Study of perpendicular exchange bias mechanism in MnPd/Co multilayers

MINISTRY OF EDUCATION AND TRAINING
HANOI UNIVERSITY OF TECHNOLOGY
INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE (ITIMS)

MASTER THESIS OF MATERIALS SCIENCE
STUDY OF PERPENDICULAR EXCHANGE BIAS
MECHANISM IN MnPd/Co MULTILAYERS
NGUYEN HUU DZUNG
Hanoi – 2007
Supervisor: Prof. D.Sc. Nguyen Phu Thuy
ii
HANOI UNIVERSITY OF TECHNOLOGY
INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE (ITIMS)
Batch ITIMS – 2005
Title of MSc Thesis:
Study of perpendicular exchange bias mechanism
in MnPd/Co multilayers
Author: Nguyen Huu Dzung
Supervisor: Prof. D.Sc. Nguyen Phu Thuy
Referees: 1. Dr. Nguyen Thang Long
2. Dr. Nguyen Phuc Duong

Abstract
The multilayers of [MnPd/Co]
10
have been investigated for the first time.
The results indicate that large perpendicular exchange bias field and magnetic

anisotropy were found in these samples below the blocking temperature T
B
~
240 K. The dependence of exchange bias on the layer thickness has also been
studied. The easy axis direction strongly depends on both the Co and MnPd
thicknesses. The origin of the perpendicular anisotropy was attributed to the
magneto-elastic effect due to the strained CoPd interfacial alloy forming at
the interface between the Co and MnPd layers. In order to explain the
perpendicular exchange bias mechanism, a phenomenological picture was put
forward in which the fluctuations of the MnPd spins at the interface play an
important role. Besides, the results show the anomalous effect related to field-
induced anisotropy, i.e. the parallel field cooling enhanced the perpendicular
anisotropy property instead of the perpendicular one.
Keywords: Perpendicular exchange bias, perpendicular magnetic anisotropy,
magnetic thin films, multilayers.


iii
TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI

VIỆN ĐÀO TẠO QUỐC TẾ VỀ KHOA HỌC VẬT LIỆU (ITIMS)
Khóa ITIMS – 2005
Tiêu đề của luận văn:
Nghiên cứu cơ chế trao đổi dịch vuông góc
trong hệ màng mỏng đa lớp MnPd/Co
Tác giả: Nguyễn Hữu Dũng
Người hướng dẫn: GS. TSKH. Nguyễn Phú Thùy
Người phản biện: 1. TS. Nguyễn Thăng Long
2. TS. Nguyễn Phúc Dương

Tóm tắt
Lần đầu tiên, hệ màng mỏng đa lớp [MnPd/Co]
10
đã được nghiên cứu.
Kết quả cho thấy độ lớn trường trao đổi dịch và năng lượng dị hướng từ
vuông góc lớn đã thu được ở dưới nhiệt độ blocking T
B
~ 240 K. Sự phụ
thuộc của hiện tượng trao đổi dịch vào chiều dày các lớp cũng đã được xem
xét. Hướng của trục dễ phụ thuộc mạnh vào chiều dày của cả hai lớp Co và
MnPd. Nguồn gốc của dị hướng từ vuông góc được gán cho hiệu ứng từ đàn
hồi do sự hình thành của hợp kim CoPd ở mặt tiếp xúc giữa lớp Co và MnPd.
Để giải thích cơ chế c
ủa hiện tượng trao đổi dịch vuông góc, một mô hình
hiện tượng luận đã được đề xuất trong đó sự thăng giáng của các spin lớp
MnPd ở mặt tiếp xúc đóng một vai trò quan trọng. Ngoài ra, hệ màng đa lớp
còn thể hiện hiệu ứng dị thường liên quan tới dị hướng cảm ứng từ trường, tức
là, quá trình làm nguội trong từ trường song song với bề mặt màng làm tăng
c
ường tính dị hướng vuông góc thay vì từ trường làm nguội vuông góc.
Từ khóa: Hiện tượng trao đổi dịch vuông góc, dị hướng từ vuông góc, hệ
màng mỏng đa lớp MnPd/Co.


