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. Foreword 2
Questions or comments?
oleg.yazyev@epfl.ch
The slides are available at
http://slideshare.net/yazyev

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. 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. 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. 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. 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. 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. 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. Role of the spacer material 10
hcp Co layers
graphene
MR = 86%
h-BN
MR = 66%
vacuum
MR = 38%

11. Role of the ferromagnetic metal 11
h-BN spacer
fcc Fe
MR = 149%
fcc Co
MR = 27%
fcc Ni
MR = 55%

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. 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. Thank you for your attention!
Questions or comments?
oleg.yazyev@epfl.ch
The slides are available at
http://slideshare.net/yazyev