Itinerant ferromagnetism in the As $4p$ conduction band of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ identified by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba$_{0.6}$K$_{0.4}$Mn$_{2}$As$_{2}$ show that the ferromagnetism (FM) below $T_{\rm{C}}\approx100$~K arises in the As $4p$ conduction band. No XMCD signal is observed at the Mn x-ray absorption edges, however, a clear XMCD signal is found below $T_{\rm{C}}$ at the As $K$ edge which increases with decreasing temperature. The XMCD signal is absent with the beam directed parallel to the crystalline \emph{\textbf{c}} axis, indicating that the ordered orbital moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant FM is associated with the As $4p$ conduction band and that distinct local-moment antiferromagnetism and itinerant FM coexist at low temperature.\\ Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences, Division of Materials Sciences \& Engineering, under Contract No. DE-AC02-07CH11358. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.