PATTERNED FERROMAGNETIC MESO AND NANO STRUCTURES GOOLAUP SARJOOSING NATIONAL UNIVERSITY OF SINGAPORE 2007 PATTERNED FERROMAGNETIC MESO AND NANO STRUCTURES GOOLAUP SARJOOSING (B. Eng(Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2007 Acknowledgements First and foremost, I would not be writing these words, had I not taken the magnetic course, taught by Associate Professor Adekunle Adeyeye, in the final year of my undergraduate degree. I would like to sincerely thank my supervisor Associate Prof Adekunle Adeyeye for giving me the opportunity to join his group and to work on the exciting topic of magnetism. He is someone who is bubbling with a contagious enthusiasm for research. His support, advice and working attitude helped me a lot in carrying out my research. I am also grateful to him for believing in me and giving me the freedom and support to pursue my own avenues in research. I would like to give special thanks to Mr Navab Singh from the Institute of Microelectronics for providing me with the deep ultra violet resist patterns used in this thesis. I would also like to thank Dr Wang Chen Chen and Ms. Jain Shika for reading through this thesis. It has been a delight to work with the current and past members of my research group: Mr Tripathy Debashish who has been a great friend and taught me a lot about half-metals! and Mr Chui Kiam Ming for forcing me to speak French. Also, I would like to thank my friends in the lab for making it a fun place to work; Mr Mambakkan Sreenivasen for always having some tid-bits to help stave away the hunger pangs at night and Mr. Cheng Xingzhi, for trying to teach me Chinese. I would like to thank my mum and dad for supporting me to pursue the PhD degree and pretending to understanding magnetism when I explained it to them. I am also grateful to my “little” brother, Avinash and my “twin” sister, Venita for their support and encouragement. Lastly, but not least, I would like to thank my flat mate Alex for putting up with me for the past years! i Table of Contents Acknowledgements i Table of Contents ii Summary vi List of Tables viii List of Figures ix List of Symbols and Abbreviations xvi Statement of Originality xviii Chapter Introduction 1.1 Why Meso-Nano Magnets? 1.1.1 Fundamental Perspective 1.2.1 Application Perspective 1.2 Challenges in the Fabrication of Meso-Nano Magnets 1.3 Focus of this Thesis 1.4 Organization of this Thesis References Chapter Theoretical Background 2.1 Introduction 12 2.2 Magnetic Energies 12 2.2.1 Exchange Energy 13 2.2.2 Zeeman Energy 13 2.2.3 Magnetic Anisotropy Energy 14 2.2.4 Magnetostatic Energy 15 2.3 Magnetization Reversal Mechanism 16 2.3.1 Coherent Rotation 17 2.3.2 Curling 20 2.4 Coupling in Multilayer Films 22 2.4.1 Pinhole Coupling 22 ii Table of Contents 2.4.2 RKKY Coupling 23 2.4.3 Néel Coupling 25 2.4.4 Interlayer Magnetostatic Coupling 26 2.5 Magneto Resistance Effect 26 2.5.1 Anisotropic Magnetoresistance 27 2.5.2 Giant Magnetoresistance/Spin Valve 28 2.6 Summary 29 References 30 Chapter Experimental Techniques 3.1 Introduction 34 3.2 Fabrication Techniques 34 3.2.1 KrF Deep Ultra Violet Lithography 36 3.2.2 Deposition Techniques 38 3.2.2.1 Evaporation 38 3.2.2.2 Sputtering 40 3.2.2.3 Lift-off 41 3.3 Characterization Techniques 41 3.3.1 Scanning Electron Microscope 44 3.3.2 Scanning Probe Microscope 43 3.3.2.1 Atomic Force Microscope 43 3.3.2.2 Magnetic Force Microscope 44 3.3.3 Vibrating Sample Magnetometer 46 3.3.4 Magnetoresistance Measurement 48 3.4 Summary 49 References 51 Chapter Magnetization Reversal in Ni80Fe20 Nanowires 4.1 Introduction 53 4.2 Sample Fabrication 54 4.3 Shape Anisotropy 56 4.4 Effect of Ni80Fe20 wire thickness 58 4.4.1 Fields Applied Along Easy Axis 58 4.4.2 Fields Applied Along Hard Axis 61 iii Table of Contents 4.5 Field Orientation Dependent Measurements 67 4.6 Angular Coercivity Variations 69 4.6.1 Modeling of curling mode of rotation 71 4.7 Magnetoresistance Measurement 73 4.8 Thickness Dependent MR Measurements 76 4.9 78 Switching Field Variations 4.10 Summary 80 References 81 Chapter Dipolar Coupling in Pseudo Spin Valve Nanowires 5.1 Introduction 84 5.2 Sample Fabrication 85 5.3 Magnetic Properties of Pseudo-Spin Valve Nanowires 86 5.4 Effect of Cu thickness on Dipolar Coupling 89 5.5 Differential Magnetization Loops 94 5.6 Minor Hysteresis Loop Measurement 97 5.7 Interaction Field 100 5.8 Summary 102 References 104 Chapter Magnetization Switching in Alternating Width Nanowires 6.1 Introduction 106 6.2 Sample Fabrication 107 6.3 Reversal Process in Alternating Width Nanowires 108 6.3.1 Fields Applied Along Easy Axis 109 6.3.2 Schematic Magnetic States 112 6.3.3 Fields Applied Along the Hard Axis 113 6.4 Effect of Ni80Fe20 Wire Thickness 114 6.4.1 Fields Applied Along Easy Axis 115 6.4.2 Fields Applied Along Hard Axis 118 6.5 Dipolar Field in Alternating Width Nanowires 121 6.6 Magnetization Reversal Mechanisms 124 iv Table of Contents 6.6.1 Reversal Modes of w1 Nanowires 125 6.6.2 Reversal Modes of w2 Nanowires 127 6.7 Summary 128 References 130 Chapter Spin State Evolution in Diamond-Shaped Nanomagnets 7.1 Introduction 132 7.2 Sample Fabrication 133 7.3 Single layer Ni80Fe20 Nanomagnets 134 7.4 Simulation For Reversal Along Major Axis 136 7.5 Remanent Spin State Configuration 141 7.6 Single layer Co Nanomagnets 142 7.7 Comparative Study of Thickness Dependence 143 7.8 Trilayer Nanomagnets 146 7.9 3-D Simulation for Reversal Along Major Axis 151 7.10 Remanent State of Trilayer Nanomagnet 154 7.11 Interlayer Coupling 156 7.12 Summary 159 References 160 Chapter Conclusion and Outlook 162 Appendix List of Publications 167 v Summary The magnetization reversal process in patterned single and multilayer nanomagnets has been systematically studied, as a function of various geometrical parameters, using a combination of characterization techniques and simulation tools. Ordered homogeneous nanomagnets were fabricated over a large area (4×4 mm2) using deep ultra violet lithography at 248 nm exposure wavelength and lift-off technique, allowing for characterization of magnetic properties using conventional magnetometers. Firstly, the magnetization reversal mechanism in Ni80Fe20 nanowire arrays as a function of wire thickness was systematically investigated. It was observed that for fields applied along the wire easy axis, a non-monotonic variation of the coercivity was observed, due to the different mechanisms of magnetization reversal dominating the switching process in the nanowire arrays. The angular dependence of coercivity was used to map the reversal mechanism in the nanowires. A cross-over from coherent rotation to curling mode of reversal was observed for thickness to width ratio > 0.5. The understanding of the reversal process was validated using theoretical modeling. Secondly, the question of how the magnetostatic interaction affects the reversal process in pseudo-spin valve (PSV) nanowire arrays was addressed. Closely packed and isolated homogeneous width Ni80Fe20(10 nm)/Cu(tCu)/Ni80Fe20(80 nm) PSV nanowires with varied Cu spacer layer thickness were studied. Marked changes in the magnetization reversal process were observed, as the Cu spacer layer thickness becomes comparable to the edge-to-edge spacing of the closely packed nanowires. This was attributed to the competition between the dipolar coupling in the neighboring nanowires and the interlayer magnetostatic coupling between the 10 nm and 80 nm vi Summary Ni80Fe20 layers. Minor loop revealed that the 10 nm Ni80Fe20 layer in the closely packed PSV nanowire experienced a larger interaction field as compared to the isolated nanowires, leading to a smaller region of anti-parallel alignment in the closely packed nanowires. Thirdly, by exploiting the width dependence of coercivity in nanowires, it has been shown that complex nanowire arrays with unique magnetic properties can be engineered. Alternating width nanowires consisting of two sets of Ni80Fe20 nanowires differentiated by their width, which are alternated in an array, were fabricated and systematically studied. The magnetization reversal process in the alternating width nanowire arrays was found to be markedly sensitive to the Ni80Fe20 wire thickness and differential width, ∆w, between the two sets of nanowires. Minor M-H loop measurements, revealed that the interaction field, was strongly dependent on the individual wire width constituting the array. Finally, a comprehensive study of the spin state evolution in diamond-shaped nanomagnets was conducted. The effect of film composition and thickness on the magnetic properties of the nanomagnets was systematically investigated. An evolution from coherent rotation to vortex mediated reversal was observed as the film thickness was increased. The onset of the vortex state was found to be strongly dependent on the film thickness and composition. By stacking layers with different modes of reversal, PSV nanomagnets with unique properties were fabricated. The understanding of the reversal process was aided using 2-D and 3-D micromagnetic simulations. MFM imaging was used to confirm the magnetic spin states of the nanomagnets. vii List of Tables Table 5.1 Effective Coercivity and interaction field for wire A, s = 35 nm 102 and