Helical spin-density waves in Fe/Cr trilayers with perfect interfaces

Despite the presence of only collinear, commensurate (C) and incommensurate (I) spin-density waves (SDWs) in bulk Cr, the interfacial steps in Fe/Cr m ultilayers are now believed to stabilize a helical (H) SDW within the Cr sp acer. Yet HSDWs were first predicted in an Fe/Cr trilayer with perfect inte rfaces when the orientation of the Fe moments does not favor C ordering: if the number of Cr monolayers is even (odd) and the Fe moments are pointing in the same (opposite) direction, then a CSDW does not gain any coupling en ergy. Under these circumstances, a simple model verifies that H ordering is indeed favored over I ordering provided that the Fermi surface mismatch is sufficiently small or the temperature sufficiently high. (C) 1999 American Institute of Physics. [S0021-8979(99)67508-9].