Applications of Spin-Polarized Electrons in Condensed Matter Physics

Transcription

1 Applications of Spin-Polarized Electrons in Condensed Matter Physics Dan Pierce Electron Physics Group, NIST Supported in part by the Office of Naval Research

2 Pierce and Meier, PR B 13, (1976) Principle of GaAs Polarized e - Source Optical excitation of spin polarized electrons For positive helicity σ + light, the theoretical polarization of the electrons photoexcited by bandgap radiation is = N N = 1-3 N + N 1+3 = -50% P Polarization is easily and rapidly reversed by switching helicity of light

3 Negative Electron Affinity (NEA) GaAs Surface treatment gives world s best photoemitter Source characteristics: P ~ 30% for bulk wafer I = 20 µa/mw continuous or pulsed operation low initial energy and energy spread high figure of merit = P 2 I

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5 Charlie s Hippie Days

6 Pierce, Celotta, Wang, Unertl, Galejs, Kuyatt, Mielczarek, RSI 51, (1980) Spin Polarized Electron Gun Spin polarized e - scattering apparatus Measure I I A= I + I M

7 Polarized e - Gun for SLAC Parity Violation Experiment Beam polarization, 35% to 45% Beam current, 1 ma cw to 15 A peak

8 Complete Polarized e - Injection System for SLAC Parity Violation Experiment

9 Technical Support in the Early Days

10 Development of Measurement Techniques Using Spin-Polarized Electrons Spin-polarized electron gun (NEA GaAs) Spin polarized electron scattering Spin polarized low energy electron diffraction (SPLEED) Spin polarized inverse photoemission spectroscopy (SPIPES) Spin polarized electron energy loss spectroscopy (SPEELS) Spin polarized low energy electron microscopy (SPLEEM) Electron spin polarization analyzer Spin polarized photoemission Spin polarized Auger electron spectroscopy Spin polarized secondary electron emission Scanning Electron Microscopy with Polarization Analysis (SEMPA)

11 Surface Sensitivity Surface sensitive due to short electron mean free paths Inelastic mean free path especially short in transition metals with many d holes Spin dependent attenuation length in ferromagnets at low energy Pappas et al, PRL 66, 504 (1991)

12 Pierce, Celotta, Wang, Unertl, Galejs, Kuyatt, Mielczarek, RSI 51, (1980) Spin Polarized Electron Gun Spin polarized e - scattering apparatus Measure I I A= I + I M

13 Spin Polarized Low Energy Electron Diffraction (SPLEED) First surface magnetization measurement with SPLEED I, I AA AB θ A(H) A(T) A=A(E,θ,H,T) Test for true magnetic effect by measuring field and temperature dependence Celotta, Pierce, Wang, Bader, Felcher, PRL 43, 728 (1979)

14 Spin-Polarized Electron Scattering Measure low temperature surface magnetization Ni 40 Fe 40 B 20 B s /B b =3 Thermal excitation of spin waves M( T) =- 1 BT32 / +... B Theory: M( 0) B surface =2 Experiment: B B s >2 b bulk Results explained by extending theory to include weaker exchange coupling of surface to bulk, J S <J B Pierce, Celotta, Unguris, Siegmann, PR B 26, 2566 (1982)

15 Spin-Polarized Electron Scattering Determine surface critical exponent A lot of theory but few experiments for magnetic critical behavior at surfaces A ex t β, where t=[1-(t/t C )]. Find β 0.78 for both Ni(110) and Ni(100) surfaces, independent of E and θ Neutron measurements give β=0.38 for bulk Alvarado, Campagna, Hopster, PRL 48, 51 (1982)

16 Unguris, Seiler, Celotta, Pierce, Johnson, Smith, PRL 49, 1047 (1982) Spin Polarized Inverse Photoemission Spectroscopy Spin dependent bandstructure of unfilled states Energy, angle (k), and spin-resolved inverse photoemission Photon detector sensitivity Measure hw = 97. ± 035. ev F I n -n A= 1 J HG P K n + n 0 cosa A B Large minority spin peak just above Fermi level from the d-holes responsible for ferromagnetism in Ni J A B

