Antiferromagnetic ordering in the absence of structural distortion in Ba (Fe_1-x Mn_x )₂ As₂

Neutron and x-ray diffraction studies of Ba (Fe_1-x Mn _x)2 As₂ for low doping concentrations (x≤0.176) reveal that at a critical concentration, 0.102<x<0.118, the tetragonal-to- orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of (1 2 1 2 1) persists. Among all of the iron arsenides this observation is unique to Mn doping, and unexpected because all models for "stripelike" antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of Ba (Fe _1-x T M_x) 2 As₂ compounds (TM=transitionmetal), and models for magnetic ordering in the iron arsenide compounds.