Study of perpendicular exchange bias mechanism in MnPd/Co multilayers

Study of perpendicular exchange bias mechanism in MnPd/Co multilayers

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

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

Keywords: Perpendicular exchange bias, perpendicular magnetic anisotropy,

magnetic thin films, multilayers

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

Từ khóa: Hiện tượng trao đổi dịch vuông góc, dị hướng từ vuông góc, hệ

màng mỏng đa lớp MnPd/Co

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

LIST OF NOTATIONS

AF Antiferromagnet(s)/ Antiferromagnetic

T Measurement temperature

LIST OF FIGURES

Fig 1-1 Schematic diagram of the spin configuration of an

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

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

Fig 2-3 Schematic diagram of glancing incident θ/2θ scan

Fig 3-1 X-ray diffraction spectra of [MnPd(10 nm)/Co(x nm)]10multilayers, (a) x = 2.5 nm, (b) x = 3.5 nm, (c) x = 4.5

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,

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,

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

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

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

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

Fig 3-12 Magnetization – temperature curve of [MnPd(10 nm)/Co(3.5 nm)]10 multilayer in the presence of a field

Fig 4-1 The Co thickness dependence of perpendicular and

parallel exchange bias fields (HE), coercitivity (HC),

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

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

CONTENTS

Chapter 1 Introduction

1.1 Background 3

Chapter 2 Experimental

2.1 Introduction 15

2.3.8 Magnetic force microscope & atomic force microscope 22

Chapter 3 Experimental results

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

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

4.3.4 Temperature dependence of magnetization in MnPd/Co

multilayers 51

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

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

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 J = HMt(where M

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

FM AF FM AF

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

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

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 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 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

spin-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, NixCo1-xO [24-26] except FeF2, MnF2 [27, 28] Oxidized film systems give usually large exchange bias, e.g., the largest interfacial energy ever found is in Fe3O4/CoO bilayers (JK = 2.2 erg/cm2) [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 FeF2 and MnF2, 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

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/cm2

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

value up to 1.3 erg/cm2 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 TC > TN and the anomalous dependence of the exchange bias field which increased with

temperature until TC [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

a) Macroscopic

Malozemoff Mauri

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

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/FeF2 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

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 FeF2-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

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 FeF2 [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,

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

2.2 Sample preparation

Thin films of MnPd/Co multilayers were deposited onto single crystal Si(111) substrates at ambient temperature by an Alcatel SCM 400 RF sputtering system at the ITIMS

Antiferromagnetic layers were prepared from a composite target (see Fig 2-1) The target is a circular Pd

Mn

Pd

Fig 2-1 Schematic view of the MnPd

target used in the present thesis

target with sectorial Mn pieces glued conductively to it In the present thesis, the area compositions of Mn and Pd on the target were about 60:40, respectively Meanwhile, a circular Co target was used to prepared ferromagnetic layers

The RF sputtering system has two power sources used for two targets The targets were placed in its positions in the deposition chamber and after that, the deposition chamber was pumped out until the pressure inside was less than 5 × 10-6 mbar Samples were fabricated in Ar gas The gas flow was regulated by a mass flow controller and kept at a constant rate during the deposition Sputtering process was carried out in the condition of the Ar pressure kept at about 5 × 10-3 mbar No external magnetic field was applied in the deposition and substrates were at ambient temperature

The samples used in the present thesis are Si/[MnPd/Co]10 multilayer thin films The MnPd and Co layers were deposited alternately onto single crystal Si(111) substrates (see Fig 2-2) at the power of 150 W for the MnPd target and 300 W for the Co target The corresponding deposition rates for MnPd and Co layers are 2.3 × 10-2 nm/s and 2.8 × 10-2 nm/s

The compositions of MnPd layer were determined using a wavelength dispersive X-ray spectrometer (WDS) and an energy dispersive X-ray spectrometer (EDS) The results showed the Mn and Pd compositions are 11: 89, respectively

