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«Title of Dissertation: MULTI-CHANNEL SCANNING SQUID MICROSCOPY Su-Young Lee, Doctor of Philosophy, 2004 Dissertation directed by: Professor Frederick ...»

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ABSTRACT

Title of Dissertation: MULTI-CHANNEL SCANNING

SQUID MICROSCOPY

Su-Young Lee, Doctor of Philosophy, 2004

Dissertation directed by: Professor Frederick C. Wellstood

Department of Physics

I designed, fabricated, assembled, and tested an 8-channel high-Tc scanning

SQUID system. I started by modifying an existing single-channel 77 K high-Tc scanning SQUID microscope into a multi-channel system with the goal of reducing the scanning time and improving the spatial resolution by increasing the signal-to-noise ratio S/N. I modified the window assembly, SQUID chip assembly, cold-finger, and vacuum connector. The main concerns for the multi-channel system design were to reduce interaction between channels, to optimize the use of the inside space of the dewar for more than 50 shielded wires, and to achieve good spatial resolution.

In the completed system, I obtained the transfer function and the dynamic range (Φmax ~ 11Φ0) for each SQUID. At 1kHz, the slew rate is about 3000 Φ0/s. I also found that the white noise level varies from 5 µΦ0 /Hz1/2 to 20 µΦ0 /Hz1/2 depending on the SQUID. A new data acquisition program was written that triggered on position and collects data from up to eight SQUIDs. To generate a single image from the multi- channel system, I calibrated the tilt of the xy-stage and z-stage manually, rearranged the scanned data by cutting overlapping parts, and determined the applied field by multiplying by the mutual inductance matrix. I found that I could reduce scanning time and improve the image quality by doing so.

In addition, I have analyzed and observed the effect of position noise on magnetic field images and used these results to find the position noise in my scanning SQUID microscope. My analysis reveals the relationship between spatial resolution and position noise and that my system was dominated by position noise under typical operating conditions. I found that the smaller the sensor-sample separation, the greater the effect of position noise is on the total effective magnetic field noise and on spatial resolution. By averaging several scans, I found that I could reduce position noise and that the spatial resolution can be improved somewhat.

Using a current injection technique with an x-SQUID, and (i) subtracting high- frequency data from low-frequency data, or (ii) taking the derivative of magnetic field Bx with respect to x, I show that I can find defects in superconducting MRI wires.

MULTI-CHANNEL SCANNING SQUID

MICROSCOPY

by Su-Young Lee Thesis submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree

–  –  –

Advisory Committee:

Professor Frederick C. Wellstood, Chair/Advisor Professor James R. Anderson Professor Richard L. Greene Professor Christopher J. Lobb Professor Colin Philips ©Copyright by Su-Young Lee 2004 DEDICATION

–  –  –

This dissertation would not have been possible without the help and patience of many people.

I wish to thank my advisor, Professor Frederick C. Wellstood for support, guidance and patience during my research. Not only his dedication and passion to physics but also his advice for my private life always inspired me. When I had to stop my project and leave to Korea suddenly, he showed me patience and gave me lots of advices for my life. In the circumstance of his extremely busy schedule as an associate chair for undergraduate education, he always cared about students, opened his door, listened and answered my silly questions.

I would like to thank the committee, Professors James R. Anderson, Richard L.

Greene, Christopher J. Lobb, and Colins Philips for being my committee and spending time to review my thesis.

I thank the Professors in the CSR, Christopher J. Lobb for generous conversation and not hesitating over my troublesome requests, Professor Richard L.

Greene for instructing the CSR seminar and stimulating discussions, Professor Richard Webb for allowing for me to use his equipment, and Professor Steven M. Anlage for his lectures on Superconductivity.

I would like to thank Professor In-Sang Yang at Ewha Womans University for

–  –  –

physics and constant attention to me. I also thank Professor Zheong G. Khim at Seoul National University for his support and warm conversations while I worked at SNU.

I thank Dr. Sojiphong Chatraphorn for instructing me in everything from fabricating SQUIDs to building scanning SQUID microscopes. Even after he left, I learned a lot from his neatly written records. I also thank Dr. Erin F. Fleet for showing me how to operate the cryo-cooled SQUID microscope. I learned a lot from both Erin and Guy during my 1st year of lab life. I also want to thank Jan Gaudesrad for being a good lab partner and for his positive thinking and diligence.

