Pressure-Induced Magnetization in FeO: Evidence from Elasticity and Mössbauer Spectroscopy

The complete elastic tensor of Fe0.94O (w\"ustite) has been determined to 10 GPa using acoustic interferometry at GHz frequencies inside a diamond-anvil cell. The soft mode (C₄₄) elastic constant of FeO is reduced by 20% over the experimental pressure range. An unusual discontinuity in the pressure derivatives of C₁₁ and C₁₂ at 4.7±0.2 GPa corresponds to the pressure at which the onset of a magnetic ordering transition is observed by high-pressure M\"ossbauer spectroscopy and neutron powder diffraction. Our new results combined with literature structural high P-T data suggest that there is a magnetic, although still cubic, phase of FeO between ~5 and ~17 GPa.