Magnetic Microscopy of Nanostructures

323 Pages·2005·5.286 MB·English

by  W. Kuch (auth.), Professor Herbert Hopster, Professor Hans Peter Oepen (eds.)

NanoScience and Technology
NanoScience and Technology
SeriesEditors:P.Avouris K.vonKlitzing H.Sakaki R.Wiesendanger
TheseriesNanoScienceandTechnologyisfocusedonthefascinatingnano-world,meso-
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dingexpertsinthefield.Thesebookswillappealtoresearchers,engineers,andadvanced
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MagneticMicroscopy
Nano-Optoelectonics ofNanostructures
Concepts,PhysicsandDevices Editors:H.HopsterandH.P.Oepen
Editor:M.Grundmann
H. Hopster H.P. Oepen (Eds.)
Magnetic Microscopy
of Nanostructures
With179Figures
123
ProfessorHerbertHopster ProfessorHansPeterOepen
UniversityofCalifornia Universita¨tHamburg
DepartmentofPhysicsandAstronomy Institutfu¨rAngewandtePhysik
Irvine,CA92697USA undZentrumfu¨rMikrostrukturforschung
E-mail:[email protected] Jungiusstr.11
20355Hamburg,Germany
E-mail:[email protected]
SeriesEditors:
ProfessorDr.PhaedonAvouris
IBMResearchDivision,NanometerScaleScience&Technology
ThomasJ.WatsonResearchCenter,P.O.Box218
YorktownHeights,NY10598,USA
ProfessorDr.,Dres.h.c.KlausvonKlitzing
Max-Planck-Institutfu¨rFestko¨rperforschung,Heisenbergstrasse1
70569Stuttgart,Germany
ProfessorHiroyukiSakaki
UniversityofTokyo,InstituteofIndustrialScience,4-6-1Komaba,Meguro-ku
Tokyo153-8505,Japan
ProfessorDr.RolandWiesendanger
Institutfu¨rAngewandtePhysik,Universita¨tHamburg,Jungiusstrasse11
20355Hamburg,Germany
ISSN1434-4904
ISBN3-540-40186-5SpringerBerlinHeidelbergNewYork
LibraryofCongressControlNumber:2004104060
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Preface
Inrecentyears,anewfieldinsciencehasbeengrowingtremendously,i.e.,theresearch
on nanostructures. In the early beginning, impetus came fromdifferent disciplines,
likephysics,chemistry,andbiology,thatproposedthepossibilityofproducingstruc-
turesinthesub-micronrange.Theworldwideoperatingelectroniccompaniesrealized
thatthiswouldopenupnewfieldsofapplication,andtheyproposedverychalleng-
ing projects for the near future. Particularly, nanomagnetism became the focus of
new concepts and funding programs, like spintronics or magnetoelectronics. These
newconceptscreatedastrongimpactontheresearchfieldoffabricatingnanoscaled
magneticstructures.Simultaneously,ademandforappropriateanalyzingtoolswith
highspatialresolutionarose.Sincethen,thedevelopmentofnewtechniquesandthe
improvement of existing techniques that have the potential of analyzing magnetic
propertieswithhighspatialresolutionhaveundergonearenaissance.Aimingatsys-
tems in the range of some 10nm means that the analyzing techniques have to go
beyondthatscaleintheirresolvingpower.Inparalleltotheeffortsinthecommercial
sector,anewbranchhasbeenestablishedinbasicresearch,i.e.,nanomagnetism,that
is concerned with the underlying physics of the fabrication, analyzing techniques,
andnano-scaledstructures.Theprogressinoneofthesefieldsisinherentlycoupled
withbetterknowledgeorunderstandingand,hence,successintheotherfields.The
imaging technique – as a synonym for spatial resolution – plays a key role in this
triangle.