I would like to thank Dr. John Matthews for taking care of my multi-channel SQUID microscope in my absence, correcting my poor English in this thesis, and his knowledge of the magnetic inverse technique. I also want to thank Dr. Roberto C.





Ramos for being so nice and always ready to help me and everyone else in the lab.

I am indebted to my colleagues in the subbasement. I thank Dr. Andrew Berkeley for his friendship and advice on physics and electronics, even though his smartness depressed me sometimes. I am thankful to David Tobias for discussions about my experiment, offering warm chats and sneaking-up on me so I would not feel lonely in my corner of the lab. I am grateful to Hanhee Paik for her friendship and helping me understand SETs and hysteretic SQUIDs. I would like to thank Huizhong Xu for showing his enthusiasm and knowledge toward physics and helping me distinguish between Korean green tea and Chinese green tea. I want to thank Sudeep

–  –  –

me, and making the lab-atmosphere warm by just his existence. I thank Gus Vlahacos for good conversation and work on building the next generation of SQUID microscopes.

I thank Felipe Busko for helping to order the new translation stage from Newport. I would like to thank Matt Kenyon, Anders Gilbertson, Mark Gubrud, Soun Pil Kwon, and Vijay Viswanathan for many good conversations and helpful advice. I wish they all obtain great results for their experiments.

I am thankful to all my colleagues in the Center for Superconductivity Research.

I thank Dr. Matthew Sullivan and Dr. Sheng-Chiang Lee for providing scrupulous information about writing a thesis and encouraging my writing. I thank Dr. Bin Ming for initial instructions for YBCO film fabrication.

I would like to thank Doug Bensen and Brian Straughn for not only their technical help but also for many warm conversations. I want to thank the staff in CSR, Brian Barnaby, Belta Pollard, Grace Sewlall, and Cleopatra White for being nice and taking care of all the paperwork.

I would like to thank Jane Hessing in the Physics Department’s Office of Student Services for warm conversations and for taking care of all the paperwork from my starting to finishing as a graduate student. I thank Jesse Anderson in Z-order and Al N. Godinez in Physics Receiving for giving me warm smiles and chatting when I ask to fill the liquid nitrogen dewar.

–  –  –

position noise paper, and encouraging my work. I want to thank Jeonggoo Kim for his constant help and advice while I was struggling with making YBCO thin films. I thank Antonio Orozco and Harsh for many helpful comments.

I would like to thank Robin Cantor, president of Star cryo-electronics, Inc., for his quick replies and advice when I set up my SQUID system with his SQUID electronics. I thank Marilyn Kushner at U.C. Berkeley Microlab for making photolithographic masks for my SQUID chips.

I want to thank my many classmates at the University of Maryland including Kyu-yong Lee for his sincere and steady effort and Jae-Woong Hyun for friendship and concern for me. I also want to thank Jong-Won Kim, Seok-Hwan Chung, Winson, and Ynggwei for their friendship and many discussions.

I would like to thank all the members in the Superconductivity group at SNU in Korea who helped me when I visited the group for 1 year. I want to thank Dr. Jaewan Hong for showing diligence and enthusiasm on his SPM system, Seung Hyun Moon and Yonuk Chong for sharing their knowledge and interest towards physics, Ungwhan Pi for friendship and helping my soldering, Seunghee Jeon for helping me with lots of problems I had, and Burm Baek for helping me understand SQUIDs better. I am grateful to Su-Youn Lee, Jonghun Kim, Joonsung Lee, Jongho Baek, Seongjin Yun, Soohyon Phark, Bongwoo Ryu, Insu Jeon, YongSeung Kim for helping me have a good time.

–  –  –

roommate, my relative in the United States, a singer in my wedding, and an aunt for my baby. I don’t know how to fully appreciate her, but I can say my life in this foreign country was happy because of her. I want to thank Young-Chan Kim for being a model physicist and humanist. I thank him for helping me while living in the U.S., answering lots of my questions, and giving theoretical comments.

Especially, I would like to thank my parents in law in Korea for supporting and encouraging me to finish my Ph. D. I am grateful to my loving daughter Woo-Jin for being my reason to live. I thank her for her health and being a nice girl even though I could not spend as much time with her as I wanted to. I want to thank Woojin’s nanny for taking care of my baby with great love.

I would like to thank my special friend and husband, Jae-Oh Cheong for his great support and patience, for not hesitating to live alone in Korea for more than 1 year, and for being my mental center. His encouragement leads me on.

