Physics – Condensed Matter – Mesoscale and Nanoscale Physics
Scientific paper
2000-05-16
Frontiers in Magnetism, J. Phys. Soc. Jpn., vol. 69 (2000), Suppl. A., pp. 383-394
Physics
Condensed Matter
Mesoscale and Nanoscale Physics
12 pages, 13 figures, to appear in The J. of Phys. Soc. of Japan, proc. of The Workshop on Frontiers in Magnetism, October 4-7
Scientific paper
The phenomenon of Quantum Tunneling of Mesoscopic Spins is reviewed in the light of the behavior of the archetype of these systems: the molecular complex Mn12-ac. Most observations can be understood in the framework of the reduced Hilbert space dimension 2S+1=21. Due to the large spin S=10, the energy barrier preventing spin rotation is large, and as a consequence, quantum relaxation is very slow. The application of a magnetic field of a few Tesla below 1 K allows to observe tunneling (i) between the states m=-10 and m=10-n with n=8 to 11 if the field is longitudinal, or (ii) between the two ground-states m~-10 and m~10 if the field is transverse. The crossover temperature between ground-state and thermally assisted tunneling in a longitudinal field extrapolates in zero field at ~1.7 K. The observation of square root relaxation at short-times/low-temperatures and of exponential relaxation at long-times/high-temperatures, as observed previously above 1.5 K, confirms the important role of the spin bath dynamics which is out of equilibrium in the first regime and at equilibrium in the second one. In a second part of this paper a new molecule, so-called V15, with resultant spin S=1/2 is investigated. Contrary to high spin molecules, the energy barrier of low spin molecules is small or null, and the splitting between the symmetrical and anti-symmetrical states is sufficiently large to allow spin-phonon transitions during spin rotation. In low spin molecules the coupling to the environment is quite different from the one found in large spin molecules in low fields.
Barbara Bernard
Caneschi Andrea
Chiorescu Irinel
Giraud Romain
Jansen A. G. M.
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