Inthisbook,webringtogetherthestate-of-the-arttechniquesofmagneticimag-
ing.Wedonotclaimtopresentacompletesurveyofallthetechniquesthatarearound
nowadays.Theevolutionistoofasttokeeptrackofthedevelopmentduringthetime
ittakestoeditabook.Nevertheless,wehaveputtheemphasisongivingacompre-
hensivesurveyofthemagnetic imaging techniques that havealready demonstrated
thehighspatialresolutionor,atleast,havethepotentialtoobtainit.Sometechniques
are well established nowadays and are already utilized in technologically oriented
laboratories for commercial purposes. The majority of techniques presented are at
thestatusofprototypebasicresearchexperiments.
It is the scope of the book to give a deeper insight into the technology and the
understandingoftherelatedeffects.Forthelatterpurpose,amoreelaboratetheory
VI Preface
of operation seems unavoidable in some cases. Each of the techniques presented
has its strength and drawback. It is not the intention of the book to give a ranking
of the techniques with respect to certain properties, e.g., spatial resolution. It is,
however, the aspect of complementariness that is the focus of our intention. The
differenttechniquesaddressdifferentaspectsofthephysicsofnanomagnetism.Thisis
demonstratedwithexamplesfromrecentinvestigations.Thecombinationofdifferent
techniques will give the most complete information about the magnetism on the
nanometerscale.Inthissense,thebookismeantasastate-of-the-artreferencebook
forthemagneticimagingtechniquesavailable.
Chapters1and2dealwiththeapplicationofsynchrotronradiation.Thetunability
ofsynchrotronradiationallowselementalresolutionbytuningtoabsorptionedges.
This,combinedwiththeuseofcircularlypolarizedradiation(circulardichroism)and
thespatialimagingoftheemittedelectronsleadstothepossibilityofmagneticimag-
ingwithelementalresolution(Chap.1).Lineardichroismcanbeusedtodetermine
alongwhichaxismagneticmomentsarealigned,e.g.,inantiferromagneticstructures
(Chap.2).
Theapplicationofshortlaserpulsesallowsimagingbythemagneto-opticalKerr
effectwithhightemporalresolution,thusallowingmagnetizationreversalprocesses
tobestudiedindetail.ThisisdescribedinChap.3.
Thecurrentstateofelectronmicroscopiesformagneticimagingisdescribedin
Chap. 4 (Lorentz microscopy) and Chap. 5 (electron holography). Spin-polarized
electron techniques have led to two new magnetic microscopies. In SPLEEM
(Chap.6),thespindependenceoflow-energyelectrondiffractionoffmagneticsur-
facesisusedtoimagethesurfacemagnetization.Ontheotherhand,inSEMPA(or
spin-SEM)aspinpolarizationanalysisofthesecondaryelectronsisperformedasthe
primaryelectronbeaminanSEMisscannedacrossthesurface.Chapter7dealswith
the basics of SEMPA and its applications to fundamental research, while Chap. 8
discussesapplicationstomagneticstoragemedia.
Theinventionofscanningtunnelingmicroscopy(STM)hasledtoalargenumber
ofscanningprobemicroscopies.Chapter9discussesmagneticeffectsontheelectron
current due to local magnetoresistance. Spin polarized STM using magnetic tips is
onlyinitsinfancy.Itistheonlytechniquecapableoftrulyatomicresolution.Chapter
10describesthepresentsituation.Thestateandfutureprospectsofmagneticforce
microscopy (MFM) is discussed in Chaps. 11 and 12. Chapter 13 discusses other
scanningprobetechniquesformagneticimaging.
