Thomas Michely
II. Physikalisches Institut
Towards ferromagnetic graphene hybrid materials
(i) all in situ surface science approach (ii) perfect graphene of controlled morphology (iii) new graphene hybrid materials
Ir(111)
C H2
H2 C2H4
J. Coraux, A.T. N’Diaye, C. Busse, T. Michely, Nano Lett. 8 (2008) 565
2000 Å
growth at 1320 K, self-limiting, full coverage
Chemical vapor deposition with epitaxy
Ir(111)
C H2
H2 C2H4
growth at 1320 K, self-limiting, full coverage
Chemical vapor deposition with epitaxy
2000 Å
Ir(111)
C H2
H2 C2H4
growth at 1320 K, self-limiting, full coverage
Chemical vapor deposition with epitaxy
2000 Å
Origin of moiré
Ir(111)
Origin of moiré
graphene
Origin of moiré
Ir(111) and graphene
Reciprocal space view
H. Hattab, A.T. N’Diaye, D. Wall, C. Klein, G. Jnawali, J. Coraux, C. Busse, R. van Gastel, B. Poelsema, T. Michely, F.-J. Meyer zu Heringdorf, M. Horn-von Hoegen, Nano Lett. 12 (2012) 678
Gr
Binding of graphene: DFT with van der Waals interaction (vdW-DF)
semi-empirical method DFT-D using GGA vdW-DF post-processing using the JuNoLo code
htheo = 3.41 Å hexp = 3.38 Å, by XSW
Eb = 50 meV/C-atom ∆h = 0.35 Å
(10 x10) C on (9 x 9) Ir
C. Busse, P. Lazić, R. Djemour, J. Coraux, T. Gerber, N. Atodiresei, V. Caciuc, R. Brako, A.T. N'Diaye, S. Blügel, J. Zegenhagen, T. Michely, Phys. Rev. Lett. 107 (2011) 036101
Binding of graphene: DFT with van der Waals interaction (vdW-DF)
semi-empirical method DFT-D using GGA vdW-DF post-processing using the JuNoLo code
htheo = 3.41 Å hexp = 3.38 Å
Eb = 50 meV/C-atom ∆h = 0.35 Å
(10 x10) C on (9 x 9) Ir
I. Pletikosić, M. Kralj, P. Pervan, R. Brako, J. Coraux, A. T. N’Diaye, C. Busse, T. Michely, Phys. Rev. Lett 102 (2009) 0568080
ARPES: intact Dirac cone, ED = 0.1 eV
semi-empirical method DFT-D using GGA vdW-DF post-processing using the JuNoLo code
htheo = 3.41 Å hexp = 3.38 Å
E = 50 meV ∆h = 0.35 Å Enonlocal = 70 meV Elocal = - 20 meV → chemical repulsion in binding distance
(10 x10) C on (9 x 9) Ir
Binding of graphene: DFT with van der Waals interaction (vdW-DF)
Physisorption with chemical modulation
charge transfer, blue: loss of e- , red: gain of e-
slight hybridization of C(2pz) with Ir(5d3z
2-r
2)
Ir(111)
Gr
template effect epitaxial growth adsorption defect engineering intercalation
exfoliation
confined reactions
functionalization through the substrate - the template effect
1030 Å
Ir(111) – substrate homoepitaxial Ir-islands graphene flake with Ir - clusters
350 K, 0.10 ML Ir
Ir cluster growth at 350 K
Ir cluster growth at 350 K 0.03 ML 0.10 ML 0.80 ML
1.5 ML 2.0 ML
550 Å
0.01 0.1 1
10
100
s av (a
tom
s);
n (%
per
cel
l)
Deposited amount Θ (ML)
A.T. N’Diaye, S. Bleikamp, P.J. Feibelman, T. Michely, Phys. Rev. Lett 97 (2006) 215501
similar: W, Pt, Re, Rh,…
Active template
P.J. Feibelman, Phys. Rev. B 77 (2008) 165419
4-atom Ir-cluster in hcp-type area
graphene
4 layer Ir slab
local bond formation
Active template
• short bonds ≈ 2.1 Å ≈ sum of Ir and C atomic radii • more than twice C-Ir chemical bonds than adatoms ! • hcp-type prefered by 3.7 eV (0.44 eV) compared to atop-type (fcc-type)
