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Ferromagnetic microstructures in highly oriented pyrolytic graphite created by high energy proton irradiation
Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 886–890 www.elsevier.com/locate/nimb
Ferromagnetic microstructures in highly oriented pyrolytic graphite created by high energy proton irradiation D. Spemann b
a,*
€hne b, T. Butz , K.-H. Han b, P. Esquinazi b, R. Ho
a
a Nuclear Solid State Physics, University of Leipzig, Linnestr. 5, D-04103 Leipzig, Germany Superconductivity and Magnetism, University of Leipzig, Linnestr. 5, D-04103 Leipzig, Germany
Abstract In this study ferromagnetic microstructures were created in highly oriented pyrolitic graphite (HOPG) by proton microbeam irradiation. For this purpose, spots of 1, 2 · 2 and 3.5 · 3.5 lm2 were irradiated with ion fluences ranging from 3.1 · 1016 to 4.7 · 1019 cm2 using 2.25 MeV proton and 1.5 MeV helium ion microbeams. As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 · 1019 and 4 · 1022 cm3 for the proton irradiation. The irradiated spots, which were characterized with atomic force microscopy (AFM) and magnetic force microscopy (MFM), show a clear swelling of the HOPG crystal proportional to the ion fluence. Strong magnetic force gradients were found even for the lowest proton fluences. Contrary to the topography, the magnetic force gradient changes after the application of a magnetic field. This rules out that the magnetic signals arise from topographical changes. Therefore, the MFM measurements reveal the existence of ferromagnetic domains in localized, disordered HOPG regions. On the contrary, helium ion irradiation of HOPG leads to much weaker magnetic signals only, which indicates that hydrogen plays a significant role in the formation of the magnetic moments and ordering. Very recently, the existence of ferromagnetism in ion beam irradiated HOPG was confirmed by SQUID measurements. 2004 Elsevier B.V. All rights reserved. PACS: 07.78.+s; 07.79.Pk; 68.35.Dv; 75.50.Dd Keywords: Ferromagnetic carbon; Ion microbeam; Irradiation; Defects
1. Introduction Recent experimental work [1–3] and theoretical studies [4–6] suggest the existence of intrinsic ferromagnetism at room temperature in metal-free
* Corresponding author. Tel.: +49-341-97-32706; fax: +49341-97-32497. E-mail address: [email protected] (D. Spemann). URL: http://www.uni-leipzig.de/~nfp.
carbon structures. According to Ovchinnikov et al. a mixture of sp2 and sp3 hybridized carbon atoms may show a huge magnetization [4,5]. However, with the exception of [7], such a magnetic material has not been realized nor reproduced. In this study, our intention was to use 2.25 MeV proton and 1.5 MeV helium ion microbeams in order to create defects in highly oriented pyrolytic graphite (HOPG) with the hope to form a mixture of sp2 and sp3 bonds. For this purpose, spots of 1, 2 · 2 and 3.5 · 3.5 lm2 were irradiated with different ion fluences. Rutherford backscattering
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.182
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
spectrometry (RBS) and particle induced X-ray emission (PIXE) spectra recorded simultaneously with the irradiation allowed to check the purity of the irradiated spots. Furthermore, SRIM2003 Monte Carlo simulations were performed in order to calculate the defect densities produced in the HOPG. The topography and the magnetic signals from the irradiated areas were investigated using atomic force microscopy (AFM) and magnetic force microscopy (MFM), respectively.
