Fabrication and characterization of lateral spin valves

FABRICATION AND
CHARACTERIZATION OF LATERAL
SPIN VALVES

TAN WANJING

NATIONAL UNIVERSITY OF SINGAPORE
2007


FABRICATION AND
CHARACTERIZATION OF LATERAL
SPIN VALVES

TAN WANJING
(B. Eng. (Hons.), NUS)

A THESIS SUBMITTED FOR THE DEGREE OF
MASTER OF ENGINEERING
NANOENGINEERING PROGRAMME
NATIONAL UNIVERSITY OF SINGAPORE
2007


ACKNOWLEDGEMENTS

First and foremost, I would like to thank my supervisor Associate Prof.
Adekunle Adeyeye and my co-supervisor Associate Prof. John Thong for their
guidance and advice during the course of my research, without which, this thesis
would not have been possible.
I would also like to thank Mr. Wang Chenchen, Mr. Goolaup Sarjoosing, Mr

Tripathy Debashish, Mr. Chui Kiam Ming, Mr Khoo Chee Keong and Ms Jaslyn Law
for their constructive discussions and suggestions throughout this project as well as
for their friendship.
Special thanks go to Mrs Ho Chiow Mooi, Mr Goh Thiam Peng, Mrs Loh
Fong Leong and Mr Wong Wai Kong for their technical support and help.
Last but not least, I would also like to thank all those who have helped me in
one way or another and whose support and understanding have helped me throughout
this project.

i


TABLE OF CONTENTS
Acknowledgements

i

Table of contents

ii

Abstract

vi

List of Figures

viii

List of symbols and abbreviations

xiii

Chapter 1 Introduction
1.1 Background

1

1.2 What is lateral spin valve

2

1.3 Why lateral spin valve structures

3

1.4 Focus of thesis

3

1.5 Organization of thesis

4

References

6

Chapter 2 Literature Review
2.1 Introduction

7

2.2 Theory of lateral spin valves

7

2.2.1 Theory of spin injection, detection and accumulation

8

2.2.2 Theory of lateral spin valve measurement

9

2.3 Summary of work done on lateral spin valves

12

2.3.1 FM/NM/FM lateral spin valves

13

2.3.2 FM/I/NM/I/FM lateral spin valves

14

ii

2.3.3 Factors affecting magnitude of spin signal

16

2.3.4 Applications for lateral spin valves

17

2.4 Summary

18

References

19

Chapter 3 Fabrication and characterization techniques of lateral spin valves
3.1 Introduction

22

3.2 Fabrication Procedures

22

3.2.1 Optical lithography procedures

23

3.2.1.1 Pre-lithography procedures

24

3.21.2 Optical Lithography

24

3.21.3 Post lithography procedures

26

3.2.2 Electron beam lithography procedures

27

3.2.2.1 Pre-lithography procedures

29

3.2.2.2 Electron beam lithography

29

3.2.2.3 Post-lithography procedures

34

3.3 Additional steps to ensure high quality of devices

35

3.3.1 Control of evaporation flux to reduce sidewalls

35

3.3.2 Capping pads to improve electrical contact of structures

37

3.3.3 Angled evaporation for conformal step coverage

38

3.3.4 Electrochemical Reaction of Copper

40

3.4 Characterization Techniques
3.4.1 Scanning Electron Microscope

42
42

iii


3.4.2 Room temperature magnetoresistance measurement

43

3.4.3 Low temperature magnetoresistance measurement

43

3.5 Conclusion

44

References

45

Chapter 4 Characterization of lateral spin valves at room temperature
4.1 Introduction

