Electrical transport study of lateral superconductor ferromagnet hybrid devices

Electrical Transport Study of Lateral Superconductor – Ferromagnet Hybrid Devices SAIDUR RAHMAN BAKAUL (B. Sc (Hons.), BUET) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Department of Electrical and Computer Engineering National University of Singapore 2010 Acknowledgement This document is undoubtedly grateful to its supervisor professor Wu Yihong’s great guidance and supervision. In addition to that, as a creator of this thesis, I feel that not only he guided me through the difficult period of PhD study, but also he acted as my philosophical mentor during that time and helped to improve my thinking process. A number of occasions came when I really felt frustrated and exhausted because of the complexity and load of the PhD study and it was Prof. Wu who helped me to stand up and move from the dark towards the daylight. I express my heartiest gratitude and acknowledge all his supports and efforts behind this thesis. I wish to express sincere thanks to my co-supervisor Dr. Han Guchang, especially for his help in analyzing results, fruitful discussions and support with equipments. I am also deeply grateful to Dr. Li Kebin who guided me as a co- supervisor during my first year of PhD study. I would like to extend my warmest thanks to all the staffs of Data Storage Institute, specially An Lihua, Luo Ping, Dr. Guo Zaibing and Dr. Qiu Jinjun for their great support in performing experiments. At the same time, I am indebted to all my group mates and colleagues in Information Storage Materials Laboratory for their friendly behavior and collaboration, especially in adjusting the booked timeslots for equipments. Special thanks go to my fellow colleagues Dr. A. K. Debnath, Dr. Md. A. Hossain, Dr. M. Haque, Wu Baolei, Dr. Wang Haomin, Catherine, Dr. Maureen, Dr. Sunny, Dr. Randal, Dr. Bala and Dr. Takashi who not only extended their helping hands in research, but also provided company and entertainment. National University of Singapore Acknowledgement I owe most sincere gratitude to my lovely family in Bangladesh who were and always will be beside my side. The support from my father Sekandar Ali Bakaul, mother Ferdousi Bakaul and brother Mahmud has always been tremendous. I would like to express special gratitude to my beloved Mahjabeen for being patient during the difficult time period. Lastly, I am grateful to the almighty Allah for providing me the capability to complete this thesis. National University of Singapore ii Contents Acknowledgement . i Contents . iii Summary viii List of Figures . x List of Tables . xiv List of Symbols and Abbreviations xv Chapter . 1. Introduction . 1.1 Superconductivity and ferromagnetism . 1.2 Motivations and objectives 1.3 Organization of this thesis Chapter . 11 2. Literature Review 11 2.1 Basic concepts of superconductor 11 2.1.1 Hallmarks of superconductivity 12 2.1.2 Ginzburg- Landau theory and characteristic lengths in superconductor13 2.1.3 Type I and II superconductor 14 2.1.4 Frozen magnetic flux in superconductor 15 2.1.5 Cooper pairs 16 2.1.6 Different types of pairing in superconductors 16 2.1.7 Andreev reflection and crossed Andreev reflection . 17 2.2 Superconductor- normal metal junction 19 2.3 Superconductor-ferromagnet junction . 21 iii Contents 2.3.1 Junction with homogeneous magnetization . 21 2.3.2 Junction with inhomogeneous magnetization 22 2.3.2.1 Domain wall mediated enhancement of Tc . 22 2.3.2.2 Domain wall and inhomogeneous magnetization induced long range odd triplet superconductivity (LRTS) . 24 2.3.2.3 2.4 Domain wall induced crossed Andreev reflection (CAR) 26 Device structure for superconductor- ferromagnet junction . 28 2.4.1 Point contact 29 2.4.2 Vertical heterojunction 32 2.4.3 Vertical multilayer 33 2.4.4 Lateral junction . 34 2.5 BTK model . 35 2.5.1 Spin polarized BTK model . 36 2.5.2 Applicability of BTK model in diffusive junction . 37 2.6 Basic concepts of ferromagnetism . 37 2.6.1 Anisotropic magnetoresistance . 38 2.6.2 Magnetic states in FM under applied field . 40 2.6.3 Ferromagnetic disk 41 2.6.4 Magnetization reversal process in FM disk with vortex magnetization state . 42 2.7 Summary . 46 Chapter . 48 3. Experimental Methods . 48 3.1 Introduction 48 3.2 Fabrication techniques . 48 3.2.1 Substrate preparation and cleaning . 48 National University of Singapore iv Contents 3.2.2 3.2.2.1 Electron beam lithography (EBL) . 49 3.2.2.2 Optical lithography – Laser writer 53 3.2.3 3.3 Nanoscale device patterning . 49 Film deposition technique: high vacuum sputtering . 53 Measurement apparatus 54 3.3.1. Scanning Electron Microscopy (SEM) 54 3.3.2 Scanning probe microscopy (SPM) 55 3.3.2.1 Atomic force microscopy (AFM) 55 3.3.2.2 Magnetic force microscopy (MFM) 56 3.3.3 Low temperature electronic transport and magnetoresistance measurement system 57 3.3.4 3.4 Challenges in electronic measurement: electrostatic discharge (ESD) . 