Paramagnetic defects and exchange coupled spins in pristine ultrananocrystalline diamonds

Electron paramagnetic resonance (EPR) and magnetic susceptibility measurements were done on nanodiamond samples fabricated by the detonation method and purified by acids. Comprehensive acid treatment leads to the reduction of EPR signals of magnetic impurities and revealing two weak and narrow EPR lines with g1 = 4.26, ▵Hpp = 2.9 mT and g2 = 4.00, ▵Hpp = 1.4 mT at T = 4 K, separated by the distance of 10.4 mT. The origin of this new doublet EPR signal observed in the well purified sample is discussed. The magnetic susceptibility behavior and doublet EPR signal (g ∼ 4) suggest the weak antiferromagnetically exchange coupling of S = 1/2 paramagnetic defects as well as the presence of isolated dimers with S = 1.