**Molecular Nanomagnets and Nanoscopic Systems**

**Ferric Wheels**

Quantum coherent tunnelling of the magnetic moment in molecular magnets was a focal point of the experimental and theoretical search for macroscopic quantum phenomena. The "ferric wheel" systems Fe

*N*, where organometallic crystals were synthesised with*N*= 6, 8, 10, 12 and 18 magnetic Fe(III) ions in ring geometry, were a particularly good place to look: these molecules have antiferromagnetic coupling between spins*s*= 5/2 on each iron site, show a ground state with vanishing total spin,*S = 0*, at zero field, and because of an effective uniaxial magnetic anisotropy admit the possibility of at least mesoscopic quantum phenomena -- in the form of coherent tunnelling of the Néel vector. At the time of our studies, there was something of a gulf between theoretical predictions of coherence and the physical measurements possible in the organic chemistry laboratories synthesising the samples. Thus our analyses of the static [1] and dynamic [2] properties of 6-, 8- and 10-membered ferric wheels were performed very explicitly to emphasise experimental characterisation of physical properties and measurements displaying the unique signature of quantum coherent tunnelling phenomena.**Manganese clusters**

This gulf was closed by the diligent efforts of some very talented experimental physicists working in the "interdisciplinary" grey zone between synthetic chemistry and theoretical physics. Accurate inelastic neutron scattering measurements were performed to locate all magnetic excitations up to 40 meV in Mn12-acetate, perhaps the best characterised of all nanomagnets. We formed a collaboration involving chemists, experimentalists and numericists to extract [3,4] a comprehensive and consistent set of interaction parameters

*(J1,J2,J3,J4)*for the Mn12-acetate molecule. Our results showed a remarkably classical type of ground state in this weakly frustrated system of "quantum" (*s = 3/2,2*) spins, and the sceptic could certainly take issue with the notion of a system with total spin*S = 10*being described as "macroscopic" ...**Nanoscopic Rings**

A "spin system" is one in which the electrons are fully localised and charge plays no role. We also included the charge degrees of freedom, and their interplay with spin, for one-dimensional models motivated by certain organometallic materials. We used the nontrivial topology of the Aharonov-Bohm ring, exploiting the threaded flux to probe quantum interference effects and thus exploring the realms of nanoscopic physics and nanodevices. Under these circumstances we

- observed electron fractionalisation [5], or nanoscopic spin-charge separation, and
- detected topological phase transitions [6], exemplified by the insulator-insulator transition in the ionic Hubbard model, which remain well-defined even in a finite (indeed, very small) system.

*Magnetization in Molecular Iron Rings*

B. Normand, X. Wang, X. Zotos and D. Loss,

Phys. Rev. B**63**, 184409 (2001).*Spin Dynamics and Coherent Tunnelling in the Molecular Magnetic Rings Fe6 and Fe8*

A. Honecker, F. Meier, D. Loss and B. Normand,

Eur. Phys. J. B**27**, 487 (2002).*Exchange Interactions and High-Energy Spin States in Mn12-acetate*

G. Chaboussant, A. Sieber, S. Ochsenbein, H.-U. Güdel, M. Murrie, A. Honecker, N. Fukushima and B. Normand,

Phys. Rev. B**70**, 104422 (2004).*Exchange Constants and Spin Dynamics in Mn12-acetate*

A. Honecker, N. Fukushima, B. Normand, G. Chaboussant and H.-U. Güdel,

J. Mag. Mag. Mater.**290**, 966 (2005).*Spin-Charge Separation in Aharonov-Bohm Rings of Interacting Electrons*

K. Hallberg, A. A. Aligia, A. P. Kampf and B. Normand,

Phys. Rev. Lett.**93**, 067203 (2004).*Detection of Topological Transitions by Transport through Molecules and Nanodevices*

A. A. Aligia, K. Hallberg, B. Normand and A. P. Kampf,

Phys. Rev. Lett.**93**, 076801 (2004).