Magnetotransport and magnetooptical properties of ferromagnetic nanostructures

MAGNETOTRANSPORT AND MAGNETOOPTICAL PROPERTIES OF FERROMAGNETIC NANOSTRUCTURES SHIKHA JAIN NATIONAL UNIVERSITY OF SINGAPORE 2009 MAGNETOTRANSPORT AND MAGNETOOPTICAL PROPERTIES OF FERROMAGNETIC NANOSTRUCTURES SHIKHA JAIN (M. Eng, NATIONAL UNIVERSITY OF SINGAPORE) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgements I feel deeply indebted to several people who have contributed in different ways towards the work accomplished in this thesis. First and foremost, I would like to express my sincerest gratitude towards my supervisor, Assoc. Prof. Adekunle Adeyeye for giving me the opportunity to work on this topic. His constant motivation, support, guidance and encouragement in all aspects varying from research to personal life, have made my candidature a truly enriching experience. I would also like to express my appreciation towards ISML lab officers, Ms. Loh Fong Leong and Mr. Alaric Wong, and Ms. Ah Lian Kiat from MOS Device Laboratory, for their help and support during my candidature. During the course of my PhD, I have had the privilege of working closely with the students and staff in Assoc. Prof. Adekunle’s group, which has benefited me immensely. I would like to thank Dr. Navab Singh for providing the templates of ferromagnetic nanodots patterned using deep ultra violet (DUV) lithography. I would also like to acknowledge Dr. Wang Chenchen and Dr. Ren Yang for their help in setting up the magnetooptical kerr effect (MOKE) magnetometry. I would also like to thank Dr. Goolaup, Dr. Sreenivasan, Kaushik and Shyam for all the enjoyable moments we have shared in ISML. I would like to thank my entire family in India for all their support, faith and advice during my stay in Singapore. I especially owe this thesis to my late father-inlaw who always believed in me. Finally, but most importantly, I would like to mention my pillar of strength; my husband Debashish. For proof reading this thesis and for your unwavering help, emotional support and understanding in all matters; in the lab and at home – thank you so much! i Table of Contents Acknowledgements i Table of Contents ii Summary viii List of Tables x List of Figures xi List of Symbols and Abbreviations Statement of Originality xviii xx Chapter Introduction 1.1 Background 1.2 Motivation 1.3 Focus of Thesis 1.4 Organization of Thesis References Chapter Theoretical Background 10 2.1 Introduction 10 2.2 Magnetization reversal in ferromagnetic dots 10 2.2.1 Vortex state in submicron dots 11 2.2.2 Topological mapping of vortex magnetization 13 2.2.3 The ‘rigid’ vortex model 14 2.3 Magnetization reversal in ferromagnetic rings 17 ii Table of Contents 2.3.1 Magnetization states 2.4 Spin Dependent Transport Phenomenon 18 21 2.4.1 Anisotropic Magnetoresistance Effect 21 2.4.2 Giant Magnetoresistance Effect 22 2.5 Coupling Mechanism in Multilayer Films 25 2.5.1 Interlayer Exchange Coupling 25 2.5.2 Pin Hole Coupling 27 2.5.3 Néel Coupling 27 2.6 Exchange bias 28 2.6.1 Basic phenomena of exchange bias 28 2.6.1 Exchange bias in ferromagnetic nanostructures 30 2.7 Summary 32 References 33 Chapter Experimental Techniques 41 3.1 Introduction 41 3.2 Fabrication Processes 41 3.2.1 Ultraviolet Photolithography 41 3.2.2 KrF deep ultra violet lithography 43 3.2.3 Electron beam lithography 46 3.2.4 Electron beam evaporation 49 3.2.5 Lift off and wire bonding 51 3.3 Characterization Techniques 51 3.3.1 Scanning electron microscope 51 3.3.2 Room temperature Magnetotransport measurement 54 iii Table of Contents 3.3.3 Low temperature Magnetotransport measurement 55 3.3.4 Magnetooptical kerr effect 57 References 62 Chapter Vortex chirality control and configurational 63 anisotropy in permalloy nanomagnets 4.1 Introduction 63 4.2 Motivation 63 4.3 Experimental Details 64 4.4 Modified ‘rigid’ vortex model 65 4.4.1 Calculation of Interaction Energy (Wint) 65 4.4.2 Calculation of Annihilation field (HA) and Nucleation field (HN) 73 4.5 Variation of HN and HA as a function of