bond angles: Xea, Xeb, Yfb, Yfc, Zgc, Zgd, Zga = 111°, 107°, 107°, 110°, 107°, 109°, 108° eaα, ebβ, fbβ, fcγ,gcγ, gdγ, gdδ,gdα = 106°,108°,108°,105°,106°,111°,106° tetrahedral bond angle 109,5° rehybridzation from sp2 to sp3
Cluster Induced Rehybridization
Cluster Induced Rehybridization - XPS
Gr/Ir(111) Gr/Ir(111) + 0.75 ML Pt
O intercalation –lifting the rehybridization
0 L
1200 Å
0.5 ML Gr/Ir(111) 0.3 ML Pt at 300 K 1 x10-7 mbar O2 400 K
E. Grånäs, J. Knudsen, U. Schröder, T. Gerber, C. Busse, M.A. Arman, K. Schulte, J.N. Andersen, T. Michely, ACS Nano 6 (2012) 9951
6 L
1200 Å
0.5 ML graphene 0.3 ML Pt at 300 K O2 at 400 K
O intercalation –lifting the rehybridization
19 L
1200 Å
0.5 ML graphene 0.3 ML Pt at 300 K O2 at 400 K
O intercalation –lifting the rehybridization
44 L
1200 Å
0.5 ML graphene 0.3 ML Pt at 300 K O2 at 400 K
O intercalation –lifting the rehybridization
95 L
1200 Å
0.5 ML graphene 0.3 ML Pt at 300 K O2 at 400 K
O intercalation –lifting the rehybridization
145 L
1200 Å
0.5 ML graphene 0.3 ML Pt at 300 K O2 at 400 K
O intercalation –lifting the rehybridization
A.T. N’Diaye, T. Gerber, C. Busse, J. Myslivecek, J. Coraux, T. Michely, New J. Phys. 11 (2009) 103045
2D cluster lattice
0.5 µm
Ir70/Gr/Ir(111)
2D cluster lattices - SXRD
D. Franz, S. Runte, C. Busse, S. Schumacher, T. Gerber, T. Michely, M. Mantilla, V. Kilic, J. Zegenhagen, A. Stierle, Phys. Rev. Lett. 110 (2013) 065503.
before Ir after Ir
K-scans
Reciprocal lattice sketch
Magnetism of cluster lattices templated by Gr/Ir(111)
4 - 150 atoms narrow size-distribution
equal-distance 2.5 nm moderate interaction cluster-Gr
inert Gr support
• collective cluster response • superparamagnetism • magnetic cluster coupling
Ir Pt W
Re Fe Au
Universality of approach: cluster materials 350 K, sav = 17-400
Cluster arrays of ferromagnetic 3d metals by seeding
Ir, 300 K, , sav = 9 + Fe, 300 K, sav = 61
A.T. N’Diaye, T. Gerber, C. Busse, J. Myslivecek, J. Coraux, T. Michely, New J. Phys. 11 (2009) 103045
+ Co, 300 K, sav = 40
Per
XAS and XMCD of Pt15Co28
Pt15Co28 total electron yield, 5 T, 10 K, ID08, Grenoble
C. Vo-Van, S. Schumacher, J. Coraux, V. Sessi, O. Fruchart, N.B. Brookes, P. Ohresser, T. Michely, Appl. Phys. Lett. 99 (2011) 142504; S. Schumacher, PhD thesis, Cologne 2014
Per
No anisotropy of Pt15Co28
magnetization loops for Pt15Co28
ms = 1.5 ±0.2 µB (bulk ms = 1.62 µB) ml/ms = 0.15 µB (bulk ml/ms = 0.095 µB) results independent of incidence angle → no anisotropy • different from clusters on metals (e.g. Weiss et al. 2005, Gambardella et al. 2003) • similar results for Pt12Fe22 and Ir13Co26 • spin-orbit interaction of seed layer irrelevant
Per
T-dependent magnetization amd Langevin fits for Pt15Co28
• no hysteresis • susceptibility decreases with T → superparamagnetic clusters
• isotropic magnetism → Langevin fit
• m = 107 ± 6 µB → m/atom = 3.8 ± 0.2 µB - unphysical → cluster coupling
Per
Large superspin of Pt15Co28 by cluster coalescence ?
