Quadrupole and magnetic mechanisms of 209Bi spin-lattice relaxation in Bi4Ge3O12

The nuclear spin-lattice relaxation in the Bi4Ge3O12 single crystal was studied by the Bi-209 NQR spin-echo technique. The recovery curves were measured for all Bi-209 NQR transitions of nuclear spin I = 9/2 in a large temperature range 10-230 K. The experimental relaxation curves were described in terms of the single effective spin-lattice relaxation time T-1*. The temperature dependence of 1/T-1* was close to a T-n-law with n = 2.4-2.7. A theoretical treatment was given for the nuclear spin-lattice relaxation in the multi-level system for the case of a single-axial crystalline electric field. Both quadrupole and magnetic mechanisms of relaxation were taken into account. In separating the quadrupole and magnetic contributions, an idea of Rega (Rega T 1991 J. Phys.: Condens. Matter 3 1871) was used. To extract the temperature dependences of the parameters W-1, W-2 and W-M the fitting procedure for calculated and experimental relaxation times was used. High-accuracy measurements of the recovery curves enable us to conclude that the magnetic mechanism contributes noticeably to the spin-lattice relaxation in Bi4Ge3O12 at T less than or equal to 50 K.