Nghiên cứu cơ chế trao đổi dịch vuông góc trong hệ màng mỏng đa lớp MnPd/Co

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 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, 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 - 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/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 f