Ultrafast reduction in exchange interaction by a laser pulse: Alternative path to femtomagnetism

TL;DR

This paper proposes that ultrafast laser-induced reduction of exchange interaction is the main mechanism behind femtomagnetism, explaining rapid demagnetization and spin switching observed experimentally, especially in spin-frustrated systems.

Contribution

It introduces a new theory that links laser-induced exchange interaction reduction to femtomagnetism, providing a potential explanation for experimental phenomena over the past seven years.

Findings

Exchange interaction reduction occurs almost instantaneously after laser pulse.

Delay in spin response is due to finite spin wave propagation.

Spin can be directly switched in spin-frustrated systems.

Abstract

Since the beginning of femtomagnetism, it has been hotly debated how an ultrafast laser pulse can demagnetize a sample and switch its spins within a few hundred femtoseconds, but no consensus has been reached. In this paper, we propose that an ultrafast reduction in the exchange interaction by a femtosecond laser pulse is mainly responsible for demagnetization and spin switching. The key physics is that the dipole selection rule demands two distinctive electron configurations for the ground and excited states and consequently changes the exchange interaction. Although the exchange interaction change is almost instantaneous, its effect on the spin is delayed by the finite spin wave propagation. Consistent with the experimental observation, the delay becomes longer with a stronger exchange interaction pulse. In spin-frustrated systems, the effect of the exchange interaction change is even more dramatic, where the spin can be directly switched from one direction to the other. Therefore, our theory has the potential to explain the essence of major observations in rare-earth transition metal compounds for the last seven years. Our findings are likely to motivate further research in the quest of the origin of femtomagnetism.