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Osservazione diretta degli effetti di taglia finita in catene di spin antiferromagnetiche.

Measurements of the spin density distribution of two eight-spin molecular rings using polarized neutron diffraction have been recently published in Nature Communication. This study was conceived by scientists of the Department of Physics and Earth Sciences of Parma in collaboration with an international team of physicists and chemists. The polarized neutron diffraction experiments were performed at the Institutes Laue-Langevin and Léon Brillouin.

Antiferromagnetic (AF) Heisenberg spin chains have received intense theoretical and experimental attention from the point of view of fundamental physics and applications in the field of spintronics and quantum computation. As finite AF chains are expected to show different magnetic behaviour depending on their length and topology, their accurate experimental characterization is of crucial importance for controlling and manipulating their quantum magnetic states. Until now a direct and quantitative measurement of the spatial distribution of the magnetization of such small structures has not been achieved, even with the most advanced microscopic techniques. 

A physical realization of such chains is provided by molecular nanomagnets (MNMs), which are among the first examples of real spin systems of finite size. Their topology and magnetic interactions can be engineered at the synthetic level. In particular, AF homometallic molecular rings like Cr8 are model systems to investigate the magnetic properties of chains, as their periodic boundary conditions allow one to link their properties to infinite AF chains. On the other hand, the introduction of a non-magnetic impurity in the homometallic ring as in Cr8Cd breaks the cyclic symmetry, with an open boundary condition resulting in an effective model system for an open chain.  The different topology of the two ‘open’ and ‘closed’ rings is expected to have an effect on how the magnetic moment is distributed among the different ions.

The scientists of the molecular magnetism group of the Department of Physics and Earth Sciences have theoretically investigated the magnetic behaviour of the Cr8 and Cr8Cd rings.  Their calculations, based on a quantum spin Hamiltonian, predict in Cr8 a  uniform spin-flop configuration and in Cr8Cd a non-uniform and staggered distribution of the spin moments along the chain. From polarized neutron diffraction (PND) measurements it was possible to determine the experimental spin distribution of Cr8 and Cr8Cd.  The complexity of the PND experiment is rewarded by the unique possibility of measuring with great accuracy the distribution and magnitude of the local spin density, which is not yet achievable with other techniques. The analysis of the data presented in this work has given quantitative experimental confirmation of the theoretical predictions for the spin arrangement in finite chains of AF-coupled magnetic ions and has shown how the parity of the chain and the boundary condition have a marked effect on the spin structure. This is a general result that can be extended to any short chain of AF-coupled magnetic ions.

Bibliographical reference: T. Guidi, B. Gillon, S.A. Mason, E. Garlatti, S. Carretta, P. Santini, A. Stunault, R. Caciuffo, J. van Slageren, B. Klemke, A. Cousson, G.A. Timco & R.E.P. Winpenny, Nature Communications6, 7061 (2015).

Pubblicato Venerdì, 15 Maggio, 2015 - 16:04 | ultima modifica Venerdì, 15 Maggio, 2015 - 16:04