STUDY OF MAGNETIC NANOSTRUCTURES FABRICATED BY NANOSPHERE LITHOGRAPHY VERMA LALIT KUMAR NATONAL UNIVERSITY OF SINGAPORE 2007 STUDY OF MAGNETIC NANOSTRUCTURES FABRICATED BY NANOSPHERE LITHOGRAPHY VERMA LALIT KUMAR (M. Sc. Electronics, University of Delhi, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATONAL UNIVERSITY OF SINGAPORE 2007 Acknowledgements Acknowledgements Firstly I would like to express my deep and sincere gratitude to my supervisor Dr. Vivian Ng for her invaluable guidance, advices and counseling during my PhD. candidature. It was a great pleasure to me to conduct this research under her supervision. Now my skills as a researcher have been well trained. Her patience and assurance at times of crisis will be remembered lifelong. Without her invaluable advices and support, this thesis would not have seen the light of the day. I would also thank my co-supervisor Prof. C. S. Bhatia for his moral support. Many thanks will also be given to the lab officer Ms Loh Fong Leong and Mr. Alaric Wong for helping and giving assistance during my stay in ISML. I would also like to express my gratitude to my colleagues and friends in ISML lab for their valuable help and friendship. I wish I would never forget the company I had from my fellow research scholars of ISML. I also want to thank my parents, who taught me the value of hard work by their own example. I would like to share this moment of happiness with my brother, parents, and my Grandma. They rendered me enormous support during the whole tenure of my research. Last but not least, I would like to thank Almighty God, Who always showered His kindness to me at every moment of my life. The financial support of National University of Singapore is gratefully acknowledged. Lalit Kumar Verma i Table of Contents Table of Contents Acknowledgements i Table of Contents ii Summary vii List of Figures ix List of Tables xx List of Abbreviations xxi Chapter Introduction 1.1. Background 1.2. Literature review and motivation 1.3. Objectives 10 1.4. Organization of thesis 10 Chapter Self assembly of polystyrene nanoparticles 12 2.1. Theory of polystyrene self assembly 14 2.2. Multiple menisci effect 16 2.3. Effect of surface tension 19 2.4. Effect of surfactant concentration 23 Chapter Etching characterization of nanoparticles 26 3.1. Argon etching 27 3.2. Oxygen etching 28 3.3. Tetrafluorocarbon etching 33 ii Table of Contents 3.4. Effect of surfactant nonuniformity 38 Chapter Fabrication methods of nanostructures 41 4.1. Hexagonal packing of nanoparticles 42 4.1.1. Triangular nanostructures 42 4.1.2. Dumbbell patterns 44 4.1.3. Zigzag nanowires 46 4.1.4. Embedded dots 51 4.1.5. Disk patterns 54 4.2. Cubic packing of nanoparticles 56 4.2.1. Asteroids and chains 57 4.2.2. Disk patterns 59 Chapter Characterization of triangular patterns 61 5.1. Theory of magnetization 61 5.2. Micromagnetic simulation 67 5.3. Characterization of permalloy triangles 71 5.3.1. Magnetic domain study 73 5.3.2. Simulation of spin states 74 5.3.3. Effect of aspect ratio 76 5.3.4. Hysteresis measurement on embedded dots 79 5.3.5. Magnetization reversal in embedded dots 80 Chapter Characterization of dumbbell nanostructures 6.1. Magnetic domain study 85 86 iii Table of Contents 6.2. OOMMF Simulation 87 6.3. Magnetization reversal process 92 6.4. Comparison with triangular patterns 97 6.5. Size dependent switching 98 Chapter Characterization of zigzag nanowires 7.1. Symmetric nanowire 101 102 7.1.1. Magnetic domain study 103 7.1.2. Simulation of spin states 105 7.2. Asymmetric nanowire 108 7.2.1. Simulation of spin states 109 7.2.2. Magnetic domain study 111 a. Magnetization along long axis 112 b. Magnetization along short axis 114 c. Magnetization along minor arm 115 d. Magnetization along major arm 117 Chapter Characterization of astroid patterns 8.1. Single astroid element 120 121 8.1.1. Magnetic domain study 121 8.1.2. Simulation of spin states 123 8.2. Chain of astroids 127 