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Magneto-Transport and Optical Control of Magnetization in Organic Systems: from Polymers to Molecule-Based Magnets

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Magneto-Transport and Optical Control of Magnetization in Organic Systems: from Polymers to Molecule-Based Magnets MAGNETO-TRANSPORT AND OPTICAL CONTROL OF MAGNETIZATION IN ORGANIC SYSTEMS: FROM POLYMERS TO MOLECULE-BASED MAGNETS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Kadriye Deniz Bozdag, M.S. Graduate Program in Physics The Ohio State University 2009 Dissertation Committee: Arthur J. Epstein, Adviser Ezekiel Johnston-Halperin Julia S. Meyer Thomas Humanic c Copyright by Kadriye Deniz Bozdag 2009 ABSTRACT Organic systems can be synthesized to have various impressive properties such as room temperature magnetism, electrical conductivity as high as conventional metals and magnetic field dependent transport. In this dissertation, we report comprehensive experimental studies in two different classes of organic systems, V-Cr Prussian blue molecule-based magnets and polyaniline nanofiber networks. The first system, V-Cr Prussian blue magnets, belongs to a family of cyano-bridged bi-metallic compounds which display a broad range of interesting photoinduced mag- netic properties. A notable example for optically controllable molecule-based magnets is Co-Fe Prussian blue magnet (Tc s 12 K), which exhibits light-induced changes in between magnetic states together with glassy behavior. In this dissertation, the first reports of reversible photoinduced magnetic phenomena in V-Cr Prussian blue analogs and the analysis of its AC and DC magnetization behavior are presented. Optical excitation of V-Cr Prussian blue, one of the few room temperature molecule-based magnets, with UV light (λ = 350 nm) suppresses magnetization, whereas subsequent excitation with green light (λ = 514 nm) increases magnetization. The partial recov- ery effect of green light is observed only when the sample is previously UV-irradiated. Moreover the photoinduced state has a long lifetime at low temperatures (τ > 106 s at T = 10 K) indicating that V-Cr Prussian blue reaches a hidden metastable state upon illumination with UV light. The effects of optical excitation are maintained up ii to 200 K and completely erased when the sample is warmed above 250 K. Results of detailed magnetic studies and the likely microscopic mechanisms for the photo illumination effects on magnetic properties are discussed. The second organic system, polyaniline nanofiber networks, was synthesized via dilute polymerization and studied at low and high electric and magnetic fields for tem- peratures 2 K - 250 K for their magneto-transport behavior. We observed large mag- netoresistance (up to 55 % at H = 8 T and T = 3 K) in polymer networks composed of nanofibers with an average diameter of about 80 nm. A crossover from positive MR to negative MR is observed at s 87 K. The positive and negative MR are attributed to two competing mechanisms; shrinkage of the localized electron wavefunction and suppression of quantum interference of electron wavefunctions propagating along dif- ferent current paths in the hopping process by the applied magnetic field. In addition to temperature dependence of magnetoresistance, dependencies on morphology of the nanofibers and applied electric field are observed. Detailed DC electrical transport results of various polyaniline nanofiber samples and possible mechanisms responsible for the magneto-transport behavior are discussed. iii Dedicated to my love Doruk and my parents, Oya and Haydar Duman. iv ACKNOWLEDGMENTS First of all, I would like to thank my adviser Dr. Arthur J. Epstein for his support and valuable advice. His enthusiasm and motivation kept me going and inspired me for the quest for the best and novel science. It has been a real pleasure to work with him. I would like to also thank Dr. Nan-Rong Chiou for preparing high-quality polyani- line nanofibers and Dr. Joel Miller and his students Amber C. MacConnel, William W. Shum, Kendric J. Nelson at University of Utah for supplying me Prussian blue samples for my experiments. Without these samples, none of this work would have been possible. I also want to thank Dr. Jung-Woo Yoo whom I have learned quite a lot in my PhD life. I am also thankful to former group members Dr. Jeremy Bergeson and Dr. Derek Lincoln who helped me in the lab anytime I needed. I also wish to thank Dr. Vladimir Prigodin for all insightful discussions. I am also grateful to all present and past group members Chia-Yi Chen, Bin Li,Chi-Yueh Kao, Austin Carter, Lynette Mier, Timi Adetunji, June Hyoung Park, Jesse Martin, Raju Nandyala, Jen- Chieh Wu and Yong Min who created a great and joyful environment to work in. In particular, I would like to thank Louis Nemzer and Mark Murphey for their help with latex and FIB. I also would like thank my graduate committee members Dr. Ezekiel v Johnston-Halperin, Dr. Julia S. Meyer and Dr. Thomas