Irvine,U.S.A. H.Hopster
Hamburg,Germany H.P.Oepen
May2003
Contents
1 ImagingMagneticMicrospectroscopy
W.Kuch ........................................................ 1
1.1 MicrospectroscopyandSpectromicroscopy–AnOverview ........... 2
1.1.1 ScanningTechniques..................................... 2
1.1.2 ImagingTechniques ..................................... 3
1.2 Basics........................................................ 6
1.2.1 X-RayMagneticCircularDichroism........................ 6
1.2.2 PhotoelectronEmissionMicroscopy........................ 9
1.3 AboutDoingXMCD-PEEMMicrospectroscopy .................... 11
1.3.1 Experiment............................................. 11
1.3.2 DataAnalysis........................................... 13
1.4 SpecificExamples.............................................. 15
1.4.1 UltrathinfccFeFilms.................................... 15
1.4.2 SpinReorientationTransitioninCo/NiBilayers.............. 19
1.5 SummaryandOutlook .......................................... 23
References ......................................................... 24
2 StudyofFerromagnet-AntiferromagnetInterfacesUsingX-RayPEEM
A.Scholl,H.Ohldag,F.Nolting,S.Anders,andJ.Sto¨hr .................. 29
2.1 Introduction................................................... 29
2.2 PhotoemissionElectronMicroscopy .............................. 31
2.2.1 X-RayAbsorptionSpectroscopy ........................... 32
2.2.2 X-RayMagneticLinearDichroism(XMLD)................. 33
2.2.3 X-RayMagneticCircularDichroism(XMCD) ............... 34
2.2.4 TemperatureDependenceofX-RayMagneticDichroism....... 34
2.2.5 Experiment............................................. 35
2.3 AntiferromagneticStructureofLaFeO ThinFilms.................. 37
3
2.4 ExchangeCouplingattheCo/NiO(001)Interface ................... 41
2.4.1 AngularDependenceofDomainContrastinNiO(001) ........ 42
2.4.2 PolarizationDependenceofDomainContrast ................ 43
VIII Contents
2.4.3 CouplingBetweenCoandNiO–AFMReorientation .......... 44
2.4.4 InterfacialSpinPolarizationinCo/NiO(001) ................ 46
2.5 Summary ..................................................... 47
References ......................................................... 48
3 TimeDomainOpticalImagingofFerromagnetodynamics
B.C.ChoiandM.R.Freeman........................................ 51
3.1 Introduction................................................... 51
3.1.1 HistoricalBackgroundofTime-ResolvedTechniques ......... 52
3.2 Instrumentation................................................ 54
3.2.1 PhysicalPrincipleofMagneto-OpticEffect.................. 54
3.2.2 Time-ResolvedExperiments .............................. 56
3.2.3 ExperimentalApparatus .................................. 57
3.3 RepresentativeResultsinThinFilmMicrostructures................. 61
3.3.1 PicosecondTime-ResolvedMagnetizationReversalDynamics.. 61
3.3.2 PrecessionalMagnetizationReversal
andDomainWallOscillation .............................. 63
3.4 ConclusionandOutlook ........................................ 64
References ......................................................... 65
4 LorentzMicroscopy
A.K.Petford-LongandJ.N.Chapman ................................. 67
4.1 Introduction................................................... 67
4.2 ExperimentalRequirements...................................... 68
4.2.1 BasicInstrumentalRequirements .......................... 68
4.2.2 SpecimenRequirements .................................. 69
4.3 BasicTheory.................................................. 70
4.4 ImagingModesinLorentzMicroscopy ............................ 71
4.4.1 FresnelMode ........................................... 72
4.4.2 FoucaultMode.......................................... 73
4.4.3 Low-AngleElectronDiffraction ........................... 74
4.4.4 DifferentialPhaseContrast(DPC)Imaging .................. 76
4.4.5 ElectronHolography..................................... 80
4.4.6 In-situMagnetizingExperiments–
UseoftheTEMasaLaboratory ........................... 80
4.5 ApplicationtoSpin-ValvesandSpinTunnelJunctions ............... 82
4.6 SummaryandConclusions ...................................... 85
References ......................................................... 85
5 ElectronHolographyofMagneticNanostructures
M.R.McCartney,R.E.Dunin-Borkowski,andD.J.Smith ................. 87
5.1 Introduction................................................... 87
5.2 BasisofElectronHolography .................................... 89
5.2.1 TheoreticalBackground .................................. 89
Contents IX
5.2.2 ExperimentalSetup...................................... 92
5.2.3 PracticalConsiderations .................................. 93
5.2.4 Applications............................................ 94
5.3 ApplicationstoMagneticMaterials ............................... 95
5.3.1 FePtThinFilms......................................... 95
5.3.2 NdFeBHardMagnets .................................... 96
