Spin-density waves and reorientation effects in thin epitaxial Cr films covered with ferromagnetic and paramagnetic layers

We report about synchrotron and neutron-scattering studies investigating incommensurate spin-density waves (I-SDW's) in epitaxially grown thin Cr(001) films, including surface and interface effects. These studies show that thin ferromagnetic cap layers of Fe, Ni, and Co with a thickness of only 2--3 nm have a strong effect on the propagation and orientation of the I-SDW's in Cr. For thick Cr films there exist essentially only transverse I-SDW's propagating parallel to the film plane with the spins oriented normal to the plane and at right angles to the in-plane magnetization of the ferromagnetic cap layers. With decreasing Cr thickness a different transverse I-SDW grows at the expense of the in-plane ones, now propagating normal to the plane and with spins parallel or antiparallel to the film magnetization. At a Cr thickness of about 250 {}, the transverse out-of-plane I-SDW completely dominates the phase diagram of Cr. All other domains are suppressed and a spin-flip transition does not occur above 10 K in strong contrast to bulk. For in-plane propagation of the I-SDW we find a coexisting commensurate spin-density wave (C-SDW) which vanishes during the reorientation to out-of-plane propagation with Cr thickness. Finally, for Cr thicknesses well below the period of the I-SDW, the Cr can only order as a C-SDW. The behavior of the SDW's in thin Cr films with ferromagnetic cap layers can be understood in terms of competing interactions at the rough interfaces inducing frustration and by finite-size and strain effects. We have also investigated the effect of Cu and Pd cap layers on the SDW. The Cu cover is similar to a Cr/vacuum interface, whereas the effect of the Pd cover is intermediate between the ferromagnetic layers and Cu.