Humanic. I want to thank all of my friends in Columbus who made my PhD life a lot smoother. I gratefully acknowledge the financial support from the Department of Physics through teaching associateship and summer quarter fellowship program, Materials Research Science and Engineering Center (MRSEC) through Center for Emergent Materials (CEM) fellowship and DEO, AFOSR and Institue for Material Research (IMR) through grants for research associateship, travel and facility usage. I am also thankful to Jenny Finnell who handled and solved our administrative problems and former graduate studies chairman Dr. Thomas Humanic and secretary Brenda Mellett for their care and help with departmental issues. Finally, I want to thank my family; my parents Oya and Haydar and my siblings Zumrut, Irem and Kaya for their continuous and endless support, encouragement and love despite the distance separating us. I wish to specially to thank my husband Doruk who has been with me at all times. I always feel your encouragement, love and kindness. vi VITA August 7, 1980 . Born - Burhaniye, TURKEY 2003 . .B.S. Physics, Bogazici University, Istanbul, TURKEY 2003-2004 . Graduate Teaching Associate, University of Cincinnati, Cincinnati, OHIO 2004-2006 . Graduate Teaching Associate, The Ohio State University, Columbus, OHIO 2007 . .M.S. Physics, The Ohio State University, Columbus, OHIO 2007-2008 . Graduate Research Associate, The Ohio State University, Columbus, OHIO 2008-present . .CEM Fellow, The Ohio State University, Columbus, OHIO PUBLICATIONS Research Publications K. Deniz Bozdag, N.-R. Chiou, V. N. Prigodin, A. J. Epstein \Temperature, Mag- netic and Electric Field Dependence of Magneto-Transport for Polyaniline Nanofiber Network". Synthetic Metals, 2009. vii FIELDS OF STUDY Major Field: Physics Studies in: Magnetism in Organic Magnets Magneto-transport in Polymer Nanofibers Field Effect Transistor, Spin-valves and Spin-leds with Organic Magnets viii TABLE OF CONTENTS Page Abstract . ii Dedication . iv Acknowledgments . .v Vita......................................... vii LIST OF FIGURES . xiii Chapters: 1. Introduction . .1 1.1 Optical Control of Magnetism . .1 1.2 Magneto-Transport in Polyaniline Nanofibers . .3 1.3 Outline . .6 2. Background on Magnetism, Molecule-Based Magnets and Photoinduced Magnetism . .8 ix 2.1 Magnetism in Solids . .8 2.1.1 Isolated Magnetic Moments . .8 2.1.2 Atoms in Solids . 15 2.1.3 Magnetic Interactions . 20 2.1.4 Magnetostatic (dipole-dipole) interactions . 26 2.1.5 Anisotropy . 27 2.1.6 Domain Walls . 30 2.1.7 Magnetic Ordering . 32 2.2 Molecule-based Magnets . 43 2.2.1 Prussian Blue Analogs . 44 2.3 Photoinduced Magnetism . 48 2.3.1 Co-Fe Prussian Blue . 50 2.3.2 M[TCNE]x x ∼ 2 M = V, Mn . 56 3. Background on Conducting Polymers . 61 3.1 Conducting Polymers . 61 3.1.1 Electromagnetic Response . 71 3.1.2 Electronic Structure of Polymers . 75 3.1.3 Transport in Conducting Polymers . 82 4. Experimental . 96 4.1 Sample Preparation . 96 4.1.1 Polyaniline Nanofibers . 96 x 4.1.2 V-Cr Prussian Blue . 97 4.2 Experimental Tools . 98 4.2.1 Elemental Analysis . 98 4.2.2 Imaging and patterning: Photolithography, FIB, SEM . 99 4.2.3 DC conductivity . 103 4.2.4 DC magnetization: SQUID magnetometer . 104 4.2.5 AC susceptibility: PPMS susceptometer . 107 4.2.6 Sample Illumination: Laser and fiber optics . 108 4.2.7 UV-Vis Spectrometer . 110 4.2.8 XPS . 110 5. Optical Control of Magnetism in V-Cr Prussian Blue . 113 5.1 Magnetic Properties . 113 5.2 Optical Control of Magnetic Properties . 128 5.3 Summary and Discussion . 138 6. Magneto-transport in Polyaniline Nanofiber Networks . 139 6.1 Resistivity . 139 6.2 Magneto-transport . 146 6.2.1 Temperature Dependence . 146 6.2.2 Morphology Dependence . 150 6.2.3 Electric Field Dependence . 153 6.3 Summary and Discussion . 156 xi 7. Discussions and Future Work . 159 7.1 Optical Control of Magnetism . 159 7.2 Magneto-transport for Polyaniline Nanofiber Networks . 160 7.3 Future Work . 162 BIBLIOGRAPHY . 164 xii LIST OF FIGURES Figure Page 1.1 Molecular structures of (top) emeraldine base PANI and (bottom) CSA.5 2.1 Magnetic moment versus H/T graph for spin values s = 3=2; 5=2 and 7=2 in which the data fits well to the brillouin function [47] . 12 2.2 Magnetic susceptibilities of diamagnetic and paramagnetic systems [43] 14 2.3 The angular distribution of d orbitals. The levels dz2 and dx2−y2 are grouped as eg levels. The remaining levels dxz, dxz and dyz are grouped as t2g levels [42, 46] . 16 2.4 The overlap of different d orbitals with the ligands. dxy orbital has lower energy compare to dx2−y2 due to smaller overlap and electrostatic interaction [43] . 17 xiii 2.5 Splitting of the d orbitals in octahedral (left) and tetrahedral (right) crystal fields . 18 2.6 Electronic configuration for high-spin (HS) and low-spin (LS) states under octahedral crystal field (left). Energy curves for LS and HS states with respect to metal-ligand coordinate (right) [46].
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