Thermal activation magnetization dynamics

Below that temperature this thermally activated motion is frozen out. There is a characteristic change in the solid-state lineshape that clearly demonstrates the effect. Nuclear magnetic resonance can in such cases provide important information about molecular structure and dynamics in solids. 

It has been proposed that in the magnetization dynamics of singledomain particles there is a characteristic crossover temperature T below which the escape of the magnetization from the metastable states is dominated by quantum barrier transitions, rather than by thermal over barrier activation. Above T the escape rate is given by the rate of the thermal transitions, determined by the Boltzman factor, = 1/exp(—t/Z/cT), where U is the barrier separating two metastable states. In a thermally activated regime it should vanish when the temperature approaches zero. 

In addition to the determination of the chemical and/or Knight shift and the spin-lattice relaxation time in the laboratory frame (Ti), there is another important NMR observable - the spin-spin relaxation time (T2). While the chemical and/or Knight shift contains essentially static structural information, the temperature and/or magnetic field dependence of the relaxation times, both Ti and T2, are related to the dynamics of the observed nucleus. Ti measures the rate at which the spin system returns to thermal equilibrium with its environment (the lattice) after a perturbation, while T2 measures the rate of achieving a common spin temperature within the spin system. Both 7i and T2 provide exceptionally important information on motions, and can cover the timescale from 10 to 10 s. Moreover, the temperature dependence of these motions provides important thermodynamic information in the form of activation energies for ligand motion on the catalyst surface. 

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