Double Helix of Icosahedra Structure and Spin Glass Magnetism of the δ-Co2.5Zn17.5–xMnx (x = 0.4–3.5) Pseudo-Binary Alloys

We have synthesized δ-Co_2.5Zn_17.5–xMn_x (x = 0.4–3.5) pseudo-binary alloys of 10 different compositions by a high-temperature solid-state synthetic route, determined their crystal structures and the Mn substitution pattern, and estimated the existence range of the δ-phase. The alloys crystallize in two chiral enantiomorphic space groups P6₂ and P6₄, where the basic atomic polyhedron of the chiral structure is an icosahedron and the neighboring icosahedra share vertices to form an infinitely long double helix along the hexagonal axis (like in the δ-Co_2.5Zn_17.5 parent binary phase). The alloys are pure δ-phase up to the Mn content x ≈ 3.5. The Mn atoms partially substitute Zn atoms at particular crystallographic sites located on the icosahedra. The study of magnetism was performed on the Co_2.5Zn_17.1Mn_0.4 alloy with the lowest Mn content. Contrary to the expectation that structural chirality may induce the formation of a nontrivial magnetic state, a spin glass state with no relation to the structural chirality was found. The magnetic sublattice contains all of the necessary ingredients (randomness and frustration) for the formation of a spin glass state. Typical out-of-equilibrium dynamic phenomena of a spin system with broken ergodicity were detected below the spin freezing temperature T_f ≈ 8 K.

In a search for structural chirality-induced nontrivial magnetic states, we have synthesized the δ-Co_2.5Zn_17.5−xMn_x (x = 0.4−3.5) pseudo-binary chiral alloys and determined their crystal structures, Mn substitution pattern, and magnetic properties. A spin glass state with typical broken ergodicity phenomena was found, being in no relation to the structural chirality.