Vortex dynamics studied by time-resolved X-ray microscopy [Elektronische Ressource] / Kang Wei Chou
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Vortex dynamics studied by time-resolved X-ray microscopy [Elektronische Ressource] / Kang Wei Chou

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Max-Planck-Institut für Metallforschung Stuttgart Vortex Dynamics Studied by Time-Resolved X-ray Microscopy Kang Wei Chou Dissertation an der Universität Stuttgart Bericht Nr. 201 August 2007 VORTEX DYNAMICSSTUDIED BYTIME-RESOLVED X-RAY MICROSCOPYVON DER FAKULTÄT MATHEMATIK UND PHYSIK DER UNIVERSITÄT STUTTGARTZUR ERLANGUNG DER WÜRDE EINES DOKTORS DER NATURWISSENSCHAFTEN(DR. RER. NAT.) GENEHMIGTE ABHANDLUNGVORGELEGT VONKANG WEI CHOUAUS MOUSCRON (BELGIEN)HAUPTBERICHTER: PROF. DR. G. SCHÜTZMITBERICHTER: PROF. DR. M. MEHRINGTAG DER MÜNDLICHEN PRÜFUNG: 3. AUGUST 2007MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNGSTUTTGART 2007“Experiments are that what you cannot explain,theory is something you don’t understand”´— ALEKSANDAR PUZICACKNOWLEDGEMENTSMy deepest gratitude to Gisela Schütz, Hermann Stoll and Bartel Van Waeyen-berge, without whom this work would never have been started, and AleksandarPuzic,´ whose gentle guidance and advice it would certainly never havebeen completed. . . thank you!A sincere token of my appreciation goes to my co-examinator, Michael Mehring,and chairman, Günter Wunner, for their willingness to handle the “matter” sowell and accurately under the existing time pressure.I would also like to thank the various people who provided me with useful andhelpful assistance.

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Published 01 January 2007
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Max-Planck-Institut für Metallforschung
Stuttgart

