Magnetic properties of graphite nanostructures in carbon microspheres

Carbon microspheres with interesting magnetic properties are prepared by solid-phase pyrolysis using as a precursor the metal-free phthalocyanine H$_2$(C$_{32}$N$_8$H$_{16}$). By changing conditions of pyrolysis an average diameter $\it d$=2-3.8$\mu$m carbon microspheres, consisting of graphitized nanocrystallites with a thickness of 5-15 graphene layers of 5-20nm width with sufficiently narrow size distribution are formed. In particular, at $\it T_{pyr}$=700$^{0}$C and $\it t_{pyr}$=60min carbon microspheres have average $\it d$=3.4$\pm$0.15$\mu$m. Magnetic characteristics of samples are investigated by vibrational magnetometer in temperature range $T$=5-300$\mu$m and magnetic fields up to $H$=80kOe, as well as with the ESR method. Analysis of magnetization $\it M$ vs $\it H$ and $\it T$ show presence of paramagnetic centers with concentration $\it n$=3${\times}$10$^{19}$spin/g and temperature-independent diamagnetism with susceptibility, $\kappa_{dia}$=1.5${\times}$10$^{-6}$ emu/g Oe. Parameters of ESR spectrum are: $\it g$-factor 2.0031, intensity ${\sim}$5${\times}$10$^{19}$spin/g and narrow linewidth of 0.8Oe due to strong exchange. Paramagnetism in carbon microspheres is apparently driven by edge uncompensated spins of nanographitic crystallites and impurity nitrogen atoms.