wire B, s = 185 nm as a function of the Cu spacer layer thickness Table 6.1 Effective Coercivity and interaction field for alternating width nanowire arrays as a function of the Cu spacer layer thickness viii 123 Spin State Evolution in Diamond-Shaped Nanomagnets magnetization, corresponding to region E in Fig 7.10. This loss in magnetic moment is due to the formation of a local vortex in the top layer as seen in Fig 7.11(e). The bottom layer on the other-hand adopts the “C-state”. The anti-parallel alignment between the magnetization of the bottom and top layer is conserved. The moments of the bottom layer are oriented clockwise, opposing the anti-clockwise chirality of the local vortex in the top layer. Further decrease in the external field, causes the vortex to move towards the bottom edge of the structure, where annihilation takes place as seen in Fig 7.11(f), corresponding to region F in Fig 7.10. At annihilation, the moments of the top layer align along the negative field direction, whereas the moments of the bottom layer adopt a general positive saturation direction. As the external field is reduced further, the magnetic moments of the bottom layer align perpendicular to the direction of applied field, pointing in the negative x-direction, as seen in Fig 7.11(g). The magnetic moments of the top layer are aligned along the negative y-direction. This spin configuration for the two layers is similar to that observed in Fig 7.11(b), except that the moments are now pointing in the respective negative direction. By reducing the field to negative saturation field, the moments of the top and bottom layers are aligned along the field direction as shown in Fig 7.11(h). This spin configuration corresponds to region H on the calculated M-H loop, Fig 7.11. 7.10 Remanent State of Trilayer Nanomagnet Direct observation of the magnetic state of the PSV nanomagnet was obtained using MFM, using similar method as described in section 7.5. The sample was magnetized by first applying a field of 1.5 kOe along the major axis (θ = 0°) and then the field was reduced to Oe. Shown in Fig 7.12, is the MFM image of the remanent state of the 154 Spin State Evolution in Diamond-Shaped Nanomagnets diamond-shaped Ni80Fe20(10nm)/Cu(2nm)/Ni80Fe20(40nm) tri-layer nanomagnets over an area of x µm2. The MFM image reveals the magnetic state of the topmost 40 nm Ni80Fe20 thick layer only. Both the flux closure state with circular spin configuration and the “S-spin” state are present at remanence. The structures with the “S-spin” configuration are enclosed by the dark circles in Fig 7.12. The spin configurations observed are in agreement with the experimental results discussed in section 7.9 and is attributed to the switching field distribution among the nanomagnets. The percentage of nanomagnets adopting the vortex state does not correspond to our quantitative estimate of 52%. This difference may be attributed to the fact that the MFM is a localized measurement which was conducted over an area of × µm2, whereas the fabricated nanomagnets cover an area of × mm2. y µm x Applied Field θ Figure 7.12: MFM image of the Ni80Fe20(10nm)/Cu(2nm)/Ni80Fe20(40nm) thick diamondshaped Ni80Fe20 nanomagnet at remanent state. The dark circles represent structures with the “S-spin” configuration. 155 Spin State Evolution in Diamond-Shaped Nanomagnets The nanomagnets adopting the flux closure configuration are characterized by either a white or black dot at their geometrical center, indicating the presence of either a spinup or spin-down vortex within the individual nanomagnets. 7.11 Interlayer Coupling We have investigated the effect of Cu interlayer film thickness in which tCu, was varied from nm to 20 nm while keep the FM layer thicknesses and lateral dimensions of the PSV structures fixed. Shown in Fig. 7.13 are the representative hysteresis loops of the tri-layer diamond-shaped nanomagnet as a function of the spacer film thickness for fields applied along the major axis (θ = 0°). It was observed that the magnetization curves are strongly dependent on the spacer layer thickness tCu, due to the nature of the coupling mechanism. A tCu is increased from nm, the magnitude in the decrease of magnetization prior to Hnt increases. This results in a distinct two-step M-H loop with a plateau-like region for tCu ≥ nm, as shown in Fig 7.13. Interestingly for tCu ≥ nm, the M-H loops display a reversible region as the field is decreased from saturation to HR. The evolution of the hysteresis curve as the spacer layer thickness is increased, may be attributed to the interlayer coupling mechanisms present at different spacer layer thicknesses. The effective coupling between the FM layers is determined by the competition between the magnetostatic interaction [20-22], Néel coupling due to roughness, direct interaction through pinholes in the spacer layer and the interlayer exchange coupling [11-13]. As the exchange length of Ni80Fe20 is around 5.7nm [21], we expect that for tCu = nm, the interlayer exchange coupling [11-13] dominates the reversal process. For spacer layer thickness, tCu ≥ nm, it is expected that the magnetostatic coupling between the FM layers to dictate the reversal process. 156 Spin State Evolution in Diamond-Shaped Nanomagnets (a) Hnt 0.5 -0.5 tCu = nm (b) Hnt 0.5 HR D Magnetization (Norm.) -0.5 tCu = nm (c) Hnt 0.5 HR -0.5 tCu = 10 nm (d) Hnt 0.5 HR -0.5 tCu = 20 nm -1 -1000 -500 500 1000 Applied Field (Oe) Figure 7.13: M-H loops of Ni80Fe20(10 nm)/Cu(tCu nm)/Ni80Fe20(40 nm) Diamond-shaped nanomagnets, as function of Cu spacer layer thickness for fields applied along θ = 0°. 157 Spin State Evolution in Diamond-Shaped Nanomagnets To better understand the effect of the Cu spacer layer thickness on the magnetic properties of the PSV diamond-shaped nanomagnets, angular measurement of the hysteresis loops were carried out. The nucleation field, Hnt, corresponding to the formation of a local vortex in the top 40 nm Ni80Fe20 layer was extracted. Shown in Fig 7.14, is the nucleation field, Hnt, as a function of the orientation of the applied field, for different Cu spacer layer thicknesses. The nucleation field is found to be strongly dependent on the orientation of the applied field and the Cu film thickness. Top Layer Nucleation Field (Oe) 250 200 150 100 nm nm 10 nm 20 nm 50 Figure 7.14: 30 60 90 120 150 Field Orientation (θ°) 180 Nucleation field, HnT of the Ni80Fe20(10 nm)/Cu(tCu nm)/Ni80Fe20(40 nm) nanomagnets, as a function of field orientation, θ, for different Cu spacer layer thicknesses. For tCu = nm, in the range 0°≤ θ ≤ 50°, the nucleation field varies between 45 Oe to 70 Oe. for fields applied along θ = 60°, a sudden increase in the nucleation field, resulting in a peak of 255 Oe, is observed. A U-shaped curve is obtained, for the field orientation corresponding to, 60° ≤ θ ≤ 120°, with a local minimum in the nucleation field is obtained for fields applied along θ = 90°. For field orientation of θ > 120°, the nucleation field decreases and reaches an almost constant value of 50 Oe. For tCu ≥ 158 Spin State Evolution in Diamond-Shaped Nanomagnets nm, the angular nucleation field display a bell-shaped curve, with the maximum nucleation field occurring for field orientation θ = 90°. Interestingly, the angular variation of the nucleation field is highly sensitive to the dominating interlayer coupling mechanism. 7.12 Summary The spin state evolution in single layer and PSV diamond-shaped nanomagnets has been investigated. An evolution from coherent spin rotation to vortex mediated reversal was observed, as the nanomagnets film thickness is increased. The onset of vortex mediated reversal was found to be dependent on the type of ferromagnetic material due to the competition between the magnetic anisotropy and magnetostatic energy term. By combining layers with spin rotation and vortex mediated reversal, PSV nanomagnets with unique magnetic properties were fabricated. Interestingly, the angular variation of the nucleation field for the PSV nanomagnets is highly sensitive to the dominating interlayer coupling mechanism. 2-D and 3-D OOMMF simulation have been used to better understand the reversal processes in the nanomagnets. The magnetic spin configuration in the nanomagnets was directly observed using MFM. 159 Spin State Evolution in Diamond-Shaped Nanomagnets References [1] N. Singh, S. Goolaup and A. O. Adeyeye, Nanotechnology, 15, 1539 (2004) [2] M. Rahm, M. Schneider, J. Biberger, R. Pulwey, J. Zweck, D. Weiss and V. Umansky, Appl. Phys. Lett., 82, 4110 (2003) [3] J. A. Osborn, Phys. Rev., 67, 351 (1945). [4] OOMMF is available at http://math.nist.gov [5] S. L. Whittenburg, N. Dao and C. A. Ross, Physica B, 306, 44 (2001) [6] R. P. Cowburn, D. K. Koltsov, A. O. Adeyeye, M. E. Welland and D. M. Tricker, Phys. Rev. Lett., 83, 1042 (1999). [7] M. Steiner and J. Nitta, Appl. Phys. Lett., 84, 939 (2004). [8] E. C. Stoner and E. P. Wohlfarth, Philos. Trans. R. Soc. London, 240, 74 (1948). [9] M. Schnieder and H. Hoffmann, J. Appl. Phys., 86, 4539 (1999). [10] P. J. H. Bloemen, M. T. Johnson, M. T. H. van de Vorst, R. Coehoorn, J. J. de Vries, R. Jungblut, J. aan de Stegge, A. Reinders and W. J. M. de Jonge, Phys. Rev. Lett., 72, 764 (1994) [11] S. S. Parkin, N. More and K. P. Roche, Phys. Rev. Lett. 64, 2304 (1990) [12] J. Barnas, J. Magn. Magn. Mater. 