Edouard LESNE Non-Equilibrium Spin Accumulation Phenomena at the LaAlO3/SrTiO3(001)

238 Pages·2015·8.47 MB·English

THE´SE DE DOCTORAT DE
l’UNIVERSITE´ PIERRE ET MARIE CURIE
´
Ecole Doctorale Physique et Chimie des Mat´eriaux
Th´ese pr´esent´ee par
Edouard LESNE
Intitul´ee
Non-Equilibrium Spin Accumulation Phenomena
at the LaAlO /SrTiO (001)
3 3
Quasi-Two-Dimensional Electron System
Pour obtenir le grade de
Docteur `es Sciences de l’Universit´e Pierre et Marie Curie
Sp´ecialit´e
Physique
Th`ese soutenue publiquement le 25 septembre 2015
devant le jury compos´e de :
Prof. Agn´es Barthe´le´my Directrice de th´ese
Prof. Michel Hehn Rapporteur
Dr. Fabio Miletto Granozio Rapporteur
Dr. Laurent Vila Examinateur
Dr. Nicolas Bergeal Examinateur
Prof. William Sacks Pr´esident du Jury
Dr. Manuel Bibes Invit´e
Dr. Henri Jaffre´s Invit´e
Th`ese pr´epar´ee `a l’Unit´e Mixte de Physique CNRS/Thales associ´ee a` l’Universit´e Paris-Sud

Lead-in
AnimportantgoalofthisworkhasbeentoassessthepotentialoftheLaAlO /SrTiO (001)
3 3
quasi-two-dimensional electron system (q2DES), as a prototypical oxide-based platform, for
the prospect of achieving the transport and control of spin-based information. A prerequisite
to this long-term challenging goal is to demonstrate the generation and detection of non-
equilibrium spin accumulation at the LaAlO /SrTiO (001) heterointerface. In this thesis, we
3 3
explore the possibility to generate and detect spin accumulation in this nonmagnetic oxide
q2DES. This is achieved by means of (i) electrical tunneling spin injection combined with
the Hanle effect, and (ii) through spin pumping experiments combined with the electrical
control of the spin-orbit interaction within the q2DES.
Introduction
The current strategy consisting of scaling down the size of charge-based electronic devices
is reaching its fundamental limits in terms of integration capabilities and performances.
While alternative device architectures are being sought, a new −beyond CMOS− paradigm
is required to tackle these stringent limitations [1, 2]. To this end, spintronics (or spin-
electronics) has emerged as a very active field of research that aims at developing a new
generation of devices whose operation relies on the manipulation of the electronic spin degree
of freedom [3, 4]. Spintronic devices have already had a considerable technological impact,
as highly sensitive magnetic field sensors implemented as readout heads in magnetic data
storage devices [5, 6].
Ultimately, spintronic devices, such as the craved and actively sought spin field-effect
transistor (spin-FET) [7, 8], which do not require charge currents to operate, will lead to
improved energy efficiency (lower power consumption), enhanced data processing speed,
and increased integration of memory and logic with substantial impacts on information
technology. To successfully incorporate spin-based electronics into existing semiconductor
technology, technical issues, as well as fundamental obstacles to allow efficient generation,
transport, manipulation, and detection of spin-based information need to be resolved. It is
therefore desirable to seek materials that have the tunability and functionalities to enable
new types of spintronic devices.
Owing to their remarkable multifunctional and strongly correlated character, oxide ma-
terials already provide such building blocks for charge-based devices such as ferroelectric
i
field-effect transistors, as well as for spin-based two-terminal devices like magnetic tunnel
junctions, with giant responses in both cases, to be exploited in oxide-based spintronics and
electronics architectures [9, 10]. The extraordinary properties of ternary oxides are possibly
best exemplified by the discovery of a high-temperature superconducting ground state in
cuprates (copper oxides) [11], or by the colossal magnetoresistance behavior found in man-
ganites [12].
