Relaxation type magnetic loss

Relaxation is an inherent property of all nuclear spins. There are two predominant types of relaxation processes in NMR of liquids. These relaxation processes are denoted by the longitudinal (Ti) and transverse (T2) relaxation time constants. When a sample is excited from its thermal equihbrium with an RF pulse, its tendency is to relax back to its Boltzmann distribution. The amount of time to re-equilibrate is typically on the order of seconds to minutes. T, and T2 relaxation processes operate simultaneously. The recovery of magnetization to the equilibrium state along the z-axis is longitudinal or the 7 relaxation time. The loss of coherence of the ensemble of excited spins (uniform distribution) in the x-, y-plane following the completion of a pulse is transverse or T2... 

A second type of relaxation mechanism, the spin-spm relaxation, will cause a decay of the phase coherence of the spin motion introduced by the coherent excitation of tire spins by the MW radiation. The mechanism involves slight perturbations of the Lannor frequency by stochastically fluctuating magnetic dipoles, for example those arising from nearby magnetic nuclei. Due to the randomization of spin directions and the concomitant loss of phase coherence, the spin system approaches a state of maximum entropy. The spin-spin relaxation disturbing the phase coherence is characterized by T. ... 

Since its first description in 1971 [35], gel-phase NMR was applied to peptide chemistry by Manatt and coworkers [36, 37], These authors used 13C NMR to determine the extent of chloromethylation of crosslinked polymers and 19F NMR to monitor protection-deprotection reactions. These two nuclei are the most commonly used in these types of studies, mainly because of their significant chemical shift dispersion, which can alleviate in part the resolution loss due to the non ideal linewidth obtained in the gel state. Apart from restricted molecular motion, that shortens T2 because of an efficient transverse relaxation, other sources of line-broadening derive from magnetic susceptibility variations within the sample (due to the physical heterogeneity of the system) and residual dipolar couplings. 

Spin-spin relaxation is a process in which there is no net loss or gain of energy, but the spins lose phase coherence. The basis of this type of relaxation is the transfer of energy from one nucleus to another via the fluctuation magnetic fields. As one spin is excited to a higher state, another is relaxed back to the lower state. The net result of these transitions is that the phases of particular types of spins spread out or dephase. As the spins continue to dephase, they will eventually cancel one another resulting in the loss of the signal. 

Jamal et al. determined the eomplex dielectric permittivity and magnetic permeability of NR-Ni nanocomposites in the X band of mierowave frequencies from 7 to 12 GHz. The real part of dielectrie permittivity does not change with the frequency but shows a steady increase with the inerease in loading of Ni nanoparticles. The dieleetrie loss shows an inerease with the frequency in general and for NR based eomposites there is a shallow relaxation peak around 8.25 GHz. In both type of eomposites, the dieleetrie loss increases... 

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