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 Mn-12 based Acetate, ([Mn12O12(CH3COO)(16)(H2O)(4)]center dot 2CH(3)COOH center dot 4H(2)O(1) and Stearate, [Mn12O12(CH3(CH2)(16)COO)(11)(CH3COO)(5)(H2O)(4)] (2) complexes has been carried out. Detailed magnetization (M) measurements as a function of temperature (T similar to 1.8-10 K), magnetic field (H similar to 0 to +/- 40 kOe) and time (t) have been performed on relatively fresh samples (1A and 2A) and samples aged for similar to 4 weeks (1B and 2B) of both Mn-12-Acetate and Mn-12-Stearate. The blocking temperatures (T-B) extracted from the measured M(T) lie between similar to 3.0 and 3.4 K for all the four samples. In all cases, below T-B, 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 Mn-12-Acetate exhibits a large increase in the magnetization drop near zero-field, but the estimated anisotropy energy barrier (U) remains unchanged similar to 71 K. Whereas, in the case of Mn-12-Stearate ageing results in a change of U from similar to 52 K (2A) to similar to 35 K (2B). The results are discussed in terms of possible ageing induced changes in the structural and chemical environment of the SMMs. (C) 2017 Elsevier B.V. All rights reserved.