2. Experimental 2.1. Sample preparation ZYA grade HOPG samples from Advanced Ceramics Co. with a content of metallic impurities well below 1 lg/g, as determined from PIXE, and a rocking-curve width of 0.4 was irradiated with a 2.25 MeV proton and a 1.5 MeV helium ion microbeam using the high energy ion nanoprobe LIPSION at the University of Leipzig. Beam spot diameters of 1 · 1 and 2 · 2 lm2 for the proton and 3.5 · 3.5 lm2 for the helium ion irradiation were chosen. The microbeam was directed onto the HOPG surface parallel to the c-axis without beam scanning (apart from line scans) leading to the
887
formation of micron-sized spots with enhanced defect density. The total deposited charge was calculated from the RBS yield for each spot. Spots separated by 20 lm were prepared for ion fluences ranging from 3.1 · 1016 to 4.7 · 1019 cm2 at beam currents of 200 pA for protons and 3.1 · 1016 – 3.1 · 1017 cm2 at beam currents of 100 pA for helium ions. As can be seen from Fig. 1 some of the irradiated spots are clearly visible in an optical microscope. This is due to the swelling of the HOPG crystal in and around the irradiated spot caused by the hydrogen or helium implantation at medium and high ion fluences. However, no exfoliation of the surface layer was observed even at the highest fluence. 2.2. Ion beam analysis at LIPSION The RBS and PIXE measurements were performed simultaneously with the irradiation of the spots. The X-ray spectra were recorded with an EG&G Ortec HPGe IGLET-X detector (active area: 95 mm2 , energy resolution: DEX ¼ 148 eV at 5.9 keV). The backscattered ions were detected at a backscattering angle h ¼ 170 using a Canberra PIPS detector. The RBS spectra were analyzed using the simulation code RUMP [8] including non-Rutherford cross-sections for carbon [9]. The analysis of the PIXE spectra were performed using GeoPIXE II [10]. Besides the analysis of the irradiated spots, large areas of HOPG samples from the same batch were analyzed with PIXE regarding the impurity content. On all samples the concentrations of the metallic impurities were found to be <1 lg/g. 2.3. Atomic force microscopy and magnetic force microscopy
Fig. 1. Optical micrograph of the HOPG surface after a proton microbeam irradiation. In general, two spots were irradiated with the same ion fluence and several ion fluences were used. Left: 2 · 2 lm2 spots, right: 1 · 1 lm2 . Additionally, 20 lm lines were irradiated with both beam spot sizes.
The irradiated spots and their surroundings were investigated at room temperature simultaneously by atomic force microscopy (AFM) and magnetic force microscopy (MFM) using a nanoscope III scanning probe microscope from digital instruments operated in ‘‘tapping/liftTM ’’ scanning mode. The scanning probes were batch fabricated silicon cantilevers with pyramidal tips coated with
888
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
a magnetic CoCr film alloy magnetized normal to the sample surface (i.e. z-direction), making the MFM sensitive to the second derivative of the zcomponent of the sample stray field. The typical error and reproducibility in the phase shift obtained, which is proportional to the force gradient, was ±0.1. All measurements were performed on the sample as prepared, i.e. before the application of an external magnetic field, (B ¼ 0) and after a magnetic field of 1 kOe was applied parallel to the c-axis of the HOPG in +z- (B-up) and )zdirection (B-down).
3. Results and discussion 3.1. Defect densities in the irradiated spots In order to calculate the defect densities created in the irradiated spots SRIM2003 Monte Carlo simulations [11] were performed with full damage cascades using a displacement energy of Ed ¼ 35 eV for the creation of a Frenkel pair in graphite. Fig. 2 shows the number of defects per ion and 1 lm depth interval obtained for 2.25 MeV protons in HOPG. Since MFM is only sensitive to the near surface region (depths <1 lm), the defect densities were calculated from the number of defects in the near surface region under the following assumptions: (i) no annealing of defects occurred during irradiation and (ii) the damaged region is of the
same size as the irradiated region. For proton fluences ranging from 3.1 · 1016 to 4.7 · 1019 cm2 the corresponding defect densities are between 3 · 1019 and 4 · 1022 cm3 . Whereas only 0.02% of the carbon atoms are displaced at the lowest fluence of 3.1 · 1016 cm2 , the highest fluence of 4.7 · 1019 cm2 results in a displacement of 35% of the carbon atoms, i.e. the defect densities span three orders of magnitude up to complete amorphization. According to the SRIM2003 simulations the 2.25 MeV protons come to rest at a depth of 46 lm from the graphite surface in a layer of 1.75 lm width (FWHM of the implantation profile). Since hydrogen is fully trapped in HOPG up to a concentration of 0.45 H/C without noticeable diffusion [12], all implanted protons will be trapped immediately where they come to rest in the HOPG for ion fluences up to 8.7 · 1018 cm2 . For the higher fluences a short-range diffusion in regions of lower hydrogen concentration followed by subsequent trapping is expected. For helium ion fluences ranging from 3.1 · 1016 to 3.1 · 1017 cm2 the corresponding defect densities in the near surface region are between 8 · 1020 and 8 · 1021 cm3 , which is in between the defect densities created by proton irradiation. Recent theoretical studies [13] show that our approach of forming a mixture of sp2 and sp3 hybridized carbon atoms via the creation of ion beam induced defects in HOPG is likely to be successful. 3.2. AFM and MFM measurements
Fig. 2. The number of defects per ion and 1 lm depth interval for 2.25 MeV protons in HOPG obtained from SRIM2003 Monte Carlo simulations.
