  Conventional magnetic domain walls are characterized by the reorientation of
local moments. However, what occurs at the boundary of itinerant magnets is
largely unknown. Here using spin-sensitive scanning tunneling microscopy, we
investigated the microscopic boundaries of spin-density-wave (SDW) state in a
prototypical itinerant anti-ferromagnet of Cr. We find at the boundary of two
incommensurate SDW domains, the spins display finite-scale decay rather than
reorientation. A novel double-Q SDW is generated with a second-order charge
modulation. In commensurate SDW domains, a clear SDW gap is observed. Screw
dislocations induced novel "half" vortex and anti-vortex that are connected by
antiphase domain wall. This domain wall is characterized by vanishing spin
density, where intriguing SDW in-gap states emerge, resembling the Andreev
bound states in superconductors. All these unique SDW boundary structures can
be viewed as consequences of local interference of two SDW, either with
different Q or reversed phases. Therefore, our study revealed a new category of
magnetic domain wall, the "interference wall", with a mechanism rooted in
itinerant nature.
