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µSR investigations of ball milled graphite

µSR investigations of ball milled graphite. M. Riccò 1 , M. Mazzani 1 , D. Pontiroli 1 , M. Belli 1 , F. Rossi 2 , L. Nasi 2. 1 Dipartimento di Fisica and CNISM, Università di Parma, Parma, Italy 2 CNR-IMEM Institute, Parma, Italy. Muons source. -Spallation source (flux 100-300Mev/h)

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µSR investigations of ball milled graphite

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  1. µSR investigations of ball milled graphite M. Riccò1, M. Mazzani1, D. Pontiroli1, M. Belli1, F. Rossi2, L. Nasi2 1Dipartimento di Fisica and CNISM, Università di Parma, Parma, Italy 2CNR-IMEM Institute, Parma, Italy

  2. Muons source -Spallation source (flux 100-300Mev/h) -Surface (Arizona) muons (Eµ=4 MeV) -Stopping range 0.15±0.01 g/cm2 (0.2 mm in Cu, 1.5 mm in H2O) -Pulsed source (max. frequency 6 MHz – 440G)

  3. Transverse geometry • Muons localise at an interstitial site and precess under the effect of the local field plus the applied field, with  = ·(Bloc+H). • The signal decay can be due either to spin-spin relaxation (local field inhomogeneities) or spin-lattice effects (fluctuating fields). • Very similar to an NMR experiment.

  4. Schematic LF (ZF) µSR experiment Longitudinal geometry • Muons (I = ½, = 2.2 µs), localise at an interstitial site and precess at the local field, with  = ·Bloc. • ZF - method of detecting weak internal magnetism, that arises due to ordered magnetic moments, or random fields that are static or fluctuating with time. • Applying an external LF field one can “freeze” the muon spin direction, and discriminate between static and dynamic local fields.

  5. µSR in matter The µ binds to an electron Insulators  H

  6. µSR in matter Electrons screen the µ Coulomb field (Yukawa) Metals γµ=136 kHz/mT Is a paramagnetic center (or its spin state) stable in a metal? NO

  7. µSR in magnetic matter • In zero applied field the muon precesses in the local magnetic hyperfine field (HF). • The external applied field sums to HF and affects the magnetic phase. • Not possible in an NMR experiment. Robust magnet→high precession frequency

  8. Muonium reactions: radicals Stable in: semiconductors, quartz, saturated hydrocarbons, endohedral fullerenes…. Reacts with: unsaturated and aromatic hydrcarbons etc. Radical: 1-The HF interaction is rather reduced (precession frequencies are observable) 2- The HF interaction can be anisotropic

  9. Semimetals? Graphite? Just like a (non magnetic) metal? n low n(RT) ≈ 3 1018 cm-3 (threshold 3 1022) T dependent Knight Shifta High T relaxation of the polarization → NOT KORRINGA from b) a) J. Chakhalian, R. F. Kiefl et al. PRB 66 (2002) 155107 b) S. F. J. Cox, M. Heggie, et al., Journal of Physics: Condensed Matter 13, 2169 (2001).

  10. µSR in Graphite DFT calc.+ local spin density AIMPRO S. F. J. Cox, M. Heggie, et al., Journal of Physics: Condensed Matter 13, 2169 (2001).

  11. Known issues New issues

  12. in-plane vacancies edges Turbostratic disorder Ball Milled Graphite • Precursor: powder graphiteRW-A(SGL Carbon)mesh 240, content of Fe <0.2 ppm, previously treated in vacuum 1 night at 800°C. Milled for 40 min. at 50 Hz in Ar atmosphere (<1 ppm H2O, O2)

  13. TEM of Ball Milled Graphite 10 nm

  14. µSR in Milled Graphite Zero Applied Field λ=0.33µ-1 (T independent @T>5K )

  15. µSR in Graphite Longitudinal Field Applied The decay is due to fast fluctuating local fields

  16. -µ Origin? Zig-Zag edges: 1-The muon attached to a terminal zig-zag carbon interacts with nearest dangling bonds 2- The paramagnetic electrons quickly exchange (depolarize) with the edge localized state Korringa: 1-The high density of defects increases the density of carriers 2- Implies a sizeable Knight Shift → TF experiment

  17. Korringa relaxation? =4.1% The relaxation is not Korringa

  18. Other sources of decay? Formation of in plane defects: vacancies + Mu µ in a diamagnetic environment → →not decaying fraction In agreement with spin-polarized DFT calculations: P. O. Lehtinen, A. S. Foster et al. PRL 93 187202.