58 Summary . 60 Chapter . 61 4. Study of Superconductor-Normal Metal-Superconductor Junction 61 4.1 Introduction 61 4.2 Experimental details . 61 4.3 Results of electrical transport measurement 63 4.4 Discussions . 63 4.4.1 Blonder-Tinkham-Klapwijk (BTK) model for SC-NM interface . 64 4.5 Effect of magnetic field and bias current on dV/dI characteristics . 68 4.6 Summary . 70 Chapter . 71 5. Study of Superconductor-Rectangular Ferromagnet-Superconductor Junction . 71 5.1 Introduction . 71 5.2 Experimental details . 71 National University of Singapore v Contents 5.3 Film deposition technique 72 5.4 Resistance network analysis of the device 75 5.5 Maxwell and Sharvin resistance for the interface . 79 5.6 Proximity effect in superconductor-ferromagnet-superconductor device 82 5.7 Magnetic field dependence of the differential conductance spectra . 83 5.7.1 Key features observed . 83 5.7.2 BTK model for spin polarized case 84 5.7.3 Effect of spin dependent barrier strength on the transport characteristics of SC- FM interface 87 5.7.4 Finite bias conductance dip . 95 5.8 Effect of magnetic field on zero bias conductance (ZBC) 97 5.9 Summary . 103 Chapter . 104 6. Study of Lateral Superconductor-Ferromagnet Disk-Superconductor Junction . 104 6.1 Introduction 104 6.2 Experimental details . 105 6.3 Differential resistance characteristics under zero field 106 6.4 Temperature dependence of zero bias resistance 108 6.5 Effect of magnetic field on dV/dI characteristics 111 6.6 Magnetoresistance characteristics at low field regime . 116 6.6.1 Micromagnetic state of FM disk under external field 116 6.6.2 Current distribution in FM disk and its effect on magnetoresistance 117 6.6.3 Theoretical calculation of anisotropic magnetoresistance . 119 6.6.4 Probing magnetization reversal process in FM disk by low field magnetoresistance of SC-FM-SC device . 120 National University of Singapore vi Contents 6.6.5 Absence of long range triplet supercurrent in SC- FM disk- SC device . 129 6.7 Magnetoresistance at high field regime and effect of frozen flux 130 6.8 Summary 135 Chapter . 136 7. Conclusions and Recommendations 136 7.1 Conclusions 136 7.2 Recommendations for future work 138 References . 140 National University of Singapore vii Summary The interplay between superconductivity and inhomogeneous magnetization can give rise to many interesting and new physical phenomena such as crossed Andreev reflection (CAR) and long range odd triplet supercurrent (LRTS) which are still not well understood by the research community. One of the most promising platforms to study these kinds of phenomena is lateral devices consisting junctions made of superconductor (SC) and rectangularly patterned ferromagnet (FM). The underlying reason is that the rectangularly patterned FM can be a source of domain walls which provide inhomogeneous magnetization right at the SC–FM interface. One of the central questions motivating this thesis is whether such kind of devices can be used to detect existence of CAR and LRTS. We performed magnetoresistance (MR) measurements and found that these kinds of phenomena indeed play a role to determine the electronic transport characteristics of SC – inhomogeneous FM interface. In addition to this, we studied electrical transport characteristics of SC- FM disk- SC devices and the effect of FM disk’s magnetization reversal process on it. Here we show that the sensitivity of SC-FM interface’s transport characteristic to stray field can be used to probe magnetization reversal process in FM disk and detect places with inhomogeneous magnetization. By performing micromagnetic modeling of the FM disk and analyzing the MR characteristics of SC- FM disk- SC devices at below and above superconducting transition temperature (Tc) we have been able to point out the onset and annihilation field of the vortex core in FM disk. The formation of vortex state during magnetization reversal causes a dramatic reduction of the stray field distribution near the edge of the disk which produces a sudden rise in Andreev conductance of SCFM interface at a temperature below Tc. This results in a sudden drop in MR which viii Summary enables us to find out the vortex onset field from electronic transport data. Results from micromagnetic modeling and spin polarized BTK simulation are in good agreement with these experimental observations. National University of Singapore ix Chapter 7. Conclusions and Recommendations 7.1 Conclusions In this thesis we have studied the electronic transport properties of lateral superconductor- inhomogeneous ferromagnet- superconductor devices. This enables us to probe the interplay between inhomogeneous magnetization and superconductivity. Moreover, by virtue of zero resistance in superconductor and changing the electrode gap length, we measured magnetoresistance from different portions of ferromagnetic disk. By studying electronic transport property of