Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction

Abstract

We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe₂/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 TbFe₂ surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, if a SiO₂ glass capping layer covers the excited TbFe₂ 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.

FIG. 1.

Characterization of sample 1 via X-ray diffraction: (a) slice of the reciprocal space map shown in (b) at q_x = 0 (black line). The blue and orange lines in (a) correspond to the probed reciprocal slice when using the convergent beam of the X-ray focusing optic and area detector at the lab-based diffraction setup at a fixed angle of incidence. (c) and (d) depict the temporal evolution of the Nb and TbFe₂ peak at 13.3 mJ/cm², respectively, with the fitted peak position indicated by dashed lines. (e) Schematic depiction of the uncapped sample structure.

FIG. 2.

Transient strain signatures of sample 1 without SiO₂ capping: (a) and (b) display transient strains extracted from the average peak shift via Gaussian lineshape fits and the simulated strain response using the udkm1Dsim toolbox as lines. The dashed line in (a) corresponds to a model with a full single-crystalline TbFe₂ layer whereas the solid line takes a disordered TbFe₂ layer at the TbFe₂/Nb interface into account. Inset (c) depicts the transient strain pulse in the Nb layer normalized to the different excitation fluences.

FIG. 3.

Time-resolved signals from the SiO₂ capped sample structure: (a) spatio-temporal strain simulation result that highlights the occurrence of multiple echoes from bipolar and unipolar strain pulses. Horizontal dashed lines indicate the layer interfaces of the schematic sample geometry displayed in (e). (b) Comparison of the strain signal from UXRD measurements and udkm1Dsim toolbox simulations. (c) Time-resolved all-optical MOKE signal S: [S(H_up) − S(H_down)]. The background subtracted signal shows pronounced peaks when strain pulses traverse the SiO₂/TbFe₂ interface, which are marked by vertical dashed lines. The field-independent polarization change [S(H_up) + S(H_down)] shown in (d) is dominated by oscillations of the time-resolved Brillouin scattering signal of the strain pulses within the SiO₂ capping.

FIG. 4.

Analysis of the strain pulse signatures: (a) comparison of the coherent phonon strain contribution seen in the MOKE and UXRD signal, scaled to the maximum amplitude and shifted to overlap in time. (b) Comparison of the initial asymmetric bipolar strain pulse in the capped sample 2 and the symmetric bipolar strain in the uncapped sample 1, to an exponential fit with a time-constant of 4.56 ps. (c) Evolution of the strain pulse after passing the SiO₂ layer multiple times.