Remarkably, a large variety of ternary oxides crystallizes in the perovskite ABO crystal
3
structure, and which can exhibit a wide range of ground states spanning from paramag-
netic insulators and semiconductors to metals and superconductors, from dielectrics and
piezoelectrics to ferroelectrics, and from antiferromagnetic to ferromagnetic, and even mul-
tiferroic materials. Additionally, perovskite materials possess the desired closely matched
lattice parameters which makes them naturally suitable for heteroepitaxy. Recent advances
in materials synthesis have made possible the experimental realization of such oxide hetero-
structures, where two or more complex oxides are combined with atomic-scale precision. In
more exotic cases, such an approach may result in emergent properties absent in the parent
bulk compounds [13–15].
Until now, the lack of suitable oxide-based channel materials and the uncertainty of elec-
trical spin injection conditions in these compounds has however prevented the exploration of
similar giant responses in oxide-based lateral spin transport structures. The striking discov-
eryin2004,byOhtomoandHwang,ofanemergent interfacialquasi-two-dimensionalmetallic
conduction between two perovskite band insulators, LaAlO and SrTiO , has provided such
3 3
a conducting oxide channel [16].
Outline of this Thesis
The first chapter of this dissertation is dedicated to a general introduction to the field
of spintronics, with an emphasize on the fundamentally important concepts related to spin-
dependent conduction in magnetic metallic multilayers, and spin-dependent tunneling in
magnetic tunnel junctions. We also present some of the relevant mechanisms (viz. spin-orbit
interactions, and the hyperfine interaction) responsible for the loss of spin information in
solid state materials. Overall, this first chapter gives an overview of the historically prom-
inent works, exemplified by the discovery of giant magnetoresistance in 1988 [17, 18], and
basic concepts, which have shaped the field of spintronics, and contributed to its flourishing
development over the past three decades.
In chapter two, we give a topical overview of the LaAlO /SrTiO (001) heterointerface
3 3
and its hallmark properties. We recall, and critically discuss, various mechanisms (polar
catastrophe, oxygenvacancies, cationicsubstitutions, etc)thathavebeenproposedtoexplain
the origin of the observed emergent interfacial metallicity. The quasi-two-dimensional nature
of the conduction is discussed. This opens up the possibility of efficient electrostatic field-
effect, which has been exploited to tune the two-dimensional superconducting ground state
in this system [19, 20]. At last, we present results of the growth optimization of LaAlO
3
thin films on SrTiO (001) substrates by the pulsed laser deposition technique. The hence
3
realized heterostructures exhibit state-of-the-art crystalline and electrical properties (with
ii
high-electron mobility), as inferred from structural and magnetotransport characterizations.
Inchapterthree,weaimtoassesstheefficiencyofelectricalspininjectioninLaAlO /SrTiO
3 3
heterostructures. We first recall fundamental results of electrical spin injection theory, and
introduce the central concept of non-equilibrium spin accumulation. We then present exper-
imental results of electrical spin injection in the LaAlO /SrTiO system. In a three-terminal
3 3
detection scheme, we make use of the Hanle effect to probe the magnitude of spin accumu-
lation signals in Co/LaAlO /SrTiO tunnel junctions. We report on large amplification of
3 3
the spin signal, with respect to theory, which we discuss in terms of enhanced spin lifetimes
on localized states. We further demonstrate a large modulation of the detected spin signal
by electrostatic field-effect, which evidences the successful generation of spin accumulation
inside the q2DES. These results are discussed in the framework of a spin-conserving sequen-
tial tunneling model. Finally, we identify the very large tunnel barrier resistance, arising
from the 4 unit cell thick LaAlO layer, as a basic obstacle for the prospect of electrical
3
spin detection in lateral spin valve structures. This in turn motivates the investigations of
chapter four.
