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, [-MnIII-ON-NiII-NO-MnIII-(OPh)2-]n, possessing a structural C2 axis on the NiII ion and an inversion center at the midpoint of MnIII-(OPh)2-MnIII 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/kB{approximately equal to}-19 K) between the MnIII and NiII ions via the -ON- linkage and the weak ferromagnetic interaction (J'/kB = +0.43 to +0.49 K) between the [Mn-ON-Ni-NO-Mn] trimer units via the -(OPh)2- linkage in addition to the strong anisotropy of the trimer unit (D/kB = -2.6 to -3.0 K), all three compounds can be viewed at low temperatures as chains of ferromagnetically coupled Ising-like anisotropic ST = 3 units. The combination of DC and AC susceptibility measurements revealed their single-chain magnet behavior (Tau0{approximately equal to} 1 X 10-10 s, Delta/kB{approximately equal to} 59 K). The observed energy barrier (Delta) is smaller than Delta/kB{approximately equal to} 70 K found in the original series of MnIIbetaNi single-chain magnets possessing D/kB{approximately equal to}-2.5 K and J'/kB{approximately equal to} +0.7 K, and is significantly larger than Delta/kB{approximately equal to} 21 K of the MnIIbetaNi single-molecule magnets possessing D/kB = -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.