NQR and NMR studies on the electron-spin correlation time in paramagnetic insulators.

Abstract

The $^{35}\mathrm{Cl}$ NQR and $^{1}\mathrm{H}$ NMR spin-lattice relaxation times, ${\mathit{T}}_{1\mathit{Q}}$ and ${\mathit{T}}_{1\mathit{M}}$, respectively, in paramagnetic Ni(${\mathrm{H}}_{2}$O${)}_{6}$${\mathrm{SnCl}}_{6}$ crystals were measured as a function of temperature. Their temperature variations depend on the correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$ of the unpaired electron spins in the paramagnetic ${\mathrm{Ni}}^{2+}$ ions, and ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$ was found to be governed mainly by the electronic spin-lattice relaxation originating from the spin-phonon interaction above approximately 250 K and by electron-spin flips caused by the exchange interaction below approximately 150 K. The temperature dependence of ${\mathit{T}}_{1\mathit{Q}}$ and ${\mathit{T}}_{1\mathit{M}}$ at high temperatures shows that the electronic spin-lattice relaxation time is determined by the Raman process. The correlation time of the electron flips was evaluated to be 1.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ s from ${\mathit{T}}_{1\mathit{Q}}$. The exchange parameter between the nearest electronic spins and the paramagnetic Curie temperature were estimated to be 0.013 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 0.075 K, respectively. It was shown that reliable information on the electron-spin dynamics can be obtained from ${\mathit{T}}_{1\mathit{Q}}$ and ${\mathit{T}}_{1\mathit{M}}$.