Soft Magnetic Materials and
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`Devices on Energy Applications
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`Xing Xing
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`The Department of Electrical and Computer Engineering
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`Northeastern University
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`Boston, Massachusetts
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`A thesis submitted in partial fulfillment of the requirements for the degree of
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`Doctor of Philosophy in the field of Electrical Engineering
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`July, 2011
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`Ex.1021 / IPR2022-00117 / Page 1 of 160
`APPLE INC. v. SCRAMOGE TECHNOLOGY, LTD.
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`Devices on Energy Applications
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`Xing Xing
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`The Department of Electrical and Computer Engineering
`
`Northeastern University
`
`Boston, Massachusetts
`
`A thesis submitted in partial fulfillment of the requirements for the degree of
`
`Doctor of Philosophy in the field of Electrical Engineering
`
`July, 2011
`
`Ex.1021 / IPR2022-00117 / Page 1 of 160
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`1 Abstract
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`The fast development of wireless communication system in recent years has been
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`driving the development of the power devices from different aspects, especially the
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`miniaturized volume and renewable power supply, and etc. In this work, we studied the
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`high frequency magnetic properties of the soft magnetic material -- FeCoB/Al2O3/FeCoB
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`structures with varied Al2O3 thickness (2nm to 15nm), which would be applied in to the
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`integrated inductors. Optimized Al2O3 thickness was found to achieve low coercive field
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`and high permeability while maintaining high saturation field and low magnetic loss.
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`Three types of on Si integrated solenoid inductors employing the FeCoB/Al2O3
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`multilayer structure were designed with the same area but different core configurations.
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`A maximum inductance of 60 nH was achieved on a two-sided core inductor. The
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`magnetic core was able to increase the inductance by a factor of 3.6 ~6.7, compared with
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`the air core structures.
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`Vibration energy harvesting technologies have been utilized to serve as the
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`renewable power supply for the wireless sensors. In this work, two generations of
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`vibration energy harvesting devices based on high permeability magnetic material were
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`designed and tested. The strong magnetic coupling between the magnetic material and the
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`bias magnetic field leads to magnetic flux reversal and maximized flux change in the
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`magnetic material during vibration. An output power of 74mW and a working bandwidth
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`of 10Hz were obtained at an acceleration of 0.57g (g=9.8m/s2) for the 1st generation
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`design, at 54Hz. An output voltage of 2.52 V and a power density of 20.84 mW/cm3 were
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`demonstrated by the 2nd generation design at 42 Hz, with a half peak working bandwidth
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`of 6 Hz.
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`2 Acknowledgement
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`I would like to take this opportunity to express my appreciation for my advisor Dr.
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`Nian Sun, who has provided me the opportunity to pursue research on magnetic material
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`and energy applications. He has been constantly guiding, supporting and encouraging me,
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`without which I could not have been writing this dissertation. I sincerely appreciate all of
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`his advice during my four-year PhD study, because it helped me master the techniques of
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`solving different kinds of engineering problems and also made me grow.
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`I would also like to thank my colleagues in the Sun Group at Northeastern
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`University. Dr. Guomin Yang and Dr. Ming Liu helped me a lot in my first year by
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`teaching me basic research techniques. Miss Ogheneyunume Obi has offered me help on
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`all kinds of issues, from paper review to preparing the graduation documents, and they
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`are so many that I could only recall a small part. These colleagues (Jing Lou, Ziyao Zhou,
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`Shawn Beguhn, Qi Wang, Zhijuan Su, Jing Wu, Xi Yang, Ming Li) always gave me their
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`hands when I mostly needed them and they made my four year graduate study an
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`enjoyable one.
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`Most importantly, I would like to cherish my gratitude for my parents, who have
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`always been supporting and loving me unconditionally, and they made me grown up as
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`an honest and happy person. I would also like to thank my love, Xiaoyu Guo, who has
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`been supporting me and tolerating everything of mine.
