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.

许多磁性材料中出现的殷钢行为长期以来一直引起材料科学的兴趣。在这里，我们不仅展示了 FePt 连续薄膜的因瓦行为，甚至还展示了平衡加热时 FePt 纳米颗粒的负热膨胀。然而，在飞秒 X 射线衍射实验中，这两个样品在激光加热时均表现出明显的瞬态膨胀。我们表明，粒状微观结构对于支持通过泊松效应产生的面内膨胀产生的面外收缩应力至关重要，该应力增加了由自旋无序驱动的单轴收缩应力。我们通过用第一个激光脉冲使磁激励饱和，然后检测对第二个脉冲的纯扩展响应来证明自旋贡献。收缩自旋应力在与我们观察到的铁磁有序恢复相同的 100 ps 时间尺度上重新建立。FePt 纳米系统机械响应的有限元建模证实了动力学的形态依赖性。