Magnetic Instability of Ultrathin fccFexNi1−xFilms

The “invar effect” in ${\mathrm{Fe}}_x{\mathrm{Ni}}_{1-x}$ alloys occurs when the Fe content approaches 65%. At this point, the magnetization falls to zero, and a martensitic structural transformation from a fcc to a bcc lattice occurs. This paper addresses the question: “What happens if the structural transformation is suppressed in an ultrathin alloy film?” We present results to this effect, showing the variation of the magnetization with changing composition in ultrathin films grown on Cu(100). We find a new low-spin, ferromagnetic phase of matter, which is a sensitive function of the atomic volume.