magnet ic read head Layered magnetic systems magnet ic RAM soft - PowerPoint PPT Presentation

Magnetic imaging by LEEM, X- PEEM, X- ray microscopy, and X- ray holography Wolf gang Kuch, Freie Universit ät Berlin basics: • x-ray absorpt ion det ect ion schemes • cat hode lens: working principle, resolut ion • phot oelect ron emission microscopy (P EEM) • low energy elect ron microscopy (LEEM) • magnet ic t ransmission x-ray microscopy (M-TXM) advanced chapt ers: • imaging by magnet ic linear dichroism • elect ron energy f ilt ering in P EEM • t ime-resolved magnet ic imaging • aberrat ion correct ion • imaging x-ray holography

Layered magnetic systems magnet ic read head

Layered magnetic systems magnet ic RAM soft magnetic layer word line tunnel barrier hard magnetic layer bit line G. Reiss et al., Phys. Bl. 54 (1998) 339

Layered magnetic systems giant magnetoresistance (GMR) tunnel magnetoresistance (TMR) spin torque transfer metallic conductivity tunneling current momentum transfer by spin polarised e– (sensor, hard disk read head) (sensor, magnetic RAM) (fast switching) spin transistor spin transistor with tunnel barrier E E B B metallic ferromagnet non-magnetic metal insulator C C semiconductor (logical devices, taking advantage of charge and spin)

Layer- resolved inf ormation f rom XMCD magnetization parallel to x-rays magnetization antiparallel to x-rays 6 ML Co/5 ML Cu/15 ML Ni/Cu(001) L 3 s – circular polarization absorption L 3 L 2 L 2 Ni Co 760 780 800 820 840 860 880 900 photon energy (eV) Co Cu Ni

Synchrotron radiation needed

Detection methods f or x- ray absorption 1.) “Tot al elect ron yield” e- sample h n e – h n Is e- – ≈ proport ional absorpt ion – surf ace sensit ive ( l ≈ 20 Å) 2.) Transmission detector sample h n h n h n – real “absorpt ion” – only very t hin subst rat es

I maging the x- ray absorption elect ron opt ics 1.) “Tot al elect ron yield” e- sample e – h n Is e- – ≈ proport ional absorpt ion – surf ace sensit ive ( l ≈ 20 Å) 2.) Transmission detector sample h n h n h n x-ray opt ics – real “absorpt ion” – only very t hin subst rat es

Optical imaging: I deal lens 1 1 1 = + p q f lens P = q p p q Q f P F F Q f ocal plane: image plane: beams st art ing under beams st art ing at same ident ical angles meet posit ion meet

Cathode lens f or electron emission microscopy elect rost at ic t riode lens H. Seiler, “Abbildung von Oberf lächen”, Bibliographisches I nst it ut , Mannheim (1968) elect rost at ic t et rode lens G. Schönhense, J . Phys.: Cond. Mat t . 11 (1999) 9517

Cathode lens f or electron emission microscopy real st art ing angle a 0 elect rost at ic t et rode lens virt ual st art ing angle a ¢ k || = k sin a 0 = ¢ k sin ¢ a r a 2mE 0 2m(E 0 + eU ex ) k = k = ¢ contrast aperture h h fi sin a 0 eU ex + 1 = sin ¢ E 0 a a 0 + U ex HV eU ex a 0 – sample ª E 0 a ¢ a ' virtual sample DW µ 1 accept ed solid angle sample is part of opt ical syst em E 0

Photoelectron spectrum using a cathode lens binding energy (eV) averaged image intensity (arb. units) 80 60 40 20 0 300 10 ML Fe pattern on W(001) U ex = 3.4 keV 250 h n = 95 eV 200 2r a = 150 m m 150 Fe 3p W 4f Fe 3d 100 ¥ 25 50 0 0 20 40 60 80 kinetic energy (eV) a 0 : 18° 8° 2° eU ex a 0 ª E 0 a ¢ DW µ 1 accept ed solid angle E 0

Aberrations in optical imaging d s spherical aberrat ion a d s = 1 C s a 3 2 d c chromat ic aberrat ion a D E d c = C c a E dif f ract ion error d D d D ª 1 a l 2 a C s C c magnet ic ª f ª f elect rost at ic ª 10f ª 4f

Resolution limit spherical aberrat ion t heoret ical resolut ion d s = 1 C s a 3 (magnet ic t riode, 25 kV/3 mm, E = 2 2.5 eV, D E = 0.25 eV ) chromat ic aberrat ion D E d c = C c a E d/nm dif f ract ion error d D ª 1 l 2 a E. Bauer, Surf . Rev. Let t . 5 (1998) 1275 2 + d c 2 + d D 2 a µ r A d = d s

Cathode lens: Flat samples required S. A. Nepij ko et al., Ann. Phys. 9 (2000) 441

Electrostatic photoelectron emission microscope (PEEM) CCD camera f luorescent screen channelplat e f irst use: E. Br üche, Z. Phys. 86 (1933) 448; J . P ohl, Zeit schr . f . t echn. Physik 12 (1934) 579 proj ect ion lenses HV + phot ons –

PEEM contrast: work f unction work f unct ion cont rast f rom coarse-grained Au Hg lamp ( h n = 4.9 eV ) H. Seiler, “Abbildung von Oberf lächen”, Bibliographisches I nst it ut , Mannheim (1968)

PEEM contrast: topographic J . St öhr and S. Anders, I BM J . Res. Develop. 44 (2000) 535

PEEM contrast: spectroscopic element al chemical J . St öhr and S. Anders, I BM J . Res. Develop. 44 (2000) 535

PEEM contrast: spectroscopic magnet ic

XMCD- PEEM: separate magnetic and topographic inf ormation I( s + ) I( s - ) I( s + ) - I( s - ) Co/ Ni/ Cu(001) I( s + ) + I( s - ) W. Kuch, FUB, K. Fukumot o, J . Wang, MP I -MSP , C. Quit mann, F. Nolt ing, 20 µm T. Ramsvik, P SI -SLS, unpublished.

