Possibility of Magnetic Imaging Using Photoelectron Emission Microscopy with Ultraviolet Lights

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Esther Williamson
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1 Possibility of agnetic Imaging Using Photoelectron Emission icroscopy with Ultraviolet Lights Institute for olecular Science Toshihiko Yokoyama and Takeshi Nakagawa

XCD PEE and UV LD/CD PEE XCD PEE UV LD PEE G. K. L. arx, H. J. Elmers, and G. Schönhense, Phys. Rev. Lett.

Strong CD >10% due to large LS coupling in the core shell Easily applicable to ultrathin film Need Third Generation

2 XCD PEE and UV LD/CD PEE XCD PEE UV LD PEE G. K. L. arx, H. J. Elmers, and G. Schönhense, Phys. Rev. Lett. 84 (2000) Hg arc lamp Ni(8L)/Co(15L)/Cu(001) Omicron HP C.. Schneider et al. Strong CD >10% due to large LS coupling in the core shell Easily applicable to ultrathin film Need Third Generation Synchrotron Light Sources Time resolving power limited by the SR source pulses (>> 1ps) polycrystalline Fe thick (~100 nm) film Very small LD 0.37% due to small LS coupling in the valence band Difficult to apply to ultrathin film No UV CD PEE images reported CD contrast should be similar In-laboratory experiments Ultrafast time resolution using lasers

3 Purpose of this work 1) Try to find out substantially improved contrast in UV-visible photoelectric CD Cs-coated magnetic thin films to reduce the work function Energy dependence of the CD asymmetry by changing the work function (=Cs amount). T. Nakagawa and T. Yokoyama, Phys. Rev. Lett. 96 (2006) hν Circularly polarized Ni Cu(001) e CD of photoelectric current Cs 2) To eliminate the possibility of the Cs effect (i) Gd deposition instead of Cs work function ~ 3.8 ev, can be excited by a HeCd laser 3) LD Trial of LD for in-plane magnetized films (ii) Clean films using FEL at UVSOR-II Photon energy up to 6 ev

4 UV-visible CD & LD Experimental Setup Work function variation during Cs dosage UHV ~10-10 Torr, Electromagnet 2500 Oe Laser: Diode (CW, 5mW) 635nm, 1.95eV Diode (CW, 10mW) 405nm, 3.06eV HeCd (CW, 10mW) 325nm, 3.81eV Φ 1.8~5.3 ev (Ni) Ti:sapphire 2nd (100fs) 400nm, 3.10eV FEL UVSOR-II ( mW) ~230nm, ~5.4 ev tunable

5 Results of Perpendicular Ni/Cu(001) agnetization curves 635nm Azimuthal angle χ dependece of a quarter-wave plate CD asymmetry A left right I I A = I left I right + left cos2χ linear Cs Ni Cu(001) Cs ~0.2 L Ni 12L 635nm right A ~cos2χ CD max. ~9%!!

6 Work Function Dependence in Ni/Cu(001) CD asymmetry HeCd LD Ti:S 2nd At hν~φ HeCd Cs ~0.06 L Ti:S ~0.10 L LD ~0.20 L Total Electron Yield HeCd LD HeCd LD aximum CD asymmetry 10~29%!! Strong CD only at hν~φ

7 Thickness Dependence in Ni/Cu(001) Comparison with Kerr results 20 L CD ~20% 6 L CD ~0.6% Perpendicular magnetization CD max. ~1% / L!! due to magnification by the presence of reflected lights In-plane magnetization CD max. ~0.1% / L due to compensation by the presence of reflected lights

EF +Φ hω 2 < Ekn< EF Ekn > EF ( eff ) Ve 3 αβ ˆ ˆ 2 2 α β 8π m hω n n σ ( ω) = dkknp kn kn p knδ( E E hν) kn kn Calculated CD asymmetry The summation over n

