v
LIST OF NOTATIONS
θ Angle between incident X-ray and crystal plane (hkl)
AF Antiferromagnet(s)/ Antiferromagnetic
AFM Atomic force microscope
at.% Atomic percent
EDS Energy dispersive spectrometer
FC Field cooling
fct Face centered tetragonal structure
FESEM Field emission scanning electron microscope
FM Ferromagnet(s)/ Ferromagnetic
hcp Hexagonally close packed structure
H External magnetic field
H
C
Coercitive force (Coercitivity)
H
E
Exchange bias field
H
FC
Cooling field
J
K
Unidirectional anisotropy (exchange bias coupling)
energy
K
eff
Effective magnetic anisotropy
K
S
Surface/interfacial anisotropy
K
U
Uniaxial magnetic anisotropy energy
K
V
Volume anisotropy
M Magnetization
MFM Magnetic force microscope
M
S
Saturation magnetization of ferromagnetic layer
RF Radio frequency
SEM Scanning electron microscope
vi
T Measurement temperature
T
B
Blocking temperature
T
C
Curie temperature
t
Co
Ferromagnetic layer thickness
t
MnPd
Antiferromagnetic layer thickness
T
N
Néel temperature
VSM Vibrating sample magnetometer
WDS Wavelength dispersive spectrometer
XRD X-ray diffraction
ZFC Zero field cooling
vii
LIST OF FIGURES
Fig. 1-1. Schematic diagram of the spin configuration of an
FM/AF bilayer at different states (After [20]). 5
Fig. 1-2. Schematic diagram of the spin structures assumed in
some of the proposed models within each category. 10
Fig. 1-3. Schematic view of spin configuration of FePt/FeMn
multilayer based on modified Malozemoff model (After
N.N. Phuoc et al. [59]). 14
Fig. 2-1. Schematic view of the MnPd target used in the present
thesis. 15
Fig. 2-2. Schematic view of [MnPd/Co]
N
multilayer structure
used in the present thesis. 17
Fig. 2-3. Schematic diagram of glancing incident θ/2θ scan X-
ray diffraction configuration. 18
Fig. 3-1. X-ray diffraction spectra of [MnPd(10 nm)/Co(x nm)]
10
multilayers, (a) x = 2.5 nm, (b) x = 3.5 nm, (c) x = 4.5
nm. 24
Fig. 3-2. Cross-sectional view of [MnPd(10 nm)/Co(7.5 nm)]
10
as-deposited multilayer. 25
Fig. 3-3. MFM image of [MnPd(10 nm)/Co(3.5 nm)]
10
as-
deposited multilayer. 26
Fig. 3-4. Schematic diagram of measurement configurations for
samples at 120K. Here, the measurement field direction
(H) is the same as the cooling field (H
FC
). 27
viii
Fig. 3-5. Parallel and perpendicular hysteresis loops measured at
T = 120 K for [MnPd(10 nm)/Co(x nm)]
10
(x = 2.5, 3.5,
4.5, 5.5, 7.5, 10 nm) multilayers. 28
Fig. 3-6. Parallel and perpendicular hysteresis loops measured at
T = 120 K for [MnPd(y nm)/Co(3.5 nm)]
10
(y = 3.5, 5.5,
7.5, 10, 15.5, 30 nm) multilayers. 29
Fig. 3-7. Schematic diagram of measurement configurations at
room temperature. Here, H
FC
denotes the cooling field
direction and H denotes measurement field directions.
Note that all samples were measured in two different
directions. 31
Fig. 3-8. Parallel and perpendicular hysteresis loops measured at
room temperature for [MnPd(10 nm)/Co(x nm)]
10
(x =
2.5, 3.5, 4.5, 5.5, 7.5, 10 nm) multilayers cooled in the
field perpendicular to the plane. 32
Fig. 3-9. Parallel and perpendicular hysteresis loops measured at
room temperature for [MnPd(10 nm)/Co (x nm)]
10
(x =
2.5, 3.5, 4.5, 5.5, 7.5, 10 nm) multilayers cooled in the
field parallel to the plane. 33
Fig. 3-10. Parallel and perpendicular hysteresis loops measured at
room temperature for [MnPd(10 nm)/Co(x nm)]
10
(x =
2.5, 3.5, 4.5, 5.5, 7.5, 10 nm) multilayers cooled in the
zero field. 34
Fig. 3-11. Parallel and perpendicular hysteresis loops measured at
room temperature for [MnPd(10 nm)/Co(x nm)]
10
(x =
2.5, 3.5, 4.5, 5.5, 7.5, 10 nm) as-deposited multilayers. 35
ix
Fig. 3-12. Magnetization – temperature curve of [MnPd(10
nm)/Co(3.5 nm)]
10
multilayer in the presence of a field
of 2500 Oe. 36
Fig. 4-1. The Co thickness dependence of perpendicular and
parallel exchange bias fields (H
E
), coercitivity (H
C
),
unidirectional anisotropy constant (J
K
). 40
Fig. 4-2. The MnPd thickness dependence of perpendicular and
parallel exchange bias fields (H
E
), coercitivity (H
C
). 42
Fig. 4-3. (a) The plot of the product of K
eff
and t
Co
versus t
Co
and
(b) the plot of K
U
versus t
Co
of [MnPd(10 nm)/Co(x
nm)]
10
(x = 2.5, 3.5, 4.5, 5.5, 7.5, 10 nm) multilayers at
120K. 45
Fig. 4-4. Anisotropy energies of [MnPd/Co]
10
multilayers which
were treated at different conditions. (a) Plot of the
product of K
eff
and t
Co
versus t
Co
and (b) plot of K
U
versus t
Co
at room temperature. 47
Fig. 4-5. Schematic diagram of multilayer structure after
annealing. 49
Fig. 4-6. Schematic view of spin configurations of MnPd/Co
multilayer: (a) perpendicular-to-the-plane easy axis and
(b) parallel-to-the-plane easy axis. 54
x
CONTENTS
Preface 1
Chapter 1 Introduction
1.1 Background 3
1.2 Overview on exchange bias 6
1.3 Previous studies on perpendicular exchange bias 12
Chapter 2 Experimental
2.1 Introduction 15
2.2 Sample preparation 15
2.3 Experimental techniques 18
2.3.1 Glancing incident X-ray diffraction 18
2.3.2 Field emission scanning electron microscope 18
2.3.3 Stylus-method profilemetry 19
2.3.4 Energy dispersive X-ray spectrometer 19
2.3.5 Wavelength dispersive X-ray spectrometer 20
2.3.6 Magnetization hysteresis loops 21
2.3.7 Magnetization – temperature curve 22
