Temperature evolution of exchange-driven magnetism of non-conducting glass-like carbon

Magnetic properties of glass-like carbon (GLC) obtained by pyrolysis of furfuryl alcohol at 600°C are studied by continuous wave electron paramagnetic resonance (EPR) in the temperature range of 4.3–300 K. The pulsed EPR is used for measuring the spin relaxation times and the spin multiplicity of the observed paramagnetic centers. Nutation experiments detect only doublet spin states in our samples. Complex non-Curie temperature dependences of paramagnetic susceptibility for samples prepared under different conditions (degassing, annealing and storage time) are observed. The strong influence of preparation conditions is results from nanoporosity detected by small-angle X-ray scattering. The wide-angle X-ray scattering reveals that average basic units of GLC600 consist of stacks of three graphene-like strongly defected planes. It is shown that the temperature dependences of the EPR parameters are caused by temperature changes in the number of paramagnetic centers and are explained by the disintegration of exchange-coupled pairs. These dependences for samples with different degrees of degassing demonstrate that the exchange coupling is destroyed by adsorbed gases. Although the probabilities of triplet and singlet ground states of these pairs are the same, the triplet states are not detected in either continuous wave or nutation EPR experiments, perhaps due to the large fine splitting.