Theoretical Study On The Magnetic Properties Of Single - Stranded Magnets

In2001, Gatteschi and co-workers reported the first one-dimensional magnetic chain,[Co(hfac)2(NITPhOMe)], in which the ion Co and the organic radical were arranged alternately. The effective energy barrier of this new magnetic chain is up to107cm-1. Then, a lot of chemists participated in the research of single-chain magnets (SCMs), and many kinds of SCMs were reported. But, the energy barriers of these SCMs are all less than107cm-1. Therefore, synthesizing SCMs with higher energy barrier has becoming the common goal for experimental and theoretical chemists.Until now, the expressions of magnetic anisotropy and correlation energy barriers of SCMs in the regime between the Ising limit and the Heisenberg limit (｜D/J｜≈4/3) were still not clear. Thus, we would investigate the origin of the relaxation barriers of this kind of SCMs. We selected a series of SCMs:[MnⅢ(5-TMAMsalen)M(CN)6](5-TMAMsalen2-=N,N’-ethylenebis(5-trimethylammoniomethylsalicylideneiminate)(M=FeⅢ, MnⅢ, CrⅢ),[MnⅢ(3,5-Cl2saltmen) NiⅡ(pao)2(phen)](3,5-Cl2saltmen2-=N,N’-(1,1,2,2-tetramethylethylene)bis(3,5-dichlorosalicylideneiminate); pao-=pyrid-ine-2-aldoximate; phen=1,10-phenanthroline) and [Fem(bpca)(CN)3MnⅢ(5-Me-salt-men)](bpac=bis(2-pyridylcarbonyl)amidate;5-Me-saltmen2-=N,N’-(1,1,2,2-tetra-methylethylene)bis(5-methylsalicylideneiminate)). Density functional theory (DFT) and complete active space configurations of second order perturbation theory (CASPT2) were used to calculate the zero-field splitting parameters, D, the exchange coupling constants, J, and so on. Calculations show that the obtained J and D are all close to the experimental values. For Mn-Fe or Mn-Mn’, it’s possible for Mn and M to flip its spin first considering the larger magnetic anisotropy energy barrier of Mn (3AA-Mn>4｜J｜SmMnSM) and the smaller one of M (M=Fe, Mn’)(3△A-m<｜J｜SmMnSM). Thus, the magnetic anisotropy energy barrier of Mn-Fe or Mn-Mn’is close to the average value of Mn and M ((△A-Mn+△A-M)/2), and the correlation barrier is equal to the average value of the obtained correlation barriers in two different spin-flip situations (△ξ=3｜J｜SmMnSM). For Mn-Ni or Mn-Cr, the spins of Mn and M (M=Ni, Cr) will flip together under the strong intrachain antiferromagnetic interactions (3△A-M<3△A-Mn<4｜J｜SmMnSM). Therefore, the magnetic anisotropy barrier of Mn-Ni or Mn-Cr mainly comes from Mn (△a=△A-Mn), and the correlation barrier relates to the angle of the flipped spin on M (△ξ=2｜J｜SmMnSM(cosθ-cos180)).