Ultrafast laser generated strain in granular and continuous FePt thin films

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.

Spin stress contribution to the lattice dynamics of FePt

“…Similarly, a granular FePt film was prepared at a slightly higher substrate temperature of 650°C by adding approximately 30 volume % of carbon to the sputtering target. X-ray reflectivity measured the sample thicknesses to be about d = 9.5 nm, where the continuous film is covered with an additional 1-nm layer of oxidized Al ( 27 ). According to scanning electron microscopy images of similarly prepared samples (see Supplementary Materials), the size distribution of the FePt nanograins segregated in a carbon matrix within the granular film is centered at approximately 8 nm.…”

“…The possibility to grow magnetic, oriented nanograins with a high degree of structural order makes this material an ideal candidate for studying the lattice using time-resolved diffraction techniques. In a recent ultrafast x-ray diffraction (UXRD) study, we have found a short-lived lattice contraction along the short out-of-plane c axis of the tetragonal unit cell of a nanogranular FePt film on a substrate, whereas continuous epitaxial thin films merely expanded under otherwise identical excitation conditions (27). Previously, ultrafast electron diffraction experiments had reported a transient c-axis contraction and in-plane expansion for freestanding FePt nanograins (11).…”

“…The sample structures are schematically depicted as insets (C) and (D) in Fig. 1, and their properties and the measurement technique have been described in a previous publication (27).…”

Reduced Thermal Conductivity in Ultrafast Laser Heated Silicon Measured by Time-Resolved X-ray Diffraction

“…During the past decade, synchrotron based time-resolved X-ray diffraction (TRXD) has become a versatile probe for characterizing various non-equilibrium phenomena, including the transmission of heat across an interface [14], charge carrier propagation across thin-film interfaces [15] and grain boundaries [16], and the production of anisotropic strain in bulk crystal [17] and multiferroic thin film [18][19][20]. Additionally, laser-based table-top X-ray diffraction methods have enabled probing dynamics at ultrafast time-scales, such as unexpected anisotropic strain development from nanograin film on substrate [21] and a nanoscale transport mechanism in metallic multilayer systems [22]. More recently, X-ray free-electron laser has provided means to resolve ultrafast phase transition phenomena [23,24] that were inaccessible at storage-ring based light sources.…”

Finite-size effects in ultrafast remagnetization dynamics of FePt

“…The investigated continuous and granular FePt films are grown in the ordered L1 0 phase onto MgO (100) oriented substrates, which aligns the easy magnetic axis out of plane. A more detailed description of the growth conditions and the structural properties of the samples can be found in Ref [22]. In particular, we mention that the granular film consists of segregated FePt-nanograins embedded in amorphous carbon with a size distribution of FePt particles centered at approximately 10 nm (see Fig.…”