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Spin resolved Inverse Photoemission Franco Ciccacci Dipartimento di Fisica – Politecnico di Milano

Spin resolved Inverse Photoemission Franco Ciccacci Dipartimento di Fisica – Politecnico di Milano. PhotoEmission versus Inverse PhotoEmission. E. E. E. e -. E kin. h v. h v. Electron energy. vacuum level. vacuum level. F. Fermi level. Fermi level. E B. empty states.

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Spin resolved Inverse Photoemission Franco Ciccacci Dipartimento di Fisica – Politecnico di Milano

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  1. Spin resolved Inverse Photoemission Franco Ciccacci Dipartimento di Fisica – Politecnico di Milano Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  2. PhotoEmission versus Inverse PhotoEmission E E E e- Ekin hv hv Electron energy vacuum level vacuum level F Fermi level Fermi level EB empty states isochromat spectrum occupied states DOS DOS electron in photon out photon in electron out sample sample (direct) PhotoEmission Spectroscopy Inverse PhotoEmission Spectroscopy • same matrix element <i|A·p|f> • same cross section behaviour s(E) • Yph/Yel =(lel/lph)2 ~ 10-5 in the UV PES ==> IPES time reversal Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  3. Isochromat UV IPES e- (E,k) Ein,2 Ein,1 hv q band-pass photon detector hv = 9.4 ± 0.3 eV Energy Efin,2 E – EF Efin,1 EF 0 kph << kBZ band structure l ~ 5- 10 Å  surfaces, thin films k Intensity experimentally controlled parameters electronic structure: resolution in monochromatic electron beam energy collimated beam + single crystal kparallel beam spin polarization spin Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  4. e E F p E i GaAs h v h v CsO x Ni(110) q = 20° photocathode vacuum level F E f i n E INTENSITY (arb.units) F 0 1 2 E - E (eV) F Spin resolved IPES Spin polarized electron gun: GaAs polarized source first measurements:J. Unguris, A. Seiler, R.J. Celotta, D.T. Pierce, P.D.Johnson, N. Smith, Phys. Rev. Lett. 49, 1047 (1982) review: M. Donath, Surface Sci. Rep. 20, 251 (1994) F. Ciccacci, Phys. Scrip. T66, 190 (1996)  U. Kolac, M. Donath, K. Ertl, H. Liebl, V. Dose, Rev. Sci. Instrum. 59, 1933 (1988) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  5. Experimental apparatus IPES H magnetic coils energy analyzer (CMA) quartz microbalance LEED/Auger Fe P UHV air GaAs X-ray source (XPS) ion gun hv spin rotator circularly polarized light (810 nm) effusion cells (MBE) SrF2 window sample preparation chamber photocathode preparation chamber measurement chamber photon detector (hv = 9.4 + 0.3 eV) G. Chiaia, S. De Rossi, L. Mazzolari, F. Ciccacci, Phys. Rev. B 48, 11298 (1993) • F. Ciccacci, E. Vescovo, G. Chiaia, S. De Rossi, M. Tosca • Rev. Sci. Instrum. 63, 3333 (1992) • M. Finazzi, A. Bastianon, G. Chiaia, F. Ciccacci • Meas. Sci. Technol (J. Phys. E) 4, 234 (1993) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  6. Systems investigated clean surfaces - adsorbates “negative” exchange splitting of image states in Fe(001) adsorbate-induced enhancement of spin dependent effects atFe(001)-p(1x1)O thin magnetic films evolution of electronic and magnetic structure: Fe on Ag(001), Au(001), and Cu(001) antiferromagnetic films: Cr/Ag(001) and Cr/Fe(001) metastable films: hcp Co on Fe(001) thin oxide films: NiO on Ag(001) and Fe(001) half-metals: LaMnSrO and LaMnSrO/SnTiO interfaces Fe homoepitaxial growth non-magnetic filmson ferromagentic substrates spin dependent quantum well states: Ag on Fe(001) interface states: Pt on Fe(001) electron mean free path: V on Fe(001) localized f-states: Ce on Fe(001) magnetic coupling in multilayers oscillatory exchange coupling: Fe/Cr/Fe(001) exchange bias: Fe/NiO/Fe(001) magnetic-metal / semiconductor interfaces Fe/GaAs(001) and Fe/ZnSe(001) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  7. Systems investigated (examples) clean surfaces - adsorbates “negative” exchange splitting of image