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

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

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)

Magnetic imaging by LEEM, X- PEEM, X- ray microscopy, and X- ray holography

Wolf gang Kuch, Freie Universit ät Berlin

magnet ic read head Layered magnetic systems

Layered magnetic systems magnet ic RAM

soft magnetic layer tunnel barrier hard magnetic layer bit line word line

• G. Reiss et al., Phys. Bl. 54 (1998) 339

giant magnetoresistance (GMR) (sensor, hard disk read head) metallic conductivity tunnel magnetoresistance (TMR) tunneling current (sensor, magnetic RAM) spin torque transfer momentum transfer by spin polarised e– (fast switching)

Layered magnetic systems

metallic ferromagnet non-magnetic metal insulator semiconductor spin transistor E B C spin transistor with tunnel barrier E B C (logical devices, taking advantage of charge and spin)

Layer- resolved inf ormation f rom XMCD

absorption 900 880 860 840 820 800 780 760 photon energy (eV) Co Ni magnetization parallel to x-rays magnetization antiparallel to x-rays L3 L2 L3 L2 s– circular polarization 6 ML Co/5 ML Cu/15 ML Ni/Cu(001)

Co Cu Ni

Synchrotron radiation needed

hn hn sample detector

– real “absorpt ion” – only very t hin subst rat es 1.) “Tot al elect ron yield” – ≈ proport ional absorpt ion – surf ace sensit ive (l ≈ 20 Å)

hn sample e- e- Is

hn e–

2.) Transmission

hn

Detection methods f or x- ray absorption

e–

elect ron opt ics x-ray opt ics

I maging the x- ray absorption

hn hn sample detector

– real “absorpt ion” – only very t hin subst rat es 1.) “Tot al elect ron yield” – ≈ proport ional absorpt ion – surf ace sensit ive (l ≈ 20 Å)

hn sample e- e- Is

2.) Transmission

hn

Optical imaging: I deal lens

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

elect rost at ic t et rode lens

• G. Schönhense, J . Phys.: Cond. Mat t . 11 (1999) 9517

elect rost at ic t riode lens

• H. Seiler, “Abbildung von Oberf lächen”, Bibliographisches I nst it ut , Mannheim (1968)

Cathode lens f or electron emission microscopy

Cathode lens f or electron emission microscopy

elect rost at ic t et rode lens sample is part of opt ical syst em

ra

real st art ing angle virt ual st art ing angle

a0 ¢ a k|| = k sina0 = ¢ k sin ¢ a k = 2mE0 h ¢ k = 2m(E0 + eUex) h

a' HV a0 contrast aperture sample + – virtual sample

Uex

fi sina0 sin ¢ a = eUex E0 +1 a0 ¢ a ª eUex E0 DW µ 1 E0

accept ed solid angle

Photoelectron spectrum using a cathode lens

a0 ¢ a ª eUex E0 DW µ 1 E0

accept ed solid angle

300 250 200 150 100 50 averaged image intensity (arb. units) 80 60 40 20 kinetic energy (eV) 80 60 40 20 binding energy (eV) Fe 3p Fe 3d W 4f ¥ 25 10 ML Fe pattern on W(001)

hn = 95 eV

Uex = 3.4 keV 2ra = 150 mm a0 : 18° 8° 2°

chromat ic aberrat ion spherical aberrat ion dif f ract ion error ds dc dD Cs Cc

magnet ic elect rost at ic

ª 10f ª f ª 4f ª f ds = 1 2 Csa3 dc = Cc DE E a dD ª 1 2 l a a a a

Aberrations in optical imaging

t heoret ical resolut ion

(magnet ic t riode, 25 kV/3 mm, E = 2.5 eV, DE = 0.25 eV)

• E. Bauer, Surf . Rev. Let t . 5 (1998) 1275

d = ds

2 + dc 2 + dD 2

a µ rA chromat ic aberrat ion spherical aberrat ion dif f ract ion error ds = 1 2 Csa3 dc = Cc DE E a dD ª 1 2 l a

Resolution limit

d/nm

• S. A. Nepij ko et al., Ann. Phys. 9 (2000) 441

Cathode lens: Flat samples required

Electrostatic photoelectron emission microscope (PEEM)

CCD camera f luorescent screen channelplat e phot ons HV + – proj ect ion lenses

f irst use:

• E. Br üche, Z. Phys. 86 (1933) 448;

J . P

• hl, Zeit schr . f . t echn. Physik

12 (1934) 579

PEEM contrast: work f unction

work f unct ion cont rast f rom coarse-grained Au

• H. Seiler, “Abbildung von Oberf lächen”, Bibliographisches I nst it ut , Mannheim (1968)

Hg lamp (hn = 4.9 eV)

PEEM contrast: topographic

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

PEEM contrast: spectroscopic

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

element al chemical

PEEM contrast: spectroscopic

magnet ic

Co/ Ni/ Cu(001)

I(s +) I(s -) I(s +) -I(s -) I(s +) +I(s -)

• W. Kuch, FUB,
• K. Fukumot o, J . Wang,

MP I -MSP ,

• C. Quit mann, F. Nolt ing,
• T. Ramsvik, P

SI -SLS, unpublished.

