Dynamics of negatively charged divacancies in neutron irradiated type Ib diamond

Thermal neutron irradiation of synthetic type Ib diamond, followed by annealing, results in a number of paramagnetic point defect centers. One of these is the composite defect designated W29. Previous EPR measurements have shown that the W29 center (S = 3/2) is a charged divacancy [VV]− with spin Hamiltonian parameters similar to those of the R4/W6 neutral divacancy center [VV]0 (S = 1) found in irradiated high purity type IIa diamond. The present EPR linewidth and spin-lattice relaxation rate measurements, made as a function of temperature on the W29 center, show that an Orbach two-phonon process, with a gap of ~ 20 meV, plays a dominant role in determining the relaxation behavior above 20 K. Below 20 K a direct single phonon process provides the spin-lattice relaxation mechanism and the relaxation times become long, of the order of milliseconds. The linewidth behavior with temperature is accounted for in terms of changes in the spin dynamics.