separation s for tNiFe = 80 nm 75 4.6 Variation of HN and HA with Ni80Fe20 thickness 80 4.7 Summary 83 References 84 Chapter Magnetoresistance behaviour of Mesoscopic rings 86 5.1 Introduction 86 5.2 Motivation 86 5.3 Theoretical analysis 87 5.3.1 Two-point probe configuration 88 5.3.2 OOMMF simulations for M-H and R-H curves 91 5.3.3 Four-point probe configuration 97 iv Table of Contents 5.4 Experimental validation 98 5.4.1 Rectangular ring 101 5.4.2 Elliptical ring 104 5.5 Summary 106 References 108 Chapter Non-local probe technique for mapping the spin 110 states in multilayer rings 6.1 Introduction 110 6.2 Motivation 110 6.3 GMR response for rectangular and elliptical rings 111 6.3.1 Basic concept of ring wire hybrid structure 111 6.3.2 Device fabrication 113 6.3.3 Experimental validation for Elliptical Ring 114 6.3.4 GMR response for Rectangular Ring 117 6.3.5 Effect of ring width 120 6.4 Synchronous transport measurement technique 122 6.4.1 Basic concept 122 6.4.2 Device fabrication 123 6.4.3 Effect of ring shape 124 6.4.4 Low field GMR responses 128 6.5 Low temperature GMR behaviour 131 6.5.1 T = 250 K 131 6.5.2 T = 150 K 134 6.5.3 T = 50 K 136 v Table of Contents 6.5.4 Temperature dependence of GMR ratio and Δδ 138 6.6 Summary 140 References 142 Chapter Magnetostatic coupling in PSV elliptical rings 144 7.1 Introduction 144 7.2 Motivation 144 7.3 Experimental methods 145 7.4 Symmetrically and asymmetrically coupled elliptical rings 146 7.5 Minor loop GMR responses 153 7.5.1 Symmetrically coupled elliptical ring 153 7.5.2 Asymmetrically coupled elliptical ring 156 7.6 Field orientation dependence 158 7.7 Stability and reproducibility of switching fields 161 7.8 Low Temperature MR Behavior of elliptical rings with coupled magnetic 164 elements 7.9 Summary 166 References 167 Chapter Exchange biased nanorings 169 8.1 Introduction 169 8.2 Motivation 169 8.3 Experimental details 170 8.4 GMR responses for unbiased triangular ring 171 8.4.1 Field orientation dependence 171 vi Table of Contents 8.4.2 Low field GMR responses 8.5 GMR responses of exchange biased triangular ring 176 179 8.5.1 Effect of temperature 180 8.5.2 Effect of cooling field direction 189 8.6 Summary 192 References 194 Chapter Conclusion 196 9.1 Overview 196 9.2 Summary of results 197 9.3 Future Work 200 List of Publications 202 vii Summary Ferromagnetic (FM) nanostructures have attracted intense research interest over the recent years, due to their potential in practical applications such as magnetic random access memories (MRAM), magnetic sensors and logic devices. In this thesis, a systematic investigation of magnetooptical and magnetotransport properties of lithographically defined FM nanostructures is presented. Firstly, a systematic control of vortex chiralities in specifically arranged FM nanodots is achieved by varying the in-plane magnetic field and lattice configurations. This can be attributed to the induced configurational anisotropy in the dot geometries which favour specific vortex chirality combinations as a function of applied field. Further, the effect of the dot thickness on the inter-dot spacing for fixed dot diameter was also studied. Secondly, a resistor network model has been developed to characterize the magnetoresistance (MR) behaviour of individual ring structures using both two-point and four-point probe configuration. It has been shown that when the contact probes are patterned directly on the ring structure, the complex parallel configurations of various segments of the ring can be simplified into a series of serial resistors comprising both constant and field dependent variable components. Experimental validation of the model was achieved by investigating the magnetization reversal process in individual rectangular and elliptical ring structures using magnetotransport technique. A good agreement between theoretical and experimental results was obtained. Thirdly, this thesis addresses various issues of fabricating contact probes directly on the ring, such as strong dependence of MR on contact geometry and shortcircuiting effect. Therefore, a universal non-local technique for probing the MR viii Chapter VIII Exchange biased Nanorings Fig. 8.10 GMR response for exchange biased triangular ring as a function of temperature when the cooling field of +5 kOe (shown as solid curve) is applied [20]. 