  The magnetic structure of a Cr monolayer on a W(110) substrate is
investigated by means of first-principles calculations based on the
noncollinear spin density functional theory (DFT). As magnetic ground state we
find a long-period homochiral left-rotating spin spiral on-top of an
atomic-scale anti-ferromagnetic order of nearest neighbor atoms. The rotation
angle of the magnetic moment changes inhomogeneously from atom to atom across
the spiral. We predict a propagation direction along the crystallographic [001]
direction with a period length of 14.3 nm, which is in excellent agreement with
a modulation of the local anti-ferromagnetic contrast observed in
spin-polarized scanning tunneling microscope experiments by Santos et al. [New
J. Phys. 10, 013005 (2008)]. We identify the Dzyaloshinskii-Moriya interaction
(DMI) as origin of the homochiral magnetic structure, competing with the
Heisenberg-type exchange interaction and magneto-crystalline anisotropy energy.
From DFT calculations we extract parameters for a micromagnetic model and
thereby determine a considerable inhomogeneity of the spin spiral, increasing
the period length by 6% compared to homogeneous spin spirals. The results are
compared to the behavior of a Mn and Fe monolayer and Fe doublelayer on a
W(110) substrate.