I want to thank my brothers Seok Lee for his prayers and sacrifice and Hyunsung Lee and his family for being strong supporters and showing their enthusiasm as doctors, and my sweet sister Seojin for showing faith toward me and giving me warm memories of my childhood. Finally, I thank my father Hong-ha Lee, and my mother Bok-young Seo for showing belief in their daughter and endless love and support for 32 years. I could not be here without their sacrifice and support. The words “thank you”

–  –  –

LIST OF FIGURES

LIST OF TABLES

Chapter 1. Introduction

1.1 A Brief History of SQUIDs

1.2 History of scanning SQUID microscopy

1.3 Motivation

1.4 Organization of the thesis

Chapter 2. dc SQUID Overview

2.1 Theory of Josephson junctions

2.1.1 Equations of motion for a Josephson junction

2.1.2 Josephson junction with βc á1 (Non-hysteretic I-V curve)

2.1.3 Josephson junction with βc à1 (Hysteretic I-V curve)

2.2 dc SQUID

2.3 Noise in the dc SQUID

2.3.1 1/f noise in low-Tc SQUID

2.3.2 White noise in low-Tc SQUID

2.3.3 Noise in high-Tc SQUIDs

2.4 SQUID applications

2.4.1 Non-Destructive Evaluation

2.4.2 Biomagnetic studies

2.4.3 Geophysics

2.5 Conclusion

Chapter 3. Design of a Multi-channel High-Tc SQUID Chip

3.1 High-Tc SQUID chip

3.2 Bare SQUID vs. coupled SQUID

viii

3.3 SQUID orientation

3.4 Crosstalk between SQUIDs and its calibration

3.5 Other concerns for design

3.6 Conclusion

Chapter 4. Chip Fabrication and Testing

4.1 YBCO thin film fabrication

4.1.1 Deposition of YBCO and Au thin films using Pulsed Laser Deposition.... 54 4.1.2 Testing YBCO films

4.2 Photolithography

4.3 Measurements of 8 SQUIDs on a Chip

4.3.1 I-V characteristics

4.3.2 The parameters of SQUIDs from the I-V curves

4.3.3 Evaluation of multi-channel SQUID chip

4.4 Conclusion

Chapter 5. Scanning SQUID Microscope Design and Construction

5.1 The old high-Tc single-channel SQUID microscope

5.1.1 Vacuum window

5.1.2 SQUID chip assembly

5.1.3 Cold-finger

5.1.4 Window manipulator

5.1.5 Wiring and dewar

5.2 Modifications for multi-channel SQUID microscope

5.3 Vacuum window

5.3.1 Window and nose cone (window assembly)

5.3.2 Calculation of bending of thin window

5.3.3 Assembling the nose cone and vacuum window

5.4 SQUID chip assembly

5.4.1 SQUID chip preparation

ix 5.4.2 SQUID chip assembly

5.5 Cold-finger and connector box

5.5.1 Design of cold-finger

5.5.2 Design of connector box

5.5.3 Assembly including wiring

5.6 Leak check

5.7 Conclusion

Chapter 6. Multi-Channel SQUID Electronics and Data Acquisition

6.1 Overall measurement scheme for 8-channel SQUID system

6.2 SQUID Electronics and its performance

6.2.1 Flux locked loop SQUID electronics

6.2.2 The transfer function Mf /Rf

6.2.3 Dynamic range

6.2.4 Slew rate

6.2.5 Flux noise measurement

6.2.6 Crosstalk

6.3 New xy translation stage

6.4 Data Acquisition program

6.4.1 Software for controlling the multi-channel system and collecting data.... 133 6.4.2 Time trigger vs. position triggering

6.5 Demonstration of multi-channel system

6.5.1 Height alignment

6.5.2 φ-calibration

6.5.3 Crosstalk correction

6.5.4 Another test scan

6.6 Conclusion

Chapter 7. The Effect of Position Noise on Imaging

7.1 Introduction

x

7.2 Theory of position noise

7.2.1 Non-accumulated position noise

7.2.2 Accumulated position noise

7.3 Measurement of position noise

7.3.1 Position noise results

7.3.2 Position noise criteria

7.4 How to reduce position noise

7.5 Applications of position noise results

7.6 Conclusion

Chapter 8. The Effect of Position Noise on Spatial Resolution

8.1 Introduction

8.2 Magnetic inverse technique

8.3 Analytical relation between z and s including position noise



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