  In itinerant magnetic systems, a spin density wave (SDW) state can be induced
by Fermi surface nesting and electron-electron interaction. It may intertwine
with other orders such as charge density wave (CDW), while their relation is
still yet to be understood. Here via spin-polarized scanning tunneling
microscopy, we directly observed long-range spin modulation on Cr(001) surface,
which corresponds to the well-known incommensurate SDW of bulk Cr. It displays
6.0 nm in-plane period and anti-phase behavior between adjacent (001) planes.
Meanwhile, we simultaneously observed the coexisting CDW with half the period
of SDW. Such SDW/CDW have highly correlated domain structures and are in-phase.
Surprisingly, the CDW displays a contrast inversion around a density-of-states
dip at -22 meV, indicating an anomalous CDW gap opened below EF. These
observations support that the CDW is a secondary order driven by SDW. Our work
is not only a real-space characterization of incommensurate SDW, but also
provides insights on how SDW and CDW coexist.
