Broken symmetries in the crystalline and magnetic structures of<i>γ</i>-iron

Abstract

It is by now well established that in antiferromagnetic \ensuremath{\gamma}-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 \ensuremath{\gamma}-Fe, stabilized by different types of constraints. We show that in near face-centered-cubic \ensuremath{\gamma}-Fe, stabilized by three-dimensional constraints, the magnetic ground state is a spin-spiral with propagation vector $\stackrel{\ensuremath{\rightarrow}}{q}=2\ensuremath{\pi}/a\ifmmode\times\else\texttimes\fi{}(0.2,0,1)$ at an equilibrium atomic volume of $\ensuremath{\Omega}=10.63{\mathrm{\AA{}}}^{3},$ very close to the propagation vector ${q}_{\mathrm{exp}}=2\ensuremath{\pi}/a\ifmmode\times\else\texttimes\fi{}(0.1,0,1),$ determined experimentally, but at considerably lower volume than the atomic volume of the \ensuremath{\gamma}-Fe precipitates in Cu on which the experiments were performed $(\ensuremath{\Omega}=11.44{\mathrm{\AA{}}}^{3}).$ At these larger volumes our calculations predict an helical spin solution at $\stackrel{\ensuremath{\rightarrow}}{q}=2\ensuremath{\pi}/a\ifmmode\times\else\texttimes\fi{}(0,0,0.6)$ to be the ground state. Epitaxially stabilized \ensuremath{\gamma}-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.