From Bulk to Nano: The Many Sides of Magnetism

187 Pages·2008·5.509 MB·English

by  Dr. Carmen-Gabriela Stefanita (auth.)

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Carmen-Gabriela Stefanita
From Bulk to Nano
The Many Sides of Magnetism
With 53 Figures
123
Dr.Carmen-GabrielaStefanita
NanoDotTek
Burlington,MA01803,USA
E-mail:[email protected]
SeriesEditors:
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UniversityofVirginia Universita¨tOldenburg,FachbereichPhysik
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In memory of my grandparents,
whose lifelong dedication made it all possible
Preface
The inspiration for this book can be traced back many years to two
major works that influenced the author’s outlook on applied physics:
Ferromagnetismus by R. Becker, W. D¨oring (Springer, Berlin 1939), and
Ferromagnetism by R.M. Bozorth (IEEE Press, New York 1951). The former
work is a collection of lectures held in the 1930s for ‘technicians’ attending
a technical college. The German language in which the work was originally
written was extremely convenient for the author of this present book, as it
was for a long time the only comfortable technical language in an English
speaking environment. Later on, upon encountering the work by Bozorth,
it was a relief to see the clarity and eloquence of the subjects presented in
English, despite the impressive thickness of the book. Bozorth’s work still
constitutes a practical review for anyone in a multidisciplinary industry who
comes across the various manifestations of magnetism. The popularity of
both works is so enduring that they are regarded as highly academic, and
yet extremely readable, a reference in their own right, still attracting many
readers these days in industry and academia.
The field of magnetism progressed immensely in the twentieth century,
and shows no signs of slowing down in the present one. It has become so
vast that it is quite often viewed only in its parts, rather than as a whole. In
today’smyriadofapplications,especiallyonananoscale,andtheirchangeable
implications mostly on a macroscale, it often seems that different aspects
of reported work on magnetism are scattered and unrelated. Furthermore,
the many atomic theories found in all major books on magnetism employ
complex mathematical language that makes it less obvious how a theoretical
description involving, e.g. spin can be associated with actual experimental
observations.
The diverse expressions of magnetic phenomena on more than one scale,
andtheapparentconfusioncreatedbytheoverwhelmingliteraturethattreats
disparate accounts of magnetism individually without placing them in a
broader context, have led to the writing of this book. Based on the author’s
ownstruggleandexperienceinsiftingthroughandorganizingthevastamount
VIII Preface
ofinformation,thisworkaddressestherelationshipbetweenindividualtopics
in magnetism, trying to make the connection between magnetic phenomena
on various scales more understandable. Nevertheless, the author makes no
claims that the book comes even close to the work of the masters mentioned
earlier. The intention of this author is only to show how the different sides of
magnetism come together. For this reason, the focus of the book is only on
a few selected topics that the author believes are more representative of the
broader subject.
The book has an introductory chapter on some basic concepts in mag-
netism. A few of these are later ‘picked up’ in subsequent chapters, while
others are not mentioned again. Nevertheless, just highlighting them once
draws the reader’s attention to their existence and hints of their usefulness.
Thesecondchapterisanunderpinningofmagneticnondestructivetechniques,
inparticularmagneticBarkhausennoise,regardedbymanyasmerelyalabo-
ratorynondestructiveevaluationmethod.Inanycase,thevaluableresultsand
understandinggainedthroughithaveprovedusefultomoreindustrialnonde-
structive techniques such as Magnetic Flux Leakage and Remote Field Eddy
Current. In the third chapter, the author takes a closer look at combined
phenomena with wide industrial applications. The simple fact that optics
andmagnetismorpiezoelectricityandmagnetostrictioncancoexisthasamaz-
ingconsequencesinmanymultidisciplinaryareas.Furthermore,thesesubjects
may recur in other established fields of magnetism, as implied in subsequent
chapters. The fourth chapter goes deeper into the origins of ferromagnetism,
showingthattheseconstitutethefoundationofemergingsemiconductorelec-
tronics spin-offs (Chap.5), as well as the recording heads in our everyday
computers. The controversial and yet extremely promising field of spintronics
isbrieflydescribedinChap.5,whilesometrendsinmagneticrecordingmedia
are tackled in Chap.6.
Magnetism is used across many disciplines because of its rich implications
in physics, chemistry, biochemistry, and the various areas of engineering. The
author has undertaken to illustrate the various subfields in magnetism in a
manner that anyone with a basic familiarity with modern physics can follow,
regardless of their specialty. By no means is this book intended to be a com-
prehensive inclusion of all aspects of magnetism, nor does it have any claims
that it treats the various areas in an exhaustive manner. On the contrary,
this work is primarily intended to link the different areas of magnetism by
showinghowvariousphenomenafitintoabroaderpicture.Itsgoalistobring
together a broad field in such a way that it provides a starting point for a
graduate student or an experienced researcher for tackling a complex issue
with maximum efficiency.
Collecting many sides of magnetism into a single volume had to be
unavoidably selective; it is just an attempt at trying to spark an interest in
this extended subject while keeping it together. Sometimes, this work has
attempted to clarify the nature of macroscopic magnetic phenomena and
how, in some cases, they can be traced back to a nanoscale. These days, the
Preface IX