5.4 MagneticNanostructures........................................ 97
5.4.1 CoSpheres ............................................. 97
5.4.2 MagnetotacticBacteria ................................... 99
5.4.3 PatternedNanostructures ................................. 100
5.5 Outlook ...................................................... 106
References ......................................................... 107
6 SPLEEM
E.Bauer ........................................................ 111
6.1 Introduction................................................... 111
6.2 PhysicalBasisofBeam-SpecimenInteractions...................... 112
6.3 ExperimentalSetupandProcedure................................ 119
6.4 Applications .................................................. 123
6.4.1 SingleLayers ........................................... 123
6.4.2 NonmagneticOverlayers ................................. 128
6.4.3 Sandwiches............................................. 129
6.4.4 OtherTopics............................................ 132
6.5 Summary ..................................................... 133
References ......................................................... 134
7 SEMPAStudiesofThinFilms,Structures,
andExchangeCoupledLayers
H.P.OepenandH.Hopster.......................................... 137
7.1 Introduction................................................... 137
7.2 Instrumentation................................................ 139
7.2.1 Basics:SecondaryElectronEmission ....................... 139
7.2.2 Spin-PolarizationAnalyzer................................ 143
7.2.3 ElectronColumn ........................................ 145
7.2.4 PolarizationVectorAnalysis .............................. 147
7.3 CaseStudies .................................................. 148
7.3.1 UltrathinFilms.......................................... 148
7.3.2 FilmswithPerpendicularMagnetization .................... 149
7.3.3 FilmswithIn-PlaneMagnetization ......................... 155
7.3.4 ExchangeCoupledFilms ................................. 157
7.3.5 DecorationTechnique .................................... 160
7.3.6 ImaginginMagneticFields ............................... 162
7.4 Conclusions................................................... 164
References ......................................................... 164
X Contents
8 Spin-SEMofStorageMedia
K.Koike ........................................................ 169
8.1 Introduction................................................... 169
8.2 HDDRecordingMedia ......................................... 170
8.3 ObliquelyEvaporatedRecordingMedia ........................... 172
8.4 Magneto-OpticalRecordingMedia ............................... 176
8.5 ConcludingRemarks ........................................... 178
References ......................................................... 179
9 HighResolutionMagneticImaging
byLocalTunnelingMagnetoresistance
W.Wulfhekel .................................................... 181
9.1 Introduction................................................... 181
9.2 ExperimentalSetup ............................................ 185
9.3 MagneticSwitchingandMagnetostrictionoftheTip................. 187
9.4 MagneticImagingofFerromagnets ............................... 189
9.5 MagneticSusceptibility ......................................... 195
9.6 TheContrastMechanism........................................ 196
9.7 ConclusionsandOutlook........................................ 200
References ......................................................... 201
10 Spin-PolarizedScanningTunnelingSpectroscopy
M.BodeandR.Wiesendanger....................................... 203
10.1 Introduction................................................... 203
10.2 ExperimentalSetup ............................................ 205
10.3 ExperimentsonGd(0001) ....................................... 206
10.4 DomainandDomain-WallStudiesonFerromagnets ................. 210
10.5 SurfaceSpin-StructureStudiesofAntiferromagnets ................. 215
Conclusions ........................................................ 222
References ......................................................... 222
11 MagneticForceMicroscopy:
ImagesofNanostructuresandContrastModeling
A.Thiaville,J.Miltat,andJ.M.García ................................ 225
11.1 Introduction:TheMagneticForceMicroscope ...................... 225
11.2 PrincipleofMFM.............................................. 226
11.2.1 MFMLayout ........................................... 226
11.2.2 ModesofOperation...................................... 228
11.3 GalleryofNanostructuresMFMImages ........................... 230
11.3.1 UltrathinFilms.......................................... 230
11.3.2 Nanoparticles........................................... 231
11.3.3 Nanowires.............................................. 232
11.3.4 PatternedElements ...................................... 232
11.4 MFMContrastinAbsenceofPerturbations ........................ 234