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 temperatures Tc 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. PACS numbers: 75.50.Pp,75.30.Ds Recent advances to develop nanographitic systems have led to a renewed interest on their electrical properties worldwide [1]. A single layer of graphite, the twodimensional (2D) graphene, appears to have quantum properties at room temperature[2] as well as rectifying electronic properties [3,4]. On the other hand, some of those properties were already observed in highly oriented pyrolytic graphite (HOPG) of low mosaicity, as the quantum Hall effect[5] and de Haas -van Halphen quantum oscillations even at room temperature [6]. The twodimensional properties of the graphene planes in graphite open up the possibility of using nanometer to micron size regions of graphite in new integrated devices with spintronic properties either through the use of ferromagnetic electrodes, e.g. spin-valves, and/or making graphite itself magnetic. In fact this has been a topic of study in the last years and reports exist showing magnetic hysteresis in blank graphite [7] but especially in proton bombarded graphite [8]. Severe limitations in the sensitivity and reproducibility of standard magnetometers added to annealing effects during bombardment, hindered the identification of a critical temperature T c as well as the characteristics and dimensionality of the ferromagnetic signals. The aim of this work is to show that specially prepared highly oriented pyrolytic graphite (HOPG) samples show ferromagnetic order with T c 350 K and the magnetization temperature dependence is in good agreement with a 2D anisotropic Heisenberg model (2DHM) and the presence of spin waves excitations [9,10,11].For the experiments we used four pieces of a HOPG sample grade ZYA, samples 1 to 4 (mass: 12.8, 12.5, 10.1, and 6 mg respectively) irradiated by a 2.25 MeV proton micro-beam (sample 4: 2.0 MeV, 0.8 mm broad beam) perpendicular to the graphite planes. With the micro-beam we produced several thousands of spots of ∼ 2 µm diameter each and separated by 5 µm (sample 1) or 10 µm (samples 2 and 3) distance, similarly to the procedure used in Ref. 12. Samples 1 and 2 were irradiated at 110 K whereas samples 3 and 4 at room temperature. Further irradiation parameters for sam-ple 1 (2,3,4) were: 51375 (25600,25600,6) spots, fluence: 0.124 (0.08,0.13,0.3) nC/µm 2 , total irradiated charge 46.9 (44.8,37.4,900) µC, and 1 nA proton current (100 nA for sample 4). The pieces we have irradiated showed an iron concentration (the only detected magnetic impurity) within the first 35 µm of ∼ (0.4±0.04) µg/g (< 0.1 ppm).Previous experiments [8] showed ferr...

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 π states, also hydrogen-mediated electronic states exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top ≈10 nm of the irradiated sample where the actual magnetization reaches ≃ 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.

a b s t r a c tWe 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 trigger magnetic order by irradiation of graphite are discussed in view of recently published theoretical work.

In this work we have investigated the changes of the magnetic properties of highly oriented pyrolytic graphite samples after irradiation either with ∼ 3 × 10 14 protons or 3.5 × 10 13 . . . 3.5 × 10 14 iron ions with energies in the MeV range. Our results show that iron and proton irradiations can produce similar paramagnetic contributions depending on the implantation temperature. However, only protons induce a ferromagnetic effect.

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 obtained 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.Comment: Invited contribution at ICACS2006 to be published in Nucl. Instr. and Meth. B. 8 pages and 6 figure