Although controversy exists regarding the precise origin of the metallic conduction at
LaAlO /SrTiO (001)interfaces,itwasuniversallyfoundthataq2DESonlyappearedbeyond
3 3
an LaAlO thickness threshold of four unit cells, dubbed the critical thickness, and which has
3
become a hallmark feature of the system [21]. In chapter four, we study the effect of metallic
capping layers on the conduction properties of LaAlO /SrTiO heterostructures. Through
3 3
magnetotransport, and X-ray absorption spectroscopy experiments, we demonstrate that the
thickness threshold for the onset of a q2DES can be reduced to just one unit cell of LaAlO
3
whenametallicfilmofcobaltisdepositedontopofit. Ab initio calculationsindicatethatCo
modifies the electrostatic boundary conditions and induces charge transfer toward the Ti 3d
bands, hence the interfacial conductivity. Through a systematic magnetotransport study of
LaAlO /SrTiO heterostructures with various metal capping layers, we tentatively identify
3 3
the possible mechanisms (electrostatics, chemistry defects, redox reactions, etc.) driving the
formation of the q2DES in those systems.
In the fifth and last chapter of this dissertation, we explore the possibility to generate
non-equilibrium spin accumulation in Ni Fe /LaAlO /SrTiO systems by resorting to spin
79 21 3 3
pumping experiments. Magnetization dynamics causes the transfer of a constant flow of
angularmomentum, viz. aspin current, fromtheferromagneticpermalloylayer(spinsource)
to the adjacent q2DES (spin sink). We find that the induced spin accumulation is converted
into a sizeable charge current, which we ascribe to the inverse Edelstein effect, and which
derives from a Rashba-like spin-orbit interaction within the q2DES. We further demonstrate
a large modulation of the effect in back-gating experiments. Our findings expand the field
of interest from planar charge transport to the exploration of spin transport phenomenon in
a nonmagnetic two-dimensional oxide-based systems with electronic correlations.
iii
Table of Contents
Lead-in i
1 An Introduction to the Field of Spintronics 1
1.1 Ferromagnetism in Solids: Fundamental Concepts . . . . . . . . . . . . . . . 2
1.1.1 The Electron Spin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Symmetry Exchange and the Direct Exchange Interaction . . . . . . 3
1.1.3 Theory of Itinerant Ferromagnetism: An Introduction . . . . . . . . . 4
1.2 Spin-Dependent Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1 The Two-Current Model . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Giant Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 Spin-Dependent Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.1 An Introduction to Electron Tunneling . . . . . . . . . . . . . . . . . 11
1.3.2 Tunneling Spin Polarization . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.3 Tunneling Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . 15
1.4 Spin-Orbit Interaction and Spin Relaxation . . . . . . . . . . . . . . . . . . . 19
1.4.1 The Spin-Orbit Interaction . . . . . . . . . . . . . . . . . . . . . . . . 19
1.4.2 Mechanisms of Spin Relaxation . . . . . . . . . . . . . . . . . . . . . 20
1.4.2.1 Electron Spin in a Fluctuating Magnetic Field . . . . . . . . 21
