Magnetic properties of carbon nanodisk and nanocone powders

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 superconducting quantum interference device in magnetic fields $-5<\mu_{0}H<5\:\mathrm{T}$ for temperatures in the range $2\leq T<350\:\mathrm{K}$. Measurements of the magnetization $M$ versus temperature $T$ and magnetic field $\mu_{0}H$ for these carbon samples show diamagnetism and paramagetism with an additional ferromagnetic contribution. The ferromagnetic magnetization is in agreement with the calculated magnetization from Fe impurities as determined by the particle-induced x-ray emission method ($<75\:\mu\mathrm{g/g}$). Magnetization measurements in weak magnetic fields show thermal hysteresis, and for strong fields the magnetization $M$ decreases as $M\sim aT^{-\alpha}$ with $\alpha<1$, which is slower than the Curie law ($\alpha=1$), when the temperature increases. The magnetization $M$ versus magnetic field $\mu_{0}H$ shows paramagnetic free-spin $S=\frac{1}{2}$ and $\frac{3}{2}$ behaviors for temperatures $T=2\:\mathrm{K}$ and $15\leq T\leq50\:\mathrm{K}$, respectively. A tendency for localization of electrons was found by electron spin resonance when the temperature $T$ decreases ($2<T<40\:\mathrm{K}$). The magnetic properties in these carbon cone and disk powder samples are more complex than a free-spin model predicts, which is apparently valid only for the temperature $T=2\:\mathrm{K}$.