total electron yield, 5 T, 10 K, ID08, Grenoble STM, 300 K, Cologne
per unit cell: Pt11Co22 28% coleascence → sav = Pt15Co28
C. Vo-Van, S. Schumacher, J. Coraux, V. Sessi, O. Fruchart, N.B. Brookes, P. Ohresser, T. Michely, Appl. Phys. Lett. 99 (2011) 142504; S. Schumacher, PhD thesis, Cologne 2014
Unpinning of small Pt clusters by CO
PCO < 2 .10-10 mbar
PCO = 5 .10-9 mbar
0.1 ML Pt, 300 K
380 Å 1650 Å
clean growth is decisive
Per
T-dependent magnetization amd Langevin fits for Pt15Co28
• no hysteresis • susceptibility decreases with T → superparamagnetic clusters
• isotropic magnetism → Langevin fit
• no saturation at high field → non-collinear spins in cluster (?) 1. superspin alignment; 2. internal spin alignment • m = 107 ± 6 µB → m/atom = 3.8 ± 0.2 µB - unphysical → cluster coupling
Larger Co clusters
Rh cluster arrays
Phys. Rev. B 49 (1994) 12295
Magnetism of supported Rh clusters
Honolka et al. [1]: m < 0.02 µB/atom for Rh/Ag001) and Rh/Pt(997) at 5 T and 5 K Barthem et al. [2]: m = 0.067 µB/atom for 220 atom clusters in Al2O3 at 17 T and 2.3 K, illdefined size distribution exchange enhanced Pauli paramagnetism Sessi et al. [3]: m up to 0.4 µB/atom for Rh in Ar on Ag(001) at 5 T and 10 K, illdefined size distribution
[1] Honolka et al. Phys. Rev. B 76 (2007) 144412. [2] Barthem et al., Phys. Rev. Lett 109 (2012) 197204 [3] Sessi et al., Phys. Rev B 82 (2010) 184413
Combining a spin injector with the spin conductor Gr
Properties of EuO
• rocksalt structure, O2+, Eu2+ [Xe] 4f7 5d0 6s0
• semiconductor Eg = 1.12 eV • ferromagnetic TC = 69 K • exchange split conduction band 2∆Eex ≈ 0.6 eV • bottom of conduction band close ≈ 100% spin-polarized • unstable • no microspopy • plenty of contradictions
DOS [1]
[1] N. J. C. Ingle and I. S. Elfimov, Physical Review B 77, 121202 (2008)
Properties of EuO
[1] R. Schiller and W. Nolting, Phys. Rev. Lett. 86, 3847 (2001)
surface layer bulk
ss
Spi
n po
lariz
atio
n
Normalized gate voltage
H. Haugen, D. Huertas-Hernando, A. Brataas, Phys. Rev B 77 (2008) 115406
proximity induced exchange splitting ∆ ≈ 5 meV
(EuO)
Spin filtering with graphene
EF
Spin polarization by tunneling
[1]
> 90% spin polarization for 3nm thick 90% EuO 10% to Eu2O3 barrier [2]
[1] J. S. Moodera, T. S. Santos, T. Nagahama, J. Phys.: Cond. Mat. 19 (2007) 165202 [2] T. S. Santos, J. S. Moodera, K.V. Raman, E. Negusse, J. Holroyd, J. Dvorak, M. Liberati, Y. U. Idzerda, E. Arenholz, Phys. Rev. Lett. 101 (2008) 147201.
