NOVEL MAGNONIC CRYSTALS AND DEVICES: FABRICATION, STATIC AND DYNAMIC BEHAVIORS DING JUNJIA (M. Eng, Huazhong University of Science and Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. ____________________ Ding Junjia January 2014 Acknowledgements The final outcome of this thesis received a lot of guidance and assistance from many people and I am extremely fortunate to have got this all along the completion of my PhD study. While it is impossible to acknowledge all of those people here, I will always remember them. I would like to acknowledge several people in particular. First and foremost, I would like to express my sincerest gratitude and appreciations to my supervisor Prof. Adekunle O. Adeyeye for giving me the opportunity to work on this topic. Without his unwavering dedication, encouragement, support and guidance in all aspects varying from research to personal life, it is impossible for me to finish this thesis in four years. Thanks Prof. Adekunle for his time to read, modify and comment on all my previous research papers and several versions of this thesis. I would like to give special thanks to Prof. Mikhail Kostylev from the University of Western Australia for his great help in the theory work of 1Dimensional Magnonic Crystals and for his reading and comments on my thesis. I would also like to express my appreciation towards ISML lab supervisor Assoc. Prof. Vivian Ng, lab officers Ms. Loh Fong Leong, Mr. Alaric Wong and Ms. Xiao Yun for their help and support during my candidature. It has been a delight to work with the current and past members of Prof. Adekunle’s group and ISML: Dr. Shikha Jain for teaching me all the nanofabrication skills and helping in setting up the Ferromagnetic Resonance spectroscopy. Dr. Tripathy Debashish who taught me film deposition technique and helped me for the antidot papers. Dr. Navab Singh from the Institute of Microelectronics for providing the deep ultra violet resist patterns used in this thesis. Dr. Ren Yang and Mr. Liu Xinming for their help in magnetooptical kerr effect measurement. Mr. Shimon Goei for his help in OOMMF simulation and I ! Acknowledgements tasty coffee. Dr. Naganivetha Thiyagarajah and Dr. Wu Baolei for their help in EBL process. Dr. Shyamsunder Regunathan for his help in SEM. I would also like to thank Dr. Xin Yi, Ms. Ria, Ms. Chen Ji, Dr. Borja, Dr. Dezheng, Dr. Xuepeng, Dr. Ajeesh, Dr. Sankha, Mr. Kaushik, Mr. Sagaran, Mr. Siddharth, Mr. Jae-Hyun, Mr. Wang Ying and Dr. Lu Hui for all the enjoyable moments we have shared in ISML. In addition to the people already mentioned, friends and colleagues outside of ISML have also made my time as a PhD candidate a rich and memorable one. Thanks to all my friends for their help and encouragement. I would like to thank my entire family and all my friends in China for all their support, faith and advice during my stay in Singapore. Lastly, but not least, I would like to thank Ms. Guo Li for her endless support and encouragement over the last two years. II! ! Table of Contents Acknowledgements I! Table of Contents III! Summary . VII! List of Figures X! List of Symbols and Abbreviations . XIX! Statement of Originality XXI! Chapter Introduction . 1! 1.1! Background 1! 1.2! Motivation . 3! 1.1.1.! 1-D MCs . 4! 1.1.2.! 2-D MCs . 5! 1.1.3.! Binary MCs . 6! 1.1.4.! Applications of MCs . 7! 1.3! Focus of Thesis 9! 1.4! Organization of Thesis . 10! Chapter Theoretical Background .11! 2.1! Introduction . 11! 2.2! Magnetization Reversal in Ferromagnetic Nanostructures . 11! 2.3! 2.2.1! Magnetic Energies in Nanostructures . 12! 2.2.2! Magnetization Reversal in Ferromagnetic Nanowires . 14! 2.2.3! Magnetization Reversal in a Ferromagnetic Antidot Array 16! 2.2.4! Magnetization Reversal in a Ferromagnetic Nanomagnet . 19! Ferromagnetic Resonance Phenomenon 21! 2.3.1! Theory of Ferromagnetic Resonance 21! 2.3.2! Dynamic Micromagnetism Simulation Method . 25! III ! Table of Contents 2.4! Summary 26! Chapter Experimental Details . 28! 3.1! Introduction . 28! 3.2! Patterning Techniques 28! 3.3! 3.4! 3.2.1! Ultraviolet (UV) Photolithography . 28! 3.2.2! Deep Ultraviolet Lithography (DUL) . 30! 3.2.3! Electron Beam Lithography (EBL) 32! Deposition Techniques . 35! 3.3.1! Electron-Beam Evaporation and Sputtering . 35! 3.3.2! Self-aligned Shadow Deposition 37! 3.3.3! Lift-Off Process 42! Characterization Techniques 42! 3.4.1! Scanning Electron Microscope . 42! 3.4.2! Scanning Probing Microscope 44! 3.4.3! Magneto-Optical Kerr Effect 45! 3.4.4! FMR Spectroscopy . 47! Chapter 1-Dimensional Magnonic Crystals . 50! 4.1! Introduction . 50! 4.2! Homogeneous-width Nanowire Arrays . 50! 4.3! 4.4! 4.2.1! Variation of the Width of Isolated Nanowires 52! 4.2.2! Homogeneous Width Arrays of Dipole-coupled Wires . 59! Alternating-width Nanowire Arrays 62! 4.3.1! Ferromagnetic Ground State . 65! 4.3.2! Anti-ferromagnetic Ground State . 67! 4.3.3! Tunable Disorder State . 79! Summary 87! Chapter 2-Dimensional Magnonic Crystals . 89! 5.1! Introduction . 89! IV! ! Table of Contents 5.2! Variation of Hole Diameter of Nanoscale Antidot Arrays . 90! 5.3! Antidot Array with Alternating Hole Diameters 95! 5.4! Ni80Fe20 Anti-ring Nanostructures . 106! 5.5! 5.4.1! 30 nm-thick Anti-ring Array . 108! 5.4.2! Effect of the Nanostructure Thickness . 113! Summary 123! Chapter Binary Magnonic Crystals 124! 6.1! Introduction . 124! 6.2! Ni80Fe20 Nanomagnets . 124! 6.3! 6.4! 6.2.1! Isolated Ni80Fe20 Nanomagnets 127! 6.2.2! 1-Dimensional Linear Chain of Ni80Fe20 Nanomagnets . 130! Binary Nanomagnets . 132! 6.3.1! Static Magnetic Properties 133! 6.3.2! Effects of Magnetostatic Coupling . 140! 6.3.3! Dynamic Properties 142! Summary 146! Chapter Magnonic Logic Applications . 147! 7.1! Introduction . 147! 7.2! Magnetic Logic Based on a Meander-type Ni80Fe20 Nanowires Arrays . 147! 7.3! 7.4! 7.2.1! Experimental Details 148! 7.2.2! Dynamic Response of the Device . 150! 7.2.3! Realization of XOR and NOT Logic Operation . 157! Binary Nanomagnets for Logic Applications 159! 7.3.1! Experimental Details 159! 7.3.2! Magnetic Response of the Binary Nanomagnets 160! 7.3.3! Manipulating the Magnetic Ground States . 166! Summary 169! V! ! Table of Contents Chapter Conclusion 171! 8.1! Overview . 171! 8.2! Summary of Results . 171! 8.3! Future Work . 174! References . 176! Appendix . 186! Journal Publications . 186! Conference Proceedings . 189! VI! ! Summary In the last decade, magnonic crystals (MC), conceived as the magnetic analogue of photonic crystals, have attracted a lot of interest due to their potential use in a wide range of applications such as microwave resonators, filters and spin wave logic devices. There are many challenges that need to be addressed before the full potential of MC based devices is realized, such as the lack of a systematic investigation of dynamic responses in tailored ferromagnetic nanowire (NW) arrays (1-Diemsional MCs) and 2-Dimensional (2-D) MCs, the method of fabrication of bi-component magnonic crystals consisting of two contrasting ferromagnetic materials and the application of the MCs in logic schemes. In this thesis, a comprehensive study of the static and dynamic magnetic properties of various types of MCs is presented. Firstly, the properties of tailored 1-D MCs consisting of NWs with different configurations have been systematically investigated. Alternated arranged nanowires with two different widths have been introduced to control the magnetization ground state in the MCs. By comparing to the normal nanowires array with a stripe width uniform across the whole array, a perfect antiparallel magnetization state has been realized in the presented engineered nanowires. We have imaged directly the parallel magnetization and anti-parallel magnetization ground states using magnetic force microscopy. A simple analytical model has been suggested to explain the experimental data. Secondly, a