N bilayers x = 2.5 – 10 nm

y = 3.5 – 30 nm N = 10 bilayers

MnPd y (nm)

Co x (nm) Co x (nm)

Co x (nm)

Co x (nm)

MnPd y (nm)

MnPd y (nm) MnPd y (nm)

Si substrate

Fig 2-2 Schematic view of [MnPd/Co]N multilayer structure used in the present thesis

2.3 Experimental techniques

2.3.1 Glancing incident X-ray diffraction

In order to analysis the structure of sample, θ/2θ scan X-ray diffraction

was carried out using a PANalytical-Philips X’pert Pro system at Hanoi University of Technology A Cu target is used as the X-ray source A double-crystal monochromator is used to obtain monochromatic and collimated Cu

Kα1 radiation (λ=0.154056) The incident X-ray and the sample were fixed

The incident angle of the X-ray beam was of 1 degree with respect to the

sample surface Meanwhile, the detector rotated so that the θ/2θ scan

configuration was preserved during the measurements In the present thesis,

the angle 2θ was from 25 to 70 degrees

Diffracted beam

Sample Incident beam

Fig 2-3 Schematic diagram of glancing incident θ/2θ

scan X-ray diffraction configuration

2.3.2 Field emission scanning electron microscope

Cross-section images were observed by a field emission scanning electron microscope (FESEM) The best resolution of the system is up to 2 nm (standard mode), 3 to 5 times better than conventional SEM Because a field emission source provides narrower probing electron beams at low temperature

and high energy with acceleration voltage from 0.5 to 30kV (variable at 0.1 kV/step) The magnification of the system is in the range of X 20 - X 800000

In present thesis, observations of cross-section were carried out by a

Hitachi FE-SEM S4800 microscope system at the Institute of Materials

Science, Vietnamese Academy of Science and Technology After the sample was broken into half, they were immediately used to view

2.3.3 Stylus-method profilemetry

The stylus method consists of measuring the mechanical movement of a stylus as it is made to trace the topography of a film-substrate step The film thickness is directly read out as the height of the resulting step-contour trace The profilemeter used in this thesis is called Alpha-step model with the vertical resolution of about 1 Å The Alpha-Step IQ is guaranteed step height repeatability which makes it easier to precisely determine the thickness of thin films, roughness, etch depth in a wide extending below 8 nm and tall step height up to 2 mm The performance is due to modern ultra-low noise electronics and precision mechanical components The stylus scanning motion provides exceptional stability for extremely repeatable measurements

To determine deposition rate for a material, a single layer with a substrate step was prepared in a specific time The single layer was measured for three times in order to receive the mean thickness Hence, one can calculate the deposition rate for the material In this thesis, the deposition rates for MnPd and Co are respectively 2.3 × 10-2 nm/s and 2.8 × 10-2 nm/s These thickness measurements were carried out at the Institute of Materials Science, Vietnamese Academy of Science and Technology

film-2.3.4 Energy dispersive X-ray spectrometer (EDS)

X-rays emitted from a sample under electron bombardment are collected with a liquid nitrogen-cooled solid state detector and analyzed via computer according to their energy Typically, the computer programs used in EDS will display a real time histogram of number of X-rays detected per channel (variable, but usually 10 electron volts/channel) versus energy expressed in KeV

Using EDS, all of the energies of the characteristic X-rays incident on the detector are measured simultaneously and data acquisition is therefore very rapid across the entire spectrum However, the resolution of an EDS detector is considerably worse than that of a WDS spectrometer Besides, it is very difficult to determine precisely elements and its compositions if there is only a small amount of one of the overlapped elements

In practice, EDS is most often used for qualitative elemental analysis, simply to determine which elements are present and their relative abundance Depending on the specific needs of the investigations, quantitative results may be advised to use the electron microprobe In some instances, however, the area of interest is simply too small and must be analyzed by TEM (where EDS is the only option) or high resolution SEM (where the low beam currents used preclude WDS, making EDS the only option) In this thesis, the sample

composition was analyzed by a Hitachi FESEM S4800 microscope system

integrated EDS at the Institute of Materials Science, Vietnamese Academy of Science and Technology

