Experimental evidence for two-dimensional magnetic order in proton bombarded graphite

Abstract

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...

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