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`Contents
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`1 Abstract
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`2 Acknowledgement
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`3 List of Tables
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`4 List of Figures
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`1 Introduction
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`1.1 BASIC MAGNETIC CHARACTERISTICS
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`1.2 HYSTERESIS LOOP AND COERCIVITY
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`1.3 ANISOTROPY
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`1.4 SATURATION MAGNETIZATION
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`1.5 PERMEABILITY
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`1.6 SOFT MAGNETIC MATERIAL
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`1.7 DEVICE APPLICATIONS OF SOFT MAGNETIC MATERIALS
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`REFERENCES
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`2 Experimental Methods
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`2.1 PHYSICAL VAPOR DEPOSITION (PVD)
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`2.2 VIBRATING SAMPLE MAGNETOMETER (VSM)
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`2.3 FERROMAGNETIC RESONANCE (FMR) DETECTION
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`2.4 HIGH FREQUENCY PERMEABILITY MEASUREMENT
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`REFERENCES
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`3 Soft Magnetic Thin Film: FeCoB/Al2O3/FeCoB Structure with Varied
`Al2O3 Thickness
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`3.1
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`INTRODUCTION
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`3.2 EXPERIMENTAL
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`3.3 HYSTERESIS LOOPS
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`3.4 FMR SIGNALS
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`3.5 PERMEABILITY
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`3.6 CONCLUSION
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`REFERENCES
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`4 Integrated inductors with FeCoB/Al2O3 Multilayer Films
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`4.1
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`INTRODUCTION
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`4.1.1 QUALITY FACTOR
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`4.1.2 SELF-RESONANCE FREQUENCY
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`4.1.3 DIFFERENT TYPES OF INTEGRATED INDUCTOR
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`4.1.4 THE MAGNETIC CORE
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`4.2 PROTOTYPE AND LAYOUT DESIGN
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`4.3 THEORETICAL MODEL
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`4.3.1
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`INDUCTANCE
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`4.3.2 RESISTANCE AND SKIN EFFECT
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`4.3.3 LAMINATION CORE LOSS
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`4.3.4 PARASITIC EFFECTS
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`4.3.5 QUALITY FACTOR OF THE SOLENOID TYPE INDUCTORS
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`4.4 ON-CHIP FABRICATION
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`4.5 ON WAFER MEASURING
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`4.5.1 TESTING PLATFORM
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`4.5.2 CALIBRATION
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`4.5.3 PARAMETER EXTRACTION
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`4.6 RESULT AND ANALYSIS
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`4.6.1 NUMBER-OF-TURN DEPENDENCY
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`4.6.2 CORE STRUCTURE DEPENDENCY
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`4.6.3 CONCLUSION
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`REFERENCES
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`5 Soft Magnetic Material Applied Vibration Energy Harvesting - 113 -
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`5.1
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`INTRODUCTION
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`5.1.1 AVAILABLE ENERGY SOURCES
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`5.1.2 VIBRATION ENERGY HARVESTING MECHANISMS
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`5.1.3 KEY MATERIALS OF VIBRATION ENERGY HARVESTING DEVICES
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`5.2 WIDEBAND VIBRATION ENERGY HARVESTER WITH HIGH PERMEABILITY MATERIAL
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`5.2.1
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`INTRODUCTION
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`5.2.2 BASIC MECHANISM
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`5.2.3 PROTOTYPE AND TESTING SYSTEM
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`5.2.4 THEORETICAL MODEL
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`5.2.5 RESULTS AND DISCUSSION
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`5.2.6 NONLINEAR EFFECT
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`5.2.7 ADVANTAGE OF HIGH PERMEABILITY VIBRATION ENERGY HARVESTING
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`5.2.8 SUMMARY
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`5.3 HIGH OUTPUT VIBRATION ENERGY HARVESTER WITH HIGH PERMEABILITY MATERIAL
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`5.3.1
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`INTRODUCTION
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`5.3.2 BASIC MECHANISM
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`5.3.3 THEORETICAL ANALYSIS
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`5.3.4 PROTOTYPE AND TESTING SYSTEM
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`5.3.5 RESULTS AND DISCUSSION
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`5.3.6 ADVANTAGES OF THE 2ND
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` GENERATION HIGH PERMEABILITY VIBRATION ENERGY HARVESTER AND
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`SUMMARY
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`REFERENCES
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`6 Conclusion and Future Work
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`3 List of Tables
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`Table 1.1 Major families of soft magnetic materials with typical properties. ...... - 24 -
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`Table 1.2 Representative commercially available magnetic amorphous metals. . - 25 -
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`Table 3.1 FMR Resonance Field along Easy and Hard Axis at 9GHz, In-plance
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`Anisotropy, 4p Ms and Exchange Coupling Coefficient for Each Sample ... - 46 -
