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

Abstract

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 (saltmen²⁻ = 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^III₂Ni 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.