  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
$T=2\:\mathrm{K}$.