111, L215 (1992) [13] P. Bruno, Europhys. Lett. 23, 615 (1993) [14] R. Kergoat, M. Labrune, J. Miltat, T. Valet and J. C. Jacquet, J. Magn. Magn. Mater., 121, 339 (1993) [15] W. Folkerts and S. T. Purcell, J. Magn. Magn. Mater. 111, 306 (1992) 160 Spin State Evolution in Diamond-Shaped Nanomagnets [16] N. Dao, C. A. Ross, F. J. Castano, M. J. Donahue and S. L. Whittenburg, Mater. Res. Soc. Symp. Proc., 731, W5.7.1 (2002) [17] M. Hwang, M. C. Abraham, T. A. Savas, H. I. Smith, R. J. Ram and C. A. Ross, J. Appl. Phys., 87, 5108 (2000) [18] M. El-Hilo, J. Appl. Phys., 84, 5114 (1998) [19] S.L. Whittenburg, N. Dao and C. A. Ross, Physica B, 306, 44 (2001) [20] F.J. Castano, Y. Hao, C. A. Ross, B. Vogeli, Henry I. Smith and S. Hartani, J. Appl. Phys. 91, 7317 (2002) [21] Xiaobin Zhu, P. Grutter, Y. Hao, F. J. Castano, S. Hartani, C. A. Ross, B. Vogeli and Henry I. Smith, J. Appl. Phys. 93, 1132 (2003) [22] C. A. Ross, F. J. Castano, E. Rodriguez, S. Haratani, B. Vogeli and Henry I. Smith, J. Appl. Phys, 053902 (2005) [21] J. K. Ha, R. Hertel and J. Kirschner, Phys. Rev. B., 67, 224432 (2003) 161 If I have seen further it is by standing on the shoulders of giants. Isaac Newton Chapter Conclusion and Outlook In the course of this study, a comprehensive investigation of the effect of various geometrical parameters on the magnetization reversal process in patterned single and multilayer nanostructures has been conducted. Large area ordered homogeneous nanomagnet arrays were fabricated using DUV lithography and lift-off technique. The effect of geometrical parameters such as film thickness, lateral dimension and edge-toedge spacing were systematically studied. Quantitative information about the magnetic properties of the nanomagnet arrays was obtained using a vibrating sample magnetometer. Spin dependent transport properties were measured using DC magnetoresistance measurement. The spin configurations of the nanomagnets were imaged using magnetic force microscopy. To substantiate the experimental results, micromagnetic simulations and theoretical modeling were performed. Firstly, the evolution in the magnetization reversal process in ferromagnetic Ni80Fe20 nanowires has been extensively studied. The reversal process is found to be strongly influenced by the thickness of the Ni80Fe20 wire. A non-monotonic variation for the easy axis coercivity was observed as the Ni80Fe20 thickness was increased. This 162 Conclusion has been attributed to the competition between different modes of magnetization reversal processes in the nanowires. A cross-over from coherent rotation dominated reversal to curling mode was observed as the thickness to width ratio exceeded 0.5. The experimental results were substantiated with theoretical predictions of the coherent and curling models. A good correlation between the magnetic and transport properties was also established. Secondly, the effect of magnetostatic interaction in pseudo-spin valve structures has been systematically investigated. Closely packed and isolated homogeneous width Ni80Fe20(10 nm)/Cu(tCu)/Ni80Fe20(80 nm) nanowire arrays with varied Cu spacer layer thickness were investigated. Minor loop measurements revealed that the 10 nm Ni80Fe20 layer in the closely packed PSV nanowires experiences a larger interaction field as compared to the isolated PSV nanowires. This results in a smaller region of anti-parallel alignment of magnetization in the closely packed PSV. When the Cu spacer layer thickness becomes comparable to edge-to-edge spacing (s) of the closely packed wires, s = tCu = 35 nm, marked changes in the magnetization reversal process was observed. This has been attributed to the competition between the dipolar coupling between the neighboring nanowires and interlayer magnetostatic coupling between the thick and thin Ni80Fe20 layers. Thirdly, it has been shown that complex magnetic system with unique behaviour can be engineered using lithography techniques. Laterally engineered nanowire arrays were fabricated and studied. By alternating two sets of Ni80Fe20 nanowires of different width into an array, alternating width nanowires were patterned. A systematic study of the magnetization reversal in the alternating width nanowires as a function of the film 163 Conclusion thickness was performed. The individual switching, of the two sets of nanowires comprising the alternating width array, resulted in a two-step M-H loop with a plateaulike region. The coupling field between adjacent wires was found