This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity con
tamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.
The carrier density in tens of nanometers thick graphite samples (multi-layer-graphene, MLG) has been modified by applying a gate voltage ($V_g$) perpendicular to the graphene planes. Surface potential microscopy shows inhomogeneities in the carrier
density ($n$) in the sample near surface region and under different values of $V_g$ at room temperature. Transport measurements on different MLG samples reveal that under a large enough applied electric field these regions undergo a superconducting-like transition at $T lesssim 17$ K. A magnetic field applied parallel or normal to the graphene layers suppresses the transition without changing appreciably the transition temperature.
We present a x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon $pi$ states, also hydrogen-mediated electronic states exhibit a net spin polar
ization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top $approx$10 nm of the irradiated sample where the actual magnetization reaches $ simeq 15$ emu/g at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin as the one found in proton bombarded graphite.
We investigate the dependence of the electrical resistivity of $sim 60 $nm thick single crystalline graphite samples on the defect concentration produced by proton irradiation at very low fluences. We show that the resistivity decreases few percent a
t room temperature after inducing defects at concentrations as low as $sim 0.1 $ppm due to the increase in the carrier density, in agreement with theoretical estimates. The overall results indicate that the carrier densities measured in graphite are not intrinsic but related to defects and impurities.
We discuss recently obtained data using different experimental methods including magnetoresistance measurements that indicate the existence of metal-free high-temperature magnetic order in graphite. Intrinsic as well as extrinsic difficulties to trig
ger magnetic order by irradiation of graphite are discussed in view of recently published theoretical work.
In this work we show that for a quasi-2D system of size $Omega$ and thickness $t$ the resistance goes as $(2rho/pi t)ln(Omega/W)$, diverging logarithmically with the size. Measurements in highly oriented pyrolytic graphite (HOPG) as well as numerical
simulations confirm this relation. Furthermore, we present an experimental method that allows us to obtain the carriers mean free path $l(T)$, the Fermi wavelength $lambda(T)$ and the mobility $mu(T)$ directly from experiments without adjustable parameters. Measuring the electrical resistance through microfabricated constrictions in HOPG and observing the transition from ohmic to ballistic regime we obtain that $0.2 mu$m $lesssim l lesssim 10 mu$m, $0.1 mu$m $lesssim lambda lesssim 2 mu$m and a mobility $5 times 10^4$ cm$^2$/Vs $ lesssim mu lesssim 4 times 10^7$ cm$^2$/Vs when the temperature decreases from 270K to 3K. A comparison of these results with those from literature indicates that conventional, multiband Boltzmann-Drude approaches are inadequate for oriented graphite. The upper value obtained for the mobility is much larger than the mobility graphene samples of micrometer size can have.
High resolution magnetoresistance data in highly oriented pyrolytic graphite thin samples manifest non-homogenous superconductivity with critical temperature $T_c sim 25 $K. These data exhibit: i) hysteretic loops of resistance versus magnetic field
similar to Josephson-coupled grains, ii) quantum Andreevs resonances and iii) absence of the Schubnikov-de Haas oscillations. The results indicate that graphite is a system with non-percolative superconducting domains immersed in a semiconducting-like matrix. As possible origin of the superconductivity in graphite we discuss interior-gap superconductivity when two very different electronic masses are present.
We have prepared magnetic graphite samples bombarded by protons at low temperatures and low fluences to attenuate the large thermal annealing produced during irradiation. An overall optimization of sample handling allowed us to find Curie temperature
s $ T_c gtrsim 350$ K at the used fluences. The magnetization versus temperature shows unequivocally a linear dependence, which can be interpreted as due to excitations of spin waves in a two dimensional Heisenberg model with a weak uniaxial anisotropy.
In this work we have studied systematically the changes in the magnetic behavior of highly oriented pyrolytic graphite (HOPG) samples after proton irradiation in the MeV energy range. Superconducting quantum interferometer device (SQUID) results obta
ined from samples with thousands of localized spots of micrometer size as well on samples irradiated with a broad beam confirm previously reported results. Both, the para- and ferromagnetic contributions depend strongly on the irradiation details. The results indicate that the magnetic moment at saturation of spots of micrometer size is of the order of $10^{-10}$ emu.
In this article we shortly review previous and recently published experimental results that provide evidence for intrinsic, magnetic-impurity-free ferromagnetism and for high-temperature superconductivity in carbon-based materials. The available data
suggest that the origin of those phenomena is related to structural disorder and the presence of light elements like hydrogen, oxygen and/or sulfur.
