Making a case for femto- phono- magnetism with FePt

TL;DR

This paper introduces femto-phono-magnetism, demonstrating how phonon excitations coupled with spin and charge can control magnetic order at ultrafast timescales, with simulations showing significant spin moment loss in FePt within 40 femtoseconds.

Contribution

It reveals the role of coupled spin-phonon modes in ultrafast magnetism, highlighting the nuclear system's active role in femtosecond spin dynamics, a novel insight beyond traditional models.

Findings

Strong non-adiabatic spin-phonon modes dominate early spin dynamics.

Activation of phonon modes causes up to 40% additional spin moment loss.

Minority spin-current enhances charge transfer and spin flips.

Abstract

In the field of femtomagnetism magnetic matter is controlled by ultrafast laser pulses; here we show that coupling phonon excitations of the nuclei to spin and charge leads to femto-phono-magnetism, a powerful route to control magnetic order at ultrafast times. With state-of-the-art theoretical simulations of coupled spin-, charge-, and lattice-dynamics we identify strong non-adiabatic spin-phonon coupled modes that dominate early time spin dynamics. Activating these phonon modes we show leads to an additional (up to 40\% extra) loss of moment in FePt occurring within 40 femtoseconds of the pump laser pulse. Underpinning this enhanced ultrafast loss of spin moment we identify a physical mechanism in which minority spin-current drives an enhanced inter-site minority charge transfer, in turn promoting increased on-site spin flips. Our finding demonstrates that the nuclear system, often assumed to play only the role of an energy sink aiding long time re-magnetisation of the spin system, can play a profound role in controlling femtosecond spin-dynamics in materials.