The multilayers of [MnPd/Co]10have 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 TB~ 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
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MINISTRY OF ND 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
Supervisor: Prof. D.Sc. Nguyen Phu Thuy Hanoi – 2007 EDUCATION A
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 TB ~
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 TB ~ 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 của 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
cường 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
HC Coercitive force (Coercitivity)
HE Exchange bias field
HFC Cooling field
JK Unidirectional anisotropy (exchange bias coupling)
energy
Keff Effective magnetic anisotropy
KS Surface/interfacial anisotropy
KU Uniaxial magnetic anisotropy energy
KV Volume anisotropy
M Magnetization
MFM Magnetic force microscope
MS Saturation magnetization of ferromagnetic layer
RF Radio frequency
SEM Scanning electron microscope
vi
T Measurement temperature
TB Blocking temperature
TC Curie temperature
tCo Ferromagnetic layer thickness
tMnPd Antiferromagnetic layer thickness
TN 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 (HFC). 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, HFC 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 (HE), coercitivity (HC),
unidirectional anisotropy constant (JK). 40
Fig. 4-2. The MnPd thickness dependence of perpendicular and
parallel exchange bias fields (HE), coercitivity (HC). 42
Fig. 4-3. (a) The plot of the product of Keff and tCo versus tCo and
(b) the plot of KU versus tCo 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 Keff and tCo versus tCo and (b) plot of KU
versus tCo 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
HE. 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 JK = HEMStFM (where MS
- 4 -
is the saturation magnetization and tFM 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,
HE = 0, is usually denoted as blocking temperature (TB).
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 TN < T < TC, the FM spins line up with the field, while the
AF spins remain random (see Fig 1-1-(a)). When cooling to T < TN, in the
presence of the field (so-called cooling field which is denoted as HFC 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)
HFC
Field cooling
H
M
O
HE
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/Fe2O3 [11], Fe/Fe3O4 [12]. Recently, the number of studies on exchange
bias in small particles has been reduced because most of the applications
using this effect ar