Quasi-static strain governing ultrafast spin dynamics

TL;DR

This paper demonstrates that quasi-static strain, generated by lattice thermal expansion after ultrafast photo-excitation, significantly influences ultrafast spin dynamics, challenging previous thermal effect assumptions and providing a new analysis approach.

Contribution

It quantitatively proves the existence of QSS and decouples its effects from thermal effects, revealing QSS's dominant role in ultrafast spin dynamics.

Findings

QSS exists and can be quantitatively measured.

QSS plays a governing role in ultrafast spin dynamics.

Decoupling thermal and strain effects improves understanding of photo-induced phenomena.

Abstract

The quasi-static strain (QSS) is the product generated by the lattice thermal expansion after ultrafast photo-excitation and the effects of thermal and QSS are inextricable. Nevertheless, the two phenomena with the same relaxation timescale should be treated separately because of their different fundamental actions to the ultrafast spin dynamics. By employing ultrafast Sagnac interferometry and magneto-optical Kerr effect, we quantitatively prove the existence of QSS, which has been disregarded, and decouple two effects counter-acting each other. Through the magnetoelastic energy analysis, rather we show that QSS in ferromagnets plays a governing role on ultrafast spin dynamics, which is opposite to what have been known on the basis of thermal effect. Our demonstration provides an essential way of analysis on ultrafast photo-induced phenomena.