to be strongly dependent on the Ni80Fe20 film thickness and the larger width wire constituting the array. The angular variation of the coercive field was used to map the reversal process of the two sets of nanowire arrays. For thin films, the reversal process is mediated by coherent rotation, whereas for thick films the curling mode of rotation dominates. Finally, a complete and systematic study of the magnetic properties of diamondshaped nanomagnets was carried out. By stacking layers with different reversal modes, novel structures with unique properties were fabricated. The magnetization reversal process of the single layer dot array was found to evolve from coherent rotation to vortex mediated reversal as the film thickness was increased. The film thickness for the onset of vortex mediated reversal was found to be strongly dependent on the type of ferromagnetic material, due to the competition between the intrinsic magnetic anisotropy and magnetostatic energy. PSV nanomagnets with unique magnetic properties were fabricated by combining ferromagnetic layers with spin rotation and vortex mediated reversal. The angular variation of the nucleation field for the PSV nanomagnets was found to be highly sensitive to the dominating interlayer coupling mechanism. To better understand the reversal processes in the nanomagnets the experimental results were substantiated with 2-D and 3-D OOMMF micromagnetic simulation, and the magnetic spin configurations were confirmed by MFM imaging. 164 Conclusion Future Work In this thesis numerous novel findings in large area nanomagnets have been reported. There are numerous promising avenues which can be further explored in the study of nanomagnet arrays. The study of the transport properties in nanomagnets has seen a dramatic growth in recent years, due to the possibility of exploiting the spin properties of electron. Traditionally the transport properties of nanostructures are measured by the magnetoresistance (MR) technique. The MR measurement is carried out by applying a current and recording the change in voltage, via bond pads that are patterned on the nanostructures. A conventional two-terminal method, for measuring the transport property in a single nanowire is illustrated schematically in Fig 8.1. V Current Direction Ferromag netic Wire II+ Figure 8.1: Schematic illustration for conventional MR measurement For the large area nanostructures that have been studied in this thesis, a different approach may be adopted to measuring the spin-dependent transport phenomena. The regions in between the nanomagnets may be filled with a thin non-magnetic conductive layer (e.g Au or Cu), whereby creating a pseudo-continuous film. A matrix consisting of the nanomagnets, dots and wire, in a thin conductive material is illustrated in Fig 8.2. For MR measurement in the dot array, the current may be 165 Conclusion applied along arbitrary directions. In the case of the wire arrays, the current can be applied along the wire width, as depicted in Fig 8.2(b) for characterization of novel properties. Moreover, with the growing interest in hybrid devices, consisting of ferromagnetic and superconducting structures, the conductive layer may be replaced by a superconducting layer. Ferromagnetic Nanostructures I (b) (a) Non-Magnetic Conducting Material Figure 8.2: Illustration of large area nanomagnets with thin film coating, (a) dot arrays and (b) wire arrays 166 Appendix List of Publications This is an up-to date list of publications, resulting from the work carried out on large area magnetic nanostructures. Journals Main Contribution 1. S. Goolaup, A. O. Adeyeye, N. Singh, G. Gubbiotti, “Magnetization switching in alternating width nanowire arrays”, Physical Review B, 75, 144430 (2007) 2. S. Goolaup, A. O. Adeyeye and N. Singh, “Magnetization Reversal Mechanisms in Diamond-shaped Co Nanomagnets,” Physical Review B, 73, 104444 (2006). 3. S. Goolaup, A. O. Adeyeye and N. Singh, "Dipolar coupling in closely packed Pseudo Spin Valve Nanowire Arrays," Journal of Applied Physics, 100, 114301 (2006). 4. S. Goolaup, A. O. Adeyeye and N. Singh, "Magnetoresistance of Closely packed Ni80Fe20 Nanowires," Thin Solid Films, 505, 29, (2006). 