  We demonstrate the in-line holography for soft x-ray vortex beam having an
orbital angular momentum. A hologram is recorded as an interference between a
Bragg diffraction wave from a fork grating and a divergence wave generated by a
Fresnel zone plate. The obtained images exhibit fork-shaped interference
fringes, which confirms the formation of the vortex beam. By analyzing the
interference image, we successfully obtained the spiral phase distribution. The
results demonstrate that the in-line holography technique is promising for the
characterization of topological magnets, such as magnetic skyrmions.