XMCD- PEEM: separate magnetic and topographic inf ormation

20 µm

• 0.15
• 0.10
• 0.05

0.00 0.05 0.10 0.15 contrast 60 40 20

• 20

incidence azimuth (deg)

XMCD- PEEM: vectorial inf ormation by variation of incidence direction

• W. Kuch, F. Of f i, L. I . Chelaru,

J . Wang, K. Fukumot o,

• M. Kot sugi, MP

I -MSP (unpublished) Co/ FeMn/ Cu(001) Co L3 hn

10 mm hn [110] hn hn Cu(001) 15 ML Ni 6.5 ML Cu 3 ML Co

Ni Co

Layer- resolved magnetic images

INi µ e- ¢

t / lNid ¢

t = (1- e-tNi / lNi )

tNi

Ú

absorption 900 880 860 840 820 800 780 760 photon energy (eV) Co Ni L3 L2 L3 L2 s– circular polarization 6 ML Co/5 ML Cu/15 ML Ni/Cu(001)

Ni

tNi e-tCo / lCo

Co

tCo e-tCu / lCu

Cu

tCu INi µ

Attenuation of secondary electron yield

l ª 2 nm

1 10 Co layer thickness (nm) 1 10 Cu layer thickness (nm) 2 3 5 2 0.5 2 3 5 0.5 Co/Cu on Ni 10000 3000 1000 500 200 100 50 20 10 5 4 3 2 1.5 1e5

Attenuation of secondary electron yield

exposure t ime f or same noise level as wit hout overlayers Ni Cu Co

5 ML FeNi 15 ML Co 15 ML FeMn Cu(001) 5 ML Cu [100] [100] [010] [010]

as grown 10 mm Fe L3 Co L3 hn

XMCD- PEEM: layer- resolved magnetic imaging

• L. I . Chelaru, F. Of f i, M. Kot sugi, and W. Kuch, MPI -MSP (unpublished)

5 ML FeNi 15 ML Co 15 ML FeMn Cu(001) 5 ML Cu [100] [100] [010] [010]

25 Oe 10 mm Fe L3 Co L3

H

hn

XMCD- PEEM: layer- resolved magnetic imaging

• L. I . Chelaru, F. Of f i, M. Kot sugi, and W. Kuch, MPI -MSP (unpublished)

5 ML FeNi 15 ML Co 15 ML FeMn Cu(001) 5 ML Cu [100] [100] [010] [010]

340 Oe 10 mm Fe L3 Co L3

H

hn

XMCD- PEEM: layer- resolved magnetic imaging

• L. I . Chelaru, F. Of f i, M. Kot sugi, and W. Kuch, MPI -MSP (unpublished)

• 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”)

XMCD- PEEM

sample

• bjective lens

magnetic sector field spin-polarized electron gun illumination column imaging column imaging unit CCD camera spin-manipulator

spin- polarized low energy electron microscopy (SPLEEM)

(Elmit ec LEEM 3)

spin- polarized low energy electron microscopy (SPLEEM)

SPLEEM

spin manipulat ion magnet ic cont rast

• Th. Duden and E. Bauer, Surf . Rev. Let t . 5 (1998) 1213

• T. Duden and E. Bauer, PRL 77 (1996) 2308

SPLEEM: example

magnet izat ion “wrinkle” in Co/ W(110)

in-plane

• ut-of-plane

tilt angle topography

• K. L. Man et al., PRB 65 (2001) 024409

2.13 ML 2.20 ML 2.33 ML 2.87 ML 3.13 ML 3.29 ML 3.60 ML 3.83 ML

SPLEEM: another example

during deposit ion of Fe/ Cu(001), growt h rat e: 0.080 ML/ min E = 1.8 eV

20 18 16 14 12 10 8 6 4 2 20 18 16 14 12 10 8 6 4 2

• W. Kuch, K. Fukumot o, J . Wang, MP

I -MSP ,

• C. Quit mann, F. Nolt ing, T. Ramsvik, PSI -SLS, unpublished.