systematic investigation of the static and dynamic response of 2-D MCs constituted by an antidot and an anti-ring array has been performed. For a homogeneous antidot array with square lattice geometry, two main resonance modes were observed for the field applied along the lattice edge. It is also observed that the frequencies of all modes can be systematically tuned by VII ! Summary varying the antidot diameter. A new design of antidot arrays with alternating “hole” diameters has been introduced to further control the spin wave (SW) modes in the MCs. The resonance modes and profiles are markedly modified due to the existence of modulated demagnetizing field distributions. In anti-ring arrays, it was observed that the FMR response of the anti-rings is highly sensitive to the nanostructure magnetization state for a fixed film thickness. The dynamic behavior of the surrounding rectangular antidot can be modified by controlling the magnetization state of the central elliptical nanomagnet. It was also found that both static and dynamic responses of the structure are adjustable by changing the film thickness. The MOKE and MFM results show that the central nanomagnets remain in the saturated state for smaller sample thicknesses, while a multi-domain state or vortex state can be observed for thicker nanostructures. Thirdly, a “self-aligned shadow deposition” technique has been introduced to fabricate bi-component MC consisting of two contrasting ferromagnetic materials (binary MC). High-quality Ni80Fe20/Ni80Fe20 and Ni/Ni80Fe20 binary elliptical  nanostructures  arranged  in  three  different  configurations were prepared using a simple self-aligned shadow deposition method. We have also demonstrated that our technique can be applied to other structures, such as binary and thickness modulated nanowires. The static and dynamic properties of the binary MCs were investigated using a combination of MOKE and broadband FMR spectroscopy. 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Ding,  ε.  Kostylev  and  A.  O.  Adeyeye.  “εagnonic  Crystal  as  a  Medium with Tunable Disorder on  a  Periodical  δattice”  Physical Review Letters, 107, 047205 (2011). [4] J. Ding,  ε.  Kostylev  and  A.  O.  Adeyeye.  “εagnetic  Hysteresis  of  Dynamic Response of One-Dimensional Magnonic Crystals Consisting of Homogenous and Alternating Width Nanowires Observed with Broadband Ferromagnetic Resonance” Physical Review B, 84, 054425 (2011). [5] J. Ding, ε. Kostylev and A. O. Adeyeye.  “Broadband Ferromagnetic  Resonance Spectroscopy of Permalloy Triangular Nanorings” Applied Physics Letters, 100, 062401 (2012). [6] J. Ding, ε. Kostylev and A. O. Adeyeye. “Realization of a εesoscopic  Reprogrammable Magnetic Logic Based on a Nanoscale Reconfigurable εagnonic Crystal” Applied Physics Letters, 100, 073114 (2012). [7] J. Ding, D. Tripathy and A. O. Adeyeye. “Dynamic Response of Antidot  Nanostructures with Alternating Hole Diameters” Europhysics Letters, 98, 16004 (2012). [8] J. Ding and A. O. Adeyeye. “Ni80Fe20/Ni Binary Nanomagnets for Logic 186 ! Appendix Applications” Applied Physics Letters, 100, 073114 (2012). [9] X. M. Liu, J. Ding and A. O. Adeyeye. “εagnetization Dynamics and  Reversal Mechanism of Fe Filled Ni80Fe20 Antidot  Nanostructures”  Applied Physics Letters, 100, 242411 (2012). [10] J. Ding and  A.  O.  Adeyeye.  “Binary  Ferromagnetic  Nanostructures:  Fabrication,  Static  and  Dynamic  Properties”  Advanced Functional Materials, 23, 1684 (2013). [11] K. L. Livesey, J. Ding, N. R. Anderson, R. E. Camley, A. O. Adeyeye, ε.  P.  Kostylev  and  S.  Samarin.  “Resonant  frequencies  of  a  binary  magnetic nanowire” Physical Review B, 87, 064424 (2013). [12] D. Cimpoesu, J. Ding, L. Stoleriu, A. O. Adeyeye, A. Stancu and L. Spinu.  “Angular  Resonant Absorption Curves in Magnetic Nanowire Arrays” Applied Physics Letters, 102, 232401 (2013). [13] S. Saha, S. Barman, J. Ding,  A.  O.  Adeyeye  and  A.  