2.3.8 Magnetic force microscope & atomic force microscope 22
Chapter 3 Experimental results
3.1 Introduction 23
3.2 Crystallographic structure 23
3.2.1 Glancing incident X-ray diffraction 23
3.2.2 Cross-section observation 25
3.3 Magnetic properties 25
xi
3.3.1 Domain observation 26
3.3.2 Magnetization hysteresis loops at low temperature 26
3.3.3 Magnetization hysteresis loops at room temperature 30
3.3.4 Temperature dependence of magnetization in MnPd/Co
multilayers 36
Chapter 4 Discussions
4.1 Introduction 37
4.2 Crystallographic structure 37
4.2.1 Glancing incident X-ray diffraction 37
4.2.2 Cross-section observation 38
4.3 Magnetic properties 38
4.3.1 Domain observation 39
4.3.2 Thickness dependence of exchange bias 39
4.3.2.1 Co thickness dependence of exchange bias 39
4.3.2.2 MnPd thickness dependence of exchange bias 41
4.3.3 Perpendicular magnetic anisotropy in MnPd/Co
multilayers 43
4.3.3.1. Perpendicular anisotropy at low temperature 44
4.3.3.2. Perpendicular anisotropy at room temperature 46
4.3.3.3. Effect of annealing on perpendicular anisotropy 46
4.3.3.4. Anomalous field induced anisotropy 50
4.3.4 Temperature dependence of magnetization in MnPd/Co
multilayers 51
4.4 Explanation of exchange bias coupling mechanism 52
Conclusions and further direction 56
References 58
- 1 -
PREFACE
Exchange bias has been studied extensively for over half of a century but
most of the research has been carried out in the configuration called parallel
exchange bias. In this configuration, the cooling field and the measurement
field are applied in the plane. Beside parallel exchange bias, there has been
very little work carried out in the perpendicular configuration with the cooling
field and the measurement field along the film normal. Perpendicular
exchange bias is recently of renewed interest because it is relevant in the
quest for a better understanding of the microscopic origin of the exchange
bias phenomenon and it might lead to wide applications in magnetic sensors,
perpendicular recording media, perpendicular magnetic read heads and also
magnetic random access memories (MRAMs).
In this thesis, the studies on perpendicular exchange bias in [MnPd/Co]
10
multilayers are reported for the first time. Since the objective of the present
thesis is to study the perpendicular exchange bias mechanism, the approach is
to investigate both the parallel and perpendicular exchange biases. Besides,
perpendicular anisotropy of the samples at low and room temperatures is also
investigated due to its important contribution to the effect.
The present thesis consists of 4 chapters.
Chapter 1 is to give an overview on exchange bias in both theoretical and
experimental research; and also previous studies on perpendicular exchange
bias.
Chapter 2 focuses on the sample preparation and experimental
techniques. Some descriptions on the apparatuses and measurements that were
used in the present thesis are introduced.
- 2 -
Chapter 3 represents the experimental results. The aim and configurations
of measurements and also sample processing procedures are given.
Chapter 4 is to discuss the results of crystallographic and magnetic
properties of [MnPd/Co]
10
multilayers. The behavior of exchange bias in both
the parallel and perpendicular directions will be summarized. After that, based
on that result and the magnetic anisotropy behavior of the samples, we try to
give a phenomenological picture to explain the perpendicular exchange bias
coupling mechanism.
Finally, conclusions and further direction as well as the list of references
are given at the end of the thesis.
- 3 -
Chapter 1
1. INTRODUCTION
1.1 Background
Nowadays, magnetic materials play an important role in the information
technology oriented social. There are various applications using magnetic
materials such as magnetic recordings, magnetic sensors, magnetic heads, and
electronic motors. It is of particular interest to note that through rapid
technological developments in recent years, thin films and multilayers have
received much attention.
Among studies on magnetic materials, the exchange bias coupling between
ferromagnetic (FM) and (AF) materials is of great interest. Since discovered
in 1956 by Meiklejohn and Bean [1], there have been many studies published
in the literature on this effect because of various applications such as spin
valves, magnetic read heads, magnetic random access memories (MRAMs).
Although it has been studied extensively, physical origin of this effect is still
in controversy.
Exchange bias effect is a phenomenon observed in a system consisting of
antiferromagnetic and ferromagnetic materials, in which the magnetization
hysteresis loop is shifted along the field axis after the sample undergoing the
so-called field cooling process through the Néel temperature of the
antiferromagnetic material. In other words, its characteristic signature is the
shift of the center of the hysteresis loop from its normal position at H = 0 to
H
E
. However, in order to compare different types of exchange bias systems
often rather than using the loop shift itself, the so-called unidirectional
anisotropy energy or exchange bias coupling energy J
K
= H
E
M
S
t
FM
(where M
S
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