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`Table 4.1 Performance summary of the various bulk/surface micromachined RF
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`inductors fabricated on silicon. ..................................................................... - 54 -
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`Table 4.2 The Design Matrix of the on-Chip Integrated Inductors ...................... - 96 -
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`Table 5.1 Comparison of different types of available ambient energy sources and
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`their performance ........................................................................................ - 114 -
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`Table 5.2 Comparison of several key figures of merit for different vibrating energy
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`harvesting mechanisms ............................................................................... - 119 -
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`Table 5.3 Comparison between different materials applied in vibration energy
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`harvesting system ........................................................................................ - 125 -
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`Table 5.4 The magnetostriction constants of different soft magnetic materials. - 127 -
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`4 List of Figures
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`Fig. 1.1 A typical magnetization hysteresis loop of magnetic materials. ............. - 20 -
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`Fig. 1.2 A typical hysteresis loop of the magnetic thin film ................................. - 22 -
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`Fig. 1.3 Chronological summary of major developments of soft magnetic materials. -
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`23 -
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`Fig. 2.1. Schematic of magnetron sputtering ........................................................ - 28 -
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`Fig. 2.2 A picture of the PVD system. .................................................................. - 29 -
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`Fig. 2.3 A schematic picture of VSM. .................................................................. - 30 -
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`Fig. 2.4 A picture of the VSM system. ................................................................. - 31 -
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`Fig. 2.5 The processional motion of the moment M around the static magnetic field
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`H, before and after the dynamic component h applied. ................................ - 32 -
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`Fig. 2.6 The FMR signal on (a) the absorbed power vs. static field plot and (b)
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`dP/dH vs. field plot. ...................................................................................... - 33 -
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`Fig. 2.7 The permeability measurement system insists of a network analyzer and a
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`coplanar waveguide. ..................................................................................... - 34 -
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`Fig. 3.1 (a) Sandwich structure schematic (b) Coercivity of non-annealed sandwich
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`structures decreases as the insulating alumina thickness increases, indicating
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`reduced exchange coupling between adjacent FeCoB layers. ...................... - 39 -
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`Fig. 3.2 (a) The saturation field as a function of the insulator layer thickness, (b) In-
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`plane hysteresis loop for sandwich with Al2O3 thickness of 2nm and (c) 15nm,
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`before magnet anneal applied. The saturation field decreases as Al2O3 layer
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`gets thicker. ................................................................................................... - 40 -
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`Fig. 3.3 Hysteresis loops of (a) sandwich structure with Al2O3 thickness of 3nm after
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`annealing, and (b) all samples in large field scale, after magnetic anneal. A
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`coercive field as low as 0.5Oe was obtained for the trilayer FeCoB(100nm)/
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`Al2O3(3nm)/ FeCoB(100nm). High 4p Ms value of over 1.5 Tesla was obtained
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`for all samples. .............................................................................................. - 41 -
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`Fig. 3.4 FMR signal along easy axis of annealed 200nm single layer FeCoB film and
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`sandwich layer with Al2O3 of 3nm, at 8.5GHz. ............................................ - 43 -
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`Fig. 3.5 FMR signals along the easy axis for all samples at (a) 7GHz, (b) 8GHz, (c)
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`9 GHz and (d) 10 GHz. Optical modes are marked with arrows. ................. - 44 -
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`Fig. 3.6 Permeability spectrum measured under zero fields for all samples. ....... - 48 -
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`Fig. 4.2 Equivalent energy model representing the energy storage and loss
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`mechanisms in a monolithic inductor. Note that Co=Cp+Cs. ....................... - 55 -
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`Fig. 4.3 The quality factor as a function of frequency. ......................................... - 56 -
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`Fig. 4.3 (a) Top view and (b) side view of a 3.5-turn square spiral inductor. ...... - 58 -
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`Fig. 4.4 The equivalent circuit of a spiral inductor. .............................................. - 58 -
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`Fig. 4.5 Optical microscope images of copper/polyimide based 4-turn elongated
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`spiral inductor with magnetic material fabricated in a 90 nm CMOS process. ... -
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`59 -
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`Fig. 4.6 3-D illustration of the on-chip integrated toroidal inductor. The circled
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`section represents a unit turn. ........................................................................ - 60 -