8 Band calculations Optical conductivity Theoretical Evaluation in fcc Ni Wien2k P. Blaha, K. Schwarz, G. K. H. adsen, D. Kvasnicka, and J. Luitz, Computer code Wien2k (Technische Universität Wien, Vienna, 2002). P.. Oppeneer, T. aurer, J. Sticht, and J. Kübler, Phys. Rev. B 45, (1992). EF +Φ hω 2 < Ekn< EF Ekn > EF ( eff ) Ve 3 αβ ˆ ˆ 2 2 α β 8π m hω n n σ ( ω) = dkknp kn kn p knδ( E E hν) kn kn Calculated CD asymmetry The summation over n (occupied states) is performed only in the energy range of E F +Φ hω < E kn < E F. CD asymmetry ( eff ) ( eff ) A Im σ xy Re σ xx Reproduce fairly well experimental data Close to hν~φ, CD >10%

9 Results of fcc Co and fcc & bcc Fe/Cu(001) Cs/ fcc Fe(3L)/Cu(001) hν = 1.95 ev Cs/ bcc Fe(15L)/Cu(001) hν = 3.81 ev CD max. 1% / L!! Cs/ fcc Co(15L)/Cu(001) CD max. 0.01% / L HeCd LD hν = 1.95 ev CD max. 0.03% / L Results of Ni, Co, Fe/Cu(001) Close to threshold, CD maximized Perpendicular : Strong (~1%/L) In-plane : Weak (<0.1%/L)

10 Gd deposition on Ni/Cu(001) Gd deposition on Ni/Cu(001) instead of Cs Gd Ni Cu(001) CD ~ 4% Gd 2 L Ni 10 L Φ ~ 3.8eV Photoelectric CD can be measured in Gd/Ni/Cu(001) using a HeCd laser (325 nm). We can eliminate the possibility of the Cs effect for the enhancement of photoelectric CD. cf. Polar OKE of the same system using a HeCd laser

FEL trial experiments Cs-free Ni/Cu(001) using FEL at

inherently circular polarized Strong intensity ~100-500 mw

mirrors Photon energy tuning not perfect Weaker CD, worse S/N

11 FEL trial experiments Cs-free Ni/Cu(001) using FEL at UVSOR-II λ ~ 230nm tunable Collaboration with UVSOR machine group, Prof.. Kato & Dr.. Hosaka et al. Upstream mirror Helical undulator FEL from helical undulator inherently circular polarized Strong intensity ~ mw Energy scan not easy due to limited range of the multilayer mirrors Photon energy tuning not perfect Weaker CD, worse S/N ratio Downstream mirror Absence of the electrode sample biased with a battery Worse S/N ratio

12 FEL trial experiments Cs-free clean Ni/Cu(001) using FEL at UVSOR-II Ni Cu(001) No Cs, no Gd hν = 5.41 (ev) 229.2nm CD ~ 0.5-1% hν = 5.37 (ev) 230.9nm CD ~ 3-5%!!

13 LD of in-plane magnetized films Cs/Co(5L)/Cu(001) Cs/Co(5L)/Cu(1 1 17) LD 635 nm LD ~ 0.8% I( // E) I( E) CD LD H LD I( E) I( // E) CD ~ 0.3% LD ~ 0.5% CD after suppression of reflected lights may be better.

Conclusions: Possibility of UV CD PEE We observed substantial enhancements of the photoelectric CD asymmetries especially in perpendicularly magnetized films when the photon energy was tuned to the

14 Conclusions: Possibility of UV CD PEE We observed substantial enhancements of the photoelectric CD asymmetries especially in perpendicularly magnetized films when the photon energy was tuned to the work function threshold. Although we believed that the valence band CD is too weak, UV CD PEE is possible rather in general. We are now preparing UV CD PEE experimental setup, and a video-rate measurements will be done by this summer using available lasers. No 3rd SR light sources are necessarily required. We are also planning ultrafast spin dynamics measurements using a third-order harmonics of a wavelength-variable Ti:sapphire laser. Time resolving power of fs is by far superior to those of third-generation SR light sources. Elmitec PEE Spector

Photon Energy Dependence of Contrast in Photoelectron Emission Microscopy of Si Devices

Photon Energy Dependence of Contrast in Photoelectron Emission Microscopy of Si Devices V. W. Ballarotto, K. Siegrist, R. J. Phaneuf, and E. D. Williams University of Maryland and Laboratory for Physical