Broken symmetries in the crystalline and magnetic structures ofγ-iron

It is by now well established that in antiferromagnetic γ-Fe, stabilized in the form of precipitates in a Cu matrix or by epitaxial growth on an appropriate substrate, magnetic and/or crystalline symmetries are broken. Little is known, however, on the physical effects driving the symmetry reduction, and on the interplay of crystalline and magnetic symmetry breaking. We have used a recently developed unconstrained vector-field description of noncollinear magnetism, implemented in an ab initio spin-density-functional code, to search for the magnetic and crystalline structure of γ-Fe, stabilized by different types of constraints. We show that in near face-centered-cubic γ-Fe, stabilized by three-dimensional constraints, the magnetic ground state is a spin-spiral with propagation vector $\overrightarrow{q}=2\mathrm{}\pi /a\times \mathrm{}(0.2,0,1)\mathrm{}$ at an equilibrium atomic volume of $\Omega =10.63\mathrm{}{\mathrm{\AA }}^3,$ very close to the propagation vector $q_{\exp }=2\mathrm{}\pi /a\times \mathrm{}(0.1,0,1),$ determined experimentally, but at considerably lower volume than the atomic volume of the γ-Fe precipitates in Cu on which the experiments were performed $(\Omega =11.44\mathrm{}{\mathrm{\AA }}^3).\mathrm{}$ At these larger volumes our calculations predict an helical spin solution at $\overrightarrow{q}=2\mathrm{}\pi /a\times \mathrm{}(0,0,0.6)\mathrm{}$ to be the ground state. Epitaxially stabilized γ-Fe is found to be unstable against both tetragonal distortion as well as monoclinic shear deformation, and the structural distortions suppress the formation of spin-spiral states, in agreement with experimental observations on Fe/Cu(100) films.