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We have investigated the magnetic properties of carbon powders which consistof nanodisks, nanocones, and a small fraction of carbon-black particles.Magnetization measurements were carried out using a superconducting quantuminterference device in magnetic fields $-5<\mu_{0}H<5\:\mathrm{T}$ fortemperatures in the range $2\leq T<350\:\mathrm{K}$. Measurements of themagnetization $M$ versus temperature $T$ and magnetic field $\mu_{0}H$ forthese carbon samples show diamagnetism and paramagetism with an additionalferromagnetic contribution. The ferromagnetic magnetization is in agreementwith the calculated magnetization from Fe impurities as determined by theparticle-induced x-ray emission method ($<75\:\mu\mathrm{g/g}$). Magnetizationmeasurements in weak magnetic fields show thermal hysteresis, and for strongfields the magnetization $M$ decreases as $M\sim aT^{-\alpha}$ with $\alpha<1$,which is slower than the Curie law ($\alpha=1$), when the temperatureincreases. The magnetization $M$ versus magnetic field $\mu_{0}H$ showsparamagnetic free-spin $S=\frac{1}{2}$ and $\frac{3}{2}$ behaviors fortemperatures $T=2\:\mathrm{K}$ and $15\leq T\leq50\:\mathrm{K}$, respectively.A tendency for localization of electrons was found by electron spin resonancewhen the temperature $T$ decreases ($2<T<40\:\mathrm{K}$). The magneticproperties in these carbon cone and disk powder samples are more complex than afree-spin model predicts, which is apparently valid only for the temperature$T=2\:\mathrm{K}$.

Magnetic properties of carbon nanodisk and nanocone powders