Barman,  “Timedomain Study of Spin-wave Dynamics in Two-dimensional Arrays of Bi-component Magnetic Structures”  Applied Physics Letters, 102, 242409 (2013). [14] M. Madami, S. Tacchi, G. Gubbiotti, G. Carlotti, J. Ding, A. O. Adeyeye, J. W. Klos, M. Krawczyk. “Spin Wave Dispersion in Permalloy Antidot Array With Alternating Holes Diameter”  IEEE Transactions on Magnetics, 19, 3093 (2013). [15] J. Ding, N. Singh, ε. Kostylev and A. O. Adeyeye. “Static and Dynamic Magnetic Properties of Ni80Fe20 Anti-ring Nanostructures”  Physical Review B, 88, 014301 (2013). [16] X. M. Liu, J. Ding,  G.  N.  Kakazei  and  A.  O.  Adeyeye.  “εagnonic crystals Composed of Ni80Fe20 Film on Top of Ni80Fe20 Twodimensional Dot Array” Applied Physics Letters, 103, 062401 (2013). [17] X. M. Liu, J. Ding, N. Singh, M. Kostylev and A. O. Adeyeye. 187! ! Appendix “εagnetoresistance Behavior of Bi-component Antidot Nanostructures”  Europhysics Letters, 103, 67002 (2013). [18] M. Kostylev, S. Zhong, J. Ding and A. O. Adeyeye.  “Resonance  Properties of Bi-component Arrays of Magnetic Dots Magnetized Perpendicular to Their Planes” Journal of Applied Physics, 114, 113910 (2013). [19] S. Saha, S. Barman, J. Ding, A. O. Adeyeye and A. Barman, “Tunable Magnetic Anisotropy in Two-dimensional Arrays of Ni80Fe20 Elements”  Applied Physics Letters, 103, 242416 (2013). 188! ! Appendix Conference Proceedings [1] J. Ding,  ε.  Kostylev  and  A.  O.  Adeyeye.  “Angular  Dependence  Dynamic  Properties  of  Permalloy  Nanowires”  Presented  at  IEEE International Magnetics Conference (INTERMAG 2011), Taipei, April 25-29, 2011. [2] J. Ding, ε. Kostylev and A. O. Adeyeye. “Dynamic Response of 1-D εagnonic  Crystals  Consisting  of  Alternating  Width  Nanowires”  Presented at 56th Annual Conference on Magnetism & Magnetic Materials, Scottsdale (MMM), Arizona, USA, October 30 – November 3, 2011. [3] J. Ding, ε. Kostylev and A. O. Adeyeye,  “Broadband Ferromagnetic  Resonance Spectroscopy of Permalloy Triangular Nanorings” Presented  at 56th Annual Conference on Magnetism & Magnetic Materials, Scottsdale (MMM), Arizona, USA, October 30 – November 3, 2011. [4] J. Ding,  D.  Tripathy  and  A.  O.  Adeyeye,  “Spin  Wave  εode  Transformation in Bi-Component Antidot Nanostructures” Presented at  56th Annual Conference on Magnetism & Magnetic Materials, Scottsdale (MMM), Arizona, USA, October 30 – November 3, 2011. [5] J. Ding and A. O. Adeyeye, “εagnonic  Crystals  Based  on  Binary  Ferromagnetic Nanostructure” Presented at 12th Joint MMM/Intermag Conference, Chicago, USA, January 14-18, 2013. [6] J. Ding and A. O. Adeyeye, “Ni80Fe20/Ni Binary Nanomagnets for Logic Applications”  Presented  at 12th Joint MMM/Intermag Conference, Chicago, USA, January 14-18, 2013. [7] J. Ding, N. Singh, ε. Kostylev and A. O. Adeyeye, “Static and Dynamic  Magnetic Properties of Ni80Fe20 Anti-Ring Nanostructures” Presented at  58th Annual Conference on Magnetism & Magnetic Materials (MMM), 189! ! Appendix Denver, USA, November 4-8, 2013. [8] J. Ding,  V.  Demidov,  ε.  G.  Cottam,  and  A.  O.  Adeyeye,  “Effect  of  Defect Types on the Dynamic Behavior of Ni80Fe20 Nanowires”  Presented at 58th Annual Conference on Magnetism & Magnetic Materials (MMM), Denver, USA, November 4-8, 2013. 190! ! [...]... Ding Dynamic Response of One-Dimensional Magnonic Crystals Consisting of Homogenous and Alternating Width Nanowires Physical Review B, 84, 054425 (2011) ‚ A systematic investigation of the static and dynamic response of 2-D MCs constituting of antidot and an anti-ring array [3] J Ding Journal of Applied Physics, 109, 07D304 (2011) [4] J Ding of Europhysics Letters, 98, 16004 (2012) [5] J Ding Static and. .. development of magnonic logic devices The complete set of logic devices such as NOT, XNOR and AND based on Mach Zehnder-type spin-wave interferometer devices has been proposed An obvious disadvantage of these designed devices is their macroscopic size Utilizing a patterned Ni80Fe20 stripe can significantly reduce the dimension There is a high demand in design of new logic cells for the magnonic logic devices. .. 