University of Groningen

Controlling spins in nanodevices via spin-orbit interaction, magnons and heatDas, Kumar Sourav

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Controlling Spins in Nanodevicesvia

Spin-Orbit Interaction, Magnons and Heat

Kumar Sourav Das

Curved Nanomembranesfor Topological

Quantum Computation

Zernike Institute PhD thesis series 2019-15ISSN: 1570-1530ISBN: 978-94-034-1625-0ISBN: 978-94-034-1624-3 (electronic version)

The work described in this thesis was performed in the research group Physics of Nanode-vices of the Zernike Institute for Advanced Materials at the University of Groningen, theNetherlands. This work was realized using NanoLabNL (NanoNed) facilities and is part ofthe Future and Emerging Technologies (FET) programme within the Seventh Framework Pro-gramme for Research of the European Commission, under FET-Open Grant No. 618083 (CN-TQC). This work is supported by the Zernike Institute for Advanced Materials and is (partly)financed by the NWO Spinoza prize awarded to Prof. B. J. van Wees by the Netherlands Or-ganisation for Scientific Research (NWO).

Typeset using LATEX.Cover art: An illustration of a spinning electron at the heart of a microchip, representing thevision of spintronic-based microprocessors of the future.Cover design: SVDH Media, background image from Adobe Stock.Printed by: Proefschriftmaken (www.proefschriftmaken.nl)

Controlling Spins in Nanodevicesvia

Spin-orbit Interaction, Magnons and Heat

PhD Thesis

to obtain the degree of PhD at theUniversity of Groningenon the authority of the

Rector Magnificus Prof. E. Sterkenand in accordance with

the decision by the College of Deans.

This thesis will be defended in public on

Friday 17 May 2019 at 14.30 hours

by

Kumar Sourav Das

born on 18 October 1988in Burdwan, India

SupervisorProf. B. J. van Wees

Co-SupervisorDr. I. J. Vera-Marun

Assessment committeeProf. G. E. W. BauerProf. A. FertProf. T. Jungwirth

Dedicated to my parents

Contents

1 Introduction 11.1 Spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Motivation and outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Concepts 92.1 Electrical spin injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.1.1 Spin injection from a ferromagnet into a non-magnetic material 102.1.2 Spin injection via spin-orbit effects . . . . . . . . . . . . . . . . . 10

2.2 Non-local spin valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2.1 1-dimensional diffusive spin transport . . . . . . . . . . . . . . . 152.2.2 Hanle spin precession measurements . . . . . . . . . . . . . . . 15

2.3 Thermoelectric effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.1 The Seebeck effect . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.2 The Peltier effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.3.3 The anomalous Nernst effect . . . . . . . . . . . . . . . . . . . . 19

2.4 Spin transport in a magnetic insulators . . . . . . . . . . . . . . . . . . . 202.4.1 Magnons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.4.2 The spin-mixing conductance . . . . . . . . . . . . . . . . . . . . 222.4.3 Electrical injection and detection of magnons . . . . . . . . . . . 232.4.4 Thermal magnon injection via the spin Seebeck effect . . . . . . 24

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3 Experimental methods 333.1 Device fabrication techniques . . . . . . . . . . . . . . . . . . . . . . . . 34

3.1.1 Deep-UV lithography . . . . . . . . . . . . . . . . . . . . . . . . 36

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Contents

3.1.2 Electron beam lithography . . . . . . . . . . . . . . . . . . . . . 373.1.3 Focussed ion beam etching . . . . . . . . . . . . . . . . . . . . . 373.1.4 Physical vapour deposition . . . . . . . . . . . . . . . . . . . . . 38

3.2 Measurement setups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3 Lock-in measurement technique . . . . . . . . . . . . . . . . . . . . . . 39

4 Anisotropic Hanle line shape via magnetothermoelectric phenomena 434.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524.5 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.5.1 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 534.5.2 Anisotropic magnetoresistance measurements . . . . . . . . . . 534.5.3 Hanle data fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . 534.5.4 Extended Hanle dataset . . . . . . . . . . . . . . . . . . . . . . . 554.5.5 Analytical heat diffusion model . . . . . . . . . . . . . . . . . . 554.5.6 Three-dimensional finite element simulation (3D-FEM) . . . . . 564.5.7 Additional experiments and modelling . . . . . . . . . . . . . . 58

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5 Independent geometrical control of spin and charge resistances in curvedspintronics 675.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 685.2 Non-local spin transport experiments in curved nanochannels . . . . . 695.3 Model for spin transport in inhomogeneous curved channels . . . . . . 705.4 Independent geometrical control of spin and charge resistances . . . . 745.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745.6 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.6.1 Sample fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 755.6.2 Electrical characterization . . . . . . . . . . . . . . . . . . . . . . 765.6.3 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.7 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . 785.7.1 Room temperature measurements . . . . . . . . . . . . . . . . . 785.7.2 Pure spin currents in inhomogeneous metallic channels . . . . . 795.7.3 Spin accumulation signal . . . . . . . . . . . . . . . . . . . . . . 815.7.4 Effect of changing the total thickness and/or the channel length

of a flat homogeneous channel . . . . . . . . . . . . . . . . . . . 825.7.5 Generalized advantage of a curved inhomogeneous nanochannel 85

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

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Contents

6 Temperature dependence of the effective spin-mixing conductance probedwith lateral non-local spin valves 916.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 926.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 936.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

7 Spin injection and detection via the anomalous spin Hall effect of a ferro-magnetic metal

1037.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1047.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1067.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1117.5 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

7.5.1 Ruling out the effect of interfacial exchange interaction betweenYIG and Permalloy . . . . . . . . . . . . . . . . . . . . . . . . . . 113

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

8 Efficient injection and detection of out-of-plane spins via the anomalousspin Hall effect in permalloy nanowires 1218.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1228.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1248.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1248.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1318.5 Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

8.5.1 Determination of the Py and the YIG magnetization orientations1328.5.2 Modelling the first harmonic non-local resistance with an angle-

dependent b-parameter . . . . . . . . . . . . . . . . . . . . . . . 1358.5.3 Control device with Pt injector and Pt detector . . . . . . . . . . 1368.5.4 Device fabrication details . . . . . . . . . . . . . . . . . . . . . . 1378.5.5 Interfacial exchange interaction between the Py nanowires and

the YIG thin film . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398.5.6 Spin current injection via the anisotropic magnetoresistance/the

planar Hall effect of the Py nanowires . . . . . . . . . . . . . . . 1418.5.7 Measurement of the third harmonic response of the non-local

signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1428.5.8 Reciprocity check in a control device with a Pt injector and a

Py detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

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Contents

8.5.9 Different mechanisms contributing to the second harmonic re-sponse of the non-local signal . . . . . . . . . . . . . . . . . . . . 144

8.5.10 Finite non-local signal in the fully perpendicular case (φ = 89◦) 146References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

9 Modulation of magnon spin transport in a magnetic gate transistor 1519.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1529.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1529.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1539.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Summary 159

Samenvatting 163

Acknowledgements 167

Publications 171

Curriculum Vitae 173

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