  We have observed that CoO films grown on different substrates show dramatic
differences in their magnetic properties. Using polarization dependent x-ray
absorption spectroscopy at the Co L$_{2,3}$ edges, we revealed that the
magnitude and orientation of the magnetic moments strongly depend on the strain
in the films induced by the substrate. We presented a quantitative model to
explain how strain together with the spin-orbit interaction determine the 3d
orbital occupation, the magnetic anisotropy, as well as the spin and orbital
contributions to the magnetic moments. Control over the sign and direction of
the strain may therefore open new opportunities for applications in the field
of exchange bias in multilayered magnetic films.