Spin polarization by tunneling
[1]
graphene
graphene
ACS Nano 6 (2012) 100063
(100) texture on graphite
“Single crystal“ EuO(100) on Ni(100)
6000 Å
100 nm, growth at 590 K + annealing in Eu at 670 K
220 eV
D. F. Förster, J. Klinkhammer, C. Busse, S. G. Altendorf, T. Michely, Phys. Rev B 83 (2011) 045424
Polar EuO(111) on Ir(111): a 2D-oxide in 3:4 epitaxy
3200 Å
720 K, partial bilayer
S. Schumacher, D. F. Förster, F. Hu, T. Frauenheim, T. O. Wehling, T. Michely, Phys. Rev B 89 (2014) 115410
• 14 ML EuO (3.3 nm) • three level system • (100) texture • random in plane orientation→ EuO weakly interacting with Gr
EuO(100) film on Gr/Ir(111)
300 K + annealing at 720 K
4400 Å
64 eV J. Klinkhammer, D.F. Förster, S. Schumacher, H.P. Oepen, T. Michely,, C. Busse, APL 103 (2013) 131601
filled
Oxygen vacancies
300 K + annealing at 720 K
4400 Å
-0.7 V
oxygen vacancies empty states: electronic defects filled states: Eu
+1.5 V - 0.7 V
1200 Å
J. Klinkhammer, D.F. Förster, S. Schumacher, H.P. Oepen, T. Michely, C. Busse, APL 103 (2013) 131601
300 K + annealing at 720 K
EuO(100) grains on Gr/Ir(111)
J. Klinkhammer, D.F. Förster, S. Schumacher, H.P. Oepen, T. Michely, C. Busse, APL 103 (2013) 131601
300 K + annealing at 720 K
1200 Å
300 K + annealing at 720 K
Eu intercalation layer
0.65µm
Gr/Eu/Ir(111), 720 K
S. Schumacher, D.F. Förster, M. Rösner, T. Wehling, T. Michely. Phys. Rev. Lett. 110 (2013) 086111
0.65µm
Gr/Eu/Ir(111), 720 K
S. Schumacher, D.F. Förster, M. Rösner, T. Wehling, T. Michely. Phys. Rev. Lett. 110 (2013) 086111
1.4 ± 0.1 nm n-channel
moiré
EuO(100) film on Gr/Ir(111)
300 K + annealing at 720 K
4400 Å
In situ longitudinal MOKE of 3.3 nm EuO(100)/Gr/Eu/Ir(111)
TC enhanced through Eu intercalation layer ?
