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We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe 2 /Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background-free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses if the excited TbFe 2 surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, if a SiO 2 glass capping layer covers the excited TbFe 2 layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.

We report GHz longitudinal as well as shear acoustic phonons photoexcitation and photodetection using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence a non-thermal pathway for the laser excitation of GHz phonons. We also provide experimental insights to the optical and mechanical parameters evolving across the LS/HS spin crossover temperature range. PACS numbers:Understanding how ultrafast photoinduced molecular switching in crystals couples to the lattice in optical materials is one of the key challenges in the fields of ultrafast photo-induced phase transitions or transformations and ultrafast acoustics. Systems like spin-crossover compounds exhibit, with temperature or pressure, change of the molecular spin state in the central d 4 -d 7 transition metals of the complex. They are promising candidates for diverse applications including miniature temperature sensors, displays, data storage and photonic devices [1,2]. Moreover, they can be fabricated in a vari