Direct measurements of impurity and lattice components of the nuclear spin-lattice relaxation in Al₂O₃crystals

Abstract

For 27Al nuclei in Al2O3 crystals, both nominally pure and doped with Cr3+, suppression of the impurity nuclear spin-lattice relaxation under the condition of additional stationary magnetic saturation of the NMR line has been observed for the first time. Measurements of the lattice and impurity components of the spin-lattice relaxation time have been performed. The study was carried out at 77 K for different crystal orientations. Possible mechanisms of the impurity NMR relaxation suppression are discussed.

Impurity Nuclear Spin–Lattice Relaxation Suppression and Charge Exchange of Chromium Ions in a γ-Irradiated Ruby Crystal

“…Thus, the relaxation time at strong saturation should be treated as T 1 lat . It follows from the measurements performed that T 1 lat ϭ 260 Ϯ 20 s. This time agrees well with the spin-lattice relaxation time corresponding to the lattice contribution which has been found in (19,25) for two different nominally pure Al 2 O 3 crystals (270 and 260 s). The substitution of the values of T 1 ⌺ and T 1 lat into Eq.…”

“…The measurements of the 27 Al spin-lattice relaxation time were carried out at 77 K with the NMR spectrometer in a constant field B ϭ 0.49 T. Since 27 Al nuclei have the quadrupole moment (I ϭ 5 2 ), the energy levels of 27 Al in hexagonal Al 2 O 3 crystals are not equidistant at arbitrary sample orientation in the magnetic field. As a result, the total relaxation process is a superposition of several exponential processes and the re-laxation rate is different for different pairs of spin levels (19,21). In this connection, we performed the measurements when the sample was oriented at the magic angle (1) and the energy levels were nearly equidistant.…”

“…In this connection, we performed the measurements when the sample was oriented at the magic angle (1) and the energy levels were nearly equidistant. For this orientation, the magnetization recovery follows an exponential law with a single relaxation time (19,21). The time of magnetization recovery was measured after a pulse saturation (22).…”