Strong coupling between magnetic and structural order parameters in SrFe₂As₂

X-ray and muon spin-relaxation experiments performed on SrFe₂As₂ polycrystals confirm a sharp first-order transition at T₀=205 K corresponding to an orthorhombic phase distortion and to a commensurate antiferromagnetic Fe ordering with a larger distortion and larger size of the ordered moment than reported for BaFe₂As₂. The structural and the magnetic order parameters present a remarkable similarity in their temperature dependence from T₀ down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scales with the lattice distortion when going from SrFe₂As₂ to BaFe₂As₂. Full-potential band-structure calculations confirm that the columnar magnetic order with propagation vector (1,0,1) and the orthorhombic lattice distortion are intrinsically tied to each other.