17 Running with Friends Bay to Breakers

18 Stopping to read while others catch up

19 Tough

20 Happy on top of the mountain

21 A Sinclair Grand Canyon Expedition

22 You re not tryin if your not bleedin! Determined

23 Principled Two things I can t tolerate: Incompetence and Cold Coffee

24 Married in 1983!

25 How bout them legs

26 Spin Polarized Secondary Emission High spin polarization where have high secondary intensity Fe 81.5 B 14.5 Si 4 Valence electron polarization n n n P= n + n = n = B V # Bohr magnetons # Valence electrons M =-m ( n -n ) B A B n B n V P Fe Co Ni Unguris, Pierce, Galejs, Celotta, PRL 49, 72 (1982)

27 Scanning Electron Microscopy with Polarization Analysis (SEMPA) High resolution magnetization imaging Incident Electrons From Scanning Electron Microscope Spin-Polarization Analyzer P = N -N N + N Spin-Polarized Secondary Electrons Magnetic Specimen Magnetization M= -µ B (N -N )

28 NIST SEMPA Apparatus Electron Source In-Plane Acquisition time (min) Old LaB 6 New Field Emission 10 kev 20 kev Auger Electron Energy Analyzer Polarization Analyzers Beam diameter (nm) Ion Gun Secondary & Backscatter Electron Detectors Sample RHEED Screen Metal Evaporators Out-of-Plane

29 Low Energy Diffuse Scattering Analyzer Y Input Optics Anode Drift Tube 150 ev e - A D B C End View X 4 cm MCP G2 G1 E2 E1 Au & C Targets Au evaporator

30 SEMPA Example Iron Crystal Measure the topography and two magnetization components (simultaneously but separately): Intensity M x M y Then derive the magnetization direction and magnitude: Direction θ = tan -1 (M x /M y ) Magnitude M = (M x2 + M y2 ) 1/2

31 Cobalt Vector Magnetization Out-of-plane M M contour In-plane M

32 SEMPA MFM Comparison Hard disk test sample with Au patterned overlay SEMPA Intensity M AFM MFM 350 nm

33 Interlayer Exchange Coupling Cr wedge schematic Fe Film 1-2 nm Cr Wedge 0-20 nm 500 µm Fe Whisker M Unguris, Celotta, Pierce, PRL 67, 140 (1991)

34 Fe/Cr/Fe Oscillatory Exchange Coupling Precise measurement of coupling periods M P(Fe) P(Cr) RHEED Interlayer Cr nm/layer Ag nm/layer Au nm/layer Measured period (layers) ± ± ± ± ± ± Cr Thickness (layers)

35 Leadership

36 Vision We ll get there if you can keep up with me

37 Between a rock and a hard place

38 Magnetic Depth Profiling Co/Cu Multilayers Magnetoresistance and domain structure Current through magnetic multilayer depends on magnetization alignment, i M but real domain structures can be complex: Cu Co Ion milled crater (2 kev Ar + ) Top Co layer 2 nd Co layer Top layer magnetization predominantly antiparallel to 2 nd layer Borchers, Dura, Unguris, Tulchinsky, Kelley, Majkrzak, Hsu, Loloee, Pratt, Bass, PRL 82, 2796 (1999)

39 Interlayer Domain Correlations Quantify layer-by-layer angle distributions Angle images: 1st layer 2nd layer Correlation distribution: FM AFM P( φ) φ ( )

40 Magnetic materials Air Side Wheel Side Transformer core ferromagnetic glass (Allied Signal)

41 Magnetic Nanostructures Fe nanowires M Cr Fe 213 nm Fe evaporated at grazing incidence on Cr lines fabricated by laser focused atom deposition Tulchinsky, Kelley, McClelland, Gupta, Celotta, JVST A16, (1998)

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43 Ready to retire???

44 Acknowledgements Spin-polarized electron work at NBS/NIST Many contributors, especially Bob Celotta and John Unguris Pictures of Charlie Joel Falk Jim Murray Roger Route