Short Spin-spin Relaxation Times Research Articles

Two-frequency effects in nuclear quadrupole resonance

A number of new effects which occur in the two-frequency excitation of a multilevel spin-system in nuclear quadrupole resonance are described. Three programs for observing the effect of trapping, used to detect some transitions in a multilevel system in terms of others, for an accurate determination of the asymmetry parameter of the gradient of the electric field, to identify lines of the multiplet spectrum of nuclear quadrupole resonance, and to investigate the phenomenon of cross-relaxation, are proposed. Quantitative estimates are given of the value of the trapping when two neighboring transitions are excited. The phenomenon of slow beats between signals of a two-frequency echo and the frequency of the transition ±3/2→±5/2 for η = 0 when an external magnetic field is applied is explained. The reason for the short spin-spin relaxation times in the secondary echoes is established. The experimental method and a block diagram of a two-frequency nuclear quadrupole resonance system are described.

Chlorine Nuclear Relaxation in ParamagneticK2IrCl6

A pure nuclear-quadrupole-resonance study of the temperature dependence of the chlorine nuclear relaxation times in the concentrated paramagnet ${\mathrm{K}}_{2}$Ir${\mathrm{Cl}}_{6}$ is reported. It is shown that three different relaxation mechanisms contribute to the observed spin-lattice relaxation times. For temperatures less than 40\ifmmode^\circ\else\textdegree\fi{}K, ${T}_{1}$ is dominated by the hyperfine interaction between the chlorine nuclear spins and the electron spins of the ${\mathrm{Ir}}^{4+}$ ions. At 4.2\ifmmode^\circ\else\textdegree\fi{}K, the isotopic ratio of ${T}_{1}$ values is measured to be 1.47\ifmmode\pm\else\textpm\fi{}0.03, in good agreement with the theoretical ratio 1.442 for a magnetic relaxation mechanism. For temperatures greater than 40\ifmmode^\circ\else\textdegree\fi{}K, two quadrupole relaxation mechanisms which result from torsional oscillation and hindered rotational motions of the ${[\mathrm{I}\mathrm{r}{\mathrm{Cl}}_{6}]}^{2\ensuremath{-}}$ complexes about their symmetry axes become important, the hindered-rotation mechanism dominating the relaxation at the highest temperatures studied. The echo amplitudes were observed to decay exponentially with time; they provided temperature-independent and rather short spin-spin relaxation times. The explanation is that the resonance line is substantially narrowed by the presence of the strong electron-exchange interaction and has a width determined by the hyperfine interaction. It is shown that the low-temperature ${T}_{1}$ value depends only on the hyperfine parameter $B$, whereas the ${T}_{2}$ value depends only on hyperfine parameter $A$, a behavior which results from the large asymmetry of the transferred hyperfine interaction in ${[\mathrm{I}\mathrm{r}{\mathrm{Cl}}_{6}]}^{2\ensuremath{-}}$ ions. Using a value of the exchange parameter $J$ taken from EPR and magnetic-susceptibility measurements, hyperfine parameters $A=1.62\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and $B=3.86\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ are deduced from the measured relaxation rates. These values are believed to provide better than order of magnitude estimates of the true hyperfine parameters.