Experiments on single crystals composed of nanomolecules with magnetic properties are being used to study quantum tunnelling of magnetization and to probe the boundary between quantum and classical mechanics.
Can quantum mechanics,
which determines the behaviour of light and matter at the atomic scale and below,
manifest itself on a macroscopic scale? This question,
which was first posed in the early days of quantum theory in the 1920s,
has fascinated physicists for more than 70 years.
In recent years,
a new type of material - molecules with magnetic moments that exhibit quantum tunnelling in spite of their relatively large size - has allowed physicists to answered this question.
The focus of this article is on one example of such a material,
the spin cluster referred to as "Mn12-ac". This manganese acetate was the first system to exhibit what is referred to as "thermally assisted resonant tunnelling." Mn12-ac was actually discovered in 1980,
but it took more than ten years of close collaboration between chemists and physicists to fully reveal its remarkable properties.
Bernard Barbara from the Laboratoire de Magnétisme in Grenoble,
France,
and Leon Gunther from Tufts University in the US discuss these unusual molecules in the March issue of Physics World.
Further reading
J R Friedman et al. 1996 Macroscopic measurement of resonant magnetization tunnelling in high-spin molecules Phys. Rev. Lett. 76 3830 L Gunther 1990 Quantum tunnelling of magnetisation Physics World December pp2834 L Gunther and B Barbara (ed) 1995 Proceedings of first NATO workshop on Quantum Tunnelling of Magnetization: QTM 94 (Kluwer, Dordrecht) J Hernandez et al. 1996 Field tuning of thermally activated magnetic quantum tunnelling in Mn12-Ac molecules Europhys. Lett. 35 301 T Lis 1980 Preparation, structure, and the magnetic properties of a dodecanuclear mixed valence manganese carboxylite Acta Crystallog. B36 2042 N V Prokof'ev and P C E Stamp 1998 Low-temperature quantum relaxation in a system of magnetic nanomolecules Phys. Rev. Lett. 80 5794 S Sangregorio et al. 1997 Quantum tunnelling of the magnetization in an iron cluster nanomagnet Phys. Rev. Lett. 78 4645 R Sessoli et al. 1993 Magnetic bistability in a metal-ion cluster Nature 365 141 P C E Stamp, E M Chudnovsky and B Barbara 1992 Quantum tunneling of magnetization in solids Int. J. Mod. Phys. B6 1355 L Thomas et al. 1996 Macroscopic quantum tunnelling of magnetization in a single crystal of nanomagnets Nature 383 145 W Wernsdorfer et al. 1997 Macroscopic quantum tunnelling of magnetization of single ferromagnetic nanoparticles of barium ferrite Phys. Rev. Lett. 79 4014
