Slow relaxation of the magnetization observed in an antiferromagnetically ordered phase for SCM-based two-dimensional layered compounds

Abstract

Two-dimensional layered compounds with different counteranions, [{Mn(salen)}₄C6](BF₄)₂·2(CH₃OH) (1) and [{Mn(salen)}₄C6](PF₆)₂·2(CH₃OH) (2) (salen^2- = N,N'-bis(salicylideneiminato), C6^2- = C₆H₁₂(COO)₂^2-), were synthesized by assembling [Mn(salen)(H₂O)]X (X^- = BF₄^- and PF₆^-) and C₆H₁₂(CO₂^-)₂ (C6^2-) in a methanol/2-propanol medium. The compounds have similar structures, which are composed of Mn(salen) out-of-plane dimers bridged by μ⁴-type C6^2- ions, forming a brick-wall-type network of [-{Mn₂}-OCO-] chains alternately connected via C₆H₁₂ linkers of C6^2- moieties. The counteranions for 1 and 2, i.e., BF₄^- and PF₆^-, respectively, are located between layers. Since the size of BF₄^- is smaller than that of PF₆^-, intra-layer inter-chain and inter-plane nearest-neighbor MnMn distances are shorter in 1 than in 2. The zigzag chain moiety of [-{Mn₂}-OCO-] leads to a canted S = 2 spin arrangement with ferromagnetic coupling in the Mn^III out-of-plane dimer moiety and antiferromagnetic coupling through -OCO- bridges. Due to strong uniaxial anisotropy of the Mn^III ion, the [-{Mn₂}-OCO-] chains could behave as a single-chain magnet (SCM), which exhibits slow relaxation of magnetization at low temperatures. Nevertheless, these compounds fall into an antiferromagnetic ground state at higher temperatures of T_N = 4.6 and 3.8 K for 1 and 2, respectively, than active temperatures for SCM behavior. The spin flip field at 1.8 K is 2.7 and 1.8 kOe for 1 and 2, respectively, which is attributed to the inter-chain interactions tuned by the size of the counteranions. The relaxation times of magnetization become longer at the boundary between the antiferromagnetic phase and the paramagnetic phase.