Guided acoustic waves in thin epitaxial films: experiment and inverse problem solution for NiTi

Despite the fundamental and technological importance of the elastic constants, a suitable method
for their full characterization in epitaxial films is missing. Here we show that transient grating
spectroscopy (TGS) with highly 𝑘−vector-selective generation and detection of acoustic waves
is capable of determination of all independent elastic coefficients of an epitaxial thin film
grown on a single-crystalline substrate. This experimental setup enables detection of various
types of guided acoustic waves and evaluation of the directional dependence of their speeds
of propagation. For the studied model system, which is a 3 μm thin epitaxial film of the NiTi
shape memory alloy on an MgO substrate, the TGS angular maps include Rayleigh-type surface
acoustic waves as well as Sezawa-type and Love-type modes, delivering rich information on
the elastic response of the film under different straining modes. The resulting inverse problem,
which means the calculation of the elastic constants from the TGS maps, is subsequently solved
using the Ritz-Rayleigh numerical method. Using this approach, tetragonal elastic constants of
the NiTi film and their changes with the austenite→martensite phase transition are analyzed.