Journal of Magnetic Resonance
Volume 179, Issue 2 , April 2006, Pages 250-262
Field-dependent nuclear relaxation of spins 1/2 induced by dipole–dipole couplings to quadrupole spins: LaF3 crystals as an example
Danuta Kruka, Oliver Lips
Institut für Festkörperphysik, TU Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany
Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland
A general theory of spin–lattice nuclear relaxation of spins I = 1/2 caused by dipole–dipole couplings to quadrupole spins S greater-or-equal, slanted 1, characterized by a non-zero averaged (static) quadrupole coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through dipole–dipole couplings with certain transitions of quadrupole spins. The averaged quadrupole coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual dipole–dipole coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the ‘classical’ Solomon relaxation theory.
Keywords: Nuclear relaxation; Quadrupole spins; Dipolar spins; Solid state; Ion dynamics