Magnetism in nanoscale graphite flakes as seen via electron spin resonance

Magnetic properties of a large assembly of ultrathin graphitic particles obtained by heavy sonication of graphite powder dispersed in $N$-methylpyrrolidone were measured by electron-spin resonance (ESR). The ESR signal was decomposed into one narrow and one broad component. The narrow component was associated with localized Curie-type defects. The temperature dependence of the predominant broad component points to a transition to a superparamagnetic-like state at 25 K. By performing the density-functional-theory calculations for graphene with selected extended defects (the sheet edges, zigzag chains of chemisorbed H atoms, and pentagon-octagon rows), we found considerable magnetic moments at C atoms in their vicinities. We attribute the magnetism in the graphitic particles to the localized electronic states near the defects in the network of the $\pi$ electrons of graphene. The ferromagnetic (FM) correlations among magnetic moments at carbon atoms near the edges are not able to give rise to a long-range FM order.

Figure 1

Scanning-electron-microscopy (SEM) image of the large assembly of graphitic particles. At least 66$\%$ of the particles have a surface area $⩽$1 mm${}^2$.

Figure 2

The atomic structure of (a) a small rectangular graphene flake, (b) a zigzag line of H adatoms on graphene, (c) a bare line defect containing carbon pentagons and octagons, and (d) a hydrogenated-line defect containing carbon pentagons and octagons. The H and C atoms are represented by red and gray spheres, respectively. For structures in (b)–(d), the surface unit cells are marked with black lines.

Figure 3

A typical ESR signal of the nanographite sample recorded at 100 K. The asymmetric signal is well fitted with a narrow and a broad component. The resonance field was roughly in the $H_0\perp c$ configuration.

Figure 4

The temperature dependence of the spin susceptibility corresponding to the broad component of the ESR line. The inset depicts the strong increase of both $\chi$ and $\chi *T$ below 25 K points to the onset of magnetic interactions.

Figure 5

The ESR linewidth (left axis) and $g$ factor (right axis) of the broad component of the ESR line as a function of temperature. At 25 K both quantities show the onset of magnetic correlations.

Figure 6

The isocontour plots of spin densities of (a) a small rectangular graphene flake, (b) a zigzag line of H adatoms on graphene, and (c) a hydrogenated-line defect containing carbon pentagons and octagons. Yellow and orange colors are used to represent spin densities generated from electron states with opposite spin orientations.

Figure 7

The linewidth and ($k_{B}T$)-multiplied susceptibility at low temperatures. The red curve represents the best fit ($\sim$$1/T$) for both mutually scaling dependences.

Figure 8

Resistance below 70 K of a thick film of nanographite (Fig. 1) as a function of temperature with weak nonmetallic temperature dependence. It shows a stronger increase below 25 K where the FM interaction is observed by ESR. The $R^{\prime }\left(T\right)$ dependence presented in the inset clearly suggests a change at around 25 K.