Magnetoresistive junctions based on epitaxial
                 graphene and h-BN


          Oleg Yazyev and Alfredo Pasqu...
Foreword                     2


    Questions or comments?
     oleg.yazyev@epfl.ch



   The slides are available at
  h...
Magnetoresistive junctions                                    3
                                                         M...
Spacer materials for MRJs                                              4
Giant magnetoresistance (GMR)
Example: Co|Cu|Co t...
Epitaxial graphene and h-BN                                                5
                                         Chem...
Multilayer graphene as perfect spin filter                                             6
   In ballistic transport through...
Technical details                              7
• PWSCF and PWCOND codes of the Quantum-ESPRESSO
electronic structure cal...
Atomic structure                                   8




(a) Top-view of epitaxial graphene on (111) surface of fcc Co or ...
Electronic structure                               9




        Spin-resolved density of states projected onto the spacer...
Role of the spacer material   10
hcp Co layers

   graphene
   MR = 86%



      h-BN
    MR = 66%



     vacuum
    MR =...
Role of the ferromagnetic metal   11
h-BN spacer

     fcc Fe
   MR = 149%



    fcc Co
   MR = 27%



    fcc Ni
   MR =...
Conclusions                                 12
• Epitaxial graphene and h-BN as ultrathin covalent spacer
materials for GM...
Acknowledgements                          13
My collaborator
 Alfredo Pasquarello (EPFL and IRRMA, Switzerland)

We acknow...
Thank you for your attention!



   Questions or comments?
    oleg.yazyev@epfl.ch

  The slides are available at
 http://...
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Magnetoresistive junctions based on epitaxial graphene and h-BN

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Magnetoresistive junctions based on epitaxial graphene and h-BN

  1. 1. Magnetoresistive junctions based on epitaxial graphene and h-BN Oleg Yazyev and Alfredo Pasquarello Ecole Polytechnique Fédérale de Lausanne (EPFL) Institute for Numerical Research in the Institute of Theoretical Physics Physics of Materials (IRRMA) (Focus Session J29: Spin Currents in Metals – New and Miscellaneous Topics: J29.00011) APS March meeting, Pittsburgh March 17, 2009
  2. 2. Foreword 2 Questions or comments? oleg.yazyev@epfl.ch The slides are available at http://slideshare.net/yazyev
  3. 3. Magnetoresistive junctions 3 Magnetoresistive junctoins (MRJs) – change electric resistance with the change of relative orientation of parallel magnetic moments of two (P) ferromagnetic layers separated by a non-magnetic spacer layer. Magnetoresistance ratio (MR): antiparallel (AP) Heiliger, Zahn & Mertig, Materials Today 9, 46 (2006); Fert, Rev. Mod. Phys. 80, 1517 (2008) Domains of application: • magnetic field sensing (e.g. in the reading heads of hard drives) • magnetic random access memories (MRAM) • spin transfer nano-oscillators (STNO)
  4. 4. Spacer materials for MRJs 4 Giant magnetoresistance (GMR) Example: Co|Cu|Co trialyers Co Spacer material: metal Cu MR typically < 20% and Co low electric resistance Tunneling magnetoresistance (TMR) Example: Fe|MgO|Fe trialyers Spacer material: ionic insulator MR upto 1000% and high electric resistance Quality of ferromagnet/spacer inter- faces is of primary concern Heiliger, Zahn & Mertig, Materials Today 9, 46 (2006); Fert, Rev. Mod. Phys. 80, 1517 (2008)
  5. 5. Epitaxial graphene and h-BN 5 Chemical vapor deposition (CVD) Epitaxial monolayer graphene and growth of well-ordered epitaxial layers isostructural hexagonal boron nitride on a variety of metallic substrates. (h-BN) as the ultimate thickness covalent spacer materials for MRJs. Oshima & Nagashima, J. Phys.: Condes. Matter 9, 1 (1997) Commensurate growth on transition metal fcc(111) and hcp(0001) surfaces due to the lattice constant matching: Co hcp(0001) 2.51 A Ni fcc(111) 2.49 A graphene 2.46 A h-BN 2.50 A The growth is typically self-inhibiting, i.e. stops after one monolayer Deposition of second metal layer was demonstrated. Intercalation of other metals possible.
  6. 6. Multilayer graphene as perfect spin filter 6 In ballistic transport through ordered interfaces k|| is conserved. Perfect spin filtering through transition metal|graphene interface was predicted. majority minority graphene hcp Co G0 / unit cell M K  But, will require well-ordered interfaces with multilayer (n > 3) graphene. Karpan et al., Phys. Rev. Lett. 99, 176602 (2007); Phys. Rev. B 78, 195419 (2008).
  7. 7. Technical details 7 • PWSCF and PWCOND codes of the Quantum-ESPRESSO electronic structure calculations package • Ultrasoft pseudopotentials, plane wave basis set, GGA exchange correlation density functional • We study the lowest energy structures of symmetric junctions Spacers: monolayer graphene and h-BN Metals: fcc Ni, hcp and fcc Co, fcc Fe (intercalated) • Optimistic definition of magnetoresistance ratio (current perpendicular to plane): • Quantum conductances by integrating the transmission probabilities on a grid of 64 x 64 k|| points
  8. 8. Atomic structure 8 (a) Top-view of epitaxial graphene on (111) surface of fcc Co or Ni. (b) Side-views of the lowest energy interfaces.
  9. 9. Electronic structure 9 Spin-resolved density of states projected onto the spacer atoms. Also, there are strong exchange couplings across the interface (~10-100 meV/unit cell; antiferromagnetic for Fe and Co, ferromagnetic for Ni)
  10. 10. Role of the spacer material 10 hcp Co layers graphene MR = 86% h-BN MR = 66% vacuum MR = 38%
  11. 11. Role of the ferromagnetic metal 11 h-BN spacer fcc Fe MR = 149% fcc Co MR = 27% fcc Ni MR = 55%
  12. 12. Conclusions 12 • Epitaxial graphene and h-BN as ultrathin covalent spacer materials for GMR junctions • Well-ordered interfaces and simple scalable production • Rather high magnetoresistance ratios (>100% for certain chemical compositions) • Intrinsically low electric resistance • Strong but tunable interlayer exchange coupling • Possibility for further tailoring the properties via intercalation
  13. 13. Acknowledgements 13 My collaborator Alfredo Pasquarello (EPFL and IRRMA, Switzerland) We acknowledge discussions with Harald Brune, Stefano Rusponi (EPFL, Switzerland) Alexander Smogunov (ICTP Trieste, Italy) Paul Kelly (University of Twente, The Netherlands) Those who helped me to realize this slidecast in practice: Federico Muñoz-Rojas, Joaquín Fernández-Rossier (Alicante, Spain) Axel Hoffmann (ANL), Samir Garzon (South Carolina) Donna Baudrau, Amy Flatten (APS)
  14. 14. Thank you for your attention! Questions or comments? oleg.yazyev@epfl.ch The slides are available at http://slideshare.net/yazyev

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