2.3.5 Wavelength dispersive X-ray spectrometer (WDS)

WDS was the original technique developed to precisely and accurately determine chemical compositions of micro-volumes (a few cubic microns) of "thick" specimens, and the instrument used is the electron microprobe The

key feature of the electron microprobe is a crystal-focusing spectrometer, of which there are usually 3-5 different diffracting crystals

The WDS spectrometer can acquire the high count rate of X-rays produced at high beam currents, because it measures a single wavelength at a time This is important for trace element analysis In practice, it is advantageous to use the speed of EDS for an initial survey of an unknown sample because major elements will be rapidly identified However, if trace elements are present they will not be identified, and it may be difficult to interpret complex overlaps which are common in EDS analysis Following the initial energy dispersion survey, wavelength dispersion can be used to check for overlaps and to increase sensitivity for trace elements A Jeol JXA 8800R electron probe microanalyzer at the Institute of Geology and Minerals was used in the present study

2.3.6 Magnetization hysteresis loops

Magnetization curve provides basic magnetic properties of a magnetic

material From the curve, one can estimate the saturation magnetization MS,

the coercitivity HC, the magnetic anisotropy K, the magnetization remanence MR and the exchange bias field HE Magnetic behavior can also be understood of microscopic structural properties In the present study, measurements of magnetization curves were performed using a DMS 880 VSM system at the ITIMS The magnetic field used in the present study was up to 13.5 kOe along both the parallel and perpendicular directions For these measurements, the background resulted from any source such as the sample holder and the substrate was subtracted Before each measurement, a standard Ni sample (with total magnetic moment of 3.799 emu) was always used to calibrate the system For the measurement of the hysteresis loops at low temperatures, a tube attached in the VSM and a thermocouple is placed inside the tube

together with the heating coil By evaporating liquid nitrogen and simultaneously adjusting the current for the heating coil, one can control the system with the temperature accuracy of about 5 degrees

2.3.7 Magnetization-temperature curve

Magnetization-temperature curve were carried out by a VSM system (described in the previous subsection) Temperature was controlled by evaporating liquid nitrogen (in the range of low temperature) or blowing pure nitrogen gas (in the range of high temperature), and simultaneously adjusting the current for the heating coil In the present study, the measurement was performed in the temperature range from 120 to 320 K and the step of 5 K

2.3.8 Magnetic force microscope & atomic force microscope

Observations of magnetic domains were carried out using a NT-MDT Solver magnetic force microscope (MFM) at the College of Technology, Vietnam National University, Hanoi The tip used in the present study was coated by CoCr alloy with the coating thickness of 40 nm, the curvature radius of 30-40 nm and the cone angle less than 30 degrees Before each measurement, it was magnetized along the direction perpendicular to the sample surface The same tip was used to observe the MFM and AFM images The surface roughness was determined to be less than 2 nm

The aim of the present thesis is to study the perpendicular exchange bias effect Therefore, the investigation and comparison between parallel and perpendicular exchange biases is necessary The perpendicular magnetic anisotropy is also important due to its contribution to the effect Some measurements were carried out at room temperature for a better understanding of physical origin of the perpendicular anisotropy and also perpendicular exchange bias The magnetic properties of the multilayers are discussed in the next chapter in conjunction with the structure

3.2 Crystallographic structure

3.2.1 Glancing incident X-ray diffraction

Fig 3-1 shows the θ/2θ scan X-ray diffraction pattern of [MnPd(10

nm)/Co(x nm)]10 (x = 2.5, 3.5, 4.5 nm) as-deposited multilayers.

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02 0 04 0 06 0 08 0 01 0 0 01 2 0 0

(b )(c)