Structure and Magnetism of Few-Layer Nanographene Clusters in Carbon Microspheres

The solid-phase pyrolysis method was used to synthesize carbon microspheres, consisting of clusters of few-layer nanographene and amorphous carbon. Powders of metal-free phthalocyanine and polyethylene served as precursors of the synthesized carbon microspheres. The pyrolysis products of metal-free phthalocyanine samples S-P(c) (700) and S-P(c) (900) contained 4 and 1 atom % nitrogen, respectively, replacing carbon in the graphene lattice in pyrrolic and pyridinic coordination. There are no impurity nitrogen atoms in the products of the pyrolysis of polyethylene. The S-Pc(700) sample showed strong paramagnetism with a concentration of paramagnetic centers of similar to 5 x 10(19) spin g(-1) and a temperature-independent diamagnetism susceptibility of chi(Dia) = -1 x 10(-6) emu g(-1) Oe(-1). In a temperature range of 5-300 K, ferromagnetism was also revealed with a temperature dependence similar to that of ferromagnetic cluster spin glasses, with maximum saturation magnetization, M-S(FM) 3 x 10(-2) emu g(-1), and coercive force, H-c = 400 Oe, at T-sg = 25 K. It was shown that the ferromagnetism in the S-Pc(700) sample is due to pi(p)-electrons of zigzag-type edge states as well as nitrogen impurity atoms. The experimental results are interpreted based on the temperature dependence of the spin correlation length. It was revealed that the temperature dependence of the integral of the magnetic resonance absorption intensity closely resembles the temperature behavior of the saturation magnetization of the ferromagnetic component.