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.

CEITEC BUT Brno University of Technology Purkyňova 123 612 00 Brno Czechia

FemtoNanoOptics Group Institut Lumière Matière Université de Lyon CNRS Université Lyon 1 69622 Villeurbanne France

Helmholtz Zentrum Berlin für Materialien und Energie GmbH Wilhelm Conrad 12 Röntgen Campus BESSY 2 Albert Einstein Str 15 12489 Berlin Germany

Institut für Ionenstrahlphysik und Materialforschung Helmholtz Zentrum Dresden Rossendorf Bautzner Landstrasse 400 01328 Dresden Germany

Institut für Physik Technische Universität Chemnitz Reichenhainer Str 70 09126 Chemnitz Germany

Institute of Physical Engineering Brno University of Technology Technická 2 616 69 Brno Czechia

Institute of Physics and Astronomy University of Potsdam Karl Liebknecht Str 24 25 14476 Potsdam Germany

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