Аннотации:
Magnetization M(T, B) of powder and glassy samples containing carbon nanoparticles, not intentionally doped and doped with Ag, Au and Co, is investigated at temperatures T between ∼3-300 K in magnetic fields B up to 5T. According to atomic force microscopy data, a system of carbon particles has a broad size distribution, given by the average and the maximum radii of ∼60 nm and ∼110 nm, respectively. In low fields of B≪BK, where BK ∼ 1T is the mean anisotropy field, M(T) exhibits large irreversibility or deviation of zero-field cooled and field-cooled magnetizations, which is suppressed completely at B > BK. The dependence of M(B) saturates above B ∼ 2T at T ∼ 150-300 K and contains a large paramagnetic-like response below ∼50-150 K. Hysteresis is observed already at 300 K and is characterized by a power-law temperature decay of the coercive field, Bc(T). This is described by the exponent n ≈ 0.8 and by the low-temperature values of Bc (0) increasing from ∼36-53 mT in the undoped sample and those doped with Ag and Au, up to 80 mT in the Co-doped material, yielding the blocking temperatures Tb ≈ 400-580 K. Analysis of the experimental magnetization data above suggests distribution of the magnetization close to the surface of the particles, yielding a thickness of the near-surface layer, h, filled with localized magnetic moments, μ1 ∼ μB, to be close to the average distance, a, between the moments, h ≈ a ∼ 1 nm. This is consistent with the origin of magnetism in nanocarbon being presumably due to intrinsic near-surface defects. Copyright © 2012 American Scientific Publishers All rights reserved.