iv
ACKNOWLEDGEMENTS
First and foremost, I thank my supervisor Prof. D.Sc. Nguyen Phu Thuy
for the guidance and inspiration over the last one year at the ITIMS. I would
like to thank him for his invaluable advice, comments and suggestions.
I would like to express most sincerely my gratitude to Dr. Nguyen Anh

Tuan as my co-supervisor at the ITIMS. I would like to thank him for his
guidance and valuable discussions.
I also wish to extend my warmest thanks to Dr. Nguyen Thang Long for
his useful discussions and also for MFM and AFM measurements at the
College of Technology, Vietnam National University, Hanoi; to Dr. Nguyen
Phuc Duong for reading my thesis and his feedback; to Dr. Nguyen Nguyen
Phuoc for many discussions and frank advice; to M.Sc. Do Hung Manh for
cross-section images and composition analysis at the Institute of Materials
Science, Vietnamese Academy of Science and Technology.
Besides, I also wish to extend my thank to Prof. D.Sc. Than Duc Hien for
the encouragement and the financial support from State Program on
Fundamental Research.
Thanks are further extended to all members at the ITIMS for their
encouragement and kind supports throughout the present thesis. Especially, I
thank M.Sc. Le Thanh Hung for his useful help in experiments.
Finally, I would like to thank my family and my friends for their love and
encouragement during this study.

October 2007
_________________
Nguyen Huu Dzung


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



- 4 -
is the saturation magnetization and t
FM
is the thickness of the FM layer) is
evaluated instead. The exchange bias effect is only observed below a certain
temperature. The temperature at which the exchange bias field becomes zero,
H
E
= 0, is usually denoted as blocking temperature (T
B
).
Exchange bias can be qualitatively understood by assuming an exchange
interaction at the AF-FM interface (Fig 1-1). When a field is applied in the

temperature range T
N
< T < T
C
, the FM spins line up with the field, while the
AF spins remain random (see Fig 1-1-(a)). When cooling to T < T
N
, in the
presence of the field (so-called cooling field which is denoted as H
FC
in
present thesis), due to the interaction at the interface, the AF spins next to the
FM align ferromagnetically to those of the FM (assuming that the interaction
is ferromagnetic). The other spin planes in the AF follow the AF order so as
to produce zero net magnetization (see Fig 1-1-(b)). When the field is
reversed, the FM spins start to rotate. However, the AF spins remain
unchanged due to its large anisotropy. Therefore, the interfacial interaction
between the AF-FM spins try to align parallel the FM spins. In other words,
the AF spins exert a microscopic torque on the FM spins, to keep them to
their original position (see Fig 1-1-(c)). The field needed to reverse
completely the FM spins is larger if it is in contact with the AF because an
extra field is to overcome a microscopic torque. As the field is back to its
original direction, the FM spins will start to rotate back at a smaller field
because it now exerts a torque with the same direction as the applied field (see
Fig. 1-1-(d) and Fig 1-1-(e)). The material behaves as if there is an extra
biased field; the hysteresis loop is therefore shifted along the field axis (see
the hysteresis loop in Fig 1-1). If the AF anisotropy is large, one should only
observe a shift of the hysteresis loop, while for small AF anisotropies, the
only observed effect should be a coercivity enhancement (without any loop

- 5 -


FM

AF
FM

AF
(d)
(c) (b)
(a)
H
FC
Field cooling
H
M
O
H
E
Fig. 1-1. Schematic diagram of the spin configuration of an FM/AF
bilayer at different states. (After [20])

