  The noncollinear magnetic state of epitaxial Mn monolayers on tungsten (110)
crystal surfaces is investigated by means of soft x-ray absorption
spectroscopy, to complement earlier spin-polarized STM experiments. X-ray
absorption spectra (XAS), x-ray linear dichroism (XLD) and x-ray magnetic
circular dichroism (XMCD) Mn L23-edge spectra were measured in the temperature
range from 8 to 300 K and compared to results of fully-relativistic ab initio
calculations. We show that antiferromagnetic (AFM) helical and cycloidal
spirals give rise to significantly different Mn L23-edge XLD signals, enabling
thus to distinguish between them. It follows from our results that the magnetic
ground state of a Mn monolayer on W(110) is an AFM cycloidal spin spiral. Based
on temperature-dependent XAS, XLD and field-induced XMCD spectra we deduce that
magnetic properties of the Mn monolayer on W(110) vary with temperature, but
this variation lacks a clear indication of a phase transition in the
investigated temperature range up to 300 K - even though a crossover exists
around 170 K in the temperature dependence of XAS branching ratios and in XLD
profiles.