XMCD- PEEM: vectorial inf ormation by variation of incidence direction Co L 3 Co/ FeMn/ Cu(001) h n 0.15 0.10 0.05 contrast 0.00 -0.05 W. Kuch, F. Of f i, L. I . Chelaru, -0.10 J . Wang, K. Fukumot o, -0.15 M. Kot sugi, MP I -MSP (unpublished) 60 40 20 0 -20 incidence azimuth (deg)

Layer- resolved magnetic images Ni Co 10 m m [110] h n h n h n 3 ML Co 6.5 ML Cu 15 ML Ni Cu(001)

Attenuation of secondary electron yield 6 ML Co/5 ML Cu/15 ML Ni/Cu(001) L 3 s – circular polarization absorption L 3 L 2 L 2 Co t Co Cu t Cu Ni Co Ni t Ni 760 780 800 820 840 860 880 900 photon energy (eV) t Ni e - t Cu / l Cu e - t Co / l Co e - ¢ t / l Ni d ¢ t = (1 - e - t Ni / l Ni ) I Ni µ I Ni µ Ú 0 l ª 2 nm

Attenuation of secondary electron yield Co/Cu on Ni 10 3000 500 1000 10000 200 1e5 100 50 5 Co layer thickness (nm) 20 10 3 5 2 2 4 3 Co 1 2 Cu Ni 0.5 1.5 0.5 2 3 5 1 10 Cu layer thickness (nm) exposure t ime f or same noise level as wit hout overlayers

XMCD- PEEM: layer- resolved magnetic imaging Fe L 3 as grown Co L 3 10 m m [010] [100] h n [100] 5 ML FeNi 5 ML Cu [010] 15 ML Co 15 ML FeMn Cu(001) L. I . Chelaru, F. Of f i, M. Kot sugi, and W. Kuch, MPI -MSP (unpublished)

XMCD- PEEM: layer- resolved magnetic imaging Fe L 3 25 Oe Co L 3 H 10 m m [010] [100] h n [100] 5 ML FeNi 5 ML Cu [010] 15 ML Co 15 ML FeMn Cu(001) L. I . Chelaru, F. Of f i, M. Kot sugi, and W. Kuch, MPI -MSP (unpublished)

XMCD- PEEM: layer- resolved magnetic imaging Fe L 3 340 Oe Co L 3 H 10 m m [010] [100] h n [100] 5 ML FeNi 5 ML Cu [010] 15 ML Co 15 ML FeMn Cu(001) L. I . Chelaru, F. Of f i, M. Kot sugi, and W. Kuch, MPI -MSP (unpublished)

XMCD- PEEM • element specif ic, can be used f or layer-specif icit y • needs synchrot ron radiat ion • good resolut ion • parallel imaging • moderat ely surf ace sensit ive (≈ 20...100 Å) • sensit ive t o ext ernal magnet ic f ields • in vacuum • vect orial inf ormat ion by rot at ing sample • quant it at ive spect roscopic inf ormat ion available (“sum-rule microscopy”)

spin- polarized low energy electron microscopy (SPLEEM) sample objective lens magnetic sector field illumination imaging column column spin-manipulator CCD camera imaging unit spin-polarized electron gun

spin- polarized low energy electron microscopy (SPLEEM) (Elmit ec LEEM 3)

SPLEEM spin manipulat ion magnet ic cont rast Th. Duden and E. Bauer, Surf . Rev. Let t . 5 (1998) 1213

SPLEEM: example magnet izat ion “wrinkle” in Co/ W(110) in-plane out-of-plane tilt angle topography T. Duden and E. Bauer, PRL 77 (1996) 2308

SPLEEM: another example 2.13 ML 3.13 ML 2.20 ML 3.29 ML during deposit ion of Fe/ Cu(001), growt h rat e: 0.080 ML/ min E = 1.8 eV 2.33 ML 3.60 ML 2.87 ML 3.83 ML K. L. Man et al., PRB 65 (2001) 024409

Topographic LEEM contrast at omic st eps at t he surf ace of Cu(001) 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 W. Kuch, K. Fukumot o, J . Wang, MP I -MSP , C. Quit mann, F. Nolt ing, T. Ramsvik, PSI -SLS, unpublished.

SPLEEM • surf ace sensit ive • f ast • vect orial measurement wit hout t urning sample • small f ield of view possible due t o elect ron beam f ocusing • t opographic inf ormat ion simult aneously available • condit ions f or best cont rast depend on sample • sensit ive t o ext ernal magnet ic f ields • needs UHV • not element specif ic

Zone plate as x- ray lens 2d(r 1 ) 2d(r 2 ) r } } l l dif f ract ion of x-rays of wavelengt h l t o one spot block areas of dest ruct ive int erf erence Æ slit s of widt h d d(r) ª l f 2r

Zone plate as x- ray lens inner part of a zone plat e lens. diamet er: 45 µm, out ermost zone: 35 nm wide. f rom: homepage of Cent er f or X-ray Opt ics, Lawrence Berkeley Nat ional Laborat ory

Transmission x- ray microscopy (TXM) G. Denbeaux et al., I EEE Trans. Mag. 37 (2001) 2764

M- TXM: example magnet o-opt ical st orage media 50 nm Tb 25 Fe 56 Co 19 P. Fischer et al., Rev. Sci. I nst rum. 72 (2001) 2322

M- TXM: example [Fe/ Gd] nanost ripes T. Eimüller et al., J . Phys. I V 104 (2003) 483