FM

AF
FM

AF

(e)
FM

AF
- 6 -
shift). Nevertheless, in general, both the effects can be observed
simultaneously, due to, for example, structural defects or grain size
distribution, which bring about local variations of the AF anisotropy.
Although this simple phenomenological model gives an intuitive picture, it
fails to quantitatively understand of these phenomena. In particular, the
theoretically predicted exchange bias field is much larger than the
experimental value. In an attempt to reduce this discrepancy, many models
such as planar domain wall model [2], random-field model [3-5], spin flop
model [6] put forward. However, there have not been experimental
confirmations of these models and they are therefore in controversy. It is due
to the fact that the role of the many different parameters involved in exchange
bias, such as anisotropy, interface roughness, spin configuration or magnetic
domain is far from being understood. A clear understanding of exchange bias
at the microscopic level is still lacking. Therefore, from the fundamental point
of view, the subject of exchange bias is still a hot topic for the years to come
and it is of great interest to study this phenomenon together with its associated
effects for a better understanding of physical origin.
1.2 Overview on exchange bias
So far, exchange bias has been investigated extensively both
experimentally and theoretically.
Regarding experimental research, from a view point of material form,
studies on exchange bias can be relatively divided into 3 categories: exchange
bias in particles, exchange bias in nanostructures and exchange bias in
(continuous) thin films.
Fine particles were the first type of system where exchange bias was

reported. Since its discovery, exchange bias in particles has been concentrated
on a number of materials, mainly ferromagnetic metals covered by their


- 7 -
antiferromagnetic oxides, such as Co/CoO [1, 7, 8], Ni/NiO [9], Fe/FeO [10],
Fe/Fe
2
O
3
[11], Fe/Fe
3
O
4
[12]. Recently, the number of studies on exchange
bias in small particles has been reduced because most of the applications
using this effect are in the form of thin films. Moreover, these systems are not
suitable for studies of fundamental aspects of exchange bias due to
uncontrolled distribution of the particle size and shape, difficulty to identify
the nature of the interface, stoichiometry and crystallinity of the AF material.
However, studies of FM-AF exchange interactions in fine particle systems has
still found interest in applications to improve the performance of permanent
magnetic materials (by means of an enhancement of the coercivity which
typically accompanies the hysteresis loop shift) [13-15] or to increase in the
superparamagnetic limit in magnetic recording media [16, 17]. Hence, in fine
particle systems, exchange bias studies may be particularly interesting not
only for the loop shift itself, but also for other exchange bias related
phenomena.
Today, the industrial demand to systematically reduce the size of spin-
valve and other exchange bias based devices is also fueling new research in

lithographically fabricated exchange biased nanostructures [15]. Different
kinds of nanostructured systems where exchange bias has been studied,
including artificial nanostructures (e.g., lithographically fabricated
nanostructures), chemical surface modification (e.g., oxidation, nitration or
sulfation), FM nanoparticles embedded in an AF matrix, controlled core-shell
nanoparticles, surface effects (e.g., ferromagnetic, ferrimagnetic or
antiferromagnetic particles with magnetically disordered surfaces) [15, 18-
23]. The recent advances in magnetic fine particle production and the
fabrication of magnetic nanostructures by lithographic methods have
propelled a renewed interest in nanostructures in general and exchange biased


- 8 -
ones in particular. However, exchange bias theories for nanostructures are still
lacking [15].
Although there has been some research on exchange bias in nanoparticles
in the last decades, the bulk of exchange bias research has focused mainly on
thin film systems. This is firstly due to the possibility of an increased number
of FM/AF combinations in thin films. Secondly, the greater control of the
FM/AF interface that thin films allow, in which the microstructure of both the
AF and FM layers (e.g., grain size, orientation, crystalline quality) and, to
some extent, the interface (e.g., roughness, spin structure or interface layers)
can be controlled. Finally, the fundamental role of exchange bias in spin valve
and tunneling devices has triggered the explosive increase of research in
FM/AF thin film systems.
In the point view of the AF material form, studies on exchange bias in thin
films can be divided into 2 categories: exchange bias with insulating AF films
and with metallic AF films.
Almost all the reported investigations of exchange bias with insulating AF
films involve oxides CoO, NiO, Ni

x
Co
1-x
O [24-26] except FeF
2
, MnF
2
[27,
28]. Oxidized film systems give usually large exchange bias, e.g., the largest
interfacial energy ever found is in Fe
3
O
4
/CoO bilayers (J
K
= 2.2 erg/cm
2
) [29].
However, since most of these oxidized film systems exhibit exchange bias at
low temperature, the applications based on this type are uncommon and it has
received less attention than before. Apart from oxides, the most popular
materials are FeF
2
and MnF
2
, exhibiting interesting phenomena such as
positive exchange bias, double-shifted loops (depending on temperature and
the cooling field) [28, 30].
Meanwhile, studies on exchange bias with metallic AF films focus on
alloys of Mn with transition metals such as Pd, Pt, Ir [24, 31, 32] or

