  X-ray beams with orbital angular momentum (OAM) are a promising tool for
x-ray characterization techniques. Beams with OAM have a helicity--an
azimuthally varying phase--which leads to a gradient of the light field. New
material properties can be probed by utilizing the helicity of an OAM beam.
Here, we demonstrate a novel dichroic effect in resonant diffraction from an
artificial antiferromagnet with a topological defect. We found that the
scattered OAM beam has circular dichroism at the antiferromagnetic Bragg peak
whose sign is coupled to its helicity, which reveals the real-space
configuration of the antiferromagnetic ground state. Thermal cycling of the
artificial antiferromagnet can change the ground state, as indicated by
reversal of the sign of circular dichroism. This result is one of the first
demonstrations of a soft x-ray spectroscopy characterization technique
utilizing the OAM of x-rays. This helicity-dependent circular dichroism
exemplifies the potential to utilize OAM beams to probe matter in a way that is
inaccessible using currently available x-ray techniques.
