13C nuclear magnetic resonance and electron spin resonance of amorphous hydrogenated carbon

The temperature dependences of the C-13 and H-1 magnetization recovery and spin-lattice relaxation times as well as the C-13 NMR spectra have been studied between 300 K and 4 K. The observed short and nearly temperature independent proton and C-13 spin-lattice relaxation times demonstrate that the dominant spin-lattice relaxation mechanism is spin diffusion to paramagnetic impurities. The fact that the magnetization recovery curves clearly deviate from the single-exponential form expected for the case of spin diffusion and randomly distributed paramagnetic centres demonstrates that the paramagnetic centres aggregate in clusters. Superparamagnetic freezing effects expected for such an inhomogeneous distribution are indeed seen below 50 K in the temperature dependence of the electron spin-resonance (ESR) intensity, the C-13 NMR spectra and the SQUID magnetization measurements which show a distinct magnetic hysteresis loop.