Ultrafast laser generated strain in granular and continuous FePt thin films

• Published in: Applied physics letters Vol. 113; no. 12
• Main Authors: von Reppert, A., Willig, L., Pudell, J.-E., Rössle, M., Leitenberger, W., Herzog, M., Ganss, F., Hellwig, O., Bargheer, M.
• Format: Journal Article
• Language: English
• Published: 17.09.2018
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/1.5050234

We employ ultrafast X-ray diffraction to compare the lattice dynamics of laser-excited continuous and granular FePt films on MgO (100) substrates. Contrary to recent results on free-standing granular films, we observe in both cases a pronounced and long-lasting out-of-plane expansion. We attribute this discrepancy to the in-plane expansion, which is suppressed by symmetry in continuous films. Granular films on substrates are less constrained and already show a reduced out-of-plane contraction. Via the Poisson effect, out-of-plane contractions drive in-plane expansion and vice versa. Consistently, the granular film exhibits a short-lived out-of-plane contraction driven by ultrafast demagnetization which is followed by a reduced and delayed expansion. From the acoustic reflections of the observed strain waves at the film-substrate interface, we extract a 13% reduction of the elastic constants in thin 10 nm FePt films compared to bulk-like samples.