Spin Glass Behaviour of Magnetic Carbon Nanoclusters

Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon in Ar exhibits para- and ferromagnetic behavior at low temperature. Magnetic susceptibility and electron paramagnetic resonance of carbon nanofoam samples produced at different Ar pressures demonstrate ferromagnetic behaviour at low temperatures. The results show that the degree of remanent order is strongly dependent on the magnetic history, i.e. whether the samples were cooled under zero-field or field conditions. Such behaviour is typical for a spin glass picture where the system can exist in many different roughly equivalent spin configurations. Detailed EPR studies identified three different types of independent spin systems with relaxation times 100 ns, 0.1-1 us, and 1 ms. We conclude that unpaired spins exist in the nanofoam in three quite different structural environments. Their role in the magnetic ordering is discussed