Shear instability in fcc Fe and thin Fe-Cu[001] films

Using total energy local-spin-density calculations, it is shown that fcc /spl gamma/-Fe is unstable with respect to a monoclinic shear deformation producing a nearly bcc structure. The instability persists in ultrathin iron films on Cu[001] substrate for film thicknesses above a monolayer limit. A strong correlation between the shear instability and the magnetic states is reported.