Vortex Dynamics Studied by
Time-Resolved X-ray Microscopy


Kang Wei Chou
Dissertation
an der
Universität Stuttgart

Bericht Nr. 201
August 2007 VORTEX DYNAMICS
STUDIED BY
TIME-RESOLVED X-RAY MICROSCOPY
VON DER FAKULTÄT MATHEMATIK UND PHYSIK DER UNIVERSITÄT STUTTGART
ZUR ERLANGUNG DER WÜRDE EINES DOKTORS DER NATURWISSENSCHAFTEN
(DR. RER. NAT.) GENEHMIGTE ABHANDLUNG
VORGELEGT VON
KANG WEI CHOU
AUS MOUSCRON (BELGIEN)
HAUPTBERICHTER: PROF. DR. G. SCHÜTZ
MITBERICHTER: PROF. DR. M. MEHRING
TAG DER MÜNDLICHEN PRÜFUNG: 3. AUGUST 2007
MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNG
STUTTGART 2007“Experiments are that what you cannot explain,
theory is something you don’t understand”
´— ALEKSANDAR PUZICACKNOWLEDGEMENTS
My deepest gratitude to Gisela Schütz, Hermann Stoll and Bartel Van Waeyen-
berge, without whom this work would never have been started, and Aleksandar
Puzic,´ whose gentle guidance and advice it would certainly never have
been completed. . . thank you!
A sincere token of my appreciation goes to my co-examinator, Michael Mehring,
and chairman, Günter Wunner, for their willingness to handle the “matter” so
well and accurately under the existing time pressure.
I would also like to thank the various people who provided me with useful and
helpful assistance. Without their care and consideration, this work would likely
not have matured.
First I would like to thank the whole Advanced Light Source crew who provided
an exquisit working place. Special thanks to the electronic workshop for being
there at three in the night, and the users’ office for the lovely unnamed chit chats.
To Tolek Tyliszczak, beamline scientist at the STXM, for his patience and many
valuable tricks.
I would also like to use the opportunity to address the Bielefeld and Regensburg
people for the careful and extremely fast sample preparation. Two persons I
have to thank specifically are Georg Woltersdorf and Christian H. Back for their
flexibility, great interest, and competent advice.
This is also the place to express many thanks towards the group of Joachim Stöhr.
My gratitude goes especially to Yves Acremann for his valuable time helping us
with the experimental setup, and Hans Christoph Siegmann for being who he is.
It would seem conspicuous if I wouldn’t certify the Max Planck Institute for
Metals Research, and especially the department Schütz, so here it is. To Monika
Kotz for taking care of all administrative drags with so much patience. My deep-
viiest sympathy and gratitude to Herrn Heinz-Dieter Carstanjen for his simplicity
and his unending support, and of course for proofreading my work. To the
mechanical workshop for the smaller and the bigger things. To Manfred Fähnle
and Kai Fauth, for dragging them into this strange world of vortex dynamics.
May it blow their heads away as it did for me.
Many thanks and much more respect to Richard Weber for being our deus ex
machina in solving the electronic problems during all those last evenings before
we had to leave on our long beamtime-journeys. Those handy hands and bright
mind saved us more than once.
To my fellow measuring mates: Arne Vansteenkiste, Denis Dolgos and Edward
Prabu. Do you guys also still spot bubbles and penguins everyday?
Finally, I would like to thank my close surrounding, friends and family, for
always being there.
K. W. Chou,
viiiCONTENTS
Acknowledgements vii
Contents ix
List of Abbreviations xi
Zusammenfassung xv
1 To cut a long story short 1
1.1 Applications of magnetism . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Time scales in . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Magnetic imaging techniques . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Magnetism in confined structures 7
2.1 Magnetization precession and micromagnetics . . . . . . . . . . . . 8
2.2 The magnetic microstructure . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Magnetic domains and domain walls . . . . . . . . . . . . . 12
2.2.2 thin film structures of ideally soft materials . . . . 15
2.3 Fast magnetization dynamics . . . . . . . . . . . . . . . . . . . . . . 16
2.3.1 Spin dynamics of the magnetic vortex state . . . . . . . . . . 18
2.3.2 Micromagnetic simulations . . . . . . . . . . . . . . . . . . . 20
3 X-rays: theoretical concepts and applications 21
3.1 Interaction of photons with matter . . . . . . . . . . . . . . . . . . . 22
3.2 Soft x-ray imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.1 XMCD as magnetic contrast mechanism . . . . . . . . . . . . 24
3.2.2 X-ray microscopy methods . . . . . . . . . . . . . . . . . . . 25
3.3 Synchrotron radiation . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.3.1 Undulator radiation . . . . . . . . . . . . . . . . . . . . . . . 26
3.3.2 Monochromator . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3.3 Time structure of synchrotron radiation . . . . . . . . . . . . 30
3.4 Scanning transmission x-ray microscopy . . . . . . . . . . . . . . . . 31
3.4.1 Zone plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
ix3.4.2 Detection system . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.5 Time-resolved scanning transmission x-ray microscopy . . . . . . . 37
3.5.1 Sample and stripline configuration for in-plane field excita-
tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.5.2 Excitation types . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.5.3 Experimental setup and data acquisition . . . . . . . . . . . 42
4 Characterization of ferromagnetic vortex structures 47
4.1 In-plane magnetization of a vortex structure . . . . . . . . . . . . . 47
4.1.1 Setup considerations . . . . . . . . . . . . . . . . . . . . . . . 47
4.1.2 Magnetic contrast at one specific polarization . . . . . . . . 48
4.1.3 - dichroism . . . . . . . . . . . . . . . . . . 49
4.1.4 Element specificity . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2 Out-of-plane magnetization of a vortex structure . . . . . . . . . . . 52
4.2.1 Setup considerations . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.2 Magnetic contrast of the vortex core . . . . . . . . . . . . . . 52
5 Magnetization dynamics in ferromagnetic vortex structures 55
5.1 Differential imaging of magnetic vortex structures . . . . . . . . . . 56
5.2 Gyrotropic mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2.1 Resonant behaviour under pulsed excitation . . . . . . . . . 59
5.2.2 sine excitation . . . . . . . . . . . . . . . . . . . . . 63
5.2.3 Driven oscillatory behaviour under burst excitation . . . . . 70
5.3 Dynamics in coupled ferromagnetic layers . . . . . . . . . . . . . . 73
5.4 Non-linear response of magnetic vortex structures . . . . . . . . . . 75
5.4.1 Vortex core reversal by sine excitation . . . . . . . . . . . . . 76
5.4.2 Hysteresis behaviour I: multiple levels . . . . . . . . . . . . 81
5.4.3 Vortex core reversal by burst excitation . . . . . . . . . . . . 83
5.4.4 Vortex core r – mechanism . . . . . . . . . . . . . . . 87
5.4.5 Hysteresis behaviour II: circular structures . . . . . . . . . . 91
5.4.6 Hysteresis III: breaking of symmetry . . . . . . . 94
5.4.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6 Conclusions and perspectives 103
6.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.2 Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
A The Advanced Light Source (ALS) 109
B Stripline characterization 111
x