1.4.2.2 Elliott-Yafet Mechanism . . . . . . . . . . . . . . . . . . . . 22
1.4.2.3 D’yakonov-Perel’ Mechanism . . . . . . . . . . . . . . . . . 24
1.4.2.4 Hyperfine Interaction Mechanism . . . . . . . . . . . . . . . 27
2 The LaAlO /SrTiO (001) Interface: A Topical Overview 30
3 3
2.1 The Canonical LaAlO /SrTiO (001) Heterostructure . . . . . . . . . . . . . 31
3 3
2.1.1 SrTiO and LaAlO Bulk Crystals . . . . . . . . . . . . . . . . . . . . 31
3 3
2.1.1.1 SrTiO : A Surprising Insulator . . . . . . . . . . . . . . . . 31
3
2.1.1.2 LaAlO Bulk Crystal . . . . . . . . . . . . . . . . . . . . . . 33
3
2.1.2 Discovery of a Metallic Conduction at the Interface Between
Two Band Insulators . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.2 Doping Mechanisms at LaAlO /SrTiO (001) Interfaces . . . . . . . . . . . . 36
3 3
2.2.1 The Polar Catastrophe Scenario . . . . . . . . . . . . . . . . . . . . . 36
2.2.2 Electron Doping by Oxygen Vacancies . . . . . . . . . . . . . . . . . 40
2.2.3 Cationic Interdiffusion Across the Interface . . . . . . . . . . . . . . . 43
2.2.4 A Polarity-Induced Defect Mechanism . . . . . . . . . . . . . . . . . 44
2.2.5 Can We Reconcile the Different Mechanisms? . . . . . . . . . . . . . 47
iv
2.3 Quantum Confinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.3.1 Electronic Band Structure . . . . . . . . . . . . . . . . . . . . . . . . 50
2.3.2 A Quasi-Two-Dimensional Electron System . . . . . . . . . . . . . . 51
2.3.2.1 Quasi-Two-Dimensional Conduction . . . . . . . . . . . . . 53
2.3.2.2 Electrostatic Field-Effect . . . . . . . . . . . . . . . . . . . . 55
2.4 On the LaAlO /SrTiO Ground State . . . . . . . . . . . . . . . . . . . . . . 56
3 3
2.4.1 Two-Dimensional Superconductivity . . . . . . . . . . . . . . . . . . 56
2.4.2 Which Magnetic Order at LaAlO /SrTiO Interfaces? . . . . . . . . . 59
3 3
2.5 LaAlO /SrTiO Samples Growth and Characterization . . . . . . . . . . . . 61
3 3
2.5.1 Pulsed Laser Deposition of LaAlO Thin Films . . . . . . . . . . . . 62
3
2.5.1.1 Principle of Pulsed Laser Deposition . . . . . . . . . . . . . 62
2.5.1.2 Determination of the Fluence . . . . . . . . . . . . . . . . . 64
2.5.1.3 LaAlO Growth Conditions . . . . . . . . . . . . . . . . . . 65
3
2.5.1.4 In situ Growth Monitoring by RHEED . . . . . . . . . . . . 66
2.5.2 Structural Characterizations . . . . . . . . . . . . . . . . . . . . . . . 67
2.5.2.1 Surface Morphology . . . . . . . . . . . . . . . . . . . . . . 67
2.5.2.2 Crystalline Quality . . . . . . . . . . . . . . . . . . . . . . . 68
2.5.3 Transport Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
2.5.3.1 The van der Pauw Method. . . . . . . . . . . . . . . . . . . 70
2.5.3.2 Hall Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
2.5.3.3 Results of (Magneto)Transport Experiments . . . . . . . . . 72
3 Electrical Spin Injection at the LaAlO /SrTiO (001) Interface 76
3 3
3.1 Theory of Electrical Spin Injection . . . . . . . . . . . . . . . . . . . . . . . 77
3.1.1 Spin Drift-Diffusion Equations . . . . . . . . . . . . . . . . . . . . . . 77
3.1.2 The Conductivity Mismatch Problem . . . . . . . . . . . . . . . . . . 79
3.1.3 Efficient Electrical Spin Injection Through a Tunnel Barrier . . . . . 82
3.1.4 Electrical Spin Detection in a Lateral Spin Valve . . . . . . . . . . . 87
