Magnetresistance and anomalous hall effect of Cu2MnAl, Co"MnSi and Co2MnGe Heusler alloy ...

179 Pages·2011·8.04 MB·English

by  Obaida, Mohamed Zakaria

Magnetoresistance and Anomalous Hall Effect of
Cu MnAl, Co MnSi and Co MnGe Heusler alloy
2 2 2
thin films

Dissertation to obtain the degree
Doctor Philosophiae (Doctor of Philosophy, PhD)
at the Faculty of Physics and Astronomy

Ruhr-University Bochum

Submitted by
Mohamed Zakaria Obaida

From
Cairo, Egypt

Bochum
2011
1. Supervisor Prof. Dr. K. Westerholt

2. Supervisor Prof. Dr. Dr. h. c. H. Zabel

Date of Disputation 15-12-2011
Magnetwiderstand und anomaler Hall-Effekt in
Cu MnAl, Co MnSi and Co MnGe Heusler-
2 2 2
Dünnschichtsystemen

Dissertation

zur
Erlangung des Grades eines Doktors der Naturwissenschaften
der Fakultät für Physik und Astronomie

der

Ruhr-Universität Bochum

vorgelegt von
Mohamed Zakaria Obaida

aus
Kairo, Ägypten

Bochum
2011
1. Gutachter Prof. Dr. K. Westerholt

2. Gutachter Prof. Dr. Dr. h. c. H. Zabel

Datum of Disputation 15-12-2011
Dedication
To My
Parents
Brother& Sister
Rasha
Omar & Malak

Lastly but not least
To the spirits of the martyrs of revolution, January 25
Acknowledgements

I am deeply indebted to Prof. Kurt Westerholt for giving me the opportunity to
conduct this work and who was an excellent supervisor, supporting me
throughout the entire time of my thesis, read this manuscript and taught me how
to publish my data.

I am heartily thankful to Prof. Hartmut Zabel, whose give me the chance to
work in his group. Besides, his guidance, support, interest, fruitful discussions
and critical comments from the initial to the final level were invaluable.

Moreover I would like to thank to Dr. Dirk Sprungmann and Hassan Inam for
their helpful and suggestive comments done during several steps in my work.

I would also like to thank the head of the research group, Prof. Dr. Andreas
Wieck for use of the research facilities and some useful discussion. I’d also like
to thank Arne Ludwig and Nadine Viteritti, for their help.

My special thanks to thank to all the persons responsible for the machines that
had to be utilized for the work of the thesis. Without their great care and proper
maintenance a lot from this work could not be done, especially Mr. Peter
Stauche and Mrs. Sabine Erdt-Böhm for their help and orientation they offered
me throughout the work in these years.

Furthermore, my thanks appertain to the secretaries Mrs. B. Öztmaur and Mrs.
C. Wulf for helping and organizational matters with great personal efforts to
promptly settle.

My regards and pleasing to my colleagues in department of Solid State Physics
for creating an inspiring environment and providing constant support.

I would like to thank the ministry of higher education and scientific research,
Egypt for the financial support during my PhD study here in Germany.