SC- FM interface we have been able to probe magnetization reversal mechanism in FM disk. The key contributions and important results are summarized below. (1) One of the promising platforms to observe the domain wall effect on superconductivity is our concept of lateral SC- FM rectangle-SC device. We have found that the magnetoresistance studies of this kind of samples reveal the presence of crossed Andreev reflection and/ or long range triplet supercurrent. Although it has not been possible to determine whether both of these processes are present in this kind of device, we have provided a quantitative analysis of the contribution due to crossed Andreev reflection which matches well with the range of the amplitude of the observed signal. (2) We have incorporated spin dependent barrier strength in the spin polarized version of Blonder-Tinkham-Klapwijk model to analyze electronic transport characteristics of SC- FM interface. In the SC- FM rectangle (single domain) – SC devices, where crossed Andreev reflection and long range triplet Chapter Conclusions and Recommendations supercurrent were absent, this effect was clearly observed. The simulation results agree pretty well with the experimental observations. On the other hand, in devices with multidomain ferromagnet, this effect was hindered by the presence of CAR and/ or LRTS. (3) The study on SC- FM disk- SC devices did not show any sign of long range triplet supercurrent. However, the magnetorsistance studies show that the stray field from the FM disk during magnetization reversal process affects the Andreev reflection process at the SC- FM interface. (4) By changing the superconducting electrode gap length, the SC- FM disk- SC devices provide a unique way to probe the magnetization reversal process in FM disk. It was found that upon decreasing external magnetic field from saturated state, a buckling pattern forms which generates a large stray field near the edge of the disk. The onset of buckling pattern and its switching to the vortex state were studied with the help of Andreev conductance of the SCFM interface. The magnetoresistance above superconducting temperature was analyzed in light of current distribution and anisotropic magnetoresistance effect. Interestingly, unlike reports found in literature so far, it was found that the onset of vortex state may lead to a jump in MR when applied field is in same direction as current flow. This theoretical prediction is well supported by our results. (5) Our SC- FM- SC devices were found to exhibit two strikingly different differential resistance states at zero field. The key difference between these National University of Singapore 137 Chapter Conclusions and Recommendations states is that the critical current (Ic) is dramatically lower for one state (state 2) than the other (state 1). Moreover, state exhibits a higher zero bias resistance than state 1. This has been explained as an effect of magnetic field from the frozen flux in superconductor on the Andreev reflection process at the SCFM interface. The MR curves at different dc current biases are nonhysteretic and symmetric with respect to zero field axis when the device is at state 1. In contrary to this, MR curves at state are asymmetric and show a pronounced hysteresis at high bias current. These effects indicate a coupling between applied field and frozen flux, which affects the transport characteristics of the SC- FM interface 7.2 Recommendations for future work Several interesting and novel phenomena have been found in this thesis and there exist a lot of possible branches which can be explored based on these findings. Some of such recommendations are provided below. (1) Our SC- multi-domain FM rectangle- SC devices showed sign of crossed Andreev reflection. Theoretically, the electronic signal due to such effect depends on number of domain walls present at the interface. Hence the ferromagnetic pattern can be engineered in such a way that the number of domain walls can be controllably varied and their effect on Andreev conductance can be explored. (2) The average size of the Cooper pair can be of the order of hundred nanometers. Therefore, if FM-NM-FM structure is put in between two lateral superconducting electrodes, it may act like a superconducting link. The National University of Singapore 138 Chapter Conclusions and Recommendations thickness of the FM layers can be engineered to control the coercivity and the systematic magnetoresistance study of this type of devices can differentiate between spin singlet or triplet supercurrent. Figure 7.1 Schematic of proposed superconductor-FM/ NM/ FM-superconductor device (3) Lateral junctions with superconductor and antiferromagnetic materials can be explored. As the antiferromagnetic (AF) materials contain oppositely magnetized layers, it might be possible for traditional singlet cooper pair to travel a large distance in this type of materials. This field is quite new and few theoretical and experimental works have been reported so far. Among the theoretical works, bound state generation at SC- AF interface [225], penetration of Cooper pair through SC- AF- SC device and current- phase relationship [226] have been studied. 