Fig. 3 shows AFM and MFM images (scan size: 20 · 20 lm) as well as line scans extracted from the images as indicated by the black triangles for a 2 · 2 lm2 spot irradiated with 2.25 MeV protons at a fluence of 7.5 · 1016 cm2 . As can be seen from the AFM line scans, the swelling of the HOPG crystal amounts to 3 nm at this low fluence and does not change with the application of a magnetic field (B-up, B-down) within the experimental errors. On the contrary, the features displayed in the MFM images change noticeably with the magnetic field. The irradiated spot can be seen clearly, especially in the B-up and B-down image. The extracted line scans show that the observed
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
889
Fig. 3. AFM (top left) and MFM images (scan size: 20 · 20 lm) for a 2 · 2 lm2 spot irradiated with 2.25 MeV protons at a fluence of 7.5 · 1016 cm2 . The AFM and MFM line scans shown below were extracted from the images as indicated by the black triangles. Since the sample had to be removed from the MFM for the exposure to an external magnetic field, the subsequent repositioning of the spot in the MFM was accurate to 2 lm only.
magnetic signals differ significantly from each other after a magnetic field was applied (experimental uncertainty ±0.1), which demonstrates a ferromagnetic state and rules out possible artifacts from the topography. Previous measurements with the same MFM setup on polymerized fullerene samples on pure regions and regions containing Fe-impurities have shown that an MFM phase shift of 1 and 10 corresponds to a localized magnetic moment of 1017 and 1015 A m2 , respectively [14]. From this, we can conclude that the phase shifts measured on the irradiated spots indicate a large localized magnetic moment. With increasing proton fluence the swelling increases up to 250 nm for 4.7 · 1019 cm2 and is independent of an external magnetic field. The magnetic signals do not show a simple dependence between the phase shifts and the ion fluence. In general, the maximum phase shift increases slightly with fluence for low ion fluences. For higher ion fluences the maximum phase shift decreases or remains constant with fluence. In general, the
largest phase shifts were obtained for fluences below 3.1 · 1018 cm2 . Further studies are necessary in order to learn more about the fluence dependency of the phase shift. Contrary to the protons, the 1.5 MeV helium ion irradiation results in a significant swelling of 80 nm of the HOPG even at ion fluences as low as 3.1 · 1016 cm2 . Again the swelling is independent of an external magnetic field. Whereas the irradiated spots were easily found in the topography images, they appear very weak in the MFM images. The overall phase shifts observed in the magnetic images are as low as ±0.2 and are therefore much smaller than those obtained by proton irradiation. From this we can conclude that helium ion irradiation of HOPG leads to very weak magnetic signals only and that hydrogen appears to be important for the formation of magnetic moments and ordering in HOPG created by ion bombardment. As a possible explanation for our results we refer either to the work in [15] where a sp3 –sp2 ferrimagnetic structure can arise in
890
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
a graphene plane through the existence of monoand dihydrogenated carbon atoms or to the case of mono-hydrogenated edges [16]. Very recently, the existence of ferromagnetism in ion beam irradiated HOPG was confirmed by SQUID measurements [17]. For this purpose, HOPG samples were first characterized with a SQUID magnetometer and then irradiated with a broad proton beam with a typical fluence of 2 · 1017 cm2 on a total area of 2 mm2 . After the irradiation ferromagnetic loops with coercive fields between 80 and 110 Oe and remanent magnetic moments up to mrem 106 emu were observed. The hysteresis loops were measured between 5 and 380 K and showed no significant variation with temperature indicating a Curie temperature much above 400 K.
