Based on a simple model with antiferromagnetic interactions at the Cr - Fe interfaces, we construct the magnetic phase diagram for the Cr spacer within Fe/Cr multilayers and wedges. The spin-density wave is found to undergo transitions between incommensurate (I) phases with a decreasing number of nodes n as the temperature increases for a fixed number N of Cr monolayers. In the limit , the incommensurate to commensurate (C) transition temperature from n = 1 to n = 0 nodes is independent of the Cr - Fe coupling strength but sensitively depends on the strain and doping. These results agree with recent neutron-scattering studies that the SDW inside Fe/Cr multilayers may be either C or I depending on the temperature and Cr thickness.