Magnetic Phase Separation in the Oxypnictide Sr₂Cr_1.85Mn_1.15As₂O₂

Layered Sr₂M₃As₂O₂-type oxypnictides are composed of tetrahedral M₂Pn₂ and square planar MO₂ layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr₂Cr_3–xMn_xAs₂O₂ solid solution, Sr₂Cr₂MnAs₂O₂ has been synthesized. The compound crystallizes in the I4/mmm tetragonal space group with a refined stoichiometry of Sr₂Cr_1.85Mn_1.15As₂O₂. The M(2) site within the M₂Pn₂ 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 MO₂ 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 Sr₂M₃As₂O₂ phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds.