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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 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 [...]... 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... 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. .. 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... 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... 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... 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... 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... 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, ... 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. .. 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 spinvalve 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 . particles, exchange bias in nanostructures and exchange bias in (continuous) thin films. Fine particles were the first type of system where exchange bias was. OF TECHNOLOGY INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE (ITIMS) Batch ITIMS – 2005 Title of MSc Thesis: Study of perpendicular exchange bias