magnetism at the atomic scale by scanning probe techniques · → sp-stm is advanced in periodic...

University of Hamburg K. von Bergmann

Magnetism at the atomic scale Magnetism at the atomic scale by Scanning Probe Techniques by Scanning Probe Techniques

Kirsten von BergmannKirsten von Bergmann

Institute of Institute of AppliedApplied PhysicsPhysicsUniversityUniversity ofof HamburgHamburg


Magnetism with SPMMagnetism with SPM

Spin-polarized scanning tunneling microscopy SP-STM→ density of states of a magnetic sample is spin-polarized(now used in various groups worldwide)

Spin-flip excitation spectroscopy with STM→ excitation of allowed transition adds an inelastic contribution(group of Andreas Heinrich, IBM Almaden)

Magnetic exchange force microscopy MExFM→ direct exchange interaction between probe tip and sample(U. Kaiser, A. Schwarz, R. Wiesendanger, Univ. Hamburg)


Scanning tunneling microscopyScanning tunneling microscopy

),()(d

dst eUETeU

U

I⋅⋅∝ ρρ st ⋅⋅∝ ρρ st ⋅⋅∝ ⋅⋅∝ ),()( ),()( ),()( ),()( ),()(

LDOS

local densityof states

φt

φs

tip sample

2

sample

U

I

tip

ddEeUETEeUEI

eU

⋅⋅∝0

st ),()(),( ρρ

topography:topography:

constantconstant--current modecurrent mode

scan

tip

sample

d dI/dU spectrum at point x,y

ddII/d/dUU spectroscopyspectroscopy


spinspin--polarized LDOSpolarized LDOS

spin-imbalance atthe Fermi-energy

spin-polarized current

magnetic material

University of Hamburg K. von Bergmann M. Bode, Rep. Prog. Phys. 66, 523 (2003).

spinspin--polarizedpolarized STMSTM

spinspin conservationconservationduringduring thethe

tunneltunnel processprocess

M. Julliere, Phys. Lett. 54A, 225 (1975).

mag

n. t

ipm

agn.

tip


angularangular dependencedependence

J. C. Slonczewski,Phys. Rev. B 39, 6995 (1989).

T. Miyazaki and N. Tezuka,J. Magn. Magn. Mater. 139, L231 (1995).

spin-averaged


magnetic tip material

thin film coating FM and AFM

optically pumped GaAs tips

spinspin--polarized probepolarized probe

FM AFM

→ reduction of stray field→ control over direction of magnetization


thinthin film tipsfilm tips

flashedflashed tiptip hashas radiusradius ~ 500 nm~ 500 nm→→ surfacesurface andand interfaceinterface

anisotropyanisotropy dominatedominate→→ MM dependsdepends onon thinthin film materialfilm material

M. Bode et al., Phys. Rev. Lett. 81, 4256 (1998)M. Bode, Rep. Prog. Phys. 66, 523 (2003).

2µm

FeFe--coatedcoated WW tiptipsensitive to sensitive to inin--planeplanemagnetizationmagnetization componentcomponent

CrCr--coatedcoated WW tiptipsensitive to sensitive to outout--ofof--planeplanemagnetizationmagnetization componentcomponent


modulation techniquemodulation technique

W. Wulfhekel and J. Kirschner,Appl. Phys. Lett. 75, 1944 (1999).

U. Schlickum, W. Wulfhekel, and J. Kirschner,Appl. Phys. Lett. 83, 2016 (2003).

FM tip material

sensitive to out-of-plane… … or in-plane magnetization


thin film tips vs modulation technique thin film tips vs modulation technique

U. Schlickum, W. Wulfhekel, and J. Kirschner,Appl. Phys. Lett. 83, 2016 (2003).

separation oftopography and magnetic signal

thin film tips

modulation technique

measurement of I(U)and dI/dU(U)

contains informationon structural,electronic and

magnetic properties

M. Bode, Rep. Prog. Phys. 66, 523 (2003).


SPSP--STM:STM: spectroscopyspectroscopy modemode

e.ge.g. 1.3 AL Fe on W(001). 1.3 AL Fe on W(001)2nd ML 2nd ML isis ferromagneticferromagnetic→→ 44 magneticmagnetic domainsdomains

sideside viewview

“spin-averaged“



spin-resolved


inin--planeplane sensitivesensitive magneticmagnetic tiptip

sideside viewview

K. von Bergmann et al.,Phys. Rev. B. 70, 174455 (2004).




inin--planeplane sensitivesensitive magneticmagnetic tiptipddII//ddUU mappingmapping atat UU=+50mV=+50mV

toptop viewviewMtip


SPSP--STM:STM: constantconstant--currentcurrent modemode

e.ge.g. 1.3 AL Fe on W(001). 1.3 AL Fe on W(001)1st1st ML ML isis outout--ofof--planeplane AFMAFM→→ atomicatomic scalescale magnetismmagnetism

c(2c(2××2) AFM2) AFM


c(2c(2××2) AFM2) AFM

SPSP--STM:STM: constantconstant--currentcurrent modemode

Mtip

S. Heinze et al., Science 288, 1805 (2000).D. Wortmann et al., Phys. Rev. Lett. 86, 4132 (2001).S. Heinze, Appl. Phys. A 85, 407 (2006).

2 nm

[100][100]

topographytopography

A. Kubetzka et al.,Phys. Rev. Lett. 94, 087204 (2005).


investigating single magnetic atomsinvestigating single magnetic atoms

F. Meier, L. Zhou, J. Wiebe, and R. Wiesendanger,Science 320, 82 (2008).

Co atoms on Pt(111)


magnetization curves of individual atomsmagnetization curves of individual atoms

Cr-

tip

+Bext

Cr-

tip

+Bext

F. Meier, L. Zhou, J. Wiebe, and R. Wiesendanger,Science 320, 82 (2008).







A.J. Heinrich, J.A. Gupta, C.P. Lutz, and D.M. Eigler,Science 306, 466 (2004).


spinspin--flip excitation spectroscopyflip excitation spectroscopy

C.F. Hirjibehedin, C.P. Lutz, and A.J. Heinrich,Science 312, 1021 (2006).

C.F. Hirjibehedin, C.-Y. Lin, A.F. Otte, M. Ternes, C.P. Lutz, B.A. Jones, and A.J. Heinrich,Science 317, 1199 (2007).

coupling between magnetic atomsanisotropy of magnetic atoms







magnetic exchange force microscopymagnetic exchange force microscopy

NiO: antiferromagnetic dueNiO: antiferromagnetic due

to superexchange betweento superexchange between

Ni atoms via O bridgesNi atoms via O bridges

bulk terminated surface magnetic structure bulk terminated surface magnetic structure →→ canted spin canted spin

concept of MExFM

detecting the direct magnetic

exchange interaction

short-range force between

the last magnetic tip atom and

the magnetic atomon the sample

U. Kaiser, A. Schwarz, and R. Wiesendanger, Nature 446, 522 (2007).


magnetic exchange force microscopymagnetic exchange force microscopy

chemical contrast:Ni - O = 4.5 pm

MExFM contrast:Ni↑-Ni↓ = 1.5 pm~30 pm closer

U. Kaiser, A. Schwarz, and R. Wiesendanger, Nature 446, 522 (2007).


summarysummary

→ SP-STM is advanced in periodic structures andis reaching for small structures and atoms

→ Spin-flip excitation spectroscopyexplores the quantum magnetic behavior

of small structures and atoms

→ MExFM is the choice for insulating samples

magnetism at the atomic scale by scanning probe techniques · → sp-stm is advanced in periodic...

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