popularity of nanotechnology may overshadow macro phenomena, although
they are closely connected. Nanotechnology deals with the manipulation of
materials on an atomic or molecular scale measured in billionths of a meter,
while having manifestations on an every day scale. At other times, the spot-
light of the book has been on explaining the physical nature of some basic
magnetic phenomena, while illustrating the connection with real applications
or contemporary research.
It is a pleasure to acknowledge the support and encouragement I have
received from colleagues and friends without whom I may have never writ-
ten this book. My thanks go to Profs. L. Clapham and D.L. Atherton, as
well as Drs. J.-K. Yi and T. Krause who may have long have forgotten how
it all started. More recently, Prof. S. Bandyopadhyay, and my collaborators
Drs.M.Namkung,F.Yun,andS.Pramanikhavelefttheirintellectualimprint
on this work, therefore my gratitude extends to them. I apologize to all those
who have not been named. Rest assured your influence has played a tremen-
dous role in shaping this book, and the many subjects tackled are a tribute
to your work.
Lastly,itshouldbestatedthattheauthordoesnotendorseanyofthecom-
mercial products discussed in this book. The products were only mentioned
for historical reasons, or to illustrate a principle and explain some magnetics
concepts.
Burlington, MA, Carmen-Gabriela Stefanita
July 2008
Contents
Symbols ..................................................... XVII
1 Introduction............................................... 1
1.1 Review of Certain Historic Magnetic Concepts .............. 2
1.1.1 Magnetic Susceptibility ............................ 2
1.1.2 Classification of Magnetic Materials ................. 3
1.1.3 The Concept of Magnetic Pole ...................... 5
1.1.4 Magnetic Dipoles.................................. 6
1.2 Origins of Magnetism on an Atomic Scale .................. 6
1.2.1 The Importance of Angular Momentum .............. 7
1.2.2 Magnetic Moment of a Sample of N Atoms ........... 8
1.2.3 Crystal Field vs. Spin–Orbit Coupling ............... 9
1.2.4 Magnetocrystalline Anisotropy ...................... 10
1.2.5 Magnetostriction .................................. 10
1.3 Structure-Dependent Micromagnetism ..................... 11
1.3.1 Division into Magnetic Domains..................... 12
1.3.2 Formation of Domain Walls......................... 12
1.3.3 Types of Domain Walls ............................ 13
1.3.4 Significance of Magnetic Domains and Domain Walls .. 14
1.4 Towards Technological Advancements ...................... 15
1.4.1 Design of New Magnetic Materials................... 15
1.4.2 Magnetic Quantum Dots ........................... 15
References .................................................. 16
2 Barkhausen Noise as a Magnetic Nondestructive Testing
Technique ................................................. 19
2.1 Introduction ............................................ 19
2.2 A Basic Definition of Magnetic Barkhausen Noise............ 20
2.2.1 Types of MBN Experiments ........................ 20
2.2.2 Where does MBN Originate? ....................... 21
2.2.3 Formation of Magnetic Domains..................... 22
2.2.4 MBN and 180◦ Domain Walls....................... 23
XII Contents
2.3 Stress Effects ........................................... 24
2.3.1 Elastic Stress Causes Changes in Bulk Magnetization .. 24
2.3.2 Magnetic Domains Respond to Stress ................ 24
2.3.3 Magnetic Anisotropy and MBN ..................... 25
2.3.4 Some Parameters Used in MBN Analysis ............. 25
2.3.5 Elastic Stress Influences on Magnetic Anisotropy ...... 27
2.3.6 Plastic Deformation and Magnetic Anisotropy ........ 27
2.3.7 Effects of Residual Stresses ......................... 28
2.3.8 Influence of Dislocations............................ 30
2.3.9 Selective Wall Energy Increases at Pinning Sites....... 30
2.3.10 Roll Magnetic Anisotropy .......................... 31
2.3.11 Limits in MBN Signal Increase with Plastic Stress..... 32
2.4 Effects of Microstructure on MBN ......................... 33
2.4.1 Variations in Grain Size............................ 33
2.4.2 Compositional and Phase Influences ................. 34
2.4.3 MBN Behavior in Different Materials ................ 34
2.5 Competitiveness of MBN in Nondestructive Evaluation....... 36
2.5.1 Usefulness of MBN for MFL ........................ 36
2.5.2 Need for Calibration of MBN as NDT................ 37
References .................................................. 38
3 Combined Phenomena in Novel Materials ................. 41
3.1 The Interest in Magneto-optical Media ..................... 41
3.1.1 Conventional vs. Continuous Media.................. 42
3.1.2 The Basis of Magneto-optical Effects................. 43
3.1.3 Composite Films Used in Magneto-optical Recording .. 43
3.1.4 Magnetic Recording and Optical Readout ............ 44
3.1.5 Quality of Magnetic Recording...................... 44
3.1.6 Overcoming Noise Problems ........................ 45
3.1.7 The MO Sony Disk ................................ 46
3.1.8 Magnetically Induced Super Resolution .............. 47
3.1.9 Nondestructive Optical Readout..................... 47
3.1.10 Double and Multilayer MO Disks.................... 48
3.1.11 Domain Wall Displacement Detection ................ 49
3.1.12 Magnetic Bubble Domains.......................... 50
3.1.13 Generation of a Bubble Bit of Memory............... 50
3.1.14 Driving Force for Wall Displacement................. 50
3.2 Magnetoelectric Materials ................................ 51
3.2.1 The Magnetoelectric Effect ......................... 51
3.2.2 Oxides, Boracites, Phosphates, etc. .................. 52
3.2.3 Layered Composite Materials ....................... 52
3.2.4 Product, Sum and Combination Properties ........... 53
3.2.5 PZT and Magnetostrictive Materials................. 53
3.2.6 Avoiding Ferrites.................................. 54
3.2.7 Undesired Effects of Sintering....................... 54