3.1.4.1 The Non-Local Lateral Spin Valve . . . . . . . . . . . . . . 87
3.1.4.2 Condition for Efficient Electrical Spin Detection . . . . . . . 89
3.2 Three-Terminal Spin Detection Scheme and the Hanle Effect:
Concept and Realization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.2.1 The Hanle Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.2.1.1 Historical Background: The Optical Hanle Effect . . . . . . 92
3.2.1.2 The Electrical Hanle Effect in a Three-Terminal Configuration 93
3.2.2 Fabrication of Three-Terminal Co/LaAlO /SrTiO Devices . . . . . . 96
3 3
3.3 Hanle and Inverted Hanle Effects at LaAlO /SrTiO Interfaces . . . . . . . . 99
3 3
3.3.1 Observation of a Large Hanle Spin Signal . . . . . . . . . . . . . . . . 99
3.3.2 Evidence of a Sequential Tunneling Process . . . . . . . . . . . . . . 101
3.3.3 Origin of the Inverted Hanle Effect : Random Hyperfine Fields . . . . 103
3.3.3.1 Inverted Hanle Effect Experiments . . . . . . . . . . . . . . 103
3.3.3.2 Hyperfine Interaction, and Random Magnetic Fields . . . . 105
3.3.3.3 Alternative Mechanisms for Inhomogeneous Magnetic Fields 106
3.4 Gate-Controlled Spin Injection into the LaAlO /SrTiO q2DES . . . . . . . 107
3 3
3.4.1 Electrostatic Field-Effect Modulation of the Spin Signal . . . . . . . . 108
v
3.4.2 Resonant Tunneling Through Localized Electronic States . . . . . . . 109
3.4.3 Enhancement and Gate-Tunability of Spin Accumulations . . . . . . 112
3.5 Concluding Remarks, and Perspectives . . . . . . . . . . . . . . . . . . . . . 117
4 Suppression of the Critical Thickness Threshold for Conductivity at
LaAlO /SrTiO Interfaces Using Metallic Capping Layers 120
3 3
4.1 Emergence of a q2DES for a Single LaAlO Unit Cell
3
on SrTiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
3
4.1.1 Fabrication of Hybrid Metal/Oxide Heterostructures . . . . . . . . . 121
4.1.2 Evidence of a Parallel Conduction in Co and SrTiO . . . . . . . . . 123
3
4.1.3 Signature of a q2DES from Magnetotransport
and Field-Effect Experiments . . . . . . . . . . . . . . . . . . . . . . 125
4.1.3.1 Hall Magnetoresistance of Co/LaAlO /SrTiO
3 3
Heterostructures . . . . . . . . . . . . . . . . . . . . . . . . 125
4.1.3.2 Electrostatic Field-Effect at Co/LaAlO /SrTiO
3 3
Heterointerfaces. . . . . . . . . . . . . . . . . . . . . . . . . 127
4.1.4 Cobalt Thin Films Characterization . . . . . . . . . . . . . . . . . . . 129
4.1.4.1 GlimpseintotheSurfaceMorphologyandMagneticResponse
of Co Thin Films . . . . . . . . . . . . . . . . . . . . . . . . 130
4.1.4.2 Magnetotransport Properties of Co Thin Films . . . . . . . 132
4.2 Electrostatic Boundary Conditions at Metal/LaAlO /SrTiO Interfaces . . . 133
3 3
4.2.1 Band Alignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4.2.2 Work Function of the Metal Capping Layer: a Decisive Role
in the q2DES Formation? . . . . . . . . . . . . . . . . . . . . . . . . 136
4.2.3 Density Functional Theory Calculations . . . . . . . . . . . . . . . . 139
4.2.3.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.2.3.2 Band Structure of Co/LaAlO (1uc)/SrTiO . . . . . . . . . 140
3 3
4.2.3.3 Influence of the Co and LaAlO Layers Thicknesses . . . . . 143
3
4.3 X-Ray Absorption Spectroscopy Experiments . . . . . . . . . . . . . . . . . 144