5. S. Goolaup, A. O. Adeyeye and N. Singh, "Magnetization Reversal in Diamond Shaped Pseudo Spin Valve Nanomagnets," Journal of Applied Physics, 98, 084318 (2005) 6. S. Goolaup, N. Singh and A. O. Adeyeye, "Coercivity variation in Ni80Fe20 Ferromagnetic Nanowires," IEEE Transactions on Nanotechnology, ,523 (2005) 7. S. Goolaup, N. Singh, A. O. Adeyeye, V. Ng and M. B. A Jalil, “Transition from coherent rotation to curling mode reversal process in ferromagnetic nanowires”, The European Physical Journal B- Condensed Matter, Vol. 44, 259 (2005) 8. S. Goolaup, A. O. Adeyeye and N. Singh, "Magnetic Properties of Diamond Shaped Ni80Fe20 nanomagnets," J. Phys. D: Appl. Phys., 38, 2749 (2005) 167 List of Publications Others 9. G. Gubbiotti, S. Tacchi, G. Carlotti, , N. Singh, S. Goolaup, A. O. Adeyeye and M. Kostylev, “Collective spin modes in monodimensional magnonic crystals consisting of dipolarly coupled nanowires”, Applied Physics Letters, 90, 092503 (2007) 10. N. Singh, S. Goolaup, W. Tan, A.O. Adeyeye and N. Balasubramaniam, “Micromagnetics of derivative ring-shaped nanomagnets”, Physical Review B, 75, 104407 (2007) [Also selected in the Virtual Journal of Nanoscale Science & Technology, March 26, 2007] 11. X. S. Gao, A. O. Adeyeye, S. Goolaup, N. Singh, W. Jung, F. J. Castano and C. A. Ross, “Inhomogeneities in spin states and magnetization reversal of geometrically identical elongated Co rings”, Journal of Applied Physics, 101, 09F505 (2007) 12. L. J. Qui, J. Ding, A. O. Adeyeye, J. H. Yin, J. S. Chen, S. Goolaup and N. Singh, “FePt Patterned Media Fabricated by Deep UV Lithography Followed by Sputtering or PLD”, IEEE Transactions on Magnetics, 43, 2157 (2007) 13. G. Gubbiotti, S. Tacchi, G. Carlotti, A.O. Adeyeye, S. Goolaup, N. Singh and A.N. Slavin,” Spin wave eigenmodes of square permalloy dots studied by Brillouin light scattering” , Journal of Magnetism and Magnetic Materials, 316, e338 (2007) 14. P. Vavassori, V. Bonanni, G. Gubbiotti, A. O. Adeyeye, S. Goolaup, and N. Singh, “Cross-over from coherent rotation to curling reversal mode in interacting ferromagnetic nanowires” ,Journal of Magnetism and Magnetic Materials, 316, e31 (2007) 15. A. O. Adeyeye, N. Singh and S. Goolaup, "Spin State Evolution and In-Plane Magnetic Anisotropy of Elongated Ni80Fe20 Nanorings", Journal of Applied Physics, 98, 094301 (2005) 16. G. Gubbiotti, S. Tacchi, G. Carlotti, P. Vavassori, N. Singh, S. Goolaup, A. O. Adeyeye, A. Stashkevich, M. Kostylev, "Magnetostatic Interactions in arrays of nanometric permalloy wires: A magneto-optic Kerr effect and a Brillouin light scattering study", Physical Review B, Vol 72, 224413 (2005) 17. N. Singh, S. Goolaup and A. O. Adeyeye, "Fabrication of large area Nanomagnets”, Nanotechnology, 15, 1539 (2004) Conference Main Contribution 1. S. Goolaup, A. O. Adeyeye and N. Singh, “Magneto-Transport properties of Pseudo Spin Valve Nanowires,” presented at the 2nd MRS-S Conference on Advanced Materials, 18 – 20 January 2006. 168 List of Publications 2. S.Goolaup, N. Singh and A.O. Adeyeye, “Magnetic Spin States in DiamondShaped Ni80Fe20/Cu/Ni80Fe20 Nanomagnetic Trilayer”, presented at the 5th IEEE Conference on Nanotechnology, Nagoya, Japan, July 11-15, 2005. 3. S. Goolaup, N. Singh, A. O. Adeyeye, V. Ng and M. B. A. Jalil, "Magnetic Anisotropy in magnetostatically Coupled Ni80Fe20 nanowires", presented at the 2004 IEEE conference on Nanotechnology in Munich, Germany, August 17-19, 2004. 4. S. Goolaup, N. Singh and A. O. Adeyeye, “Flux closure configuration in Ferromagnetic Diamond-shaped nanomagnets”, presented at the International Magnetics Conference, Nagoya, Japan, April 4-8, 2005. 5. S. Goolaup, N.Singh and A.O. Adeyeye, “Evolution in the Magnetoresistance of Ni80Fe20 Ferromagnetic Nanowires”, presented at the 3rd International Conference on Materials for Advanced Technologies, Singapore, July 2-8, 2005. Others 6. N. Singh, S. Goolaup and A. O. Adeyeye, "Fabrication of magnetic nanostructures using KrF lithography", presented at the 2004 IEEE conference on Nanotechnology in Munich, Germany, August 17-19, 2004. 7. P. Vavassori, V. Bonanni, G. Gubbiotti, A. O. Adeyeye, S. Goolaup, and N. Singh “Cross-over from coherent rotation to curling reversal mode in interacting ferromagnetic nanowires” presented at Joint European Magnetic Symposia, San Sebastián, Spain, June 26-30, 2006, 8. G.Gubbiotti, M. Madami, S. Tacchi, G. Carlotti, A. O. Adeyeye, S. Goolaup, N. Singh, and A. N. Slavin, “Spin wave eigenmodes of square permalloy dots studied by Brillouin light scattering” presented at Joint European Magnetic Symposia, J San Sebastián, Spain June 26-30, 2006, 9. A.O. Adeyeye, N. Singh and S.Goolaup, “Dipolar Magnetostatic Interactions in Mesoscopic Rings”, presented at the 17th International Conference on Magnetism, Kyoto, Japan, August 20-25, 2006 Chapter in Edited Book 1. A.O. Adeyeye, S. Goolaup and N. Singh, “Large area magnetic nanostructures for spintronic applications”, Magnetic Properties of Laterally Confined Nanometric Structures, Ed. Gianluca Gubbiotti , – 23 [2006], ISBN 81-7895212-2 169 [...]