  Coupling and hybridization of different elementary excitations leads to new
functionalities. In phononics and spintronics, magnetoelastic coupling between
Rayleigh-type surface acoustic wave (SAW) and spin wave (SW) has recently
attracted much attention. Quantitatively evaluating and comparing the coupled
system are essential to develop the study of the magnetoelastic SAW-SW
coupling. So far, previous studies of SAW-SW coupling have employed a quantity
called coupling strength. However, it is still challenging to compare the
coupling strength values among studies fairly because the quantity depends on
the device geometry and the applied magnetic field angle, which are not unified
among the previous studies. Here, we focus on a practical constant composed of
a magnetoelastic constant and a strain amplitude that depends only on the
material properties. We demonstrate a versatile evaluation technique to
evaluate the practical constant by combining electrical measurements and
optical imaging. An essential part of the technique is an analysis that can be
used under off-resonance conditions where SAW and SW resonance frequencies do
not match. Existing analysis can only handle the case under on-resonance
conditions. Our analysis makes it possible to observe the magnetoelastic
couplings between SAW with resonance frequencies that can be imaged optically
and SW with resonance frequencies in the gigahertz range. Our demonstrated
technique, which uses electrical and optical measurements under off-resonance
conditions, can significantly advance research on SAW-SW coupled systems.
