Biopolymers

From the point of view of both fundamental science and practical applications, of great interest is high-temperature ferromagnetism, observed in various carbon-based materials [1]. Carbon microspheres with interesting magnetic properties have been fabricated by the solid-phase pyrolysis of a metal-free phthalocyanine H2Pc=H2(C32N8H16) used as a precursor. By changing the conditions of pyrolysis we prepared carbon microspheres with a mean diameter 2-3.8 μm consisting of (layered graphite) graphitized nanocrystallites and amorphous carbon with a thickness of 5-15 graphene layers and width of 5-20 nm and sufficiently narrow size distribution. In particular, at Tpyr = 700°C and tpyr = 30 min we obtained carbon microspheres with a mean diameter d = 3.4±0.15 μm [2]. The comparison of magnetic properties in metal-free phthalocyanine and graphite-like nanostructures is provided in the broad range of temperatures and magnetic fields. Magnetic characteristics of these samples were investigated with a vibrational magnetometer in the temperature range 5-300К, as well as with electron spin resonance (ESR) spectroscopy [2]. Analysis of curves Ðœ-Т and Ðœ-Н shows the presence of paramagnetic centers with the concentration n = 3∙1019 spin/g and temperature-independent diamagnetism with susceptibility of χDia=1.2∙10-6 emu/g∙Oe. The parameters of ESR spectrum were measured at 300K: the g-factor 2.0031 and intensity ~5∙1019 spin/g. The ESR line width is only 0.8 Oe which indicates a strong exchange narrowing. Along with the paramagnetic centers and diamagnetism, we have revealed ferromagnetic hysteresis loops from T = 5 K to room temperature. The dependences of magnetizations of diamagnetic and ferromagnetic signal on H at T = 300K are presented in figures below. The maximum value of coercive force is 150 Oe at T = 300K. We also analyze the nature of the reported evidence on superparamagnetism, diamagnetism and ferromagnetic ordering in the metal-free phthalocyanine and metal-free pyrolytic carbon structures containing only p- and s-electrons.