Defect-induced ferromagnetism in graphite

We demonstrate direct evidence for ferromagnetic order at defect structures in highly oriented pyrolytic graphite with magnetic force microscopy at room temperature. Magnetic impurities have been excluded as the origin of the magnetic signal after careful analysis supporting an intrinsic magnetic behavior of carbon-based materials. The observed ferromagnetism has been attributed to originate from unpaired electron spins localized at grain boundaries. Scanning tunneling spectroscopy of grain boundaries showed intense localized states and enhanced charge density compared to bare graphite. Introduction. – Graphite has been considered as a diamagnetic material for a long time. However, recent experiments have shown that ferromagnetic order is possible in different carbon-based materials. Ferromagnetism with high Curie temperature, well above room temperature, and very small saturation magnetization has been reported in various graphitic systems [1–6]. The role of different magnetic impurities on the measured ferromagnetism has been studied in various samples of highly oriented graphite (HOPG), Kish graphite, and nature graphite [5]. The magnetization results showed no correlation with the magnetic impurity concentration [5]. Ferroor ferrimagnetic ordering was demonstrated in protonirradiated spots in highly oriented graphite [4]. Bulk ferromagnetic graphite with a high defect concentration has been prepared via chemical route reaching the saturation magnetization 0.58 emu/g [3]. Apart from that, ferromagnetism has been observed in other carbon-based materials such as polymerized fullerenes [7], carbon nanofoam [8], proton irradiated thin carbon films [9] and nitrogen and carbon ion implanted nanodiamond [10]. All these observations suggest an inherent ferromagnetic behavior of carbon-based materials. Several theoretical investigations have been carried out to explain magnetism observed in these systems. The origin of ferromagnetism was suggested to be attributed to the mixture of carbon atoms with sp and sp bonds resulting in ferromagnetic interaction of spins separated by sp centers [11]. Another theoretical calculation suggested magnetism in sp bonded carbon nanostructures, which contains a negatively curved graphitic surface introduced via the presence of sevenor eight-membered rings [?]. In nanometer scale graphite, the electronic structure is strongly affected by the structure of the edges. Fujita and coworkers proposed that the π electrons on a monohydrogenated zigzag edge might create a ferrimagnetic spin structure on the edge [12]. Recently, it has been shown in spin-polarized density functional theory calculations that point defects in graphite such as vacancies and hydrogen-terminated vacancies are magnetic [13, 14]. Three-dimensional network of single vacancies in graphite developed ferrimagnetic ordering up to 1 nm separation among the vacancies [30]. In this Letter, we report an experimental study of ferromagnetic order in highly oriented pyrolytic graphite (HOPG) arising from defect structures. A ferromagnetic signal has been observed locally with magnetic force microscopy (MFM) and in the bulk magnetization measurements using superconducting quantum interference device (SQUID). Observed ferromagnetism has been attributed to originate from itinerant electrons occupying narrow defect states of grain boundaries in the graphite crystal. A systematic study of the grain boundaries have been performed on the same samples with scanning tunneling microscopy (STM) and spectroscopy (STS). Experimental. – Samples of HOPG of ZYH quality were purchased from NT-MDT. The ZYH quality of HOPG with the mosaic spread 3.5◦ 5◦ has been chosen because it provides a high population of step edges and grain boundaries on the graphite surface. HOPG samp-1 ar X iv :0 81 0. 56 57 v1 [ co nd -m at .m tr lsc i] 3 1 O ct 2 00 8 J. Červenka and C. F. J. Flipse ples were cleaved by an adhesive tape in air and transferred into a scanning tunneling microscope (Omicron LT STM) working under ultra high vacuum (UHV) condition. The HOPG samples have been heated to 500◦C in UHV before the STM experiments. STM measurements were performed at 78 K in the constant current mode with mechanically formed Pt/Ir tips. The same samples have been subsequently studied by atomic force microscopy (AFM), magnetic force microscopy (MFM) and electrostatic force microscopy (EFM) in air using Dimension 3100 SPM from Veeco Instruments. PPP-MFMR cantilevers made by NanoSensors with a hard magnetic material Co-coating film have been used in the MFM tapping/lift mode. Results and discussion. – In figure 1, AFM, MFM and EFM images of the same area on the HOPG surface are shown. AFM topography picture in figure 1a displays a surface with a high population of step edges, surface distortions and defects. The MFM images in figure 1b and figure 1c were taken on the same place as the AFM image with a lift scan height of 50 nm, where long-range van der Waals forces are negligible and magnetic forces prevail. Magnetic signal is measured on most of the line defects, however, a step edge marked as A in figure 1a does not show a magnetic signal in the MFM image. On the other hand, two lines in the MFM image in figure 1b that are indicated as B and C do not show a noticeable height difference in the topography. The lines B and C are grain boundaries of HOPG. A detailed AFM and STM study can be found elsewhere [16,17]. In order to determine the character of the detected magnetic signal, the MFM tip has been magnetized in two opposite directions: aiming into (figure 1b) and out of the graphite surface plane (figure 1c). Since the MFM signal represents the phase shift between the probe oscillation and the driving signal due to a magnetic force acting on the tip, the dependence of the phase shift on the force gradient can be expressed by a simple form [18]