Spin triplet supercurrents in thin films of ferromagnetic CrO2
Anwar, M.S.
Citation
Anwar, M. S. (2011, October 19). Spin triplet supercurrents in thin films of ferromagnetic CrO2. Casimir PhD Series. Retrieved from https://hdl.handle.net/1887/17955
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Spin triplet supercurrents in thin films of ferromagnetic
CrO 2
Proefschrift
ter verkrijging van
de graad van Doctor aan de Universiteit Leiden, op gezag van Rector Magnificus prof.mr. P.F. van der Heijden,
volgens besluit van het College voor Promoties te verdedigen op woensdag 19 oktober 2011
klokke 13:45 uur
door
Muhammad Shahbaz Anwar geboren te Faisalabad, Pakistan
op 5 februari 1982
ii
Samenstelling van de promotiecommissie
Promotor: Prof. dr. J. Aarts Universiteit Leiden Leden: Prof. dr. C.W.J. Beenakker Universiteit Leiden
Prof. dr. ir. T.M. Klapwijk Technische Universiteit Delft Prof. dr. A. Brinkman Technische Universiteit Twente Prof. dr. E.R. Eliel Universiteit Leiden
Prof. dr. ir. T.H. Oosterkamp Universiteit Leiden
Casimir PhD series, Delft-Leiden, 2011-23 copyright c°Muhammad Shahbaz Anwar Printed by: W¨ohrmann Print Service ISBN: 978-90-8593-110-2
This work was performed with financial support from the Higher Education Commission (HEC) of Pakistan, and from de Stichting voor Fundamenteel Onderzoek der Materie (FOM) in the Netherlands.
iii
I dedicate my thesis to the memory of my father Muhammad Anwar
Contents
1 Introduction 1
1.1 Superconductivity . . . 1
1.2 Superconductivity in contact with a normal metal . . . 3
1.3 Superconductivity in contact with a ferromagnet . . . . 4
1.4 Long range proximity effect . . . 5
1.5 Motivation and outline . . . 7
2 The physics of superconductor-ferromagnet hybrids 10 2.1 Theoretical concepts: triplet superconductivity . . . 11
2.2 Critical current density in S/FL/F/FR/S junctions . . . 16
2.3 Spin active interfaces . . . 17
2.4 Chromium dioxide (CrO2) . . . 20
3 Growth and magnetic properties of CrO2 thin films 24 3.1 Introduction . . . 25
3.2 Deposition of CrO2 thin films . . . 26
3.3 Crystallography . . . 29
3.3.1 Deposition on TiO2 . . . 29
3.3.2 Deposition on sapphire . . . 30
3.4 Morphology . . . 31
3.4.1 Deposition on TiO2 . . . 31
3.4.2 Deposition on sapphire . . . 33
3.5 Magnetic properties . . . 35
3.5.1 Films on TiO2 . . . 35
3.5.2 Films on sapphire . . . 39
3.5.3 Saturation magnetization as a function of tem- perature . . . 40
3.6 Conclusions . . . 43 iv
CONTENTS v
4 Transport properties of CrO2 44
4.1 Introduction . . . 45
4.2 Sample preparation . . . 45
4.3 Results: Resistivity, magnetoresistance, Hall effect . . . 46
4.3.1 Resistivity . . . 46
4.3.2 Magnetoresistance: High field MR . . . 49
4.3.3 Anomalous Hall Effect . . . 50
4.4 Discussion . . . 52
4.5 Results: Low field MR, Rotational scans of MR, Planar Hall Effect . . . 54
4.5.1 Magnetoresistance: Low field MR . . . 54
4.5.2 Rotational scans of MR . . . 55
4.5.3 Planar Hall effect . . . 58
4.6 Conclusion . . . 60
5 Supercurrents through CrO2 61 5.1 Introduction . . . 62
5.2 Device fabrication . . . 63
5.3 Results; S contacts on CrO2 . . . 65
5.3.1 TiO2 based devices . . . 65
5.3.2 Sapphire based devices . . . 67
5.3.3 Discussion . . . 69
5.3.4 Results; N/F/S contacts . . . 74
5.3.5 Discussion . . . 76
5.4 Conclusions . . . 79
6 Anisotropic magnetothermoelectric power of ferromag- netic thin films 81 6.1 Introduction . . . 82
6.2 Thermal transport phenomena . . . 83
6.3 Experimental . . . 87
6.4 Results . . . 89
6.4.1 Py and Co . . . 90
6.4.2 CrO2 . . . 91
6.5 Discussion . . . 94
6.6 Conclusions . . . 98
A An outlook: Co-based lateral junctions 99 A.1 Introduction . . . 99
A.2 Junctions fabrication . . . 100
vi CONTENTS
A.3 Results and discussion . . . 103 A.3.1 Full Co films . . . 103 A.3.2 Co islands . . . 104 B Resistance anomaly of CrO2 based SFS devices 107
C Control experiments 110
Bibliography 111
Summary 119
Samenvatting 122
List of publications 123
Curriculum Vitae 124
Acknowledgement 125
