Spin stress contribution to the lattice dynamics of FePt
A. von Reppert, L. Willig, Jan‐Etienne Pudell, Steffen Peer Zeuschner, Gabriel Sellge, Fabian Ganss, Olav Hellwig, Jon Ander Arregi, Vojtěch Uhlíř, Aurélien Crut, Matias Bargheer
Abstract
Invar-behavior occurring in many magnetic materials has long been of interest to materials science. Here, we show not only invar behavior of a continuous film of FePt but also even negative thermal expansion of FePt nanograins upon equilibrium heating. Yet, both samples exhibit pronounced transient expansion upon laser heating in femtosecond x-ray diffraction experiments. We show that the granular microstructure is essential to support the contractive out-of-plane stresses originating from in-plane expansion via the Poisson effect that add to the uniaxial contractive stress driven by spin disorder. We prove the spin contribution by saturating the magnetic excitations with a first laser pulse and then detecting the purely expansive response to a second pulse. The contractive spin stress is reestablished on the same 100-ps time scale that we observe for the recovery of the ferromagnetic order. Finite-element modeling of the mechanical response of FePt nanosystems confirms the morphology dependence of the dynamics.