46

4.2 Experimental Processes

47

4.3 Anisotropic Magnetoresistance of the Ni80Fe20 electrodes

50

4.3.1 AMR response of rectangular electrodes

50

4.3.2 AMR response of the castellated electrode

57

4.4 Local Spin Valve Measurements

61

4.4.1 LSV measurements for Geometry 2

62

4.4.2 LSV measurements for Geometry 1

68

4.5 Non-local Spin Valve Measurements

70

4.5.1 NLSV measurement for Geometry 1

71

4.5.2 NLSV measurement for Geometry 2

73

4.5.3 Investigation of the spin relaxation length and injection polarization 78
in copper
4.6 Summary

80

References

82

iv


Chapter 5 Characterization of lateral spin valves at low temperature
5.1 Introduction

84

5.2 Local Spin Valve Measurements

85

5.2.1 Four-point and two-point local spin valve measurement
5.3 Non-local Spin Valve Measurements

85
88

5.3.1 Non-local Spin Valve measurements for different configurations

88

5.3.2 Temperature dependence of asymmetry in switching fields

91

5.3.3 Spin relaxation length and spin injection polarization in copper

94

and aluminum
5.4 Conclusion
References

99
100

Chapter 6 Conclusion and outlook
6.1 Conclusion

101

6.2 Future Work

102

References

104

v

ABSTRACT

Lateral spin valves with the separation between the spin injector and spin
detector ranging from 160nm to 450nm were successfully fabricated. Spin-dependent
transport properties such as the spin relaxation length and spin injection polarization
were also studied for copper and aluminum.
Different types of lateral spin valve geometries were fabricated and
characterized and an optimum geometry which showed good switching characteristics
and large spin injection efficiency was developed.
A systematic study of the switching processes for the individual injector and
detector electrodes was carried out using a combination of anisotropic measurements
as well as magnetic force imaging and micromagnetic simulations.
The geometrical effect of the probe configuration on the spin accumulation
signal was investigated in order to maximize the detected spin signal. We showed that
the local spin valve measurements yielded a larger signal than the non-local spin
valve configuration. However, the local spin valve signal included anisotropic
magnetoresistance effects from the injector and detector electrodes, which means that
it is not a pure spin accumulation signal. We were able to use the non-local spin valve
configuration to separate the spin and the charge current and hence remove irrelevant
magnetoresistance changes.
For the non-local measurements, we investigated two kinds of probe
geometries, namely the ‘half’ and the ‘cross’ geometry. We were able to obtain a
larger spin accumulation signal for the ‘half’ geometry due to the spatial distribution
of the spin current. A temperature dependent study was also performed for both

vi


geometries and we found the difference in the magnitude of the spin signal to be more
pronounced at low temperatures.

Non-local measurements were performed for different separations between the
spin injector and detector in order to study the spin relaxation lengths in copper and
aluminum. The spin relaxation lengths of copper and aluminum at 20 K were found
to be 2 µm and 300nm, respectively. The long spin relaxation length obtained for
copper is attributed to the high purity of our copper deposition source. A high spin
injection polarization of 8.1 % was also obtained for our lateral spin valve structure
due to the presence of a tunnel barrier which we identified to be iron oxide from our
low temperature measurements.

vii


LIST OF FIGURES
Figure 1.1

Figure 2.1

2
Schematic diagram of a (a) vertical spin valve and a (b)
lateral spin valve where FM represents a ferromagnet and NM
represents a nonmagnet
8
Schematic diagram of a ferromagnet (FM) in contact with a
nonmagnet (NM). λN and λF represents the distance which
spin accumulation exists in the nonmagnet and ferromagnet
respectively

Figure 2.2

(a) Non-local “half” probe configuration (b) Non-local

“cross” probe configuration

10

Figure 2.3

Lateral spin valves with (a) parallel magnetization and (b)
anti-parallel magnetization of the FM injector and detector,
giving rise to a high and a low output voltage respectively.

11

Figure 2.4

Schematic diagram of three Py wires bridged by a Cu wire

17

Figure 3.1

Schematic diagram showing flow of optical base pads
fabrication

23

Figure 3.2

(a) Exposure of positive resist to UV light. Yellow and pink
portion represents exposed and unexposed regions
respectively. (b) Resist profile after development. Areas

exposed by light has been removed.

