Magnetic Phase Separation in the Oxypnictide Sr 2 Cr 1.85 Mn 1.15 As 2 O 2 .

Layered Sr 2 M 3 As 2 O 2 -type oxypnictides are composed of tetrahedral M 2 Pn 2 and square planar MO 2 layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr 2 Cr 3- x Mn x As 2 O 2 solid solution, Sr 2 Cr 2 MnAs 2 O 2 has been synthesized. The compound crystallizes in the I 4/ mmm tetragonal space group with a refined stoichiometry of Sr 2 Cr 1.85 Mn 1.15 As 2 O 2 . The M(2) site within the M 2 Pn 2 slab is occupied by 42.7% Cr and 57.3% Mn, and the magnetic moments order antiferromagnetically below T N (M2) = 540 K with a C-type antiferromagnetic structure. The M(1) site within the MO 2 layers is fully occupied by Cr, and antiferromagnetic order is observed below T N (M1) = 200 K. Along c , there are two possible interplanar arrangements: ferromagnetic with the (1/2, 1/2, 0) propagation vector and antiferromagnetic with the (1/2, 1/2, 1/2) propagation vector. Magnetic phase separation arises so that both propagation vectors are observed below 200 K. Such magnetic phase separation has not been previously observed in Sr 2 M 3 As 2 O 2 phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds.