Magnetization and ${}^{13}\mathrm{C}$ NMR spin-lattice relaxation of nanodiamond powder

The bulk magnetization at temperatures of $1.8–400\mathrm{K}$ and in magnetic fields up to $70\mathrm{kOe}$, the ambient temperature ${}^{13}\mathrm{C}$ 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. ${}^{13}\mathrm{C}$ NMR $T_{1,C}$ relaxation would be unaffected by the presence of the ferromagnetic particles with the bulk magnetization of $\sim 0.01\mathrm{emu}∕\mathrm{g}$ at $300\mathrm{K}$. 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 $\sim 30$ unpaired electrons per $\sim 4.6\mathrm{nm}$ diameter nanodiamond grain.

Figure 1

Wide-angle x-ray diffraction pattern of nanodiamond powder ND-1. Data for microdiamonds are shown for reference.

Figure 2

Magnetization of nanodiamond ND-1 ($15.6\mathrm{mg}$ sample mass) measured (a) at 1.8 and $300\mathrm{K}$ in magnetic fields from $0\text{to}55\mathrm{kOe}$ and (b) at 1.8, 5, 10, 20, and $30\mathrm{K}$ in magnetic fields from $0\text{to}70\mathrm{kOe}$. The dashed lines show the initial magnetic susceptibility, $dM∕dH{∕}_0$. The inset in (a) shows an expanded view of the magnetization of ND-1 measured at $300\mathrm{K}$ for two directions of the magnetic field.

Figure 3

Magnetization of nanodiamond samples (a) ND-2 ($16.3\mathrm{mg}$ sample mass) and (b) ND-3 ($24.8\mathrm{mg}$ sample mass) measured at 1.8 and $300\mathrm{K}$. The insets show expanded views of the magnetization measured at $300\mathrm{K}$ for two directions of the magnetic field.

Figure 4

Temperature dependencies of the $M∕H$ ratio of nanodiamond samples (a) ND-1 and (b) ND-2 measured in various magnetic fields. The insets show the experimental and calculated $H∕M$ ratios for the same samples. The anomalies in the magnetization of both samples are marked by vertical arrows.

Figure 5

Temperature dependencies of the magnetization of (a) ND-1 and (b) ND-2 measured under various conditions.

Figure 6

Morphology and composition of nanodiamond ND-1: (a) SEM backscattered-electron image obtained at $\times 1500$ and (b) $\times 50000$; note that large grains consist of agglomerated nanoparticles. (c) Composition obtained by energy-dispersive spectroscopy for the area shown in (a). (d) Composition of one of the iron-bearing grains marked by $A$ in (a).

Figure 7

${}^{13}\mathrm{C}$ NMR spin-lattice, $T_{1,C}$, relaxation of nanodiamond ND-1. Data for natural and synthetic microdiamonds are shown for reference; the inset presents an expanded view of their $T_{1,C}$ relaxation curves.

Figure 8

Dependence of the $M∕M_s$ ratio vs $H∕T$ for ND-1, where $M_s$ is the saturation magnetization, estimated from the experiment in the highest magnetic field. The magnetization, $M$, has been corrected for the diamagnetic contribution. The Brillouin functions for noninteracting $1∕2$, 1, and $5∕2$ spins are shown by long-dashed, short-dashed, and dash-dotted lines, respectively. The curve for $S=1∕2$ with the average magnetic moment of ${\mu }_{\mathit{av}}=1.25{\mu }_B$ is shown by a continuous line.