25

Figure 3.3

Schematic of mask design drawn using AutoCAD

25

Figure 3.4

(a) Optical image of a die overview (b) Magnified view of
one of the six terminal base structure shown in (a)

27

Figure 3.5

Schematic diagram showing flow of EBL fabrication. In this
illustration, two lateral spin valve structures are fabricated
within the six optical base pads.

28

Figure 3.6

SEM image showing a separation of 60nm between two
Ni80Fe20 electrodes

30

Figure 3.7

Main program window of ELPHY Quantum

31

Figure 3.8

SEM image showing alignment marks of optical structures

31

Figure 3.9

SEM image of structures after 2nd EBL and 2nd evaporation

33

Figure 3.10

(a) SEM image of two completed lateral spin valve structures

33,

viii


(b) Lateral spin valve showing separation of around 350nm

34

Figure 3.11

Photo of a mounted sample after wire bonding

35

Figure 3.12

Diagram showing formation of sidewalls during evaporation

35

Figure 3.13

(a) Diagram showing how sidewalls affect the second
deposited layer (b) AFM image showing discontinuity in
Ni80Fe20 electrodes due to high sidewalls of the optical base
structure

36

Figure 3.14

SEM image of structure after passing a current of 1µA

37

Figure 3.15

IV curve of structure before 3rd EBL step showing resistance
6.7kΩ (red line) and after 3rd EBL step showing resistance of
600Ω (blue line)

38

Figure 3.16

(a) Uniform deposited film thickness resulting from
conformal deposition (b) Non-uniform deposited film
thickness resulting from non-conformal deposition

39

Figure 3.17

Breakage at the step when a current of 1µA is passed through
the copper line

39

Figure 3.18

Structures showing good step coverage after angled
evaporation

40

Figure 3.19

Opening of copper line due to galvanic corrosion

41

Figure 3.20

SEM image of die after encapsulation with 90nm oxide

41

Figure 3.21

Schematic diagram of electron-sample interaction

42

Figure 4.1

SEM images of the two different kinds of lateral spin valve
geometries fabricated and the dimensions of the electrodes.
The inset shows the overview of the devices and the number
of probe terminals available for each geometry.

48

Figure 4.2

Schematic diagram showing the two-point probe

configurations used to measure the AMR response of the
individual electrodes. The external magnetic field is applied
along the x-axis i.e. the longitudinal axis of the electrodes.

51

Figure 4.3

AMR response of the (a) 600nm width Ni80Fe20 electrode and
(b) 300nm width Ni80Fe20 electrode at room temperature. The

52

ix


black (blue) curve corresponds to the positive (negative)
sweep of the external magnetic field.
Figure 4.4

55
OOMMF simulations for a rectangular element of width
600nm. (a) shows the magnetization for a large negative field,
(b) shows the magnetization for a small negative field, (c)
shows the magnetization for a small positive field and (d)
shows the magnetization for a large positive field.

Figure 4.5

MFM image of a 300nm and 600nm width Ni80Fe20 electrode

at a remnant field of (a) -3000 Oe (b) +90 Oe (c) +3000 Oe

55

Figure 4.6

(a) AMR of the castellated Ni80Fe20 electrode in Geometry 2
at room temperature. The probe configuration is shown in the
inset. The black (blue) curve corresponds to the positive
(negative) sweep of the external magnetic field.
(b) Micromagnetic simulation of the spin states of a
castellated structure in Geometry 3 corresponding to the
labeled states in the (a). The current flow is shown only for
the first spin state A.

58

Figure 4.7

(a) Two-point probe configuration for local spin valve
measurement (b) Four-point probe configuration for local
spin valve measurement. The areas circled in red are the areas
of the Ni80Fe20 electrodes which contribute to the local spin
valve signal. The direction of the external magnetic field
applied is also shown.

