Magnetism on a Mesoscopic Scale: Molecular Nanomagnets Bridging Quantum and Classical Physics

Abstract

In recent years polynuclear transition metal molecules have been synthesized and proposed for example as magnetic storage units or qubits in quantum computers. They are known as molecular nanomagnets and belong in the class of mesoscopic systems, which are large enough to display many-body effects but small enough to be away from the finite-size scaling regime. It is a challenge for physicists to understand their magnetic properties, and for synthetic chemists to efficiently tailor them by assembling fundamental units. They are complementary to artificially engineered spin systems for surface deposition, as they support a wider variety of complex states in their low energy spectrum. Here a few characteristic examples of molecular nanomagnets showcasing unusual many-body effects are presented. Antiferromagnetic wheels and chains can be described in classical terms for small sizes and large spins to a great extent, even though their wavefunctions do not significantly overlap with semiclassical configurations. Hence, surprisingly, for them the transition from the classical to the quantum regime is blurred. A specific example is the Fe18 wheel, which displays quantum phase interference by allowing Néel vector tunneling in a magnetic field. Finally, the Co5Cl single-molecule magnet is shown to have an unusual anisotropic response to a magnetic field.

Magnetic Cluster Excitations

“…The L&E-band concept is classical in nature, though some subtleties are present, and the meaning of "classical" in this context is in fact not yet completely understood. It rather seems that in magnetic clusters of the size discussed here the line between classical and quantum physics is blurred (Konstantinidis et al, 2011). Fundamental questions hence remain.…”

“…It appears natural that the physical pictures developed there can also be extended to the short antiferromagnetic chains considered here. However, there are indications that this expectation is not fulfilled and the situation in the antiferromagnetic chains much more involved (Konstantinidis et al, 2011). A definitive answer is not available at the moment.…”

“…21). Reviews and follow-up articles on the QTNV scenario in antiferromagnetic wheels are available (Konstantinidis et al, 2011;Leuenberger et al, 2003;Meier and Loss, 2001b).…”

“…Adapted from Konstantinidis et al, 2011;Waldmann et al, , 2009. (Chiolero and Loss, 1998;Meier and Loss, 2001b).…”

“…First, it turned out that the J and D values as they appear in Eq. (73) Konstantinidis et al, 2011). Not doing so leads to e.g.…”