  A DC non-contact method for measuring the magnetostrictive strain in
thin-films is demonstrated, achieving a state-of-the-art sensitivity of 0.1
ppm. In this method, an optical profilometer is used to measure the curvature
induced in a magnetostrictively coated coverslip under a DC field through
phase-sensitive interferometry. From this the magnetostrictive stress and
strain are calculated using Stoney's formula. This addresses limitations of
conventional techniques that measure magnetostriction based on the deflection
of a cantilever under an AC field, which require complex dedicated set-ups and
are sensitive to vibrational noise. Further, it reveals information about the
anisotropy of the film and allows for the possibility of measuring multiple
samples simultaneously. The theoretical sensitivity limits are derived,
predicting a shot-noise-limit of 0.01 ppm. The method is implemented to measure
the magnetostrictive hysteresis and piezomagnetic coupling of thin-film
galfenol. Degradation in film performance is observed above a thickness of 206
nm, alongside a change in coercivity. This prompts investigation into the
growth and optimization of galfenol films for use in devices.