62

Figure 4.8

Local spin valve response at room temperature for Geometry 63
2 using the (a) two-point probe configuration and the (b) fourpoint probe configuration. The black (blue) curve corresponds
to the positive (negative) sweep of the external magnetic
field.

Figure 4.9

Schematic diagram of the magnetization states in Geometry 2
corresponding to the fields indicated in Figure 4.8 (a) and (b).

64

Figure 4.10

Schematic representation of the lateral spin valve structure
used by van Staa et al. [7] and Jedema et al. [9]

65

Figure 4.11

(a) Change in resistance as a function of external field for
Interface 1 with the probe configuration shown in (c). (b)
shows the change in resistance as a function of external field
for Interface 2 with the probe configuration shown in (d). The
black (blue) curve corresponds to the positive (negative)
sweep of the external magnetic field.

67

x


Figure 4.12

Local spin valve signal for Geometry 1 where H1 corresponds
to the switching field of the wider electrode and H2
corresponds to the switching field of the narrower electrode.
The magnetization states of the electrodes corresponding to
the respective portions of the curve are shown in the red
boxes. The black (blue) curve corresponds to the positive
(negative) sweep of the external magnetic field.

69

Figure 4.13

Schematic diagram showing the widths of the electrodes in
contact with the spin diffusion line for Geometry 1 and 2.

70

Figure 4.14

Non-local spin valve probe configuration for Geometry 1

71

Figure 4.15

Resistance change at room temperature as a function of
applied magnetic field for a Ni80Fe20/Al/ Ni80Fe20 lateral spin
valve with a separation of 240nm between the electrodes. The
black (blue) curve corresponds to the positive (negative)
sweep of the external magnetic field.

72

Figure 4.16

Non-local spin valve probe configuration for Geometry 2.
Icross (red arrows) represents the current flow for the non-local
cross configuration whereas Ihalf (blue arrows) represents the
current flow for the non-local half configuration.

74

Figure 4.17

Resistance change as a function of the external magnetic field
measured in the (a) non-local “half” configuration and the (b)
non-local “cross” configuration at room temperature.The
black (blue) curve corresponds to the positive (negative)
sweep of the external magnetic field.

75

Figure 4.18

Schematic diagram showing spatial distribution of the spin

current (blue arrows) and the effective distance traveled by
the polarized electrons (dotted red arrows). The length of the
blue arrows corresponds to the magnitude of the spin current.

76

Figure 4.19

Dependence of ∆R on the separation between the two
Ni80Fe20 electrodes measured in the non-local ‘half’
configuration for Ni80Fe20/Cu/ Ni80Fe20 lateral spin valves at
room temperature. The red curve represents the best fit based
on Equation 4.2.

78

Figure 5.1

SEM image of lateral spin valve structure used for low
temperature measurements. The probe configurations for both
the two-point probes and the four-point probes are shown.

85

xi


86

Figure 5.2

Local spin valve response at T = 10 K for Geometry 3 using
the (a) two-point probe configuration and the (b) four-point
probe configuration. The black (blue) curve corresponds to
the positive (negative) sweep of the external magnetic field.

Figure 5.3

Plot of ∆R as a function of temperature for the two-point LSV 87
configuration (blue curve) and the four-point LSV
configuration (red curve).

Figure 5.4

Non-local magnetoresistance response at T = 5 K for the (a)
‘half’ probe configuration and the (b) ‘cross’ probe
configuration. The black (blue) curve corresponds to a
positive (negative) sweep of the external magnetic field.

90

Figure 5.5

Plot of ∆R as a function of temperature for the NLSV ‘half’
configuration (red curve) and the NLSV ‘cross’ (blue curve).

91

Figure 5.6

Schematic diagram of the interface between the Ni80Fe20
electrode and the spin diffusion line. The diagram is not
drawn to scale for clarity.

