Abstract
Optical excitation of spin-ordered rare earth metals triggers a complex response of the crystal lattice since expansive stresses from electron and phonon excitations compete with a contractive stress induced by spin disorder. Using ultrafast x-ray diffraction experiments, we study the layer specific strain response of a dysprosium film within a metallic heterostructure upon femtosecond laser-excitation. The elastic and diffusive transport of energy to an adjacent, non-excited detection layer clearly separates the contributions of strain pulses and thermal excitations in the time domain. We find that energy transfer processes to magnetic excitations significantly modify the observed conventional bipolar strain wave into a unipolar pulse. By modeling the spin system as a saturable energy reservoir that generates substantial contractive stress on ultrafast timescales, we can reproduce the observed strain response and estimate the time- and space dependent magnetic stress. The saturation of the magnetic stress contribution yields a non-monotonous total stress within the nanolayer, which leads to unconventional picosecond strain pulses.
Highlights
Heavy rare earth metals are an interesting class of materials for lattice dynamics since their magnetic heat capacity Cmag and the associated entropy of magnetic excitations dSmag 1⁄4 DQmag=T are comparable to the phonon contribution over a large temperature region.[31,55,58]
The presented picosecond strain dynamics in a laser-excited heterostructure containing a rare-earth transducer shows strong magnetic contributions to the lattice response. Both the picosecond strain pulse and the thermal transport are affected by energy transfer processes to magnetic excitations
The transient strain observed in a buried detection layer directly shows the saturation of the contractive magnetic stress component, which occurs when an increasing fraction of the Dy layer is excited across its magnetic phase transition
Summary
Experiments that probe the strain response of the atomic lattice that results from the light-matter interaction of a femtosecond optical pulse with an opto-acoustic transducer material can be subsumed as picosecond ultrasonics.[1,2] They yield fundamental insights into physical processes within the laser-excited thin film, such as electronphonon coupling,[3,4,5] hot electron propagation,[6,7] and electron–hole pair generation.[8,9] This is possible because the lattice strain is the deterministic, elastic response to a physical stress that itself contains the time- and length-scales of the energy transfer processes within the transducer region. A second contractive contribution is needed that rises with an %15 ps time constant These timescales match the sub-picosecond electron-spin coupling and the subsequent phonon-spin coupling that were reported by previous demagnetization experiments in heavy rare earth elements.[37–39] For high excitation densities, we observe an additional increase in the spin-stress on a longer timescale.
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