states in Fe(001) adsorbate-induced enhancement of spin dependent effects atFe(001)-p(1x1)O thin magnetic films evolution of electronic and magnetic structure: Fe on Ag(001), Au(001), and Cu(001) antiferromagnetic films: Cr/Ag(001) and Cr/Fe(001) metastable films: hcp Co on Fe(001) thin oxide films: NiO on Ag(001) and Fe(001) half-metals: LaMnSrO and LaMnSrO/SnTiO interfaces Fe homoepitaxial growth non-magnetic filmson ferromagentic substrates spin dependent quantum well states: Ag on Fe(001) interface states: Pt on Fe(001) electron mean free path: V on Fe(001) localized f-states: Ce on Fe(001) magnetic coupling in multilayers oscillatory exchange coupling: Fe/Cr/Fe(001) exchange bias: Fe/NiO/Fe(001) magnetic-metal / semiconductor interfaces Fe/GaAs(001) and Fe/ZnSe(001) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  8. DE 12 B2 B1 D 5 H 15 DOWN spin 8 E [eV] UP spin INTENSITY [arb.units] 4 S S H' D 25 1 G DE 12 E F H' G ' Fe(001) q = 0 G-H line 25 D 25 5 G 12 G D H 0 2 4 6 E - E [eV] F Spin-resolved IPES from Fe(001) A. Santoni, F.J. Himpsel, Phys. Rev. B 43, 1305 (1991) J. Kirschner, M. Glöbl, V. Dose, H. Scheidt: Phys. Rev. Lett. 53 (1984) , 612 S. De Rossi, F. Ciccacci, J. Electron Spectrosc. Relat. Phenom. 76 (1995) , 177 J Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  9. New polarized electron sources extrapolartion to Po = 100% measured spectra: I↑ , I↓ I↑- I↓ asymmetry: A = I↑+ I↓ I↑ + I↓ (1 ± A / Po) spin resolved spectra: S ± = 2 highly polarized sources very useful (but…) F. Ciccacci, S. De Rossi, E. Pelucchi, A.Tagliaferri, Rev. Sci. Instrum 68, 1841 (1997) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  10. Fe bandstructure (*) maj. Z min. Fe(001) C 20 0 -20 Fe(001)-p(1x1)O B2 C B1 60 S 40 20 0 -20 0.0 4.0 8.0 12.0 16.0 20.0 Oxygen-induced enhancement of spin dependent effects: Fe(001)-p(1x1)O (c) Fe bandstructure (*) kz maj. k min. H12 H12 H15 H15 H H (b) Fe(001) C B2 B1 S B1 B2 S (x10) RC AC 1.0 Intensity (arb. un.) 0.5 Intensity (arb.un.) 0.0 (a) Fe(001)-p(1x1)O Asymmetry (%) (x10) X D C B2 B1 S (x10) RC 1.0 AC 0.5 0.0 0.0 4.0 8.0 12.0 16.0 20.0 E-EF (eV) E-EF (eV) AC: absorbed current RC: reflected current (*) E. Tamura and R. Feder, Phys. Rev. Lett. 57, 759 (1986) R. Bertacco and F. Ciccacci, Phys. Rev. B 59, 4207 (1999) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  11. Fe-O-Fe-O-Fe-O-Fe-O-Fe Fe substrate Oxygen assisted Fe homoepitaxial growth 14 Fe(001)-p(1x1)O Fe atom 4 0 12 Asymmetry (%) -4 Fe(001)-p(1x1)O -8 10 -12 2 6 10 14 Kinetic Energy (eV) | Asymmetry | (%) 8 6 • oxygen surfactant action • layer-by-layer growth • structure  magnetic properties 4 Kinetic Energy = 6 eV 0 1 2 3 4 Fe coverage (ML) F. Bisio, R. Moroni, M. Canepa, L. Mattera, R. Bertacco, F. Ciccacci, Phys. Rev. Lett. 83, 4868 (1999) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  12. Fe(001) Ag(001) Intensity (arb. units) Cr1ML/Fe(001) Cr1ML/Ag(001) Cr4ML/Fe(001) Cr4ML/Ag(001) E – EF (eV) Ultrathin Cr(001) films 4 ML 4 ML 3 3 2 2 1 1 Cr film Cr film Intensity (arb. units) Cr1ML/Ag(001) Cr1ML/Fe(001) Fe(001) Ag(001) Cr 1ML Cr 1ML 0 2 4 0 2 4 E – EF (eV) theory: S. Blügel, B. Drittler, R. Zeller , and P.H. Dederichs, Appl. Phys. A 49 (1989), 547 Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  13. Cr 4 ML Cr 4 ML Cr1ML/Fe(001) Cr1ML/Fe(001) Cr 3 ML Cr 3 ML Cr 2 ML Cr 2 ML Fe(001) Fe(001) Cr 1 ML Cr 1 ML Cr1ML Cr1ML Majority Minority Majority Minority 0 2 4 0 2 4 0 2 4 E – EF (eV) 0 2 4 E – EF (eV) Cr films: spin resolution Cr/Ag(001): no polarization effect Cr/Fe(001) Intensity (arb. units) G. Isella, R. Bertacco, M. Zani, L. Duò, F. Ciccacci Solid State Commun. 