- 9 -
ferromagnetic metals as Fe, Ni [33-35]. As for interfacial energy aspect, its
value in the published reports is usually in the range from 0.1 to 0.5 erg/cm
2

(lower than oxidized film systems). Recently, Imakita et al. [36] obtained the
largest exchange bias energy at room temperature in CoFe/MnIr with the J
K

value up to 1.3 erg/cm
2
capable of using for the future read heads in hard disk
drivers. Jiao et al. showed that exchange bias might exist in the Gd/Cr
bilayers and Cr/Gd/Cr trilayers regardless of the condition of T
C
> T
N
and the
anomalous dependence of the exchange bias field which increased with
temperature until T
C
[37].
As for theoretical research, many models have been proposed to
understand its mechanism and have been achieved different results with
experimental observations. The models may be classified as either
macroscopic, mesoscopic, or microscopic (see Fig. 1-2). Most of the works
have been concentrated on the discrepancy between the theoretically
predicted and experimental exchange bias field. Mauri et al. [2] proposed a

model based on the formation of a planar domain wall. The interfacial
exchange energy is thus due to the wall energy in the AF layer giving the
same order of the experimental exchange bias field in some cases.
Malozemoff [3-5] put forward a random-field arising from the random defects
at the interfaces, which are argued to be more likely in the real systems. Due
to the random-field, the AF is broken into domains. In this case, domain walls
perpendicular to the interface are energetically favorable. Therefore, the
interfacial exchange energy is also of the same order of the wall energy in the
AF. Takano, Berkowitz and coworkers proposed a model for the exchange
anisotropy of AF/FM bilayers in which the AF layer consists of (essentially
uncoupled) grains [6]. The exchange bias field is due to uncompensated
surface spins of antiferromagnetic grains. As for exchange bias in the systems


- 10 -


a) Macroscopic
Meiklejohn-Bean

Malozemoff
Mauri
Takano-Berkowitz

Koon, Schulthess-Butler

b) Mesoscopic
Fig. 1-2. Schematic diagram of the spin structures assumed in
some of the proposed models within each category.

c) Microscopic
FM

AF
FM

AF
FM

AF
FM

AF
FM




AF
- 11 -
with compensated interfacial spin, Koon [38] put forward perpendicular
coupling between the AF and the FM spins.
Schulthess and Butler [39] have shown that Koon’s perpendicular
coupling, together with uncompensated spins (similar to Malozemoff or
Takano et al. suggestions [3-5]) can explain simultaneously the loop shift and
coercivity enhancement encountered in FM–AF bilayers.
Meanwhile, Miltényi, Nowak, Misra, Beckmann et al. used Monte Carlo at
finite temperature to study a FM–AF couple with defects in the bulk of the
AF, i.e., not necessarily at the interface. They found the formation of domains
in the bulk of the AF, perpendicular to the FM–AF interface, which gave rise

to uncompensated spins at the interface, which were responsible for the
hysteresis loop shift. They also found that increasing the number of defects,
within certain limits, increases the number of AF domains, leading to larger
exchange bias [40-46].
Suess et al. [47-49] have developed a model based on perpendicular
coupling and randomly distributed exchange coupled AF grains. Interestingly,
the origin of exchange bias is found to be in the energy stored in the domain
walls between AF grains with different orientations. Lederman et al. have
recently reported that if the FM layer couples differently to each of the two
AF sublattices. It could give rise to exchange bias. Actually, using this simple
concept many of the experimentally observe defects in FM/FeF
2
bilayers can
be explained [50].
It is should be noted that all these theories are applied for the case of
parallel exchange bias phenomena. In which, 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


