  Non-collinear spin states with unique rotational sense, such as chiral
spin-spirals, are recently heavily investigated because of advantages for
future applications in spintronics and information technology and as potential
hosts for Majorana Fermions when coupled to a superconductor. Tuning the
properties of such spin states, e.g., the rotational period and sense, is a
highly desirable yet difficult task. Here, we experimentally demonstrate the
bottom-up assembly of a spin-spiral derived from a chain of Fe atoms on a Pt
substrate using the magnetic tip of a scanning tunneling microscope as a tool.
We show that the spin-spiral is induced by the interplay of the Heisenberg and
Dzyaloshinskii-Moriya components of the Ruderman-Kittel-Kasuya-Yosida
interaction between the Fe atoms. The relative strengths and signs of these two
components can be adjusted by the interatomic Fe distance, which enables
tailoring of the rotational period and sense of the spin-spiral.