116 (2000), 283 Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  14. Spintronics - Magnetoelectronics new degree of freedom (electronspin)  realisation of new electronic devices controlling the carrierspinrather than its charge integrating magnetism and semiconductor physics addingspin-up spin-downmagnetic dualism to electron hole dualism spinpolarized electron injection into semiconductors spin-dependent field effect transistor coupling semiconductor logic with non volatile magnetic memory nanometric layered magnetic structures - metallic magnetic multilayers – metal oxide junctions – half-metallic magnets - - magnetic semiconductors - ferromagnetic metal / semiconductor interfaces - - small scale magnetic elements combined with conventional semiconductor electronics Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  15. ï Cr Exchange coupling in Fe/Cr/Fe multilayers Cr  AFM ï ï Cr ï ï Cr ï ï Cr Cr/Fe interface  AFM ï ï ï Cr ï ï Fe (sub) ï ï substrate at 30 °C period: 12 ML Fe/Cr/Fe system  FM/AFM oscillations ï Fe (top) ï ï ï substrate at 350 °C period: 2 ML ï ï Cr ï ï Cr ï ï Cr ï ï Fe (sub) Unguris, R. J. Celotta, D. T. Pierce, Phys. Rev. Lett. 67 (1991), 140; ibid. 69 (1992), 1125 ï ï odd  FM even  AFM naïve model experiment Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  16. Cr Cr ï Fe (sub) ï Fe ð ï 7 ML Fe (top) ð ï Cr ï ï // Fe (sub) ï ï // AFM FM Fe/Cr/Fe trilayers – empty states 1) Cr deposition on Fe(001) 2) Fe deposition on Cr/Fe(001) Spin resolved IPES Spin resolved IPES Absorbed current G. Isella, R. Bertacco, L. Duò, F. Ciccacci, Surface Sci. 454, 909 (2000) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  17. FM/AFM coupling in Fe/Cr/Fe(001)-p(1x1)O Spin dependent absorbed current Asymmetry (%) G. Isella, R. Bertacco, L. Duò, F. Ciccacci, Mat. Sci. Forum 373, 169 (2001) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  18. Magnetic Tunnelling Junctions (MTJ) M. Viret, M. Drouet, J. Nassar, J.P. Contour, C. Fermon, A. Fert, Europhys. Lett. 39, 545 (1997) J.M. De Teresa, A. Barthélémy, A. Fert, J.P. Contour, F. Montaigne, P. Seneor, Science 286, 507 (1999) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  19. Materials with high spin polarization at Fermi level: Half-metals Mn density of states in LSMO LSMO crystal structure Jex = 2.5 – 4 eV J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, and T. Venkatesan, Nature, 392, 794 (1998) A. Chattopadhyay, A. J. Mills, S. Das Sarma, Phys. Rev. B, 61, 10738 (2000) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  20. Spin Resolved IPES from LSMO(001) zoom sample: La0.7Sr0.3MnO3(001) 350 Å thick film grown on SrTiO3(001) by Pulsed Laser Deposition (Orsay) R. Bertacco, M. Portalupi, M.Marcon, L. Duò, F. Ciccacci, M. Bowen, J.P. Contour, A. Berthélèmy, J. Magn. Magn. Mater. 242, 710 (2002) Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  21. eV EF EF+ eV EF LSMO/STO/LSMO MTJ Spin resolved IPES TEM: good epitaxy La2/3Sr1/3MnO3 SrTiO3 La2/3Sr1/3MnO3 Understanding the bias dependence of TMR Bias-dependent tunneling reflects the unoccupied DOS of the collecting Insulator/FM interface Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  22. ZnSe – 200 Ǻ ZnSe(001) GaAs(001) GaAs(001) Se protecting layer ZnSe – 200 Ǻ GaAs(001) Fe/ZnSe(001) interface sample: ZnSe(001) thin film grown by Molecular Beam Epitaxy (Paris) heating in UHV  clean ZnSe(001) surface ( 1x1 or 2x2 reconstruction ) Fe Fe epitaxy Fe deposition Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  23. Fe/ZnSe(001) A, D: ZnSe bulk states CB: conduction band minimum (1.3 eV above Femi level) Spin resolved IPES d: band bending upon interface formation (0.5 eV) CB d = 0.5 eV EF Egap = 2.8 eV Fe VB Fe film - magnetic properties: ultrathin Fe films: no polarization effect (RT and LNT) onset of polarization dependent effect(magentism): ~ 6-8 ML Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

  24. J.P. Contour A. Fert M. Bowen A. Barthèlèmy Orsay - CNRS/Thales Riccardo Bertacco Lamberto Duò Marco Marcon Marco Portalupi Milano Politecnico V.H. Etgens M. Eddrief Paris - Université VI-VII V.H. Etgens M. Eddrief Yu.A. Mamaev Yu. Yashin St. Petersburg – Techn. Univ. Yu.A. Mamaev Yu. Yashin Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center

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