Surface Magnetism: Correlation of Structural, Electronic and Chemical Properties with Magnetic Behavior

154 Pages·2010·6.542 MB·English

by  Mathias Getzlaff (auth.)

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Mathias Getzlaff
Surface Magnetism
Correlation of Structural,
Electronic and Chemical Properties with
Magnetic Behavior
123
Prof.Dr. MathiasGetzlaff
Universität Düsseldorf
Institutfür Angewandte Physik
Universitätsstr. 1
40225Düsseldorf
Germany
e-mail: [email protected]
ISSN 0081-3869 e-ISSN1615-0430
ISBN 978-3-642-14188-1 e-ISBN 978-3-642-14189-8
DOI 10.1007/978-3-642-14189-8
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To Sarah and Tim

Preface
This book is intended to give an introduction and a comprehensive overview
concerning the main areas of surface magnetism with special emphasis on rare
earth metals.
Investigations in this field require experimental techniques which are sensitive
to the topmost layers on the one hand and simultaneously to magnetic properties
on the other hand. Using additionally tools with a high lateral resolution the
visualization of magnetic domains becomes possible.
Theunderstandingofmagneticandelectronicbehaviorrequirestheknowledgeof
thestructureonamicroscopicscale.Duetothisimportantrelationshipthedepen-
denceofelectroniconstructuralpropertiesisthefirsttopic.Thiscontainsinvesti-
gationsnotonlyonrareearthmetalsbutadditionallyon3dferromagneticsystems.
It is important to keep in mind that the chemical behavior of a surface deter-
mines the surface electronic properties. Thus, variations, e.g. due to adsorbate
atoms,haveasignificantinfluence.Thisaspectwillbefocusedonasthenexttopic
with the description of selected substrate layers which were exposed to different
types of gaseous molecules.
Investigations on the surface magnetism of itinerant ferromagnetic materials,
includingtheinfluenceofadsorbatesonsurfacemagneticproperties,andmagnets
withlocalizedmomentsisthefinalandmaintopicofthisvolume.Itwillendwith
therealizationoflaterallyresolvedspinpolarizedvacuumtunnelingwhichenables
to image magnetic domains on the nanometer scale.
Acknowledgements
This work summarizes my research on the above-mentioned topics performed at
the Universities of Bielefeld, Mainz, Hamburg, and Düsseldorf.
The realization of such an extensive work as presented here is, of course,
impossible without the help of many other people. It is a pleasure to thank all of
them for their collaboration, their support and their interest.
vii
viii Preface
Firstofall,IwouldliketothankProf.Dr.R.Wiesendangerfortheopportunity
to join his group at the Institute of Applied Physics and Microstructure Research
Center at the University of Hamburg. The excellent laboratory facilities, the
extensive financial outfit, and the continuous support and encouragement he pro-
vided allowed me to develop the activities on scanning tunneling microscopy to
thatlevelasdescribedinthisreport.Ihavealsotothankallothermembersofthe
scanning tunneling microscopy group of Prof. Wiesendanger. The stimulating
atmosphere in the group contributed a lot to success of these activities.
Further, I thank Prof. Dr. H. Steidl from the University of Bielefeld for his
support concerning the spectroscopic capability on spin polarized metastable de-
excitation spectroscopy (MDS) because this unique atomic beam source opens
fascinating possibilities on surface magnetism due to its extraordinary figure of
merit. I would also like to thank Prof. Dr. W. Raith and Prof. Dr. G. Baum for
joining the group in Bielefeld.
Mythanksadditionally goes toProf. Dr.G.Schönhense fromthe Universityof
Mainz. His knowledge and fruitful discussions on photoemission with its widely
spread capabilities allowed me to investigate ‘‘surface magnetism’’ in a very
intensive way.
I thank Prof. Dr. Jürgen Braun (now at the LMU München) for a lot of
theoretical calculations giving a better understanding of magnetic dichroism
experiments.
Especially,IwouldliketothankProf.Dr.MatthiasBode(nowattheUniversity
of Würzburg) for his introduction and continuous help on the field of scanning
tunneling microscopy and spectroscopy. Further, I thank Prof. Dr. Astrid Pundt
from the University of Göttingen for her fruitful discussions on the influence of
hydrogen atoms in solids. Last but not least I thank PD Dr. Joachim Bansmann
