Direct evidence of exchange interaction dependence of magnetization relaxation in a family of ferromagnetic-type single-chain magnets

Three Mn₂Ni chains having the same repeating motif, [-Mn ^III-ON-Ni^II-NO-Mn^III-(O_Ph) ₂-]_n, possessing a structural C₂ axis on the Ni^II ion and an inversion center at the midpoint of Mn ^III-(O_Ph)₂-Mn^III have been synthesized: [Mn₂(saltmen)₂Ni(L)₂(py) ₂](A)₂ (saltmen^2- = N,N′-(1,1,2,2- tetramethylethylene)bis(salicylideneiminate); L = 1-methylimidazole-2-aldoximate (miao^-), A = ClO₄^-, 1; PF₆ ^-, 2; L = 1-ethylimidazole-2-aldoximate (eiao^-), A = ClO₄^-, 3). Due to the strong antiferromagnetic interaction (J/k_B ≈ -19 K) between the Mn^III and Ni^II ions via the -ON- linkage and the weak ferromagnetic interaction (J′/k_B = +0.43 to +0.49 K) between the [Mn-ON-Ni-NO-Mn] trimer units via the -(O_Ph)₂- linkage in addition to the strong anisotropy of the trimer unit (D/k_B = -2.6 to -3.0 K), all three compounds can be viewed at low temperatures as chains of ferromagnetically coupled Ising-like anisotropic S_T = 3 units. The combination of DC and AC susceptibility measurements revealed their single-chain magnet behavior (τ₀ ≈ 1 × 10^-10 s, Δ/k_B ≈ 59 K). The observed energy barrier (Δ) is smaller than Δ/k _B ≈ 70 K found in the original series of Mn^III ₂Ni single-chain magnets possessing D/k_B ≈ -2.5 K and J′/k_B ≈ +0.7 K, and is significantly larger than Δ/k_B ≈ 21 K of the Mn^III₂Ni single-molecule magnets possessing D/k_B = -2.4 K reported previously. The present report is the first systematic investigation of the linear exchange coupling (J′) dependence of the magnetization relaxation in a family of compounds including related Mn₂Ni single-chain magnets and single-molecule magnets.