5 research outputs found
Spatio-temporal coherent control of thermal excitations in solids
X-ray reflectivity (XRR) measurements of femtosecond laser-induced transient
gratings are applied to demonstrate the spatio-temporal coherent control of
thermally induced surface deformations on ultrafast timescales. Using gracing
incidence X-ray diffraction we unambiguously measure the amplitude of transient
surface deformations with sub-\AA{} resolution. Understanding the dynamics of
femtosecond TG excitations in terms of superposition of acoustic and thermal
gratings makes it possible to develop new ways of coherent control in X-ray
diffraction experiments. Being the dominant source of TG signal, the
long-living thermal grating with spatial period can be canceled by a
second, time-delayed TG excitation shifted by . The ultimate speed
limits of such an ultrafast X-ray shutter are inferred from the detailed
analysis of thermal and acoustic dynamics in TG experiments
Accelerating the laser-induced phase transition in nanostructured FeRh via plasmonic absorption
By ultrafast x-ray diffraction we show that the laser-induced
magnetostructural phase transition in FeRh nanoislands proceeds faster and more
complete than in continuous films. We observe an intrinsic 8 ps timescale for
nucleation of ferromagnetic (FM) domains in both types of samples. For the
continuous film, the substrate-near regions, which are not directly exposed to
light, are only slowly transformed to the FM state by domain wall motion
following heat transport. In contrast, numerical modeling of the plasmonic
absorption in the investigated nanostructure reveals a strong contribution near
the FeRh/MgO interface. On average, the absorption is larger and more
homogeneous in the nanoislands, enabling the phase transition throughout the
entire volume at the intrinsic nucleation timescale
Spin stress contribution to the lattice dynamics of FePt
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
Towards shaping picosecond strain pulses via magnetostrictive transducers
Using time-resolved x-ray diffraction, we demonstrate the manipulation of the picosecond strain response of a metallic heterostructure consisting of a dysprosium (Dy) transducer and a niobium (Nb) detection layer by an external magnetic field. We utilize the first-order ferromagnetic–antiferromagnetic phase transition of the Dy layer, which provides an additional large contractive stress upon laser excitation compared to its zero-field response. This enhances the laser-induced contraction of the transducer and changes the shape of the picosecond strain pulses driven in Dy and detected within the buried Nb layer. Based on our experiment with rare-earth metals we discuss required properties for functional transducers, which may allow for novel field-control of the emitted picosecond strain pulses