Last but not least, my parents, brother, sister; Rasha, Omar and Malak, there

are no words that say thank you enough for everything you have done for me.
Contents
Preface
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
I. Magnetoresistance Phenomena and Theoretical Background 3
1. Introduction 4
1.1. Spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Magnetoresistancephenomena . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Ordinarymagnetoresistance(OMR). . . . . . . . . . . . . . . . . . . . 6
1.2.2. Anisotropicmagnetoresistance(AMR) . . . . . . . . . . . . . . . . . . 7
1.2.3. Giantmagnetoresistance(GMR) . . . . . . . . . . . . . . . . . . . . . 9
1.2.4. Tunnelingmagnetoresistance(TMR) . . . . . . . . . . . . . . . . . . . 11
1.3. Spintronicapplications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.1. Spinvalve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3.2. ApplicationinHDDreadheads . . . . . . . . . . . . . . . . . . . . . 16
1.3.3. ApplicationinMRAM . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2. TheoreticalConceptsandLiteratureSurvey 20
2.1. Magneticmaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.1. Ferromagnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2. Quantumtheoryofmagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.1. Exchangeinteractionstypes . . . . . . . . . . . . . . . . . . . . . . . 27
2.2.1.1. Directexchangeinteraction . . . . . . . . . . . . . . . . . . 27
2.2.1.2. Indirectexchangeinteraction . . . . . . . . . . . . . . . . . 29
2.2.1.3. Super-exchangeinteraction . . . . . . . . . . . . . . . . . . 30
2.3. Ferromagneticbandstructures . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.1. TheWeissmeanfieldmodelofferromagnetism . . . . . . . . . . . . . 31
2.4. Energycontributionstoferromagneticfilms . . . . . . . . . . . . . . . . . . . 33
2.5. Magneticanisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.5.1. Magnetocrystallineanisotropy . . . . . . . . . . . . . . . . . . . . . . 35
i
2.5.2. Shapeanisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.6. Domainwalls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.7. Electrical resistivity: temperature dependence and the weak localization phe-
nomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.8. TheHalleffect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.8.1. OrdinaryHalleffect . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.8.2. TheanomalousHalleffect(AHE) . . . . . . . . . . . . . . . . . . . . 46
2.9. Heusleralloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.9.1. FerromagneticfullHeusleralloys . . . . . . . . . . . . . . . . . . . . 50
2.9.2. Halfmetallicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.10. Cu MnAl,Co MnSiandCo MnGeHeusleralloy: : surveyoftheliterature . . 56
2 2 2
2.10.1. Thefull-Heusleralloys . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.10.1.1. Cu MnAl . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2
2.10.1.2. Co MnSi . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2
2.10.1.3. Co MnGe . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2
II. Experimental Work 86
3. PreparationMethodsandMeasurementTechniques 87
3.1. Samplepreparationtechnique. . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.1.1. Thinfilmdeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.1.2. Thesputteringsystem . . . . . . . . . . . . . . . . . . . . . . . . . . 90
3.2. Filmgrowthmodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.2.1. Macroscopicmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.2.2. Microscopicmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
3.2.3. Epitaxialgrowthmechanism . . . . . . . . . . . . . . . . . . . . . . . 96
3.3. Substratechoice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
3.4. UVlithographytechnique. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.4.1. DesignoftheUVlithographymask . . . . . . . . . . . . . . . . . . . 102
3.5. SamplefabricationandpatterningwithUV-lithography . . . . . . . . . . . . . 103
3.6. Samplecharacterizationtechniques . . . . . . . . . . . . . . . . . . . . . . . . 106
3.6.1. X-RayDiffraction(XRD) . . . . . . . . . . . . . . . . . . . . . . . . 106
3.6.2. ElectricaltransportandHalleffectmeasurements . . . . . . . . . . . . 108
3.6.3. VibratingSampleMagnetometer(VSM) . . . . . . . . . . . . . . . . . 110
ii
III. Experimental Results and Discussion 113
4. ResultsandDiscussion 114
4.1. Anoverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.2. Developmentofmagnetizationonannealing . . . . . . . . . . . . . . . . . . . 116
4.3. Structuralproperties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4.3.1. X-rayreflectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4.3.2. X-raydiffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4.3.3. Hysteresisloopsmeasurements . . . . . . . . . . . . . . . . . . . . . 123
4.4. Resistivitymeasurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
4.5. Magnetoresistance(MR)measurement . . . . . . . . . . . . . . . . . . . . . . 129
4.5.1. Highfieldmagnetoresistance . . . . . . . . . . . . . . . . . . . . . . . 130
4.5.1.1. ThetemperaturedependenceoftheMR . . . . . . . . . . . 133
4.5.2. Lowfieldoranisotropicmagnetoresistance . . . . . . . . . . . . . . . 135
4.5.2.1. AMRcrossoverphenomenon . . . . . . . . . . . . . . . . . 136
4.6. AnomalousHallcoefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
4.6.1. TemperaturedependenceoftheAHE . . . . . . . . . . . . . . . . . . 141
4.6.1.1. AHEcrossover . . . . . . . . . . . . . . . . . . . . . . . . 143
4.6.2. TheoriginoftheAHE . . . . . . . . . . . . . . . . . . . . . . . . . . 144
5. Conclusions&Outlook 147
iii
Preface
Preface
Sincealongtimeagothehumanitywasfascinatedbythepropertiesofmagnetsandmag-
netization. Magnetic properties have been observed since approximately 2500 years ago in
AncientGreece[1]andithasbeenclaimedthattheChinesewereusinglodestoneincompasses
fornavigationbefore2500B.C.[2].
Thomson discovered the magnetoresistance (MR) effect in 1857[3]. The MR is defined
as the change of the resistance of a material when an external magnetic field is applied. A
magneto-resistance exists in non-ferromagnetic as well as in ferromagnetic compounds and is
usuallyquitesmall. oftheorderofonepercent,orsmallerintypicallaboratoryfields. Amuch
largerMReffect,thesocalledgiantmagneto-resistance(GMR)wasdetectedindependentlyby
GrünbergandFert[4,5]in1980s. Thiseffectcanbeobservedinthinfilmstructuresconsisting
oftwoferromagneticelectrodesseparatedbyametallic, non-magneticlayer. Itmanifestsitself
in a pronounced change of the sample’s resistance of up to more than 100 % [6] as a function
ofthemagnetizationorientationoftheadjacentferromagneticelectrodes. Theenormousimpact
of their work is underlined by the fact, that Grünberg and Fert have been awarded the Nobel
Price in Physics 2007 for this invention. The GMR was followed by the discovery (or in some
casesthere-discovery)ofsimilareffects,suchasColossalMagnetoresistance(CMR),Balliastic
Magnetoresistance (BMR), and Tunneling Magnetoresistance (TMR). Subsequent to its discov-
ery, the magnetoresistance (MR), became very popular thanks to its great potential in terms of
applicationsinelectronicsdevices[7].
Magnetoresistiveeffectsfindapplicationsinmanyfieldsofelectronicstodaynotonlyinan
ordinarymagneticfieldsensors[8],butalsousedasreadoutandstorageelementsinnon-volatile
data storage devices, such as hard disk drives (HDD) and magnetic random access memories
(MRAM) [9]. In particular, research and development related to GMR has been pushed at a
tremendous pace, so that it took less than ten years from its first demonstration in the labora-
tory to the beginning of large-scale production of HDD using GMR read heads at IBM in 1997
[10]. This thesis relates to the recently emerging and rapidly growing field of spin-electronics,
orspirntronics. Spintronicsdevicesutilizethefundamentalfactthatconductionelectronscarry
not only charge but also spin. In magnetic materials, where the spin up (majority) and spin
down(minority)electronnumbersarenotequal,anelectriccurrentinthesematerialsshouldbe
naturallybespinpolarized. Inthiscase, amagneticfieldcanbeused to manipulatespinpolar-
ized electrical currents, providing an additional channel ofinformation as well as an additional
degreeoffreedomfordesigninganovelspintronicdevices.
1