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