framework for spin-molecular dynamics that connects, on the one hand, to ab initio calculations of spin-lattice coupling parameters [3] and, on the other hand, to the magneto-elastic continuum theory. The derived Hamiltonian describes a closed system of spin and lattice degrees of freedom and explicitly conserves the total momentum, angular momentum and energy. This framework will enable the use of multi-scale modeling for investigating a broad range of spin-lattice dynamics phenomena from slow to ultrafast.

 

[1] S. R. Tauchert et al. Nature 602, 73 (2022).

[2] C. Dornes et al. Nature 565, 209 (2019).

[3] S. Mankovsky et al., Phys. Rev. Lett. 129, 067202 (2022)