Transcript spin valves
Endre Tóvári International Workshop on Electrical Properties of New Materials (IWEPNM) Kirchberg-in-Tirol, 2012. March 3-10. 1 Tóvári Endre: Kirchberg IWEPNM 2012 2 Topics • synthesis of CNT and graphene • CNT sorting and functionalization, nanopore arrays • graphene magnetism, spintronics, 1 mm optoelectronics, transport in suspended SLG, BLG and TLG, FQHE, graphene on hexagonal BN • optical conductivity, Raman spectroscopy, intrinsic properties, effect of strain, substrate, ESR • functionalization, doping Tóvári Endre: Kirchberg IWEPNM 2012 3 A few interesting presentations Growth of high-density CNT forests Liquid-induced densification of different SWNT forests bygel soaking the samples with ethanol and drying in air. Sorting of SWCNTs using multi-column chomatography structure-dependent interaction strength of SWCNTs with an allyl dextran-based gel Japanese Journal of Applied Physics 51 (2012) 01AH01 NATURE COMMUNICATIONS | 2:309 | DOI: 10.1038/ncomms1313 Tóvári Endre: Kirchberg IWEPNM 2012 4 A few interesting presentations Repeated growth and bubbling transfer of graphene with millimetre-size singlecrystal grains using platinum • • • • • • • • mm-size single-crystal SLG CVD on polycrystalline Pt µ > 7000 cm2V-1s-1 bubbling transfer: nondestructive to Pt and graphene both increasing T or low conc. of CH4: nucleation density decreases, grain size increases (ambient pressure, T>1000°C) CH4/H2 flow ratio 4/700 sccm: dominantly hexagonal grains with smooth edges (suppressed nucleation, and low stability edges are etched away by active atomic H) most grains: no reflex angle at edges, no visible boundaries under SEM no new nuclei with increasing growth time Nature Communications, 3:699 | DOI: 10.1038/ncomms1702 Raman D-band intensity map: showing grain boundary 1 mm 0,5 mm d,e,f g,h,i 100 µm d,e,f: grain boundary 10 µm 10 µm 10 µm 10 µm Tóvári Endre: Kirchberg IWEPNM 2012 5 A few interesting presentations Repeated growth and bubbling transfer of graphene with millimetre-size singlecrystal grains using platinum Pt+Gr+PMMA cathode (-): Bubbling transfer: aqueous NaOH electrolysis cell nondestructive to Pt: reusable nondestructive to graphene, transfer to SiO2 • free of metal residues • preserves the original shape • mostly monolayer • small Raman D-band (ID/IG<5%) • µ > 7000 cm2V-1s-1 100 µm 400 µm Nature Communications, 3:699 | DOI: 10.1038/ncomms1702 Tóvári Endre: Kirchberg IWEPNM 2012 6 A few interesting presentations Spin-half paramagnetism in graphene induced by point defects SQUID magnetometry graphene laminates: large collections of electronically non-interacting, parallel SLG and BLG crystallites (10-50 nm) before and after fluorination fluorination: clustering, only the atoms at cluster edge without pairs on the other C sublattice contribute ~10-3 µB/F high conc. (x≈1): lower M, but still PM (still a large number of defects in the CFx lattice) (F conc.: Raman, XPS) 350-400 keV proton irradiation or 20 MeV C4+ irrad.: ~0,1-0,4 µB/defect (defect conc. calc. with simulation!) Nature Physics Vol 8, 199, March 2012 Tóvári Endre: Kirchberg IWEPNM 2012 Review: Spin transport and relaxation in graphene • low intrinsic spin-orbit coupling (SOC) and hyperfine coupling (HFC) • key words: spin injection, diffusion (spin-polarized currents), precession in magnetic field, gate dependence, relaxation mechanisms • extrinsic spin relaxation mechanisms (τS~µs expected, ~100 ps – ns measured): Elliot-Yafet,local Dyakonov-Perel nonlocal setup setup spin valves spin diffusion is usually described by a spindependent chemical potential (µ↑ and µ↓), where a splitting of the chemical potential corresponds to the spin density in the graphene. Han, McCreary, Pi, Wang, Li, Wen, Chen, Kawakami ; Journal of Magnetism and Magnetic Materials 324 (2012) 369–381 