Specular and off-specular scattering from supermirror: Reflection of x-rays from the back side

The interface roughness of supermirrors with m=4 times the critical angle of nickel has been investigated by the reflection of x-rays from the back side of the mirrors. Reflectivity measurements from the front side do not contain useful information about the morphology of the layers because most photons are totally reflected from the top layer, which has typically a thickness of approximately 80 nm. Therefore, it is not straightforward to obtain information about the interface roughness of the layers underneath, which are decisive for the reflectivity of the supermirrors. In contrast, specular and off-specular measurements from the back side provide quantitative information on the buildup of roughness at the interfaces as well as the lateral and vertical correlation lengths of the roughness. As the intensity of laboratory x-ray sources is much higher than the intensity of neutron beams, it is possible to probe up to 8 harmonics of the supermirror sequence corresponding to m=32. We demonstrate that the sheets caused by resonant diffuse scattering off supermirrors with a high reflectivity have a lower intensity and larger lateral correlation lengths than mirrors with a low reflectivity. We show that the reflection of x-rays from the back side of supermirrors is an alternative method for their characterization.