Scattering patterns at oxygen vacancy defects
Topography at 5 K same area dI/dV map at 5 K + 1.25V +1.25V
→ surface state
J. Klinkhammer, M. Schlipf, F. Craes, S. Runte, T. Michely, C. Busse, Phys. Rev. Lett. 112 (2014) 016803.
Dispersion of surface state
Dispersion of surface state
Dispersion of surface state
2k = 2π/λ E-EF = eUbias
Comparison to DFT calculation
J. Klinkhammer, M. Schlipf, F. Craes, S. Runte, T. Michely, C. Busse, Phys. Rev. Lett. 112 (2014) 016803
projected bulk bands
calculated
• no half metallicity • surface partly overlaps with bulk
bands • magnetic surface state • ∆Eth = 0.37 eV • ∆Eexp = 0.64 eV DFT by M. Schlipf
Electronically intact Gr in contact with a ferromagnet
A. Varykhalov, J. Sanchez-Barriga, A. M. Shikin, C. Biswas, E. Vescovo, A. Rybkin, D. Marchenko, O. Rader, Phys. Rev. Lett. Phys. Rev. Lett.101 (2008) 157601
Hybridization of 3d ferromagnets with Gr
Eu intercalation
Eu intercalation
S. Schumacher, F. Huttmann, M. Petrovic, C. Witt, D. F. Förster, C. Vo-Van, J. Coraux, A. J. Martınez-Galera, V. Sessi, I. Vergara, R. Rückamp, M. Grüninger, N. Schleheck, F. Meyer zu Heringdorf, P. Ohresser, M. Kralj, T. O. Wehling,T. Michely, Phys. Rev. B 90 (2014) 235437
Stable rare earth ferromagnetism in contact with Gr
Summary Gr on Ir(111) - perfect orientation order, strictly monolayer, tunable morphology template effect - strong template effect of Gr/Ir(111) for clusters, molecules and atoms - highly regular cluster lattice arrays with 2.5 nm pitch - template effect results from rehybridzation sp2 → sp3 nanomagnetism - noninteracting superparamagnetic Co and Fe clusters seeded by Ir or Pt - no measurable magnetic anisotropy - highly regular Rh18 cluster arrays with m < 0.2 µB/atom epitaxy of EuO - excellent quality, stoichometric (100)-textured films on Gr - oxygen vacancies for atomic structure - magnetic surface state observed Gr/Eu-(√3 x√3)/Ir(111): ferromagnetic ordering Gr/Eu-(√3 x√3) /15 ML-Ni/Ir(111): ferromagnetic Eu layer at 300 K
Acknowledgements Köln: Charlotte Herbig, Felix Huttmann, Ulrike Schröder, Wouter Jolie, Antonio Martinez-Galera, Carsten Busse, Stefan Schumacher, Nicolas Schleheck,Timm Gerber, Sven Runte, Fabian Craes, Sebastian Standop, Alpha N‘Diaye, Jürgen Klinkhammer, Achim Rosch Albuquerque: Peter Feibelman Bremen: Tim Wehling, Malte Rösner Duisburg: Michael Horn-von Hoegen, Frank Meyer zu Heringdorf, Dirk Wall, Hichem Hattab, Niemma Buckanie, Claudius Klein, Jnawali Giriaj Heiko Wende, David Klar, Carola Schmitz-Antoniak Grenoble: Johann Coraux, Jörg Zegenhagen, Chi Vo-Van, Philippe Ohresser, Violetta Sessi, Olivier Fruchart, Nick Brookes Hamburg: Andreas Stierle, Dirk Franz, Hans-Peter Oepen Jülich: Stefan Blügel, Nicolae Atodiresei, Vasile Caciuc, Martin Schlipf Lund: Jan Knudsen, Elin Granas, Jesper Andersen, Karina Schulte, Zagreb: Marco Kralj, Ivo Pletikosić, Marin Petrović, Petar Pervan, Iva Šrut, Predrag Lazi¢, Enschede: Raoul van Gastel, Bene Poelsema Aachen: Markus Morgenstern, Marco Pratzer, Marcus Liebmann, Victor Geringer, Dinesh Subramaniam, Christian Pauly, Torge Mashoff Madrid: Jose Gomez-Rodrıguez Prag: Josef Mysliveček Wien: Jani Kotakoski, Florian Libisch, Joachim Burgdörfer Helsinki: Ossi Lehtinen, Arkady Krashenninikov, Harriet Åhlgren Hannover: Cristof Tegenkamp, Thomas Langer, Herbert Pfnür
3rd European Workshop on Epitaxial Graphene and 2D Materials
Bergisch Gladbach (near Cologne) 17. – 21. May 2016
eweg2d.ph2.uni-koeln.de abstract submission deadline: 15.12.2015
limited to 100 partcipants
Acknowledgements