Theoretical investigation of dynamic properties of magnetic molecule systems as probed by NMR and pulsed fields experiments

In this dissertation we theoretically investigate static and especially dynamic properties of magnetic molecules (MM\u27s), as probed by the nuclear spin lattice relaxation rate 1/T1 (first part) and pulsed fields measurements of the magnetization M( t) (second part). In the first part, we provide a general first-principles account for 1/T1, which incorporates the decay of spin fluctuations and the corresponding broadening of the discrete magnetic energy levels of MM\u27s. This is achieved by including the interaction of the electronic moments with the local deformation of the host lattice (phonons), in the Markovian regime and employing the quantum regression theorem. Within this framework, we provide a rigorous interpretation of a number of 1/ T1 experimental findings in MM\u27s. We also provide an extensive account of the model spin-1/2 tetramer V12 by analyzing magnetic susceptibility and 1/T1 data. The second part focuses on phenomena manifested in pulsed fields measurements of M(t), such as hysteresis loops and Landau-Zener-Stuckelberg (LZS) steps. First, we give a theoretical analysis of the low-T hysteresis loops and LZS steps at B ≈ 0 observed in the magnetic molecule V6. The loops are successfully reproduced by employing a generalization of the standard Bloch equation which in turn reveals the one-phonon acoustic processes as the dominant source of relaxation in this system. The origin of the US steps is attributed to the presence in V 6 of a weak intra-molecular anisotropic exchange. The small deviation from the quantum-mechanical prediction of exact magnetization reversals at B ≈ 0 is attributed to the role of the phonon heat bath (dissipative US problem). Second, we provide a general, first-principles account of all dynamic phenomena manifested in pulsed fields experiments, by extending the standard spin-lattice relaxation theory to include time-dependent (pulsed) fields. This theory accounts for: (i) hysteresis effects (including the generalized Bloch equation used for V6), (ii) the effects associated with the dissipative US problem, in the adiabatic regime and in particular, (iii) the so-called magnetic Foehn effect. We also discuss how the phonon bottleneck effect (typically occurring at T ≲ 1 K) can give rise to an enhanced Foehn effect