Field-dependent magnetic domain structure in antiferromagnetically coupled multilayers by polarized neutron scattering

We study the magnetic structure of antiferromagnetically (AF) coupled $\mathrm{Co}∕\mathrm{Cu}$ multilayers (MLs) with 10 or 40 bilayers under the influence of an external field by polarized neutron scattering in specular and off-specular geometry. We observe in the spin-flip channels off-specular intensities around the $\frac{1}{2}$-order Bragg position. Based on simulations of the measured data within the distorted-wave Born approximation we find vertically correlated domains, and their domain size evolves for a sufficiently large number of bilayers along the ML stack. Small domains most likely at the top and large domains presumably at the bottom coexist within a single ML. The small domains gradually get aligned with the applied field direction around $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, whereas the bigger domains remain AF coupled up to $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, which is well above the apparent saturation field measured by conventional magnetometry methods. Moreover, we observe a double-peak structure at the $\frac{1}{2}$-order position for the MLs with ten as well as 40 bilayers.