190 Chapter VIII Exchange biased Nanorings On comparing the bias fields obtained for the first and the last switching fields, it was observed that for the first switching (H1), the shift HE1 is -90 Oe, while for the third switching, the shift HE3 is -246 Oe. For -HFC, the bias shifts obtained were HEB(OV) = 269 Oe and HEB(VO) = 300 Oe, respectively. It is thus evident that the magnitude of shift for +HFC is significantly lower than that for -HFC. This observation can be attributed to the contribution from FM domains, which substantially influence the AFM-FM interface coupling even at temperatures where effects from thermal fluctuations can be conveniently neglected. This may be attributed to the fact that during the field cooling procedure, AFM spins lie along the positive direction. Upon sweeping the field initially towards negative direction at a finite temperature, the magnetization of the FM layer re-orients itself along the external field direction, thus reducing the AFM-FM interface coupling and consequently reducing the bias shift. On further decreasing the temperature to T = 50 K, the GMR response shows a more complex reversal mechanism (Fig. 8.10(c)). The ring undergoes reversal from forward onion to reverse onion state via three intermediate metastable states. The magnitude of bias is again smaller when compared to the bias obtained from negative HFC. Considering the last switching field of H4(↑) and H4(↓), the bias is only -109 Oe as compared to HEB(VO) = 293 Oe for -HFC. As the applied magnetic field is decreased from negative saturation (though field cooling was done in positive saturation) and the FM layer relaxes from its saturated state forming an onion state, parts of the magnetization are twisted along the field direction and the other parts in pinning direction. During reversal, randomly oriented domains in the AFM layer influence the FM layer by forming domain walls at the straight edges of the FM layer. This energetically favours multi-step reversal behaviour due to shape anisotropy. Since several small energy barriers exist due to domain walls in the FM layer, magnetization 191 Chapter VIII Exchange biased Nanorings reversal occurs via various switching steps. Fig. 8.10(d) shows that the reversal process at T = 20 K exhibits four intermediate states. Since the AFM domains stabilize almost completely at such low temperatures, exchange bias fields obtained from the two curves are almost identical. However, the dominating effect of external magnetic field over cooling field can still be observed in the form of multi-step switching behaviour. 8.6 Summary In summary, the GMR response of individual ferromagnetic and exchange biased multilayer triangular ring has been investigated in detail. For an unbiased multilayer ring, when the magnetic field is applied at an angle of θ = nπ , n=0,1,2 with respect to any edge of the triangle, the Co layer in the ring undergoes a transition directly from forward onion state to reverse onion state. However, if the magnetic field is applied in the range nπ (n + 1)π , n=0,1,2 , there is formation of an [...]... widely used to probe the MR responses of various magnetic structures It is extremely sensitive to the changes in resistivity of the magnetic layer with varying magnetic field and thus forms the basis for AMR and GMR responses 1.3 Focus of thesis In this thesis, a comprehensive study of the magnetization states in ferromagnetic (FM) nanostructures using magnetooptical and magnetotransport measurement 4 Chapter... together with deposition and lift off procedure is widely used for fabrication of magnetic structures Since it is a serial 3 Chapter I Introduction process of patterning, it forms an excellent tool for