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`Fig. 4.7 Schematic diagrams of (a) the meander type integrated inductor with
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`multilevel magnetic core and (b) the more conventional solenoid-bar type
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`inductor. The structure of the two inductor schemes is analogous. .............. - 61 -
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`Fig. 4.8 Schematic design of an integrated solenoid inductor: (a) top view and (b)
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`cross-section view. ........................................................................................ - 63 -
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`Fig. 4.9 Cross sectional SEM image of inductors integrated on an 130 nm CMOS
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`process with 6 metal levels. Two levels of CoZrTa magnetic material were
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`deposited around the inductor wires using magnetic vias to complete the
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`magnetic circuit. ............................................................................................ - 64 -
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`Fig. 4.10 Inductor design type I. Left top is a schematic top view, where the blue
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`pads indicate the vias. Left bottom is the 3-D schematic. The red rectangular on
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`the top is the top layer metal consisting the top layer of the coil, while the blue
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`rectangular is the bottom layer. The top and bottom metal are connected by vias,
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`which are in green and yellow. The right picture is the 6-layer layout design
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`done with Cadence. ....................................................................................... - 65 -
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`Fig. 4.11 The cross sectional view of inductor type I, shown in Fig. 4.10, along (a)
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`the transverse direction and (b) longitudinal direction. The yellow parts indicate
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`the metal, the dark blue parts are the magnetic cores and the light blue area is
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`the polyimide. The grey part stands for the Si substrates. ............................ - 66 -
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`Fig. 4.12 The schematic drawn of inductor type II, the top view (left top), 3-D view
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`(left bottom) and a 6-layer layout of a 10-turn structure (right). .................. - 67 -
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`Fig. 4.14 The schematic pictures of inductor type III, the top view (left top), 3-D
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`view (left bottom), and a 6-layer layout of a 10-turn device. ....................... - 68 -
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`Fig. 5.14 The layout design of an 8-turn inductor Type I (upper) and its open
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`structure with only the contour ground structure and the testing pads (lower).
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`The cross arrays in the open area are the aligning marks. ............................ - 69 -
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`Fig. 4.16 Layout design of the testing structures, including the via resistance testing
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`bars, polyimide step monitor, precision monitor and sheet resistance testing
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`pads. .............................................................................................................. - 70 -
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`Fig. 4.16 Self inductance value for a rectangular conductor versus its length and
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`width (the thickness is fixed at 1m m). ........................................................... - 72 -
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`Fig. 4.17 Equivalent circuit of an integrated inductor. ......................................... - 78 -
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`Fig. 4.18 A 10m m thick polyimide layer was coated on the Si substrate. ............. - 79 -
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`Fig. 4.19 A seedlayer was deposited with PVD, for the bottom metal. ................ - 79 -
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`Fig. 4.20 A thick photoresist layer was patterned above the seedlayer. ............... - 80 -
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`Fig. 4.21 Electric plating of the bottom coils and the removal of PR................... - 80 -
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`Fig. 4.22 The schematic view of the wafer (upper) and a zoom-in picture of the real
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`wafer (lower) after the Cr/Cu seedlayer removed. ........................................ - 81 -
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`Fig. 4.23 Polyimide layer-2 coating and patterning. ............................................. - 82 -
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`Fig. 4.24 Photoresist layer coating and patterning for the magnetic lift-off process. .. -
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`82 -
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`Fig. 4.25 Schematic view of the lift-off process (upper), and pictures of the lift-off
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`patterned cores (lower). ................................................................................ - 83 -
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`Fig. 4.26 Polyimide layer-3 was spin coated above the magnetic layer, and patterned
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`to have the via openings. ............................................................................... - 84 -
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`Fig. 4.27 (a) A seedlayer (Cr/Cu) was deposited above PI3 layer. (b) The re-sputter
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`process is able to re-distribute the seedlayer in the via openings during the
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`deposition. ..................................................................................................... - 85 -
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`Fig. 4.28 An 8 m m thick photoresist layer was coated and patterned above the
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`seedlayer ....................................................................................................... - 86 -
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`Fig. 4.29 After the top metal layer plated, the photoresist was stripped with acetone.
