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 T-0=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 T-0 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.