fabricating individual nanostructures The study of nanomagnets resides in the ability to characterize the nanostructures and extract quantitative information about the magnetic properties and the reversal processes... complete removal of the energetically unfavourable vortex core and utilizing the geometry of ring elements [29] For the successful implementation of such magnetic nanostructures in practical applications, there is a need for advancement in nanofabrication [11] and nanocharacterization tools for the exploration of their magnetic properties Aided with the increase in the processing power of computers, the... 150 K, and (b) 4 K, are shown The corresponding responses for the ring with asymmetrically placed magnetic elements at 150 K and 4 K are shown in (c) and (d), respectively [20] Fig 8.1 SEM micrograph of a triangular ring of width 300 nm and edge 171 3 µm Fig 8.2 GMR response of the triangular ring when the applied field is 173 (a) parallel to edge ‘c’, (b) makes an angle of 15° with edge ‘c’, and (c)... “Direct comparison of magnetization reversal process in rectangular and elliptical ring nanomagnets”, S Jain, C C Wang, and A O Adeyeye, Journal of Applied Physics 103, 07D904 (2008) • Development of a novel characterization technique for probing the magnetization reversal process in individual FM nanostructures independent of contact geometries [4] "Magnetoresistance behaviour of ferromagnetic nanorings... of Applied Physics 104, 103914 (2008) [9] “Magnetoresistance behavior of Elliptical Ring Nanomagnets in Close Proximity with Magnetic Elements”, S Jain and A O Adeyeye, Journal of Applied Physics 105, 07E904 (2009) • An extensive investigation of the effect of domain wall pinning on the vortex formation in triangular ring for both FM and antiferromagnetic (AFM) multilayer structures as a function of. .. thickness t and radius R c is the relative radius of the vortex core Fig 2.2 Schematic diagrams of (a) ‘c’-state and (b) ‘s’-state 12 configurations Fig 2.3 A schematic presentation of (a) a vortex state and (b) an onion 18 state Fig 2.4 Schematic illustration of the magnetic states in rectangular ring, 20 (a) onion state, (b) vortex state, and (c) horseshoe state Fig 2.5 Schematic illustration of the AMR... system Fig 4.3 (a) Variation of the interaction energy Wint (2 dot) as a function of 69 the inter-dot spacing s for θ = 0°, 45° and 90°, and tNiFe = 80nm (b-d) Simulated spin states of two dot geometry for s = 55 nm, t = 80 nm as a function of field orientation (θ) Fig 4.4 Variation of the interaction energy Wint as a function of the inter- 71 dot spacing s for (a) three dot and (b) four dot system along... 140 rectangular and circular ring structures Fig 7.1 (a) Schematic drawing of an elliptical ring in ring-wire hybrid 146 configuration SEM micrographs of elliptical rings (b) with two elliptical elements on top and bottom of the ring, and (c) with one of the two elements placed at an angle of 45° with respect to the ring Fig 7.2 GMR response for elliptical ring (a) without any coupling, and 147 (b) with... H Fecher, W Jark, and G Schonhense, J Magn Magn Mater 233, 14 (2001) [41] C A Ross, F J Castano, D Morecroft, W Jung, H I Smith, T A Moore, T J Hayward, J A C Bland, T J Bromwich, and A K Petford-Long, J Appl Phys 99, 08S501 (2006) [42] W Jung, F J Castano, and C A Ross, Appl Phys Lett 91, 152508 (2007) [43] T J Hayward, J Llandro, R B Balsod, J A C Bland, D Morecroft, F J Castano, and C A Ross, Phys . MAGNETOTRANSPORT AND MAGNETOOPTICAL PROPERTIES OF FERROMAGNETIC NANOSTRUCTURES SHIKHA JAIN NATIONAL UNIVERSITY OF SINGAPORE 2009 MAGNETOTRANSPORT AND MAGNETOOPTICAL. such as magnetic random access memories (MRAM), magnetic sensors and logic devices. In this thesis, a systematic investigation of magnetooptical and magnetotransport properties of lithographically. husband Debashish. For proof reading this thesis and for your unwavering help, emotional support and understanding in all matters; in the lab and at home – thank you so much! ii Table of Contents