Spin-lattice relaxation paramagnetic contribution

In paramagnetic materials, the relaxation frequency is in general determined by contributions from both spin-lattice relaxation and spin-spin relaxation. Spin-lattice relaxation processes can conveniently be studied in samples with low concentrations of paramagnetic ions because this results in slow spin-spin relaxation. Spin-spin relaxation processes can be investigated at low temperatures where the spin-lattice relaxation is negligible. Paramagnetic relaxation processes have... 

The paramagnetic contribution to the spin—lattice relaxation (for example, the C-2 proton of a histidyl residue of Hb) from the electronic spin of the heme iron is given by... 

From the comparison of the measured and calculated temperature dependences of the relaxation time (see Fig. 20), it follows that the inelastic phonon scattering is the most essential mechanism of the spin-lattice relaxation for Ge. It is evident that only at low temperatures T < 30K) some other mechanisms (the most probable one is the relaxation due to a small amount of paramagnetic impurities) become dominant. At T > 300/C some additional mechanism of relaxation may also exist. The interaction of the nuclear quadrupole moment with vibrations of the nearest four Ge atoms brings about the main contribution to the spin-lattice relaxation rate. The effective modulation of the EFG by the nearest bond charges is greatly reduced because of strong correlations between their displacements. As the main result of the present investigation of spin-lattice relaxation,... 

The frequency dependence of NMR spin-lattice relaxation is also a powerful tool in the study of spin dynamics in tlie conducting polymers, to reveal the microscopic dynamics of the charge carrier, polaron and/or conduction electron spins. However, one has to pay attention to one important point the localized paramagnetic spins and molecular motion which substantially contribute to the NMR 77]]. Since the relaxation caused by the spins diffusing is suppressed by a factor of D, the contribution due to the localized spins and the molecular motions could become dominant [6]. Therefore, interpretation of the NMR relaxation rate is generally useful but difficult. 

Roose et al. (1996) studied the magnetic-field dependence of the proton spin-lattice relaxation time Tj (referred to as nuclear-magnetic-relaxation dispersion) in aqueous colloidal silica containing paramagnetic Mn + ions (Figure 1.112). The experimental relaxation rate of solvent protons in aqueous colloidal silica suspensions containing Mn + ions can be expressed as a weighted mean of several contributions ... 

Another form of spin-lattice relaxation occurs when there is a quadrupolar nucleus vide infra) present in the paramagnetic species. Associated with this nucleus is an electric quadrupolar field, which will interact with gradients in the electric component of the microwave radiation. However, as this electric quadrupolar interaction is generally weak, it does not contribute much towards the relaxation. 

Bi NQR spin-lattice relaxation as a function of Thas been studied for Bi4Ge30i2 single crystals doped with minor amounts of paramagnetic atoms of Cr, Nd or Gd. A dramatic increase (up to eight-fold) in effective spin-spin relaxation time T 2 had been found previously at RT on doping. Unlike T 2, the effective spin-lattice relaxation time T i at RT differed insignificantly for doped and pure samples. At lower T, however, different Ti behaviour was observed, caused by different contributions to the... 

Using the Solomon-Bloembergen equations, together with the expression for paramagnetic contributions to nuclear spin-lattice and spin-spin relaxation rates (i ipara and / 2para) in a paramagnetic system are expressed by... 

---