Room Temperature Superparamagnetism observed in Foam-like Carbon Nanomaterials
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Room Temperature Superparamagnetism observed in Foam-like Carbon Nanomaterials Shunji Bandow, Hirohito Asano, Susumu Muraki, Takahiro Mizuno, Makoto Jinno and Sumio Iijima Department of Materials Science and Engineering, Meijo University, 1-501 Shiogamaguchi, Tenpaku, Nagoya 468-8502, Japan
ABSTRACT Magnet-attractive carbon nanopowder can be produced by a pulsed Nd:YAG laser (10 Hz) vaporization of pure carbon in a few % of H2 containing Ar gas at 1000°C. On the other hand, magnet-attractive nanopowder cannot be formed when vaporizing in pure Ar. As-grown carbon nanopowder includes a few to ten % of micron sized graphite flakes as the impurity. Removal of such flakes can be achieved by a centrifugal separation and the supernatant is checked by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Magnetization curve at 400 K is easy to saturate at low magnetic field of 10 kG, and no hysteresis is observed. This feature is explained by a superparamagnetism of finely dispersed ferro- or ferri-magnetic nanoparticles. Elementary analyses using electron energy loss spectroscopy (EELS) and atomic absorption spectroscopy (AAS) suggest that the observed strong magnetism should be an intrinsic carbon magnetism. INTRODUCTION All carbon nanomagnet was first developed by Rode et al. in 2004 by using high repetition rate (2-25 kHz) pulsed laser vaporization of carbon in pure Ar gas [1]. However, the strong magnetism was observable only at low temperatures (< ~90 K) and this magnetism was not stable in open air. Theoretically, strong carbon magnetism is associated with the zig-zag edge of graphene where the non bonding electron spins are localized in the narrow band near the Fermi level [2, 3]. The electron spins on such zig-zag edge have a tendency to orient their directions with a ferromagnetic manner and the finite number of magnetic moment will be appeared. On the other hand, the electrons on the armchair face do not indicate such magnetism. Generally, the non bonding electron spins are quite active, so that these spins immediately react with other chemical species such as O2 and H2O in air. This will result the non magnetic state. Carbon is a light element and exists widely on the earth. In addition, the strong magnets are all composed of heavy rare earth elements. Hence the carbon magnet will be pioneering and environment-friendly material. In this study, a reproducible method for making the carbon nanomagnet is introduced, and the purified product is analyzed in detail by using TEM, XRD, SQUID (superconducting quantum interference device), EELS and AAS.
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EXPERIMENTAL Sample preparation Pure carbon target for a pulsed Nd: YAG laser vaporization was produced by pressing the carbon powder (a Kojundo Kagaku, purity 99.7 % with the grain size ~5 Pm) in a 10 mm diameter cylinder at 350 kg/cm2 for 30 min. Then the pelletized carbon powder was carefully removed from the cylinder and treated by using non-magnetic tweezers. In order to get wellformed cylindrical target, it was necessary to use the carbon powde
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