Magnetic relaxation direct processes

As already observed for some isotropic polynuclear clusters [30 - 32], slow relaxation of the magnetization in an external magnetic field can occur because of the inefficient transfer of energy to the environment, for example, the helium bath, and consequent reabsorption of the emitted phonon by the spin system. The phenomenon, also known as phonon bottleneck (PB), was first introduced by Van Vleck [33]. It is characteristic of low temperatures, where relaxation is dominated by the direct process between closely spaced levels, and results from the low density of phonons with such a long wavelength to match the small energy separation... 

Fig. 4. Experimental results for the spin-lattice relaxation rate of Nd ions in lanthanum magnesium nitrate (LaMN), magnetic field normal to c-axis. The data are fitted to a combination of an Orbach process, 6.3 x lO expf—47.6/T) and a direct process, 0.3cotanh(h( /2fcr). (After Ruby et al. 1962.)...

For diamagnetic peptides in isotropic solvents, the primary mechanism of nuclear magnetic relaxation of protonated nuclei and of N nuclei at natural abundance is the dipolar interaction with the directly bound protons. At high magnetic fields, chemical shift anisotropy (CSA) also contributes to the relaxation of the heteronuclei. The rates of these relaxation processes are governed by both the internal motions and the overall rotational motion of the molecule. Consequently, characterization of and N... 

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