From quantum mechanics to spintronics Ingrid Mertig - PowerPoint PPT Presentation

From quantum mechanics to spintronics Ingrid Mertig Martin-Luther-Universität Halle, Germany Kyoto January 23, 2009

Outline • Ab initio calculations • Tunneling magnetoresistance on the sub-nanometer scale • Multiferroic interfaces and magnetoelectric coupling • Magnetic molecules • Summary Kyoto January 23, 2009

Ab initio calculations

Green function method • Kohn-Sham equation • Green’s function • Dyson equation N scaling! Kyoto January 23, 2009

The power of Green functions Surface ∞ Nanocontact Kyoto January 23, 2009

Tunneling magnetoresistance

Thanks Martin Gradhand Christian Heiliger Peter Zahn Bogdan Yavorsky Martin Gradhand Michael Czerner Martyna Pollock Peter Zahn Wowa Maslyuk Peter Bose Igor Maznichenko Michael Fechner Steven Walczak Christian Heiliger Dima Fedorov

Tunneling magnetoresistance TMR TMR =(g P - g AP )/ (g P +g AP ) M. Julliere, Phys. Lett. 54A , 225 (1975) J. S. Moodera et al., Phys. Rev. Lett. 74 , 3273 (1995) Kyoto January 23, 2009

Ab initio calculation – coherent limit E F MgO Fe Fe TMR > 1000 % W. H. Butler et al., Phys. Rev. B 63 , 054416 (2001) G. Mathon et al., Phys. Rev. B 63 , 220403(R) (2001) C. Heiliger et al., Phys. Rev. B 72 , 180406(R) (2005) Kyoto January 23, 2009

High quality MgO barriers Fe(001) (Pinned layer) MgO(001) Fe(001) (Free layer) 2 nm S. Yuasa et al., Nature Materials 3 , 868 (2004) Kyoto January 23, 2009

Development of the TMR effect > 400 % Courtesy of S. Yuasa Kyoto January 23, 2009

Role of the electrodes amorphous CoFeB MgO TMR ratio as depoited: 60% S. Parkin, MRS Bulletin 31, 389 (2006) Fe bcc after annealing: 350% . . . . . . free electron-like free electron like MgO reservoir reservoir amorphous Fe electrodes Cu-bcc Cu-bcc Kyoto January 23, 2009

TMR for amorphous Fe electrodes 1.0 ∞ Fe 0.9 TMR ratio 0.8 Δ 1 majority electrons Δ 1 minority electrons 0.7 as deposited: 60% 47% 0.2 0.0 0 1 2 3 4 5 6 number of crystalline Fe layers Kyoto January 23, 2009

TMR – 1ML of Fe and amorphous Fe electrodes 1.0 ∞ Fe 0.9 TMR ratio 0.8 0.7 570% after annealing: 350% 0.2 0.0 0 1 2 3 4 5 6 number of crystalline Fe layers Kyoto January 23, 2009

Conductance for P and AP configuration 0.3 m ) 2 Ωμ P 0.2 Δ 1 AP majority electrons conductance density (1/ Δ 5 minority electrons 0.1 ∞ Fe 0.0010 0.0005 ∞ Fe 0.0000 0 1 2 3 4 5 6 number of crystalline Fe layers C. Heiliger et al., Phys. Rev. Lett. 99, 066804 (2007) M. Gradhand et al., Phys. Rev. B 77 , 134403 (2008) Kyoto January 23, 2009

Conductance for P and AP configuration 0.3 m ) 2 Ωμ P 0.2 AP conductance density (1/ 0.1 ∞ Fe 0.0010 Δ 5 Δ 1 majority electrons 0.0005 ∞ Fe Δ 5 minority electrons 0.0000 0 1 2 3 4 5 6 number of crystalline Fe layers C. Heiliger et al., Phys. Rev. Lett. 99, 066804 (2007) M. Gradhand et al., Phys. Rev. B 77 , 134403 (2008) Kyoto January 23, 2009

Amorphous versus free electron like electrodes C. Heiliger et al., Phys. Rev. Lett. 99, 066804 (2007) M. Gradhand et al., Phys. Rev. B 77 , 134403 (2008) Kyoto January 23, 2009

Multiferroic interfaces

Thanks Igor Maznichenko Christian Heiliger Peter Zahn Bogdan Yavorsky Sergey Ostanin Martin Gradhand Michael Czerner Martyna Pollock Wowa Maslyuk Peter Bose Igor Maznichenko Michael Fechner Michael Fechner Steven Walczak Arthur Ernst Dima Fedorov

Multiferroic interfaces M Magnetic layer + + + + + + + + + + P Ferroelectric oxide - - - - - - - - - - BaTiO 3 Kyoto January 23, 2009

Magnetoelectric coupling Magnetisation External magnetic field M B M + + + + + - - - - - P E P + + + + + - - - - - External Electrical electric field polarisation Kyoto January 23, 2009

One monolayer of Fe on BaTiO 3 δ = z(O) – z(Kation) P up P down -0.07 0.02 -0.03 0.08 -0.05 0.09 0.08 -0.08 -0.09 0.09

Magnetic order of Fe on BaTiO 3 ferromagnetic P up P down Fe 2.97 μ B Fe 2.91 μ B Ti -0.10 μ B Ti -0.07 μ B -0.07 0.02 -0.03 0.08 O 0.10 μ B O 0.09 μ B -0.05 0.09 0.08 -0.08 -0.09 0.09

Charge transfer from Fe to Ti under switching P up P down Change of charge on the Fe layer: Δ q = 0.56 e Δ M = 0.13 μ B M. Fechner et al., PRB 78, 212406 (2008) Kyoto January 23, 2009

