Abstract
The effects of structural and magnetic aging in Mn12-stearate single-molecule magnets (SMMs) deposited on the surface of 300 nm spherical silica nanoparticles were investigated. It was pointed out that the molecules attached on the surface preserve its magnetic characteristics (hysteresis in M(H) loops and slow magnetic relaxation). However, the observed magnetic properties evolve over time revealing significant changes. DC susceptibility studies together with Raman spectroscopy were applied to investigate this aging effect. Measurements over 221 days period showed change of magnetic parameters: exponential decrease of coercive field from 4.5 kOe to 150 Oe (with τc = 33 days) and remnant magnetization from 0.072 emu/g to 0.0022 emu/g (with τrem = 46 days). Time-dependent magnetization measurements have indicated the presence of slow magnetic relaxations (Ueff = 33.6 K) for the fresh sample in temperature range 2.0–3.0 K. However, the aged sample (221 days after synthesis) did not show typical relaxations, but a magnetic viscosity behavior was observed. The Raman measurements revealed partial decomposition of stearate groups and possible formation of hydroxyl system over time, which explain the observed magnetic changes.
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