Scopul nostru este sprijinirea şi promovarea cercetării ştiinţifice şi facilitarea comunicării între cercetătorii români din întreaga lume.
Autori: Raluca TIRON
This thesis deals with quantum tunnelling of magnetic moments in molecular magnets and involves both numerical and especially experimental studies. The molecular magnets are formed of large magnetic molecules arranged within a crystal. All the molecules in the crystal have the same total spin, anisotropy and controlled orientation, giving a well defined mesoscopic magnet. Though the quantum physics of a single molecule is well understood, inter-molecular exchange interactions have been neglected so far. The main purpose of this work is to understand the role of inter-molecular exchange coupling in quantum tunnelling of the magnetisation in molecular magnets. From the experimental point of view, the work involved the use of various pieces of apparatus such as SQUID, micro-SQUID and micro-Hall magnetometers and dilution refrigerators. We show that intermolecular exchange interactions are not always negligible and can be used to couple SMMs. The system used is a dimerized SMM [Mn4]2 with the spins of the two Mn4 molecules coupled antiferromagentically. As each molecule acts as a bias on its neighbour, the quantum tunnelling resonances are shifted with respect to the isolated SMM. We study for the first time, quantum tunnel transitions via entangled states of the dimer and establish that the dimer really behaves as a coupled quantum system. We show that there is a distribution of the exchange coupling parameter and that the exchange coupling is nearly isotropic. We also study a three-dimensional network of exchange coupled SMMs. The intermolecular interactions are strong enough to cause a clear field bias, but too weak to transform the spin network into a classical antiferromagnetic material.
Cuvinte cheie: Magnetisation quantum tunnelling, molecular magnets, (Mn4)2 dimer, entangled states, exchange bias, many-body quantum phenomena, cross spin relaxations, nano-magnets, micro-SQUID.