Random Co(II)–Ni(II) Ferromagnetic Chains Showing Coexistent Antiferromagnetism, Metamagnetism, and Single-Chain Magnetism

A series of isomorphous compounds of general formula [Co_1–xNi_x(tzpo)­(N₃)­(H₂O)₂]_n·nH₂O (x = 0, 0.19, 0.38, 0.53, 0.68, 0.84, and 1; tzpo = 4-(5-tetrazolate)­pyridine-N-oxide) was prepared. The compounds consist of homometallic or heterometallic chains with simultaneous azide-tetrazolate bridges. The heterometallic systems feature random distribution of metal ions. All compounds across the series exhibit intrachain ferromagnetic coupling, interchain antiferromagnetic (AF) ordering, field-induced metamagnetic transition, and, except the Ni-only compound, single-chain magnetic dynamics. The AF ordering temperature, the metamagnetic critical field, and the relaxation parameters show different composition dependence. Notably, the blocking temperature for the Co-rich materials is higher than the Co-only compound, suggesting synergy between the randomly distributed Co­(II) and Ni­(II) ions in promoting slow relaxation. The results imply rich physics in the random mixed-metal systems and demonstrate the possibility of improving single-chain relaxation properties by blending metal ions.