Transition-metal dimers and physical limits on magnetic anisotropy

Recent advances in nanoscience have raised interest in the minimum bit size required for classical information storage, i.e. for bistability with suppressed quantum tunnelling and energy barriers that exceed ambient temperatures. In the case of magnetic information storage much attention has centred on molecular magnets[1] with bits consisting of ~ 100 atoms, magnetic uniaxial anisotropy energy barriers ~ 50 K, and very slow relaxation at low temperatures. In this article we draw attention to the remarkable magnetic properties of some transition metal dimers which have energy barriers approaching ~ 500 K with only two atoms. The spin dynamics of these ultra small nanomagnets is strongly affected by a Berry phase which arises from quasi-degeneracies at the electronic Highest Occupied Molecular Orbital (HOMO) energy. In a giant spin-approximation, this Berry phase makes the effective reversal barrier thicker. [1] Gatteschi, D., Sessoli, R. & Villain, J. Molecular Nanomagnets. (Oxford, New York 2006).