Magnetic properties of fullerene–thermally exfoliated graphite composites doped with sodium

The magnetic properties (field range $H$ = 0–50 kOe, temperature range $T$ = 3–300 K), and structural features of a sodium-doped carbon composite material based on fullerene $C_{60}$ and thermally exfoliated graphite (TEG) are studied. The material is obtained with different ratios of the components by sintering at a pressure of 7 GPa and $T$ = 600$^{\circ}$C, at which it is found that significant amorphization of the crystal lattice of the initial $C_{60}$ occurs. The dia-, para-, and ferromagnetic components ($M_D$, $M_{PM}$, and $M_{FM}$) were separated from the total magnetic moment of the samples under study. It is found that a sodium dopant has no effect on the magnetic properties of the composite. Analysis of the $M _{PM}(H)$ field dependences by using the Brillouin function for the fullerene-containing sample (i.e., without TEG) makes it possible to determine the quantum number of the total angular momentum of paramagnetic (PM) centers. Its value is found to be $J$ = 1, which corresponds to elementary magnetic moment $\mu_{PM}$ = 2$\mu_B$ of a PM center. The concentration of PM centers is estimated at the level of $N_{PM}\approx$ (2 - 5) $\times$ 10$^{18}$ g$^{-1}$ for most samples, including the material without TEG. The introduction of TEG into the initial composition and an increase in its proportion in the composite leads to a strong increase in the magnetic moment, which is explained by an increase in both the $J$ value and the concentration of PM centers.