7 Tóvári Endre: Kirchberg IWEPNM 2012 8 Spin valves: introduction changing in-plane magnetic field: changing magnetic polarization of Co electrodes (different widths, different coercivities: switching at different fields) nonlocal local Nature Letters Vol 448, 571 (Aug 2007) Tóvári Endre: Kirchberg IWEPNM 2012 Spin valves: introduction Transparent and tunneling contacts: difference in spin injection efficiency (1% or 26-30%) Spin relaxation: contact spacing RG: graphene spin resistance RF: FM contact’s spin resistance RJ: contact resistance PF: spin polarization in FM contact PJ: polarization if interfacial current L: contact spacing λG: spin diffusion length transparent/tunneling: relation of RJ and RG 9 Tóvári Endre: Kirchberg IWEPNM 2012 10 Spin valves: introduction Hanle spin precession in a ┴ magnetic field: spin precession sign: P or AP contact polariz. spin diffusion in the L (contact spacing) length spin precession spin relaxation L gBH S DS ~ 1 4 m Tóvári Endre: Kirchberg IWEPNM 2012 Spin transport and relaxation in graphene 𝜏S does not decrease, although D does: Au doping is effective at generating momentum scattering, but in SLG for transparent contacts charged impurity scattering is not the dominant process behind spin relaxation Tunneling contacts on SLG: 400-1000 ps Spin diffusion from Co into SLG: escape time 1 1 1 S1 spin flip esc spin flip in good tunnel barriers For pinhole and transparent contacts (50-200 ps range) the dominant spin relaxation is generated by the contacts (escape time, inhomogeneous fields, interfacial scattering) 11 Tóvári Endre: Kirchberg IWEPNM 2012 12 Spin relaxation in single-layer graphene tunnel barriers: suppress the contact-induced spin relaxation 𝜏S ~ 400-1000 ps 300 K: no correlation with D 4 K: strong correlation of 𝜏S and D in SLG with tunneling contacts: both increasing with carrier conc.; similar behaviour as a function of T D (10-2 m2/s) 𝜏S ~ D ~ 𝜏p: Elliott-Yafet spin relaxation dominant at low T in SLG: finite probability of spin-flip during a momentum scattering event (possible sources: long-range and short-range impurity scattering; at RT multiple sources are possible, such as phonons, which ruin the linear relationship; see Ref. 5) Tóvári Endre: Kirchberg IWEPNM 2012 Spin relaxation in bilayer graphene 300 K: : 𝜏S ~ 200-400 ps, and no correlation with D 4 K: 𝜏S ~ 2-6 ns, strong correlation with D: opposite behaviour with D as a function of gate voltage 𝜏S-1 ~ D ~ 𝜏p dominant spin relaxation mechanism in BLG at low T (with tunneling contacts): random magnetic Rashba fields of the Dyakonov-Pereltype (can be generated by ripples, adatoms in the graphene sheet): spin relaxation via precession in internal spin-orbit fields. (Elliott-Yafet mechanisms negligible due to enhanced screening of scatterers) 13 Tóvári Endre: Kirchberg IWEPNM 2012 14 All of the above mainly from: Han, McCreary, Pi, Wang, Li, Wen, Chen, Kawakami: Review - Spin transport and relaxation in graphene, Journal of Magnetism and Magnetic Materials 324 (2012) 369–381 Related articles (among many; mainly from van Wees’ and Kawakami’s group) : 1. Electronic spin transport and spin precession in SLG at room temperature, Nature Letters Vol 448, 571 (Aug 2007) 2. Tunneling spin injection into SLG, PRL 105, 167202 (2010) 3. Comparison between charge and spin transport in FLG, PRB 83, 115410 (2011) 4. Observation of long spin-relaxation times in BLG at room temperature, PRL 107, 047206 (2011) 5. Spin relaxation in single layer and bilayer graphene, PRL 107, 047207 (2011) 15 16 Thank you for your attention! Tóvári Endre: Kirchberg IWEPNM 2012 pictures from the first slide: • arXiv:1202.3212v1 • PRL 107, 217203 (2011) • SCIENCE VOL 334, 648 (2011) • Nature Communications, 3:699 | DOI: 10.1038/ncomms1702 http://www.iwepnm.org/2012/calendar.php 17