4. Conclusions and outlook We have produced ferromagnetic microstructures in HOPG by proton microbeam irradiation. The irradiated spots show a clear change in topography proportional to the ion fluences due to the swelling of the HOPG crystal. Furthermore, strong magnetic force gradients were found even for the lowest ion fluences. Since the magnetic force gradient changes with the application of a magnetic field while the topography does not, it can be ruled out that the magnetic signals arise from topographical changes. Therefore, the MFM measurements reveal the existence of ferromagnetic domains in localized, disordered HOPG regions. On the contrary, helium ion irradiation of HOPG leads to very weak magnetic signals only, which indicates that hydrogen plays a significant role in the formation of magnetic moments and ordering. Our experimental findings open up new fields of applications for ion micro- and nanoprobes. In the future, magnetic microstructures of arbitrary shape will be produced in HOPG by a dedicated ion beam scanning system. Furthermore, the studies will be extended to other graphitic materials, e.g. amorphous carbon films. In order to gain more insight into the physical processes involved systematic investigations of the
magnetic properties of different carbon materials irradiated at different ion fluences are necessary.
Acknowledgements The financial support by the Federal Ministry of Education and Research of the FRG within the funding scheme ‘‘Innovative regionale Wachstumskerne’’ under grant 03WKI09 and the Deutsche Forschungsgemeinschaft DFG under grant Es 86/6-3 is gratefully acknowledged.
References [1] Y. Kopelevich, P. Esquinazi, J.H.S. Torres, S. Moehlecke, J. Low Temp. Phys. 119 (2000) 691. [2] P. Esquinazi, A. Setzer, R. H€ ohne, C. Semmelhack, Y. Kopelevich, D. Spemann, T. Butz, B. Kohlstrunk, M. L€ osche, Phys. Rev. B 66 (2002) 024429. [3] T.L. Makarova, B. Sundqvist, R. H€ ohne, P. Esquinazi, Y. Kopelevich, P. Scharff, V. Davydov, L.S. Kashevarova, A.V. Rakhmanina, Nature (London) 413 (2001) 716. [4] A.A. Ovchinnikov, I.L. Shamovsky, J. Mol. Struct. (Theochem.) 251 (1991) 133. [5] A.A. Ovchinnikov, I.L. Shamovsky, K.V. Bozhenko, J. Mol. Struc. (Theochem.) 251 (1991) 141. [6] J. Gonzalez, F. Guinea, M. Vozmediano, Phys. Rev. B 63 (2001) 134421. [7] Y.M. Shulga, A.I. Boldyrev, A.A. Ovchinnikov, Chem. Phys. Lett. 189 (1992) 577. [8] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [9] E. Rauhala, Nucl. Instr. and Meth. B 12 (1985) 447. [10] C.G. Ryan, Nucl. Instr. and Meth. B 181 (2001) 170. [11] J.F. Ziegler (Ed.), The Stopping and Range of Ions in Matter, Vols. 2–6, Pergamon Press, New York, 1977– 1985. [12] R. Siegele, J. Roth, B.M.U. Scherzer, S.J. Pennycook, J. Appl. Phys. 73 (1993) 2225. [13] R.H. Telling, C.P. Ewels, A.A. El-Barbary, M.I. Heggie, Nature Mater. 2 (2003) 333. [14] K.-H. Han, D. Spemann, R. H€ ohne, A. Setzer, T. Makarova, P. Esquinazi, T. Butz, Carbon 41 (2003) 785; Addendum, Carbon 41 (2003) 2425. [15] K. Kusakabe, M. Maruyama, Phys. Rev. B 67 (2003) 092406. [16] M. Fujita, K. Wakabayashi, K. Nakada, K. Kusakabe, J. Phys. Soc. Jpn. 65 (1996) 1920; See also K. Wakabayashi, M. Fujita, H. Ajiki, M. Sigrist, Phys. Rev. B 59 (1999) 8271. [17] P. Esquinazi, D. Spemann, R. H€ ohne, A. Setzer, K.-H. Han, T. Butz, Phys. Rev. Lett. 91 (2003) 227201.
