Irreversible magnetic properties of nanocarbon

Abstract

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.