  We report the direct measurement of antiferromagnetic spin polarization at
the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray
magnetic scattering. This supports recent theoretical models suggesting that
the oxygen spin polarization is key to the magnetoelectric coupling mechanism.
The spin polarization is observed through a resonantly enhanced diffraction
signal at the oxygen K edge at the commensurate antiferromagnetic wavevector.
Using the fdmnes code we have accurately reproduced the experimental data. We
have established that the resonance arises through the spin polarization on the
oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we
have discovered that the position of the Mn3+ ion directly influences the
oxygen spin polarization.