92

Figure 5.7

Plot of HE as a function of temperature

93

Figure 5.8

Dependence of the spin valve signal, ∆R, on the electrode
spacing at a temperature of 20 K for lateral spin valves with
(a) copper and (b) aluminum as spin diffusion medium.

95

Figure 5.9

(a) SEM image of our lateral spin valve with aluminum as the
spin diffusion line (b) SEM image of the spin valve fabricated
by Jedema et al. [1]

98

xii

LIST OF SYMBOLS AND ABBREVIATIONS
aluminum

Al

aluminum oxide

Al2O3

anisotropic magnetoresistance

AMR

atomic force microscopy

AFM

carbon nanotube

CNT

cobalt

Co

copper

Cu

electron beam lithography

EBL

ferromagnetic

FM

giant magnetoresistance

GMR

gold

Au

isopropanol

IPA

leadless chip carrier

LCC

local spin valve

LSV

magnetic force microscopy

MFM

magnetic random access memories

MRAM

Magnetic tunnel junction

MTJ

magnetoresistance

MR

methyl isobutyl ketone

MIBK

multiwalled nanotube

MWNT

non-local spin valve

NLSV

non-local transverse spin signal

NLTS

xiii


nonmagnet

NM

object oriented micromagnetic framework OOMMF
polymethyl methacrylate

PMMA

scanning electron microscopy

SEM

Silicon dioxide

SiO2

tunneling magnetoresistance

TMR

xiv


Chapter One: Introduction
CHAPTER ONE
INTRODUCTION

1.1 Background
In the last decade, there has been significant interest in the magnetic and
transport properties of ferromagnetic mesostructures both from fundamental and
application viewpoints. This increasing interest is due to advances in fabrication
techniques such as electron-beam lithography as well as nano-characterization
techniques such as scanning probe microscopy and nano magneto-optical Kerr effect
microscopy.
From the application viewpoint, magnetic mesoscopic structures and
nanostructures form the basic building blocks for various spintronic applications. In
the hard disk industry, the giant magnetoresistance (GMR) effect is being exploited in
the magnetic sensors used in the read-heads of hard disks. Aggressive research work
is also being carried out on magnetic random access memories (MRAM) due to their
various advantages over conventional memories such as high speed, high storage
density and non-volatility [1].
Fundamentally, novel properties emerge as the lateral size of the
ferromagnetic mesostructures become comparable to or smaller than certain
characteristic length scales, such as the spin relaxation length and the magnetic
domain wall width. There has been active research on using magnetic mesoscopic
structures to probe these length scales and in addition, there has also been ongoing
effort to study spin-dependent electron transport and to understand the underlying
physics from which they result. In order to open up the way for more spin-based

1


Chapter One: Introduction
electronic applications of new and improved functionality, there is a need to be able
to precisely manipulate the dynamics of spin in solid state devices, and this begins
with a clear understanding of spin injection, spin accumulation and spin detection.
Since the discovery of the anisotropic magnetoresistance (AMR) effect, some

of the most well-known spin-dependent electron transport which have been studied
are the GMR effect [2, 3] and the tunneling magnetoresistance (TMR) effect [4, 5].
The structures which were used to probe the GMR and TMR effects are the
conventional vertically stacked magnetic multilayers, generally called the vertical
spin valve.

1.2 What is lateral spin valve?
In the lateral spin valve structure, the ferromagnetic structures are laid out
laterally instead of being stacked as in the vertical spin valve. Figure 1.1 (a) and (b)
shows a general schematic of a conventional vertical spin valve and a lateral spin
valve respectively.
(a)

(b)

FM
FM
NM
FM

FM
FM
Cu

Figure 1.1 Schematic diagram of a (a) vertical spin valve and a (b) lateral spin
valve where FM represents a ferromagnet and NM represents a nonmagnet

