13C spin–lattice relaxation in natural type-IIb semiconducting diamond

13C spin-lattice relaxation in natural diamond: Zeeman relaxation in fields of 500 to 5000 G at 300 K due to fixed paramagnetic nitrogen defects

13C Spin-lattice relaxation (SLR) times in the laboratory frame have been measured at room temperature as a function of field in the range of 500 to 5000 G on two natural type 1b and 1a diamonds after dynamic nuclear polarization. Each of the diamonds contains two types of fixed paramagnetic centers with overlapping inhomogeneous electron paramagnetic resonance (EPR) lines. EPR techniques have been employed to identify these defects and to determine their concentrations and relaxation times at X-band. Three different nuclear SLR paths, namely that due to electron SLR and two types of three spin processes, are discussed. The one three-spin process (TSP) (type 1) involves a simultaneous transition of two electron spins belonging to the same hyperfine EPR line and a 13C spin while the other process (type 2) involves two electron spins belonging to different hyperfine EPR lines and a 13C spin. It is shown that the thermal contact between the 13C nuclear Zeeman and electron dipole-dipole interaction reservoirs decreases with an increase in field intensity, thus forming a bottleneck in the 13C relaxation path due to the type 1 TSP. The contribution of TSP of type 1 dominates that due to electron SLR and the type 2 TSP in relaxing the 13C nuclei in type lb diamond from about 1200 to 5000 G, while for type 1a diamond it dominates from 500 up to about 2200 G. In type 1a diamond over the range 2200 to 5000 G it seems that the type 2 TSP, which involves electrons of neighboring P2 hyperfine lines, dominates that of electron spin-lattice and the type 1 TSP. Over the range 500 to about 1200 G, a field-dependent electron SLR mechanism associated with N3 centers appears to dominate the 13C SLR.