Ferromagnetic microstructures in highly oriented pyrolytic graphite created by high energy proton irradiation D. Spemann b
a,*
€hne b, T. Butz , K.-H. Han b, P. Esquinazi b, R. Ho
a
a Nuclear Solid State Physics, University of Leipzig, Linnestr. 5, D-04103 Leipzig, Germany Superconductivity and Magnetism, University of Leipzig, Linnestr. 5, D-04103 Leipzig, Germany
Abstract In this study ferromagnetic microstructures were created in highly oriented pyrolitic graphite (HOPG) by proton microbeam irradiation. For this purpose, spots of 1, 2 · 2 and 3.5 · 3.5 lm2 were irradiated with ion fluences ranging from 3.1 · 1016 to 4.7 · 1019 cm2 using 2.25 MeV proton and 1.5 MeV helium ion microbeams. As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 · 1019 and 4 · 1022 cm3 for the proton irradiation. The irradiated spots, which were characterized with atomic force microscopy (AFM) and magnetic force microscopy (MFM), show a clear swelling of the HOPG crystal proportional to the ion fluence. Strong magnetic force gradients were found even for the lowest proton fluences. Contrary to the topography, the magnetic force gradient changes after the application of a magnetic field. This rules out that the magnetic signals arise from topographical changes. Therefore, the MFM measurements reveal the existence of ferromagnetic domains in localized, disordered HOPG regions. On the contrary, helium ion irradiation of HOPG leads to much weaker magnetic signals only, which indicates that hydrogen plays a significant role in the formation of the magnetic moments and ordering. Very recently, the existence of ferromagnetism in ion beam irradiated HOPG was confirmed by SQUID measurements. 2004 Elsevier B.V. All rights reserved. PACS: 07.78.+s; 07.79.Pk; 68.35.Dv; 75.50.Dd Keywords: Ferromagnetic carbon; Ion microbeam; Irradiation; Defects
1. Introduction Recent experimental work [1–3] and theoretical studies [4–6] suggest the existence of intrinsic ferromagnetism at room temperature in metal-free
* Corresponding author. Tel.: +49-341-97-32706; fax: +49341-97-32497. E-mail address: [email protected] (D. Spemann). URL: http://www.uni-leipzig.de/~nfp.
carbon structures. According to Ovchinnikov et al. a mixture of sp2 and sp3 hybridized carbon atoms may show a huge magnetization [4,5]. However, with the exception of [7], such a magnetic material has not been realized nor reproduced. In this study, our intention was to use 2.25 MeV proton and 1.5 MeV helium ion microbeams in order to create defects in highly oriented pyrolytic graphite (HOPG) with the hope to form a mixture of sp2 and sp3 bonds. For this purpose, spots of 1, 2 · 2 and 3.5 · 3.5 lm2 were irradiated with different ion fluences. Rutherford backscattering
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.182
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
spectrometry (RBS) and particle induced X-ray emission (PIXE) spectra recorded simultaneously with the irradiation allowed to check the purity of the irradiated spots. Furthermore, SRIM2003 Monte Carlo simulations were performed in order to calculate the defect densities produced in the HOPG. The topography and the magnetic signals from the irradiated areas were investigated using atomic force microscopy (AFM) and magnetic force microscopy (MFM), respectively.
2. Experimental 2.1. Sample preparation ZYA grade HOPG samples from Advanced Ceramics Co. with a content of metallic impurities well below 1 lg/g, as determined from PIXE, and a rocking-curve width of 0.4 was irradiated with a 2.25 MeV proton and a 1.5 MeV helium ion microbeam using the high energy ion nanoprobe LIPSION at the University of Leipzig. Beam spot diameters of 1 · 1 and 2 · 2 lm2 for the proton and 3.5 · 3.5 lm2 for the helium ion irradiation were chosen. The microbeam was directed onto the HOPG surface parallel to the c-axis without beam scanning (apart from line scans) leading to the
887
formation of micron-sized spots with enhanced defect density. The total deposited charge was calculated from the RBS yield for each spot. Spots separated by 20 lm were prepared for ion fluences ranging from 3.1 · 1016 to 4.7 · 1019 cm2 at beam currents of 200 pA for protons and 3.1 · 1016 – 3.1 · 1017 cm2 at beam currents of 100 pA for helium ions. As can be seen from Fig. 1 some of the irradiated spots are clearly visible in an optical microscope. This is due to the swelling of the HOPG crystal in and around the irradiated spot caused by the hydrogen or helium implantation at medium and high ion fluences. However, no exfoliation of the surface layer was observed even at the highest fluence. 2.2. Ion beam analysis at LIPSION The RBS and PIXE measurements were performed simultaneously with the irradiation of the spots. The X-ray spectra were recorded with an EG&G Ortec HPGe IGLET-X detector (active area: 95 mm2 , energy resolution: DEX ¼ 148 eV at 5.9 keV). The backscattered ions were detected at a backscattering angle h ¼ 170 using a Canberra PIPS detector. The RBS spectra were analyzed using the simulation code RUMP [8] including non-Rutherford cross-sections for carbon [9]. The analysis of the PIXE spectra were performed using GeoPIXE II [10]. Besides the analysis of the irradiated spots, large areas of HOPG samples from the same batch were analyzed with PIXE regarding the impurity content. On all samples the concentrations of the metallic impurities were found to be <1 lg/g. 2.3. Atomic force microscopy and magnetic force microscopy
Fig. 1. Optical micrograph of the HOPG surface after a proton microbeam irradiation. In general, two spots were irradiated with the same ion fluence and several ion fluences were used. Left: 2 · 2 lm2 spots, right: 1 · 1 lm2 . Additionally, 20 lm lines were irradiated with both beam spot sizes.