(nowatUlmUniversity)fortheclosecollaborationonmagneticdichroismstudies.
ThestudiesonspinpolarizedMDSinBielefeldweresupportedbymydiploma
studentMikeWilhelmandPh.D.studentsDirkEgertandPeerRappolt.Thework
on Heavy Fermion Systems in Mainz was connected with a lot of work done by
Ulrich Kübler, Mike Wilhelm, and Bernhard Schmied who wrote his Ph.D. thesis
in this topic as well as Dr. Gerhard H. Fecher. For the activities on STM in
Hamburg I would like to thank André Kubetzka, René Pascal, Oswald Pietzsch,
Heinz Tödter and Christoph Zarnitz for all the work they have done and the
stimulating discussions.
ImaythankallthepeopleofBESSY;duetotheirhelpitwasapleasuretowork
in Berlin.
Last, but not least, I have to acknowledge the efforts of the mechanical and
electrical workshops; scientific experimentation is impossible without them.
Düsseldorf, May 2010 Mathias Getzlaff
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Experimental Aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Analysis of Electron Spin Polarization. . . . . . . . . . . . . . . 6
2.1.2 Geometric Arrangement for Dichroism Investigations . . . . 7
2.1.3 Circularly Polarized Light . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Metastable De-excitation Spectroscopy. . . . . . . . . . . . . . . . . . . . 9
2.2.1 De-excitation Mechanisms. . . . . . . . . . . . . . . . . . . . . . . 10
2.2.2 Sensitivity to Spin Polarization. . . . . . . . . . . . . . . . . . . . 11
2.2.3 Preparation of Spin Polarized Metastable Atoms. . . . . . . . 13
2.3. Scanning Tunneling Microscopy . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.1. Vacuum Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.2. Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.4 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Structural and Electronic Properties of Rare Earth Metal Systems . 21
3.1 Gadolinium as Prototype of a Rare Earth Metal . . . . . . . . . . . . . 23
3.1.1 Gd on W(110) at Submonolayer Coverage: Structural
and Local Electronic Properties . . . . . . . . . . . . . . . . . . . 24
3.1.2 Morphologies of Epitaxial Gd(0001) Films . . . . . . . . . . . 28
3.1.3 The Gd(0001) Surface State. . . . . . . . . . . . . . . . . . . . . . 30
3.1.4 ThicknessandMorphologyDependentElectronicProperties 32
3.2 Binary Alloys of Rare Earth and Transition Metals. . . . . . . . . . . 37
3.2.1 Remarks on the Preparation Procedure . . . . . . . . . . . . . . 38
3.2.2 Submonolayer Coverage Regime . . . . . . . . . . . . . . . . . . 38
3.2.3 Structural Determination as Function of Film Thickness . . 41
3.3 Heavy Fermion Systems as Ternary Intermetallics
with Extraordinary Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 45
ix
x Contents
3.3.1 Preparation of Well-ordered Surfaces . . . . . . . . . . . . . . . 45
3.3.2 Determination of Electronic Properties Using Resonant
Photoemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4 Influence of Adsorbates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.1 Unusual Adsorption Characteristics for Hydrogen on Gd Surfaces. 53
4.1.1 The Hydrogen Induced State on Gd(0001). . . . . . . . . . . . 54
4.1.2 Spatially Resolved Structural and Electronic Properties. . . 61
4.2 Hydrogen Incorporation in Gadolinium . . . . . . . . . . . . . . . . . . . 68
4.3 Coadsorption Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.3.1 Gadolinium Exposed to Carbon Monoxide. . . . . . . . . . . . 70
4.3.2 Coadsorption of Hydrogen and Carbon Monoxide. . . . . . . 71
4.4 Oxygen on Rare Earth Metal Surfaces. . . . . . . . . . . . . . . . . . . . 77
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5 Magnetic Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.1 Itinerant Ferromagnetic Materials . . . . . . . . . . . . . . . . . . . . . . . 86
5.1.1 Spin Dependent Transport Properties. . . . . . . . . . . . . . . . 86
5.1.2 Magnetic Properties of Epitaxially Grown Films . . . . . . . 88
5.1.3 Nanostructured Systems. . . . . . . . . . . . . . . . . . . . . . . . . 99
5.1.4 Influence of Adsorbates. . . . . . . . . . . . . . . . . . . . . . . . 104
5.2 Ferromagnetic Materials with Localized Magnetic Moments. . . . 113
5.2.1 Temperature Dependent Characteristics. . . . . . . . . . . . . 114
5.2.2 Magnetic Domain Imaging on the Nanometer Scale . . . . 123
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139