Topographic LEEM contrast

at omic st eps at t he surf ace of Cu(001)

• 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

SPLEEM

2d(r1) l r l

} }

2d(r2)

Zone plate as x- ray lens

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 lf 2r

f rom: homepage of Cent er f or X-ray Opt ics, Lawrence Berkeley Nat ional Laborat ory

inner part of a zone plat e lens. diamet er: 45 µm,

• ut ermost zone: 35 nm wide.

Zone plate as x- ray lens

• G. Denbeaux et al., I EEE Trans. Mag. 37 (2001) 2764

Transmission x- ray microscopy (TXM)

• P. Fischer et al., Rev. Sci. I nst rum. 72 (2001) 2322

M- TXM: example

magnet o-opt ical st orage media 50 nm Tb25Fe56Co19

• T. Eimüller et al., J . Phys. I V 104 (2003) 483

M- TXM: example

[Fe/ Gd] nanost ripes

• W. Chao et al., Nat ure 435, 1210 (2005)

New high- resolution zone plate

images of t est obj ect wit h 19.5 nm lines and spaces images of t est obj ect wit h 15.1 nm lines and spaces 25-nm zoneplat e 25-nm zoneplat e 15-nm zoneplat e 15-nm zoneplat e

• W. Chao et al., Nat ure 435, 1210 (2005)

New high- resolution zone plate

• high resolut ion
• element specif ic, can be used f or layer-specif icit y
• needs synchrot ron radiat ion
• insensit ive t o magnet ic f ields
• parallel imaging or scanning
• only in t ransmission
• does not need UHV

M- TXM

LEEM image of st eps on Si(100), FOV: 4 µm

• G. L. Kellogg, Sandia Nat l. Lab., Albuquerque

When samples st art looking at you... ...it ’s t ime f or a break!

3d eg t 2g eg t 2g

• ct ahedral

cryst al f ield magnet izat ion E

Linear magnetic dichroism in sof t x- ray absorption (XMLD)

• D. Alders et al., Phys. Rev. B 57 (1998) 11623

5 4 3 2 1 absorption Co L3

• 0.06
• 0.04
• 0.02

0.00 0.02 0.04 0.06 difference 785 780 775 770 photon energy (eV) ¥ 30

• W. Kuch et al.,

P

• hys. Rev. Let t . 92 (2004) 017201

Ni L2 NiO/ MgO(001) Co/ Cu(001)

met al

• xide

Linear magnetic dichroism in sof t x- ray absorption (XMLD)

Co L3

hn 10 mm XMCD XMLD [110] 6 ML Co/Cu(001)

XMLD as contrast mechanism in PEEM

• W. Kuch, F. Of f i, L. I . Chelaru, M. Kot sugi, J . Wang, and K. Fukumot o,

MPI -MSP (unpublished)

• H. Ohldag et al., Phys. Rev. Let t . 86 (2001) 2878

Ni XMLD Co XMCD 8 ML Co/ NiO(001)

XMLD as contrast mechanism in PEEM

e– with Ekin > e⋅UG can pass grid G (highpass) L1 L2 G MCP screen retarding field analyzer

I maging electron energy analyzers

highpass f ilt er bandpass f ilt er

EF EVac 2p3/2 hn EF EVac hn 3p Ekin electron energy electron energy min maj

Photon absorption vs. electron emission spectroscopy

phot on absorpt ion elect ron emission

Magnetic linear dichroism in photoemission (MLDAD)

• W. Kuch et al., Phys. Rev. B 51 (1995) 609

XMCD/MLDAD

PEEM: absorption vs. photoemission

XMCD absorpt ion MLDAD phot oemission sensit ivit y: Fe(001)

• W. Kuch et al., unpublished

(see also: W. Kuch et al., J . Vac. Sci. Technol. B 20 (2002) 2543)

ON

OFF ON OFF f ocal plane image plane real space k space

PEEM: imaging of the dif f raction plane

sample

Fermi surf ace mapping by PEEM

• M. Kot sugi et al., Rev. Sci. I nst rum. 74 (2003) 2754

phot on energy 95 eV

Timescales in magnetic materials Magnetic storage technology Thermally activated processes (Domain wall propagation / nucleation) Spin precession Spin-orbit interaction

10 -15

Read/Write Storage

10 -12 10 -9 10 -6 10 -3 1 109

Photoelectric interactions

Time (s)

Time- resolved PEEM

• A. Kuksov et al.,
• J. Appl. Phys. 95 (2004) 6530
• C. M. Schneider et al.,
• Appl. Phys. Let t . 85 (2004) 2562

