Spin-spiral state of a Mn monolayer on W(110) studied by soft x-ray absorption spectroscopy at variable temperature

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 scanning tunneling microscopy experiments. X-ray absorption spectra (XAS), x-ray linear dichroism (XLD) and field-induced x-ray magnetic circular dichroism (XMCD) Mn $L_{2,3}$-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 $L_{2,3}$-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.