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 $T_{\mathrm{IH}}$ is always lower than the bulk Néel temperature $T_{\mathrm{N}}$ and the nodes of the I SDW lie near the Fe-Cr interfaces. While a H SDW with a single $\pm \pi /2$ twist has lower free energy than a I SDW above $T_{\mathrm{N}}$, H SDW's with larger twists are stable between $T_{\mathrm{IH}}$ and $T_{\mathrm{N}}$.