• von Reppert, A.; Willig, L.; Pudell, J.-E.; Rössle, M.; Leitenberger, W.; Herzog, M.; Ganss, F.; Hellwig, O.; Bargheer, M.: Ultrafast laser generated strain in granular and continuous FePt thin films. Applied Physics Letters 113 (2018), p. 123101/1-5

10.1063/1.5050234
Open Access Version

Abstract:
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.