Abstract
We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Grüneisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime.
Highlights
Magnetic stresses in laser-excited materials are interesting because they potentially allow one to manipulate the ultrafast, nanoscopic strain response by changing macroscopic parameters such as temperature, laser fluence, or applied external fields
It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale
The total stress depends on the population of magnetic excitations, which is intrinsically limited by the temperature-dependent, finite integral of the magnetic heat capacity
Summary
Magnetic stresses in laser-excited materials are interesting because they potentially allow one to manipulate the ultrafast, nanoscopic strain response by changing macroscopic parameters such as temperature, laser fluence, or applied external fields. This decrease is continuous for the higher fluence and mimics the M2ðTÞ dependence We explain both observations by a Gru€neisen model, which relates the ultrafast expansion to the subsystem-specific heat capacities and Gru€neisen constants that are anisotropic for the spin system and isotropic for the combined electron-phonon system. These Gru€neisen constants are extracted from the available thermal expansion of bulk SRO by subtracting the Poisson contribution that arises from the in-plane expansion.
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