  We investigate the magnetoelastic properties of
$\mathrm{Co_{25}}\mathrm{Fe_{75}}$ and $\mathrm{Co_{10}}\mathrm{Fe_{90}}$ thin
films by measuring the mechanical properties of a doubly clamped string
resonator covered with multi-layer stacks containing these films. For the
magnetostrictive constants we find
$\lambda_{\mathrm{Co_{25}}\mathrm{Fe_{75}}}=(-20.68\pm0.25)\times10^{-6}$ and
$\lambda_{\mathrm{Co_{10}}\mathrm{Fe_{90}}}=(-9.80\pm0.12)\times10^{-6}$ at
room temperature. In stark contrast to the positive magnetostriction previously
found in bulk CoFe crystals. $\mathrm{Co_{25}}\mathrm{Fe_{75}}$ thin films
unite low damping and sizable magnetostriction and are thus a prime candidate
for micromechanical magnonic applications, such as sensors and hybrid
phonon-magnon systems.