2-D and binary MCs is reviewed in the first three section The fourth section focuses on the applications 3! ! Introduction of the MCs 1.1.1 1-D MCs The static and dynamic properties of ferromagnetic NWs of rectangular cross-section have attracted a lot of interest both from a fundamental viewpoint and because of their potential in a wide range of applications such as microwave devices [8, 19] and domain... more than one magnetic material This kinds of MCs have been theoretically predicted and experimental demonstrated to further control of the static and dynamic response Z K Wang et al [6, 16] have designed and fabricated a 1-D MC in the form of a periodic array comprising alternating contacting Co and Ni80Fe20 nanostripes Band gap tenability have been observed in such kind of MCs Nanomagnets are another... Typical band structures of PCs [5] and MCs [6] are shown in (d) and (g), respectively 1 ! Introduction Recently, there has been a growing interest in the fundamental understanding of the SW propagation in MCs because of their potential use in a wide range of applications such as microwave resonators, filters [8] and spin wave logic devices [9] A number of periodic structures have been identified as candidates... Brillouin light scattering (BLS) and ferromagnetic resonance (FMR) have been used earlier to characterize the dynamic properties of ferromagnetic nanostructures Tacchi et al [18] have reported a good agreement between BLS and broadband FMR spectroscopy for 2! ! Introduction the dynamic response of such structures A vector network analyzer (VNA) is used in the broadband FMR spectroscopy technique to... [7] MCs The band structure has also been observed in these structures Like PCs, magnonic ones are expected to possess special and interesting properties arising from their frequency band gaps as shown in Fig 1.1(g) [6] ∋()∗)+,∃−./01∗∀21 6∀5+)+,∃ /01∗∀21 !∀# !%# !3# !∃# !&# !4# !5# Fig 1.1 Typical SEM images of (a) 1-D, [2] (b) 2-D [3] and (c) 3-D [4] PCs Typical SEM images of (e) 1-D [6] and (f) 2-D... imperative to have control over the dynamic magnetic response Brillouin light scattering (BLS) [36, 42] and ferromagnetic resonance (FMR) [39, 43-47] have been used earlier to characterize the dynamic properties of antidot structures Tacchi et al [18, 48] have reported a good agreement between BLS and FMR for the dynamic response of such structures Moreover, the dynamic response of antidots has also... function of lattice symmetry [18] and orientation of an [43, 46, 48, 49] and a strong dependence was observed It has been shown that multiple resonances occur in antidot arrays for in- To achieve a consistent and experimentally tunable dynamic response in antidot arrays in the nanoscale regime, it is thus critical to understand the effect of physical dimensions on the dynamic properties Bi-component... the cells, distinct dynamic states probed by broadband ferromagnetic resonance spectroscopy are realized We show that the magnetic ordering can be manipulated to achieve logic operations by controlling the amplitude and the This proposed logic cell may be useful for downscaling magnonic logic devices IX! ! List of Figures Fig 1.1 ! Typical SEM images of (a) 1-D, [2] (b) 2-D [3] and (c) 3-D [4] PCs . NOVEL MAGNONIC CRYSTALS AND DEVICES: FABRICATION, STATIC AND DYNAMIC BEHAVIORS DING JUNJIA (M. Eng, Huazhong University of Science and Technology) . structures, such as binary and thickness modulated nanowires. The static and dynamic properties of the binary MCs were investigated using a combination of MOKE and broadband FMR spectroscopy. We. Mr. Alaric Wong and Ms. Xiao Yun for their help and support during my candidature. It has been a delight to work with the current and past members of Prof. Adekunle’s group and ISML: Dr. Shikha