Charge transfer from Ti to Fe Fe Fe P up - P down Ti Ti Kyoto January 23, 2009

Magnetic order in the Fe layer on BaTiO 3 antiferrimagnetic P up P down Fe 2MLTi -2.71 μ B Fe 2MLTi -2.36 μ B Fe 2MLBa 2.38 μ B Fe 2MLBa 2.18 μ B Fe 1ML 0.37 μ B Fe 1ML 0.33 μ B Structure: T c = 205 K � Fe in plane 2.79 � Fe between planes 1.1 M. Fechner et al., PRB 78, 212406 (2008) Kyoto January 23, 2009

Magnetic order in the Fe layer on BaTiO 3 P up P down Magnetoelectric coefficient: Kyoto January 23, 2009

Magnetic order in the Fe layer on BaTiO 3 Kyoto January 23, 2009

Magnetic molecules

Thanks Vova Maslyuk Christian Heiliger Peter Zahn Bogdan Yavorsky Martin Gradhand Michael Czerner Martyna Pollock Wowa Maslyuk Peter Bose Igor Maznichenko Michael Fechner Steven Walczak Dima Fedorov

Organometallic benzene-vanadium wires K. Miyajima, et al. Eur. Phys. J. D 34 177 (2005) Kyoto January 23, 2009

Organometallic benzene-vanadium wires ferromagnetic half-metallic 4 4 2.0 LDA+U, U=3 E 1 E 1 3 3 1.5 E 2 A 1 A 1 2 2 E 1 1.0 E 1 E 2 1 1 A 1 Energy, eV 0.5 E F 0 0 Energy (eV) E F E F 0.0 A 1 -1 -1 -0.5 E 2 minority majority -2 E 2 -2 -1.0 -3 -3 Vanadium majority minority -1.5 -4 -4 Total DOS -5 E 1 -5 -2.0 E 1 E 2 E 2 -6 -6 -2.5 A 1 A 1 -7 -7 -3.0 Δ Γ Δ Α Γ Α Γ Δ A DOS (arb.units) W. Maslyuk et al. PRL 97 , 097201(2006) Kyoto January 23, 2009

Organometallic benzene-vanadium wires Spin density Charge density W. Maslyuk et al. PRL 97 , 097201(2006) Kyoto January 23, 2009

January 23, 2009 Stretching Kyoto

Low spin - high spin transition Kyoto January 23, 2009

Low spin - high spin transition Kyoto January 23, 2009

Transport through the molecule

Transport through V n Bz n +1 between Co(100) electrodes V n Bz n+1 1.5 4.0 n=1 1.0 E F n=2 0.5 3.0 n=3 0.0 n=4 -0.5 2.0 Transmission -1.0 1.0 -1.5 -0.50 -0.25 0.00 0.25 0.50 spin-up 0.0 spin-down -1.0 G -2.0 = down 0 . 4 G -3.0 up -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 Energy (eV) Kyoto January 23, 2009

Transport through V n Bz n +1 between Co(100) electrodes V n Bz n+1 1.5 4.0 n=1 1.0 E F n=2 0.5 3.0 n=3 0.0 n=4 -0.5 2.0 Transmission -1.0 1.0 -1.5 -0.50 -0.25 0.00 0.25 0.50 spin-up 0.0 spin-down -1.0 G -2.0 = down 1 . 5 G -3.0 up -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 Energy (eV) Kyoto January 23, 2009

Transport through V n Bz n +1 between Co(100) electrodes V n Bz n+1 1.5 4.0 n=1 1.0 E F n=2 0.5 3.0 n=3 0.0 n=4 -0.5 2.0 Transmission -1.0 1.0 -1.5 -0.50 -0.25 0.00 0.25 0.50 spin-up 0.0 spin-down -1.0 G -2.0 = down 9 . 9 G -3.0 up -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 Energy (eV) Kyoto January 23, 2009

Transport through V n Bz n +1 between Co(100) electrodes V n Bz n+1 1.5 4.0 n=1 1.0 E F n=2 0.5 3.0 n=3 0.0 n=4 -0.5 2.0 Transmission -1.0 1.0 -1.5 -0.50 -0.25 0.00 0.25 0.50 spin-up 0.0 spin-down -1.0 G -2.0 = down 10 . 1 G -3.0 up -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 Energy (eV) Kyoto January 23, 2009

Bias dependence 3 3 Transmission Transmission (spin-down) (spin-up) 2 2 1 1 0 0 0.5 0.5 0.4 0.4 Bias (V) ) V 0.3 ( 0.3 s a 0.2 i B 0.2 0.1 0.1 0.0 0.0 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 Energy (eV) Energy (eV) Co(100)-V 4 Bz 5 -Co(100) Kyoto January 23, 2009

Summary • Tunneling current and TMR effect are tailored by the interface between oxide barrier and first layer of the electrodes! • Ferroelectricity changes at the surface! • We predict magnetoelectric coupling via the interface caused by charge transfer! • Organometallic contacts show pronounced spin-dependent transport Kyoto January 23, 2009

Collaborations and funding A. Ernst, J. Henk, L. Sandratski, V. Stepanyuk, MPI Halle P.H. Dederichs, R. Zeller, FZ Jülich F. Evers, A. Bagrets, FZ Karlsruhe W. Hergert, MLU Halle M. Scheffler, FHI Berlin P. Bruno, Grenoble M. Brandbyge, H. Skriver, TH Denmark J. Kudrnovsky, Prague P. M. Levy, New York University J. Staunton, University of Warwick M. Stiles, C. Heiliger, NIST Washington L. Szunyogh, TU Budapest W. Temmerman, Z. Szotek, Daresbury Laboratory P. Weinberger, TU Wien Kyoto January 23, 2009