Ferromagnetism in multilayered graphite nanostructures doped with nitrogen*

Carbon microspheres consisting of multilayered nanographite structures are prepared using solid-phase pyrolysis of metal-free phthalocyanine H₂(C₃₂N₈H₁₆). The mean diameter d=3±0.2μm of carbon microspheres have been obtained at pyrolysis T_pyr =670^oC, and t_pyr =30min. Morphology and structure of prepared samples at two temperatures T_pyr were investigated by HRTEM, X-ray diffractometry, Raman and XPS spectroscopy. Magnetic properties of samples were investigated using vibrational magnetometer and magnetic fields up to 80 kOe, as well as X-band ESR spectrometer in range of temperature, T=5-300K. We found that the temperature dependences of saturation magnetization closely follow the behavior of integrated magnetic resonance intensity. Parameters of ESR spectrum are following g-factor 2.0031, intensity ~5×10¹⁹ spin/g and narrow linewidth of 0.8Oe due to strong exchange. Maximum values of saturation magnetization M_s≈0.03emu/g and coercive force H_c=400Oe have been obtained at 25K. The results suggest that observed ferromagnetism in multilayer graphene can be attributed to nitrogen atoms and zigzag edges.