The irradiated spots and their surroundings were investigated at room temperature simultaneously by atomic force microscopy (AFM) and magnetic force microscopy (MFM) using a nanoscope III scanning probe microscope from digital instruments operated in ‘‘tapping/liftTM ’’ scanning mode. The scanning probes were batch fabricated silicon cantilevers with pyramidal tips coated with
888
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
a magnetic CoCr film alloy magnetized normal to the sample surface (i.e. z-direction), making the MFM sensitive to the second derivative of the zcomponent of the sample stray field. The typical error and reproducibility in the phase shift obtained, which is proportional to the force gradient, was ±0.1. All measurements were performed on the sample as prepared, i.e. before the application of an external magnetic field, (B ¼ 0) and after a magnetic field of 1 kOe was applied parallel to the c-axis of the HOPG in +z- (B-up) and )zdirection (B-down).
3. Results and discussion 3.1. Defect densities in the irradiated spots In order to calculate the defect densities created in the irradiated spots SRIM2003 Monte Carlo simulations [11] were performed with full damage cascades using a displacement energy of Ed ¼ 35 eV for the creation of a Frenkel pair in graphite. Fig. 2 shows the number of defects per ion and 1 lm depth interval obtained for 2.25 MeV protons in HOPG. Since MFM is only sensitive to the near surface region (depths <1 lm), the defect densities were calculated from the number of defects in the near surface region under the following assumptions: (i) no annealing of defects occurred during irradiation and (ii) the damaged region is of the
same size as the irradiated region. For proton fluences ranging from 3.1 · 1016 to 4.7 · 1019 cm2 the corresponding defect densities are between 3 · 1019 and 4 · 1022 cm3 . Whereas only 0.02% of the carbon atoms are displaced at the lowest fluence of 3.1 · 1016 cm2 , the highest fluence of 4.7 · 1019 cm2 results in a displacement of 35% of the carbon atoms, i.e. the defect densities span three orders of magnitude up to complete amorphization. According to the SRIM2003 simulations the 2.25 MeV protons come to rest at a depth of 46 lm from the graphite surface in a layer of 1.75 lm width (FWHM of the implantation profile). Since hydrogen is fully trapped in HOPG up to a concentration of 0.45 H/C without noticeable diffusion [12], all implanted protons will be trapped immediately where they come to rest in the HOPG for ion fluences up to 8.7 · 1018 cm2 . For the higher fluences a short-range diffusion in regions of lower hydrogen concentration followed by subsequent trapping is expected. For helium ion fluences ranging from 3.1 · 1016 to 3.1 · 1017 cm2 the corresponding defect densities in the near surface region are between 8 · 1020 and 8 · 1021 cm3 , which is in between the defect densities created by proton irradiation. Recent theoretical studies [13] show that our approach of forming a mixture of sp2 and sp3 hybridized carbon atoms via the creation of ion beam induced defects in HOPG is likely to be successful. 3.2. AFM and MFM measurements
Fig. 2. The number of defects per ion and 1 lm depth interval for 2.25 MeV protons in HOPG obtained from SRIM2003 Monte Carlo simulations.