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National University of Singapore 151 [...]... inside a window of bias current 134 Figure 7.1 Schematic of proposed superconductor- FM/ NM/ FM -superconductor device 139 Figure 7.2 Schematic of proposed superconductor- antiferromagnet -superconductor device 139 National University of Singapore xiii List of Tables Table 5 1 Resistance contribution from different parts of lateral SC- FM-SC devices 78 Table... effect of DW on transport characteristics of SC-FM junction Beside CAR phenomenon, another interesting issue in studying SC- FM junction is the effect of stray field from FM on the SC-FM interface’s transport characteristics Generally, researchers always try to get rid of such effect in order to study the pure interaction between superconductivity and ferromagnetism However, the sensitivity of SC-FM... understanding the results of SC-FM devices Chapter 4 will discuss about the electrical transport property of lateral SC-NM-SC devices The experimental results are analyzed by well known Blonder-Tinkham-Klapwijk model [45] A comparison with the simulation result will also be provided In chapter 5, the experimental results on lateral SC-FM rectangle-SC devices will be presented and analyzed The ferromagnetic portion... List of Figures Figure 4.1 (a) SEM image of lateral SC-NM-SC device (b) R-T curve of the lateral SC-NM-SC device 62 Figure 4.2 (a) Typical dI/dV curve observed in lateral SC-NM-SC device at 1.4 K The conductance is normalized to the conductance of the device at high bias voltage (b) Simulation result following the model of Ref.130 65 Figure 4.3 Magnetic field dependence of dV/dI... To study the effect of domain walls on lateral SC- FM interface and perform magnetoresistance experiment to detect presence of DW assisted CAR 2 To understand the effect of inhomogeneous magnetization during magnetization reversal process in FM disk on the electronic transport property of SC- FM interface 1.3 Organization of this thesis In chapter 2, a review on the past reports from other groups’ study. .. past reports from other groups’ study on SC- FM hybrid devices is provided In addition to that, a brief theoretical background on the basics of superconductivity and ferromagnetism, which are related to this thesis, will be National University of Singapore 8 Chapter 1 Introduction provided This will be followed by an overview on electrical transport study of SC-NM junction and SC- inhomogeneous FM junction... and magnetostatic energy, lies in the range of the exchange length of the material [21] The vortex core magnetization may point upward or downward and the sense of the rotation of the chirality of magnetization at other places of the disk may be clockwise or anti-clockwise The combination of these two degrees of freedom can be utilized to store two bits of data at the same time In order to utilize... shifted vertically for clarity 68 Figure 4.4 Magnetic field dependence of dV/dI at different DC bias for SC-NM-SC sample The bottom figure shows the bias dependent resistance of the sample 69 Figure 5.1 Schematic image of lateral SC- FM-SC device 72 Figure 5.2 Schematic image of the interfaces of lateral SC- FM-SC devices The sample with 90 nm thick NiFe is predicted to have continuity near... 25 Figure 2.4 (a) Schematic of spear- anvil technique (b) SEM image of a lithographically fabricated MCBJ device for Gold [After E Scheer, 2001, Ref 107] 31 Figure 2.5 Schematics of lateral SC- FM- SC devices studied to detect penetration of triplet supercurrent through half metal in Ref 93 34 Figure 2.6 (a) Device structure for measurement of MR of FM disk by normal electrode [After... Moreover, as a part of this thesis focuses on the effect of magnetization reversal process in disk shaped FM particle on SC-FM interface’s magnetotransport property, an overview on anisotropic magneto resistance (AMR) and micromagnetic simulation in FM disk will be provided The literature review chapter also covers history of different types of geometry used to study SC- FM hybrid devices Chapter 3 provides . Electrical Transport Study of Lateral Superconductor – Ferromagnet Hybrid Devices SAIDUR RAHMAN BAKAUL (B. Sc (Hons.), BUET) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF. community. One of the most promising platforms to study these kinds of phenomena is lateral devices consisting junctions made of superconductor (SC) and rectangularly patterned ferromagnet (FM) a window of bias current. 134 Figure 7.1 Schematic of proposed superconductor- FM/ NM/ FM -superconductor device 139 Figure 7.2 Schematic of proposed superconductor- antiferromagnet-superconductor