4.3.1 Principle of X-Ray Absorption Spectroscopy . . . . . . . . . . . . . . 145
4.3.2 Electronic Reconstruction Revealed by X-Ray Linear Dichroism . . . 148
4.3.2.1 XAS at the Ti L-edge . . . . . . . . . . . . . . . . . . . . . 149
4.3.2.2 Energy Hierarchy of Orbital Symmetries . . . . . . . . . . . 151
4.3.3 X-Ray Magnetic Circular Dichroism . . . . . . . . . . . . . . . . . . . 151
4.3.3.1 XMCD at the Co L-edge . . . . . . . . . . . . . . . . . . . . 152
4.3.3.2 XMCD at the Ti L-edge . . . . . . . . . . . . . . . . . . . . 155
4.4 Perspectives, and Future Works . . . . . . . . . . . . . . . . . . . . . . . . . 156
5 Efficient Spin-to-Charge Current Conversion at LaAlO /SrTiO
3 3
Interfaces 158
5.1 Spin-Transfer and Spin Pumping Effects . . . . . . . . . . . . . . . . . . . . 159
5.1.1 Magnetization Dynamics: Phenomenology . . . . . . . . . . . . . . . 159
5.1.2 Basics of Ferromagnetic Resonance . . . . . . . . . . . . . . . . . . . 160
5.1.2.1 Analogy with Electron Paramagnetic Resonance . . . . . . . 160
5.1.2.2 The Ferromagnetic Resonance Condition . . . . . . . . . . . 160
vi
5.1.3 Phenomenology of Spin-Transfer . . . . . . . . . . . . . . . . . . . . . 161
5.1.3.1 Mechanical Analogy . . . . . . . . . . . . . . . . . . . . . . 162
5.1.3.2 Spin-Transfer Torque . . . . . . . . . . . . . . . . . . . . . . 163
5.1.4 Basics of Spin Pumping . . . . . . . . . . . . . . . . . . . . . . . . . 165
5.1.4.1 Spin Current Generation
by Coherent Magnetization Precession . . . . . . . . . . . . 166
5.1.4.2 FMR-Operated Spin Battery . . . . . . . . . . . . . . . . . 166
5.1.4.3 Gilbert-Damping Enhancement . . . . . . . . . . . . . . . . 168
5.2 Spin Current-Charge Current Reciprocal Conversion
Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
5.2.1 Direct and Inverse Spin Hall Effects . . . . . . . . . . . . . . . . . . . 170
5.2.1.1 Extrinsic Mechanisms . . . . . . . . . . . . . . . . . . . . . 170
5.2.1.2 Intrinsic Mechanism . . . . . . . . . . . . . . . . . . . . . . 172
5.2.2 Direct and Inverse Edelstein Effects in a Rashba 2DES . . . . . . . . 172
5.2.2.1 Rashba Spin-Orbit Interaction . . . . . . . . . . . . . . . . . 172
5.2.2.2 Direct and Inverse Edelstein Effects . . . . . . . . . . . . . . 173
5.2.2.3 Relation with the Spin Hall Conductivity . . . . . . . . . . 176
5.3 Broadband FMR Characterization of Py/LaAlO /SrTiO
3 3
samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
5.4 Microwave Rectifying Effects in FMR-Spin Pumping
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
5.4.1 X-Band Cavity Measurement Setup . . . . . . . . . . . . . . . . . . . 180
5.4.2 Origins of the Electromotive Force . . . . . . . . . . . . . . . . . . . 181
5.4.3 Electrical Detection of Prospective IEE Signals . . . . . . . . . . . . 182
5.5 Gate-Controlled Inverse Edelstein Effect at LaAlO /SrTiO
3 3
Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
5.5.1 Evidence for an IEE Signal . . . . . . . . . . . . . . . . . . . . . . . . 183
5.5.2 Amplitude of the IEE Signal and its Gate-Dependence . . . . . . . . 186
5.5.2.1 Estimation of the Inverse Edelstein Length λ . . . . . . . 186
IEE
5.5.2.2 Complex Band Structure of SrTiO −based 2DES . . . . . . 190
3
5.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
Conclusions and Perspectives 195
vii