Fig. 3 shows AFM and MFM images (scan size: 20 · 20 lm) as well as line scans extracted from the images as indicated by the black triangles for a 2 · 2 lm2 spot irradiated with 2.25 MeV protons at a fluence of 7.5 · 1016 cm2 . As can be seen from the AFM line scans, the swelling of the HOPG crystal amounts to 3 nm at this low fluence and does not change with the application of a magnetic field (B-up, B-down) within the experimental errors. On the contrary, the features displayed in the MFM images change noticeably with the magnetic field. The irradiated spot can be seen clearly, especially in the B-up and B-down image. The extracted line scans show that the observed
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
889
Fig. 3. AFM (top left) and MFM images (scan size: 20 · 20 lm) for a 2 · 2 lm2 spot irradiated with 2.25 MeV protons at a fluence of 7.5 · 1016 cm2 . The AFM and MFM line scans shown below were extracted from the images as indicated by the black triangles. Since the sample had to be removed from the MFM for the exposure to an external magnetic field, the subsequent repositioning of the spot in the MFM was accurate to 2 lm only.
magnetic signals differ significantly from each other after a magnetic field was applied (experimental uncertainty ±0.1), which demonstrates a ferromagnetic state and rules out possible artifacts from the topography. Previous measurements with the same MFM setup on polymerized fullerene samples on pure regions and regions containing Fe-impurities have shown that an MFM phase shift of 1 and 10 corresponds to a localized magnetic moment of 1017 and 1015 A m2 , respectively [14]. From this, we can conclude that the phase shifts measured on the irradiated spots indicate a large localized magnetic moment. With increasing proton fluence the swelling increases up to 250 nm for 4.7 · 1019 cm2 and is independent of an external magnetic field. The magnetic signals do not show a simple dependence between the phase shifts and the ion fluence. In general, the maximum phase shift increases slightly with fluence for low ion fluences. For higher ion fluences the maximum phase shift decreases or remains constant with fluence. In general, the
largest phase shifts were obtained for fluences below 3.1 · 1018 cm2 . Further studies are necessary in order to learn more about the fluence dependency of the phase shift. Contrary to the protons, the 1.5 MeV helium ion irradiation results in a significant swelling of 80 nm of the HOPG even at ion fluences as low as 3.1 · 1016 cm2 . Again the swelling is independent of an external magnetic field. Whereas the irradiated spots were easily found in the topography images, they appear very weak in the MFM images. The overall phase shifts observed in the magnetic images are as low as ±0.2 and are therefore much smaller than those obtained by proton irradiation. From this we can conclude that helium ion irradiation of HOPG leads to very weak magnetic signals only and that hydrogen appears to be important for the formation of magnetic moments and ordering in HOPG created by ion bombardment. As a possible explanation for our results we refer either to the work in [15] where a sp3 –sp2 ferrimagnetic structure can arise in
890
D. Spemann et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 886–890
a graphene plane through the existence of monoand dihydrogenated carbon atoms or to the case of mono-hydrogenated edges [16]. Very recently, the existence of ferromagnetism in ion beam irradiated HOPG was confirmed by SQUID measurements [17]. For this purpose, HOPG samples were first characterized with a SQUID magnetometer and then irradiated with a broad proton beam with a typical fluence of 2 · 1017 cm2 on a total area of 2 mm2 . After the irradiation ferromagnetic loops with coercive fields between 80 and 110 Oe and remanent magnetic moments up to mrem 106 emu were observed. The hysteresis loops were measured between 5 and 380 K and showed no significant variation with temperature indicating a Curie temperature much above 400 K.
4. Conclusions and outlook We have produced ferromagnetic microstructures in HOPG by proton microbeam irradiation. The irradiated spots show a clear change in topography proportional to the ion fluences due to the swelling of the HOPG crystal. Furthermore, strong magnetic force gradients were found even for the lowest ion fluences. Since the magnetic force gradient changes with the application of a magnetic field while the topography does not, it can be ruled out that the magnetic signals arise from topographical changes. Therefore, the MFM measurements reveal the existence of ferromagnetic domains in localized, disordered HOPG regions. On the contrary, helium ion irradiation of HOPG leads to very weak magnetic signals only, which indicates that hydrogen plays a significant role in the formation of magnetic moments and ordering. Our experimental findings open up new fields of applications for ion micro- and nanoprobes. In the future, magnetic microstructures of arbitrary shape will be produced in HOPG by a dedicated ion beam scanning system. Furthermore, the studies will be extended to other graphitic materials, e.g. amorphous carbon films. In order to gain more insight into the physical processes involved systematic investigations of the
magnetic properties of different carbon materials irradiated at different ion fluences are necessary.
Acknowledgements The financial support by the Federal Ministry of Education and Research of the FRG within the funding scheme ‘‘Innovative regionale Wachstumskerne’’ under grant 03WKI09 and the Deutsche Forschungsgemeinschaft DFG under grant Es 86/6-3 is gratefully acknowledged.
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