Helical and incommensurate spin-density waves in Fe/Cr multilayers with interfacial steps

Although absent in bulk transition metals, a noncollinear, helical (H) spin-density wave (SDW) is stabilized by steps at the interfaces in Fe/Cr multilayers. Using the random-phase approximation, we evaluate the phase boundary between the H SDW and the collinear, incommensurate (I) SDW found in bulk Cr. In agreement with neutron-scattering results, the I-to-H transition temperature T_IH is always lower than the bulk Néel temperature T_N and the nodes of the I SDW lie near the Fe-Cr interfaces. While a H SDW with a single ±π/2 twist has lower free energy than a I SDW above T_N, H SDW's with larger twists are stable between T_IH and T_N.