8.2.1. Magnetic domain study 128 8.2.2. Simulation of spin states 131 8.3. Chain of astroids 135 iv Table of Contents 8.3.1 Magnetic domain study 135 8.3.2 Simulation of spin states 137 8.4. Chain of astroids 142 8.4.1 Magnetic domain study 143 8.4.2 Simulation of spin states 145 8.5. Chain of astroids 151 8.5.1 Magnetic domain study 152 8.5.2 Simulation of spin states 155 8.6. Development of vortex states 160 8.7. Magnetization reversal from vortex states 161 8.8. A Long chain of 13 astroids 164 8.8.1 Spin arrangement at remanence 164 8.8.2 Magnetization along the short axis 165 8.8.3 Magnetization along the long axis 170 8.9. Magnetic strain Chapter Characterization of trilayer triangular patterns 175 179 9.1 Fabrication process 180 9.2 Trilayer triangular patterns – set 187 9.2.1 Hysteresis measurement 187 9.2.2 Magnetoresistance measurement 189 9.3 Trilayer triangular patterns – set 196 9.3.1 Hysteresis measurement 196 9.3.2 Magnetoresistance measurement 199 9.4 Temperature dependent MR measurement 203 v Table of Contents Chapter 10 Conclusion and future work 211 References 219 vi Summary Summary This thesis presents the advancements made in the area of nanosphere lithography in the fabrication of nanostructures of different shapes and sizes. They were achieved by modifying the nanosphere mask using a combination of self assembly of nanoparticles and different ion beam etching processes. Processes were optimized for selective etching of the mask patterns using gases such as Ar, CF4, and O2. The fabrication process of novel magnetic nanostructures such as embedded dots, dumbbells, zigzags, rings and astroids were developed, and their magnetic properties were investigated. An experiment of multiple menisci was also developed to locally repair the defects in the self-assembly of nanoparticles. The effect of shape anisotropy on the magnetic properties of the novel nanostructures fabricated by nanosphere lithography is also demonstrated. OOMMF simulations were performed to substantiate the experimental results. For single layer triangular patterns, as the aspect ratio of the triangles increases, the magnetic orientations of the spins changes from in-plane to the out-of-plane, making perpendicular alignment of magnetization a preferable direction for the triangular pillars. Their magnetic states switch independently of the magnetization states of their neighboring elements during the magnetization reversal process. This demonstrates the feasibility of fabricating embedded media using a simple self-assembly process. The shape effect becomes progressively dominant when the two neighboring triangles are connected to form dumbbell patterns. It is found that the neck of the dumbbell vii Summary elements provides a path to the magnetic spins to relax further from their arrangements in the triangular patterns. The spin arrangements observed at remanence show a strong preference for the long axis alignment and not switch easily under the off-axis alignment of the applied field. This can be useful in the area of magnetic logic elements where stray fields from the neighboring elements may be present. The two neighboring dumbbells were further connected to form a zigzag nanowire, and their spin arrangements were investigated based on the asymmetry present in the nanowires. The spin arrangements vary with the alignment of the applied field and show single domains in triangular sections of the nanowire for the long axis alignments, but a long domain type of arrangement for the short axis alignments. Asymmetry in zigzag arms was created to further investigate its effect on their domain patterns. Another nanostructure investigated includes the astroidal element and its chains, which shows a great dependence of the spin arrangements in an element on the spin states of the neighboring elements of the chain. A spin strain is discovered dominating the spin arrangements progressively as the number of neighboring elements increases. The spin arrangements of the different elements studied in this thesis is very important for the understanding of the switching properties, for their applications in magnetic random memories and sensors. 