Quantum tunnelling and coherence in magnetic molecule and ions

Tuesday, 26. Februay, 13:00 h, H 0105

The search for quantum tunnelling in complex magnetic systems, started with domain walls, continued with nano-particles and really succeeded with Single Molecule Magnets. A SMM is a complex object containing several magnetic ions coupled by robust super-exchange interactions and having an energy barrier against spin reversal. The so-called Mn12-ac system is the archetype of SMMs. Despite a huge Hilbert space, resonant tunnelling between its ground-state spin components S= 10 and -10 could be observed, leading to large quantum steps in the classical magnetization-field hysteretic loop. This quantum/classical coexistence can be controlled by temperature (populating energy states closer to the top of the barrier) or applied transverse field Ht (increasing the tunnel splitting D ˜exp-aS, where a represents the ratio of longitudinal/transverse terms of the Hamiltonian), allowing an exploration of nanomagnetism between the quantum and classical limits (where D is large or null, respectively). The discovery of quantum tunnelling in Mn12-ac, later extended to other SMMs, led to a multidisciplinary research on incoherent tunnelling of large magnetic objects. Coherent spin dynamics, observed only recently at the solid-state (e.g. rare-earth ions in crystalline matrices) will now be extended to SMMs for the study of decoherence in complex systems and also for the future implementations of 2D networks of SMM qubits.

Bernard Barbara
Institut Neel, CNRS, Grenoble, France