S.-B. Choe et al., Science 304 (2004) 420

Time- resolved PEEM

• H. St oll et al., Appl. Phys. Let t . 84 (2004) 3328

Time- resolved M- TXM

e– BESSY Dt1 Dt2 Dt3 PEEM 800 ns Dt Pulse Supply time photon pulse from BESSY (50 ps width) 40 ns Dt magnetic pulse Stroboscopic time scheme

Time and layer resolved PEEM imaging

sample Cu foil x-rays J . Vogel et al., Appl. P

• hys. Let t . 82 (2003) 2299

5 mm

5 nm Fe19Ni81 4 nm Cu 5 nm Co

Time and layer- resolved PEEM imaging

fi import ance of domain wall energy f or local domain wall speed

• W. Kuch et al., Appl. P
• hys. Let t . 85 (2004) 440

Starting conf iguration

Fe Co

20 mm

4 nm Fe19Ni81

• 2. 5 nm Al2O3

7 nm Co tunnel junction

field (mT)

Fe

20 mm Layer- resolved stroboscopic magnetic microscopy

4 nm Fe19Ni81

• 2. 5 nm Al2O3

7 nm Co tunnel junction

DICTIONARY OF PHOTOGRAPHY 1889, London by E.J. Wall “In fig. 23 I am enabled, by the kindness of

• Messrs. Perken, Son, & Rayment, to give a sketch
• f the Euryscope lens, which is composed of two

symmetrical combinations of flint glass, and works at an aperture of f/6, a great gain for rapid

• work. These lenses are perfectly free from

spherical and chromatic aberration...” “... some of the finest lenses of the day; and in figs. 21 and 22 are shown two more of Steinheil's lenses, which work at f/2.5, No. 21 being for groups, No. 22 for portraits.”

Aberration correction in light optics

Aberration correction in electron optics

• O. Scherzer , Opt ik 2 (1947) 114

Round convex lenses Chromat ic aberrat ion Spherical aberrat ion

focal point focal point focal point focal point

• uter electrode

at -3750 V inner electrode at 15000 V electron trajectory

Equipot ent ial surf aces in a diode mirror electrostatic mirror

Aberration correction by electrostatic mirror

• H. Rose and D. P

reikszas, Nucl. I nst r . & Met h. A 363 (1995) 201

• R. Fink et al., J ES 84 (1997) 231

sample t ransf er opt ics energy f ilt er proj ect or E/DE = 150.000 screen correct or (t et rode mirror) elect ron gun f inal lat eral resolut ion: Dx = 2 nm energy resolut ion: DE = 100 meV

LEEM/ PEEM: improved resolution by aberration correction

“SMART” proj ect

• H. Rose and D. P

reikszas, Nucl. I nst r . & Met h. A 363 (1995) 201

• R. Fink et al., J ES 84 (1997) 231

Energy filter Detector Mirror corrector 90° sector field Measurement chamber Vibration damped frame electron gun

“SMART” project: set- up at BESSY

• D. Preikszas and H. Rose, J . Elect r . Micr. 1 (1997) 1
• Th. Schmidt et al., Surf . Rev. Let t 9 (2002) 223

LEEM/ PEEM: improved resolution by aberration correction

“SMART” t arget paramet ers:

DE a2 + DE2 a DE a + … Chromat ic aberr . 1/ a 1/ a Dif f ract ion a5 a3 + … Spherical aberr. with correction without correction Resolut ion limit

Coherent x- ray dif f raction (speckles)

Lensless domain imaging using coherent sof t x- rays

• M. Lörgen et al., BESSY-Highlight s 2003, p. 32

holographic image [Co/ Pt ] mult ilayer 1200 nm sample hole 200 nm ref erence hole

Dif f erence (RCP – LCP) FFT (Dif f erence)

Convolut ion t heorem applied t o dif f ract ion: FT(dif f ract ion) = Aut ocorrelat ion (Obj ect )

saturated lin. scale

X- ray holography: Digital image reconstruction

*

W B

* *

Resolut ion 30 - 40 nm

• S. Eisebit t et al., Nat ure 432 (2004) 885

Ref erence hole ∅ 100 nm

• W. F. Schlot t er
• Y. Acremann

FT hologram STXM image

I mage reconstruction f rom speckle hologram

[Co(4 Å)/ P t (7Å)] 50

MTXM

dipole bend: wiggler: undulat or : FEL: µ N µ nN µ n2N µ n2N2 N - number of elect rons n - number of undulat ions

Free electron laser (FEL)

FEL perf ormance

FEL project at BESSY

µm nm mm ns ps f s magnet ic domains recorded bit s grains, nanopart icles molecules at oms exchange int eract ion magnet ic anisot ropy/ spin–orbit coupling spin precession and damping t hermal act ivat ion

Exploring the nano- and f emtoworld

space time