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B, 46, pp.8659-8663 1992. 233 Publication list Conference publications 1. Verma L. K. and Ng V. “Dynamics of self assembly process”, the 1st conference on Nanotechnology (NANOTECH 2004), Singapore, 13-17 July 2004. 2. Verma L. K. and Ng V. “Fabrication of embedded media by etching of self assembled mask”, the 4th IEEE conference on Nanotechnology (IEEE- NANO 2004), Munich, Germany, 16-19 August 2004. 3. Verma L. K. and Ng V. “Study of scattering events in embedded magnetic nanostructures”, 3rd International Conference on Materials for Advanced Technologies (ICMAT 2005), Singapore, 3-8 July 2005 4. Verma L. K. and Ng V. “Temperature dependent study of Co/Cu/Co layers” the 5th IEEE conference on Nanotechnology (IEEE-NANO 2005), Nagoya, Japan, 11-15 July 2005. 5. Verma, L.K.; Ng, V., “Fabrication of magnetoresistive sensors using self assembled nanosphere mask, Magnetics Conference, 2005. INTERMAG Asia 2005. Digests of the IEEE International, 4-8 April 2005, On page(s): 927- 928 6. Verma, L.K.; Ng, V., “Magnetic Properties of Dumbbell Shaped Nanostructures” Magnetics Conference, 2006. INTERMAG 2006. IEEE International, 8-12 May 2006, pages: 942-942 7. Verma L. K. and Ng V. “Magnetic properties of triangular pseudo-spin-valve nanostructures” International Magnetic Conference, (ICM 2006), Kyoto. Japan, August 20-25, 2006. 8. Verma L. K. and Ng V. “Magnetic strain at saturation and its dependence on the magnetic history of a permalloy nanowire” International Magnetic Conference, (ICM 2006), Kyoto. Japan, August 20-25, 2006. Journal publications 9. Verma, L. K. and Ng V., “Study of scattering events in embedded magnetic nanostructures” Thin Solid Films 505 (2006) 50-53. 10. Verma, L.K. and V. Ng, “Effect of magnetic strain and the magnetic history on magnetization states of a permalloy nanowire”, Journal of Magnetism and Magnetic Materials, 310 (2007) 2442–2444. 11. Verma,,L. K. and Ng V., “Magnetic domain patterns in a zigzag nanowire” Journal of Magnetism and Magnetic Materials, 313 (2007) 317–321. 12. Verma, L. K. and Ng V., “Magnetic domain patterns in an asymmetric zigzag nanowire” Journal of Applied Physics, 103 (2008) 1-6. Under Review 13. Verma L. K. and Ng V., “Fabrication of lateral zigzag nanowires by nanosphere lithography” IEEE transactions on nanotechnology (under Review). 14. Verma L. K. and Ng V., “Domain evolution in ferromagnetic asteroidal elements and nanowires” Physical Review B (under Review). 15. Verma L. K. and Ng V., “Magnetic properties of dumbbell shaped elements fabricated by nanosphere lithography” Journal of Applied Physics (under Review). 16. Verma L. K. and Ng V., “Magnetic properties of embedded media fabricated by nanosphere lithography” Nanotechnology (under Review). To be submitted 17. Verma L. K. and Ng V., “Magnetic switching properties of a permalloy (Ni80Fe20) nanowire of asteroid shaped elements” (In preparation). 18. Verma L. K. and Ng V., “Magnetic properties of ring nanostructures fabricated by nanosphere lithography” (In preparation). [...]... fabricated by our modified nanosphere lithography methods, and further explore the limits of this technique in the area of magnetic nanostructures Magnetic properties of nanostructures are shape and size dependent and have been center of attraction recently [75-83] as fabricated by nanosphere lithography It becomes important to explore magnetic properties of these patterns of unique shapes and sizes... futuristic magnetic logic circuits and devices [4] Magnetic properties of Co nanostructures fabricated by self-assembly of nanoparticles were recently investigated by magneto optical Kerr effect (MOKE) on a localized location by Li et al [74] Haes and Van Duyne [75] studied the magnetic properties of Ni and Co dots fabricated by shadow evaporation [64] Further work in the area of magnetic nanostructures. .. out -of- plane magnetic hysteresis loops measured by VSM for the 150 nm embedded media Magnetic force micrograph (a) before application of field, (b) at remanance after magnetizing in a field of H = 4000 Oe directed into-the-plane, (c) 80 Oe out -of- plane, (d) 120 Oe out -of- plane, (e) 140 Oe out -of- plane, (f) 160 Oe out -of- plane, (g) 180 Oe out-ofplane, (h) 250 Oe out -of- plane, (i) 500 Oe out -of- plane, (j)... properties of array of nanostructures which are randomly arranged on the substrate; however it has been used by Choi et al [80] to study the hysteresis loop of CoCrPt nanodots and the effect of dot dimension on their magnetic properties was estimated Simulations of such patterns were reported by Kim et al [81] and torque measurements performed by Weeks et al [82] on Co dots fabricated by self-assembly... of the colloidal suspension Another interesting application of polystyrene nanoparticles in photonics was developed by Breen et al [59] by coating these nanoparticles with thin layers of ZnS because of the simplicity of this process Colloidal assembly came into light as a lithography technique after initial work done by Hulteen et al [60-61] A monolayer of nanospheres was used as a tool to create nanostructures. .. 4.15 Scanning electron micrograph of the top view of the nanostructure with a square hole 59 Figure 4.16 Scanning electron micrograph of array of nanostructures fabricated by overlap of two layers 59 Figure 5.1 Variation of intrinsic coercivity with particle diameter showing the regimes of single and multi-domain formation (schematic) 62 Figure 5.2 (a) Interactions among magnetic nanoparticles A, B, and... sizes of the antidot patterns Characterization of all these patterns fabricated by nanoparticles assembly is a challenge because of the random arrangement of fabricated patterns which 7 Chapter 1 Introduction limits the type of characterization methods needed to systematically investigate the properties of the new nanostructures Systems such as vibrating sample magnetometer give collective properties of. .. techniques used to modify the nanosphere mask, and transfer the mask patterns onto the substrates Chapter 4 discusses the extension of the fabrication capabilities of nanosphere lithography by presenting the fabrication of different nanostructures such as triangles, embedded pillars, dumbbell shaped patterns, zigzag nanowires, ring type of nanostructures with different shapes of holes, and astroid shape... image after applying a reverse field H = - 2000 Oe, (d) MFM image of a symmetric nanowire for comparison of spin states 118 Figure 8.1 SEM image of the astroid nanostructures fabricated by square assembly of nanoparticles 121 Figure 8.2 (a) Topography of single astroid shaped pattern, (b) MFM image of spins at remanence, (c) MFM image of spins at remanence after magnetization reversal 122 Figure 8.3... the area of magnetic nanostructures was done by Ng et al [76] by developing the monolayer assembly by thermal treatment and fabricating large area nanomagnets Recently Rybczynski et al [77] reported monolayer coverage of 1 cm2 area with a grain size of 50 µm2 and fabricated a large array of magnetic dots Zhukov et al [78, 79] fabricated antidot patterns by evaporating a thin film on nanoparticles after . STUDY OF MAGNETIC NANOSTRUCTURES FABRICATED BY NANOSPHERE LITHOGRAPHY VERMA LALIT KUMAR NATONAL UNIVERSITY OF SINGAPORE 2007 STUDY OF. Simulation of spin states 131 8.3. Chain of 3 astroids 135 Table of Contents v 8.3.1 Magnetic domain study 135 8.3.2 Simulation of spin states 137 8.4. Chain of 4 astroids 142 8.4.1 Magnetic. nanosphere lithography in the fabrication of nanostructures of different shapes and sizes. They were achieved by modifying the nanosphere mask using a combination of self assembly of nanoparticles