- 12 -
with the cooling field and the measurement field along the film normal,
namely perpendicular exchange bias.
1.3 Previous studies on perpendicular exchange bias
Parallel exchange bias has been studied for a long time, but perpendicular
exchange bias has been observed recently in the FM/AF systems with
perpendicular magnetic anisotropy. Perpendicular exchange bias is 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).
The exchange bias effect was measured for the first time in FeF
2
-CoPt
hetero-systems with perpendicular anisotropy by Kagerer et al. [51, 52] .The
exchange bias field exhibits a strong dependence on the cooling field and
temperature. Maat et al. [53] studied perpendicular exchange bias in the
system of [Co/Pt]/CoO multilayers and found that the perpendicular exchange
bias field is larger than the parallel one, which can be attributed to the
anisotropy in the CoO induced by the CoO (111)-textured growth of the films
thus producing the difference between the spin projections on the parallel and
perpendicular directions. Conversely, Marrows [54], who carried out research
on perpendicular exchange bias in [Co/Pd]/FeMn multilayers, found that the
difference of parallel and perpendicular exchange bias might not due to the
texture of the film because the discrepancy between parallel and
perpendicular exchange biases were clearly observed in the weak textured
film systems. This difference can be attributed to the fluctuations of the AF
spin at the interface, which naturally played a key role in determining any
exchange bias [54]. Garcia et al. [55] found a large anomalous enhancement


- 13 -
of perpendicular exchange bias in [Co/Pt]/FeMn by introduction of a
nonmagnetic spacer between the ferromagnetic and the antiferromagnetic
layers, which was presumably interpreted as the enhancement of
perpendicular magnetic anisotropy. Sort et al. [56] found that a temperature
range of square loops behavior on Co/Pt multilayers with perpendicular
anisotropy could be extended by using exchange bias with either FeMn or
IrMn layers. This was attributed to additional anisotropy induced to the

multilayers by exchange bias coupling. Recently, a 1/cosθ dependence of
exchange bias field on the angle θ between the applied field and the
perpendicular-plane cooling field was observed by Kim et al. [57] in
[Pt/Co]
4
/MnIr multilayers and Sun et al. [58] in FeMn/[FeNi/FeMn]
15

multilayers. This 1/cosθ dependence was ascribed to the strong out-of-plane
anisotropy. They also found that the hysteresis loops became asymmetric at
intermediate angle with a shift not only along the field axis but also along the
magnetization axis [57, 58]. It is only lately that N.N. Phuoc et al. has used
the modified Malozemoff model with the assumption of spin canting at the
interface of FM and AF layers to explain the perpendicular exchange bias
effect in [FePt/FeMn]
10
multilayers and found that the canting spins at the
interface play an important role in the effect (see Fig. 1-3) [59].
Among these studies on perpendicular exchange bias, very few materials
have been investigated, mainly Co/Pt multilayers with CoO [53], Co/Pt,
CoFe/Pt, Co/Pd multilayers with FeMn [54-56], Co/Pt multilayers with MnIr
[56, 57], Co/Pt multilayers with FeF
2
[51, 52]. In which, the multilayers are
ferromagnetic and have perpendicular anisotropy.
The exchange bias effect in MnPd/Co bilayers has received much attention
by N.N. Phuoc et al. [60], N.T. Nam et al. [61] and N.P. Thuy et al. [62]. All
these works have concentrated on parallel exchange bias in the bilayers,

- 14 -
K
AF
parallel
K
AF
perpendicular
K
AF
α
AF
FM
AF
FM
Substrate
Fig. 1-3. Schematic view of spin configuration of FePt/FeMn multilayer based on
modified Malozemoff model (After N.N. Phuoc et al. [59]).
showing that the exchange bias coupling between the Co and MnPd layers is
of huge values. However, perpendicular exchange bias in this kind of material
has never been investigated before. Therefore, a study on perpendicular
exchange bias in these systems is necessary for a better understanding of
physical origin of exchange bias and also related phenomena. We will show
in this thesis that perpendicular exchange bias can be indeed observed in the
samples produced by the multilayer thin film technique from the same
materials.