Quantum phase transitions in crystals of molecular nanomagnets

Abstract

Quantum phase transitions (QPT) have been extensively studied in recent years. Here, I describe results of experiments showing that a crystal of Fe8 single molecule magnets is an experimental realization of one of the canonical QPT models: the quantum Ising model in a transverse field. Quantum annealing, induced by the application of a transverse magnetic field along the medium magnetic anisotropy axis of Fe8, has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents we obtain agree with expectations for the mean-field universality class. By contrast, when the magnetic field is applied along the hard magnetic axis (x), we find that the ac magnetic susceptibility and the superparamagnetic blocking temperature of Fe8 oscillate as a function of the field amplitude Hx. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S = 10 and excited multiplets and therefore enables us to quantify such mixing.