2

Chapter One: Introduction
1.3 Why lateral spin valve structures?
Most of the work done on spin transport has focused on the vertical spin valve
structure. However, a serious drawback of these vertical heterostructures is the
difficulty in integrating them with other devices for future spintronic applications due
to the physical constraints in fabricating multi-terminal devices for these vertical spin
valve structures.
The lateral spin valve structure offer additional degrees of freedom to control
magneto-transport properties by tailoring shape anisotropies. Multi-terminal devices
can also be realized with the lateral spin valve structures, making it possible for the
integration of a large number of devices.
Another important use of the lateral spin valve is that it allows for non-local
spin valve measurements, whereby the spin and charge currents are separated. This is
a powerful means of detecting spin-dependent signals because irrelevant
magnetoresistance changes such as AMR and spin Hall effects can be removed [6].
This separation of spin and charge is not possible for the vertical spin valve structure.

1.4 Focus of thesis
This thesis is devoted to the study of spin-dependent electron transport
phenomena, whereby the electrical injection of spins, the transport of the spin
information in non-magnetic metals and the detection of the resulting spin will be
investigated using the lateral spin valve structure.
Various lateral spin valve devices with different geometrical parameters are
fabricated using multi-level electron-beam lithography (EBL) and deposition

3


Chapter One: Introduction
techniques. Spin injection and detection are achieved using Ni80Fe20 electrodes while

spin transport through two different kinds of metals, copper and aluminum, is
investigated.
Characterization of the lateral spin valves is carried out by magnetotransport
measurements at both room temperature and low temperatures. The geometrical
effect of the probe configuration on the spin accumulation signal is studied in order to
understand the spatial distributions of the spin current which is essential for
developing spin devices.
The spin relaxation length for copper and aluminum is measured for our
lateral spin valve structures. The spin injection polarization, which is a measure of the
efficiency of spin injection, is also determined for our structures. It is desirable for the
spin relaxation length and the spin polarization to be as large as possible and we have
found a spin relaxation length of 2 µm for copper at 20 K. This is almost double that
of what other groups have obtained [7, 8]. We were also able to achieve high spin
polarizations of up to 8.1 %, showing that we can successfully fabricate and
characterize lateral spin valve structures.

1.5 Organization of thesis
Chapter 2 gives a brief introduction to some of the fundamental
magnetotransport effects and reviews past works done on the lateral spin valve
devices, which provide a theoretical framework and background for the experimental
work presented in subsequent chapters. The various fabrication processes and
characterization techniques are presented in Chapter 3. In Chapter 4, a room

4


Chapter One: Introduction
temperature investigation of the local and non-local measurement configurations for
different types of lateral spin valve geometries is presented. The low temperature
measurements are presented in Chapter 5 whereby the spin relaxation lengths in

copper and aluminum are investigated. A discussion on the spin injection polarization
of our lateral spin valve devices is also presented. Finally, in Chapter 6, a summary of
the main observations from the data obtained is presented and suggestions for future
work are given.

5


Chapter One: Introduction
References

[1]

J. M. Slaughter, R. W. Dave, M. DeHerrera, M. Durlam, D. N. Engel, J.
Janesky, N. D. Rizzo and S. Tehrani, J. Supercond. 15, 19 (2002)

[2]

G. Binasch, P. Grunberg, F. Saurenbach and W. Zinn, Phys. Rev. B, 39, 4828
(1989)

[3]

M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P.
Eitenne, G. Creuzet, A. Friederich and J. Chazelaz, Phys. Rev. Lett, 61, 2472
(1988)

[4]

M. Julliere, Phys. Lett, 54A, 225 (1975)

[5]

J. S. Moodera, L. R. Kinder, T. M. Wong and R. Meservey, Phys. Rev. Lett,
74, 3273 (1995)

[6]

F. J. Jedema, H. B. Heersche, A. T. Filip, J. J. A. Baselmans and B. J. van
Wees,

[7]

Nature (London), 416, 713 (2002)

Y. Ji, A. Hoffmann, J. E. Pearson and S. D. Bader, Appl. Phys. Lett, 88,
052509 (2004)

[8]

