Magnetization and ¹³C NMR spin-lattice relaxation of nanodiamond powder

The bulk magnetization at temperatures of 1.8–400 K and in magnetic fields up to 70 kOe, the ambient temperature ¹³C NMR spin-lattice relaxation, T_1,C, and the elemental composition of three nanodiamond powder samples have been studied. The total magnetization of nanodiamond can be explained in terms of contributions from (1) the diamagnetic effect of carbon, (2) the paramagnetic effect of unpaired electrons present in nanodiamond grains, and (3) ferromagneticlike and (4) superparamagnetic contributions from Fe-containing particles detected in spatially resolved energy-dispersive spectroscopy. Contributions (1) and (2) are intrinsic to nanodiamond, while contributions (3) and (4) arise from impurities naturally present in detonation nanodiamond samples. ¹³C NMR T_1,C relaxation would be unaffected by the presence of the ferromagnetic particles with the bulk magnetization of ∼0.01 emu∕g at 300 K. Thus, a reduction of T_1,C by 3 orders of magnitude compared to natural and synthetic microdiamonds confirms the presence of unpaired electrons in the nanodiamond grains. The spin concentration in nanodiamond powder corresponds to ∼30 unpaired electrons per ∼4.6 nm diameter nanodiamond grain.