... Fermi surface In meso- nano magnets, the anisotropy depends on the interplay between the band structure of the parent material and the shape of the meso- nano magnets The overall anisotropy (magnetization direction) can be engineered by tailoring the shape and size of the meso- nano magnets As the magnetic material is patterned down to the meso- nano scale regime, for ordered arrays of nanomagnets, the... Introduction 1.2 Challenges in the Fabrication of Meso- Nano Magnets The ability to characterize nanostructures and extract quantitative information about the magnetic properties and the reversal mechanisms is very crucial in the study of nanomagnets As the characterization of a single nanostructure is extremely difficult, it is highly desirable to fabricate nanostructures over a macroscopic area This will... distribution in pore size and orientation, making it difficult to control the period and uniformity of the nanostructures [46] As the techniques described above have certain limitations, there is a need to explore new method to fabricate nanomagnets In this thesis, by leveraging on the lithographic process in the semiconductor micro-electronic industry, large area mesonano structures were patterned by a combination... homogeneous nanomagnets Various methods for synthesizing nanomagnets have been developed in the last few years; electron beam lithography [29-32], focused ion beam etching [33], X-ray lithography [34-36], nanoimprint lithography [37-39] and nanotemplating method [40-43] Electron beam lithography (EBL) together with deposition and lift-off technique is widely used for the fabrication of magnetic nanostructures... writing of the nanostructures onto the resist, enabling the patterning of patterns with arbitrary shapes and array configuration [30-32, 44] The major drawback of EBL is the serial nature of the patterning process As such the fabrication large area of nanostructures is time consuming and costly Also, thin resists are used to improve the resolution of the structures as such high aspect ratio nanomagnets... nm, (b) 150 nm and (c) 180 nm, for nanowires with w = 185 nm and s = 35 nm Figure 4.13 MR response for 20 nm thick Ni80Fe20 nanowire arrays (w = 185 nm and s = 35 nm) with fields applied along (a) 75 = 15° and (b) = 90°, with respect to the long axis (c) Schematic diagram of the magnetic reversal process for 20 nm thick Ni80Fe20 nanowire with fields along = 15° Figure 4.14 MR response for nanowire arrays,... Chapman and Hall [4] W F Brown Jr, J Appl Phys., 39, 993 (1968) [5] M E Schabes and H N Bertran, J Appl Phys., 64, 1347 (1988) [6] R P Cowburn, A O Adeyeye and M E Welland, Phys Rev Lett., 81, 5414 (1998) [7] R P Cowburn and M E Welland, Appl Phys Lett., 72, 2041 (1998) [8] P Vavassori, D Bisero, F Carace, A di Bona, G C Gazzadi, M Liberati, and S Valeri, Phys Rev B, 72, 054405 (2005) [9] C P Bean and. .. with width 185nm and edge-to-edge spacing = 35nm Figure 5.6 Representative M-H loops and minor loops for Ni80Fe20(10 98 nm)/Cu(tCu nm)/Ni80Fe20(80 nm) spin-valve nanowire arrays with tCu = 20 nm; (a) wire A, s = 35nm and (b) wire B, s = 185nm, and tCu = 35 nm; (c) wire A, s = 35nm and (d) wire B, s = 185nm, as a function of the reverse field, Hm Figure 5.7 Representative M-H loops and minor loops for... [2]) and domain wall width (e.g for Fe, ld 39.5 nm [3]) The magnetization state of a bulk magnetic material is usually magnetically divided into domains and the exact domain configuration is unpredictable 1 Introduction due to the many regions of energy minimum When the magnetic material is patterned down to the meso- nano scale; the number, size and orientation of the domain becomes well defined and. .. synchrotron source [34, 35] and is only a 1:1 pattern transfer technique The mask fabrication in XRL is extremely difficult and the patterns on the mask are still written using EBL Nano- templating technique, offers a cheaper alternating for fabricating large area of nanostructures By combining commercially available template and electrodeposition technique, the fabrication of large area nanomagnets is possible . PATTERNED FERROMAGNETIC MESO AND NANO STRUCTURES GOOLAUP SARJOOSING NATIONAL UNIVERSITY OF SINGAPORE 2007 PATTERNED. NATIONAL UNIVERSITY OF SINGAPORE 2007 PATTERNED FERROMAGNETIC MESO AND NANO STRUCTURES GOOLAUP SARJOOSING (B. Eng(Hons.), NUS). (a) 120 nm, (b) 150 nm and (c) 180 nm, for nanowires with w = 185 nm and s = 35 nm. 73 Figure 4.13 MR response for 20 nm thick Ni 80 Fe 20 nanowire arrays (w = 185 nm and s = 35 nm) with fields