F. J. Jedema, M. S. Nijboer, A. T. Filip and B. J. van Wees, Phys. Rev. B, 67,
085319 (2003)

6


Chapter Two: Literature Review
CHAPTER TWO
LITERATURE REVIEW
2.1 Introduction

In this chapter, various topics pertaining to lateral spin valves will be
presented. Firstly, the theory of spin injection, detection and accumulation, which
forms the underlying physics of the lateral spin valve phenomenon, will be
introduced. The two probe configurations widely used to obtain the spin valve signal
will be presented and the theory behind how the spin signal is obtained using the
lateral spin valve structure will be explained as well. Lastly, a review of the work
done by other groups pertaining to lateral spin valves will be presented, where
emphasis will be given on the spin relaxation lengths and the spin injection
polarization obtained for different materials. Factors which affect the magnitude of
the spin accumulation signal and some applications for the lateral spin valve are also
reviewed.

2.2 Theory of lateral spin valves
Lateral spin valve devices are structures which can be used for spin injection
and detection as well as for probing the spin relaxation lengths in non-magnetic
materials. A lateral spin valve device typically consists of a spin injector and a spin
detector, both of which are made of ferromagnetic material, in contact with a spin
diffusion medium, which is a non-magnetic metal. In order to understand how the
lateral spin valve structure works, it is important to first have a basic knowledge of
spin injection, detection and spin accumulation.

7


Chapter Two: Literature Review
2.2.1 Theory of spin injection, detection and accumulation
In electrical spin injection, a ferromagnetic (FM) electrode is connected to a
non-magnetic (NM) metal as shown in Figure 2.1. The current applied flows
perpendicular to the FM/NM interface and because the conductivities for spin-up and
spin-down electrons in a FM metal are not equal, the charge current in the FM is

accompanied by a spin current. When the electrons carrying the spin current crosses
the FM/NM interface into the NM metal, the electrons accumulate over a distance λN
and λF at both sides of the FM/NM interface because the conductivities for the spin-up
and spin-down electrons are equal in the NM metal [2-4] .

I

I

λF λN
FM
NM
Cu
wire
Figure 2.1 Schematic diagram of a ferromagnet (FM) in contact with a
nonmagnet (NM). λN and λF represents the distance which spin accumulation
exists in the nonmagnet and ferromagnet respectively.

The rate of spin accumulation in the NM metal depends on the rate of spin
relaxation, which is one of the important length scales for spin dependent transport.
This is a description of how far an electron can travel in a diffusive conductor before
it loses its initial spin direction. Spin relaxation lengths in metals are usually much
longer than the elastic mean free path and it is given by [5]:

8


Chapter Two: Literature Review

l sf =

v Fτ sf λ
3

-------------- (2.1)

where v F is the Fermi velocity, τ sf is the spin flip time and λ is the mean free path.
In the absence of magnetic impurities, the dominant mechanism that causes
electrons to be flipped is the spin-orbit interaction as proposed by Elliot and Yafet [6,
7]. At high temperatures, 1 τ sf increases linearly as temperature increases and this

indicates phonon-induced spin relaxation. At low temperatures, 1 τ sf is constant and
scattering is by impurities.
The presence of non-equilibrium spin accumulation in the NM metal causes
an energy splitting between the spin-up and spin-down chemical potentials of the
electrons in the NM metal. The majority spins have a higher chemical potential than
the minority spins and it is this chemical potential which is being detected as the spin
accumulation signal [8]. The goal of many spintronic devices is to maximize this spin
detection sensitivity [9].

2.2.2 Theory of lateral spin valve measurement

There are typically two types of measurement configuration that have been used
to probe spin transport in lateral spin valves. They are the non-local spin valve
(NLSV) measurement configuration and the local spin valve (LSV) measurement
configuration shown in Figure 2.2 (a) and (b) respectively.

9