Jozef Cernak, Geir Helgesen, Jozef Kovac, Arne T. Skjeltorp, Josef Voltr
We have investigated the magnetic properties of carbon powders which consist of nanodisks, nanocones and a small fraction of carbon black particles. Magnetization measurements were carried out using a supercomputing quantum interface device (SQUID) in magnetic fields $-5<\mu_{0}H<5\:\mathrm{T}$ for temperature $2\leq T<350\:\mathrm{K}$. In the temperature range $2\leq T<10\:\mathrm{K}$, magnetization $M$ versus temperature shows an abrupt transition with a different behaviour than the Curie law. At $T=2\:\mathrm{K}$, magnetization $M$ versus magnetic field $\mu_{0}H$ shows a paramagnetic free spin behavior only in the case when $\mu_{0}H/T>1$T/K which could be a signature of electron localization in high magnetic fields at low temperature. Magnetization $M$ versus $T$ and $M$ versus $\mu_{0}H$ for these carbon powders show diamagnetism with an additional ferromagnetic contribution for temperature $2\leq T<350\:\mathrm{Only about 5% of the saturation magnetization could be assigned to a low level of Fe impurities (< 100 ppm). This ferromagnetic ordering is probably an intrinsic property of the carbon powder particles due to their layered structure containing both graphitic parts and disordered phases.
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http://arxiv.org/abs/1203.0284
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