Ferromagnetic Spots in Graphite Produced by Proton Irradiation

Abstract

Due to the fine-tuning possibilities of the building blocks and demands for environmental protection, organic molecular magnetic materials composed only of light elements (C,H,N,O,S) are considered potential candidates for magnetic applications. However, for these applications it is necessary that the magnetic state (ferromagnetism or ferrimagnetism) is stable at room temperature. This was believed to be only possible in materials containing metallic 3-d or 4-f elements. As a matter of fact, recording media are in general polycrystalline, granular or amorphous ferromagnetic metals, alloys and oxides. Recent reports, however, showed weak ferromagnetic signals at room temperature and above in highly oriented pyrolytic graphite [1] (HOPG) and in polymerized fullerene. [2,3] Nevertheless, magnetism experts are highly skeptical about room-temperature ferromagnetism in carbon-based materials without magnetic ions. Recently, unusually large ferromagnetic signals were found in meteoritic graphite. [4] This large ferromagnetic signal was interpreted as induced by magnetite (Fe 3 O 4 ) inclusions in the graphite structure by a magnetic proximity effect. [4] It is interesting to note that in spite of some reports in the pastÐcasting doubts on simple interpretations of ferromagnetism in carbon-based materials in terms of ferromagnetic impurities [5] Ðit appears that prejudices against a possible intrinsic origin of the observed ferromagnetism hindered a broad and rush development of these organic ferromagnets. In this work, we produced localized ferro-or ferrimagnetic spots (a few micrometers in diameter) on clean HOPG surfaces using a proton microbeam. Our results rule out the influence of ferromagnetic impurities and open up a new field of investigation with clear implications for future applications of metal-free carbon ferromagnets.Clean surfaces of a HOPG sample (Advanced Ceramic, Fe content < 0.3 ppm, rocking-curve width= 0.4) were irradiated by 2.25 MeV protons using a microbeam applied parallel to the c-axis. Beam diameters between 1 and 2 lm, separated by 20 lm, and at different fluences and doses were chosen. The total deposited electric charge (areal) density was between 0.05 and 50 nC lm ±2 . The irradiated areas and surroundings were characterized simultaneously by atomic force (AFM) and magnetic force microscopy (MFM) at room temperature, operating in the ªTapping/Liftº scanning mode, using Si cantilevers with pyramidal tips coated with a magnetic CoCr film alloy magnetized perpendicular to the sample surface. MicroRaman characterization of the spot areas was used to determine the degree of disorder. Figure 1a shows the MFM images and Figure 1b the line scans of the topography and MFM signals obtained in three areas of the HOPG sample before irradiation. For virgin graphite samples, even though the changes in topography are significant, one obtains in general a MFM signal with a phase shift of the order of ± 0.1, which corresponds to the noise of COMMUNICATIONS

Magnetism in carbon structures

“…The measurements with the SQUID and the MFM we have done show indeed clear signs of magnetic ordering after irradiating HOPG samples [36,37] and disordered carbon films [20]. In the published literature on magnetism in carbon-based structures one realizes that apparently hydrogen (or maybe also other light atoms like oxygen) plays a role in the reported ferromagnetism.…”

“…Adapted from Ref. [37]. Bottom: Scanning height dependence of the maximum phase shift at proton irradiated spots with fluences of 0.2 nC/µm 2 and 2.2 nC/µm 2 .…”

The electronic states of a double carbon vacancy defect in pyrene: a model study for graphene

“…These defects arise in graphene or in graphitic nanostructures during defective growth and can also be created artificially by means of ion irradiation. [7][8][9][10][11][12][13][14] Their structural details can be directly observed by means of several experimental techniques such as transmission electron microscopy (TEM) [15][16][17][18][19] and scanning tunneling microscopy (STM). 20,21 Because of the occurrence of dangling bonds the defect structure will be associated with a high polyradical character with a multitude of closely spaced locally excited electronic states possessing different spin multiplicities which make their theoretical description very challenging.…”

Intrinsic Magnetism of Grain Boundaries in Two-Dimensional Metal Dichalcogenides

“…Following the advent of graphene, there has been an upsurge of interest in the intrinsic magnetism in two-dimensional (2D) crystals. While experiments have detected ferromagnetic signals in certain graphene samples [3][4][5][6][7], these systems show some significant limitations [8]. First, the vanishing band gap of graphene is undesirable for switching functionalities and controllable magnetism.…”