  19. H2 treated sample after 13 days • Small ageing effect in H2 treated sample Ageing effect • Msdecreases if the delay between milling and treatment increases 30°C H2 treated graphite 80 Starting milled graphite 60 • Relation between number of PM active centres and magnetism Ms (10-3 emu/g) 40 20 100°C 500°C 250°C 0 0 50 100 150 200 250 Time (hours)

  20. µSR in Milled Graphite Zero Applied Field λ=0.33µ-1 (T independent @T>5K )

  21. Therefore The milling creates a metastable state (lifetime 12 h) withlarge local field (>500G) → Undetectable high frequency precession due to: 1- Magnetism (not revealed by SQUID) 2- Localized hyperfine interaction (radical) 3- Localized hyperfine interaction (muonium) The application of longitudinal field can discriminate among the different possibilities

  22. LF recovery of the missing fraction Completely recovers in 500G: 1-Not compatible with muonium. 2- Adduct radical with anisotropy (armchair edges)→ Fit 3- Magnetism (80-100 G anisotropy)? Unlikely but cannot be definitely excluded

  23. Conclusions • Muons are very efficiently trapped by graphene edges • Zig-zag edges induce a slow (0.33 µsec-1) depolarization • Armchair edges strongly depolarize muons through the HF interaction with the σ radical electron. • The armchair paramagnetic states (dangling bonds) slowly disappear (mean lifetime ≈12 h) • µSR proves to be an extremely sensitive probe for the study of graphite/graphene edges.

  24. Therefore

  25. Therefore

  26. Effects of milling (RW-A, 1 hour at 50 Hz in agate jar in Ar) 1-Reduction in grain size: from 60 µm to 10 nm (Sherrer broadening of X-ray) X-Ray powder diffraction Large amounts of edges!

  27. Muon depolarization comes from edges! Zig-zag Armchair Which of the two? Both?

  28. µ depolarization by edges Origin: Fluctuating local field due to the dipolar fields of a 1D array of unpaired electrons (dangling bonds) Zero field → T1=T2

  29. µSR in H2 Treated Graphite λ=0.33µ-1 Milled λ=0.23µ-1 Milled + H2@30C λ=0.25µ-1 Milled + H2@500C

  30. Low temperature frozen state • The slow muon depolarization (σ=0.38 µs-1) is only compatible with spin cancellation at its site. • AF ordering in the zigzag case • No compatible order in the armchair case (due to asymmetric disposition) Test: Zigzag edges are known to absorb molecular hydrogen with zero energy barrier. For armchair Ea ~ 1 eV.* * N. B. Arboleda, Jr. H. Kasai et al. J. Appl. Phys. 96 (2004) 6331

  31. Thanks for attention!

  32. G D ID/IG = 0.76→ r = 5.8 nm FWHM = 55 cm-1 → r = 6 nm Raman Determination of grain size (T. Makarova) • Raman measurements of freshly-milled graphite clearly shows that • BALL-MILLING BREAKS GRAPHENE PLANESforming small crystallites. • Their size distribution range from 4 to 8 nm with a strong maximum at 6nm • 2 independent ways to estimate crystallite size S • from the relative intensity ID/ IG which is proportional to S [ref. 1,2] • from the FWHM of the G peak [ref.2] • F.Tunistra, J.L.Koenig, Journal of Chem. Phys., 53, 1126 (1970). • A.C.Ferrari, J.Robertson, Phil.Trans. of Royal Soc. London A, 362, 2486 (2004).

  33. Low temperature (1.4K) decay Below 1.6K paramagnetic fluctuations get frozen

  34. γµ=136 kHz/mT

  35. Magnetometry in ball milled graphite

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