Ageing effects on the magnetic properties of Mn12-based Acetate and Stearate SMMs

A study of ageing effects on the magnetic properties of Single-Molecule-Magnets (SMMs) of the Mn12 based Acetate, ([Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H2O (1) and Stearate, [Mn12O12(CH3(CH2)16COO)11(CH3COO)5(H2O)4] (2) complexes has been carried out. Detailed magnetization (M) measurements as a function of temperature (T ∼ 1.8-10 K), magnetic field (H ∼ 0 to ±40 kOe) and time (t) have been performed on relatively fresh samples (1A and 2A) and samples aged for ∼4 weeks (1B and 2B) of both Mn12-Acetate and Mn12-Stearate. The blocking temperatures (TB) extracted from the measured M(T) lie between ∼3.0 and 3.4 K for all the four samples. In all cases, below TB, the M-H loops exhibit hysteresis with periodic steps. Interestingly, the ageing process leads to significant changes in the magnetic response of both the complexes. With ageing the Mn12-Acetate exhibits a large increase in the magnetization drop near zero-field, but the estimated anisotropy energy barrier (U) remains unchanged ∼71 K. Whereas, in the case of Mn12-Stearate ageing results in a change of U from ∼52 K (2A) to ∼35 K (2B). The results are discussed in terms of possible ageing induced changes in the structural and chemical environment of the SMMs.