engleski

Quantum tunneling of magnetization in Mn12-acetat

Molecules provide the exciting opportunity to study magnetism on the passage from atomic to macroscopic level. One of the most interesting effects in such mesoscopic systems is the appearance of quantum tunnelling of magnetization (MQT) at low temperatures. In the last decade molecular chemistry has had a large impact in this field by providing new single molecule magnets. They consist of small clusters exhibiting superparamagnetic behaviour, similar to that of conventional nanomagnetic particles. The advantage of these new material is that they form macroscopic samples consisting of regularly arranged small identical high-spin clusters which are widely separated by organic molecules. The lack of distributions in size and shape of the magnetic clusters and the very weak intercluster interaction lead in principle to only one barrier for the spin reversal. We present detailed magnetic investigations on a Mn12-ac single crystal. In this compound the tetragonal ordered cluster consist of a central tetrahedron of four Mn4+ (S=3/2) atoms surrounded by eight Mn3+ (S=2) atoms with antiparallel oriented spins, leading to an overall spin moment of S=10. In the hysteresis loops nine different jumps at regularly spaced fields are identified in the investigated temperature range (1.5K<T<3K). At these fields the relaxation of moment due to thermal activation is superimposed by strong quantum tunnelling. In lowering the temperature the time dependence changes from thermally activated to thermally assisted tunneling.

single molecule magnet; Mn12-acetate; macroscopic quantum tunneling

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