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`The seedlayer was removed as discussed earlier. ......................................... - 86 -
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`Fig. 4.30 Picture of the on-chip inductors, Type I, type II and Type III respectively. -
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`Fig. 4.31 (a) The GSG probes are mounted on the arms of two micropositioners
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`which can be controlled by the (b) probe station. Meanwhile, they are
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`connected to the two ports of the vector network analyzer through the low loss
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`coaxial cables. ............................................................................................... - 89 -
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`Fig. 4.32 The SOLT calibration procedure includes four steps: Measuring the short,
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`open, load and through standard. .................................................................. - 91 -
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`Fig. 4.33 Three possible configurations for measuring a two-port device ZL with a
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`VNA. They are two-port measurement with the device placed in series with the
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`ports, two-port measurement with the device placed in parallel with the ports,
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`and one-port measurement, respectively from left to right. .......................... - 92 -
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`Fig. 4.34 Setup for the calculation of the scattering parameters of a two-port
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`measurement with the P
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`Fig. 4.35 The measured inductance and quality factor of inductor type I, for different
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`numbers of turns. .......................................................................................... - 98 -
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`Fig. 4.36 The measured inductance and quality factor of inductor type II, for
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`different numbers of turns. ............................................................................ - 99 -
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`Fig. 4.37 The measured inductance and quality factor of inductor type III, for
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`different numbers of turns. .......................................................................... - 100 -
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`Fig. 4.38 The inductance plots for different core types, with the same numbers of
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`turns. (a) N=6; (b) N=8: (c) N=10 (d) N=12; (e) N=14. ............................. - 104 -
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`Fig. 4.39 The quality factor plots for different core types, with the same numbers of
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`turns. (a) N=6; (b) N=8: (c) N=10 (d) N=12; (e) N=14. ............................. - 107 -
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`Table 5.1 Comparison of different types of available ambient energy sources and
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`their performance ........................................................................................ - 114 -
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`Fig. 5.1 Power-generation mode of thermoelectric material .............................. - 116 -
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`Fig. 5.2 Figure of merit ZT as a function of temperature for several bulk
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`thermoelectric materials. ............................................................................. - 117 -
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`Fig. 5.3 In-plane overlap varying. ....................................................................... - 122 -
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`Fig. 5.4 In-plane gap closing............................................................................... - 122 -
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`Fig. 5.5 Out-of-plane gap closing. ...................................................................... - 122 -
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`Fig. 5.6 Electromagnetic energy harvesting mechanism. ................................... - 123 -
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`Fig. 5.7 The section view of the schematic design of the vibration energy harvesting
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`device. Dimension of each part is: 4.4cm ×3.2cm×4cm for the solenoid,
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`1.25cm×2.2cm×1.5cm for the magnet pair including the gap in between,
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`1.3cm×1.5cm×2.5cm
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`for
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`the mounting
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`frame on one
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`side
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`and
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`0.5cm×1.5cm×0.6cm on the other. ............................................................. - 130 -
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`Fig. 5.8 Magnet pair with antiparallel magnetic moment provides closed magnetic
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`field lines, making sure the maximum magnetic flux change, from Φ to –Φ
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`during the vibration. .................................................................................... - 131 -
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`Fig. 5.9 Magnet pair with parallel magnetic moment provides repelling magnetic
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`field lines, in which case the magnetic flux changes from Φ to 0 and back to Φ
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`during the vibration. .................................................................................... - 132 -
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`Fig. 5.10. Prototype of the wideband high permeability vibration energy harvester. .. -
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`Fig. 5.11. The Schematic of the calculation of induced voltage across the solenoid. . -
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`134 -
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`Fig. 5.12 Hysteresis loop of the MuShield beam with the dimension of 4.6cm ×
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`0.8cm × 0.0254cm. ..................................................................................... - 135 -
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`Fig. 5.13. Beam shape at its maximum deflection. ............................................. - 136 -
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`Fig. 5.14 Measured and calculated results of the open circuit voltage for the energy
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`harvesting device at the mechanical vibration frequency 54 Hz, acceleration
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`0.57 g (g=9.8 m/s2). .................................................................................... - 139 -
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`Fig. 5.15 Normalized magnetic flux as a function of time and free end amplitude, at
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`vibration frequency of 54 Hz, acceleration 0.57 g. ..................................... - 140 -
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`Fig. 5.16 Measured and calculated frequency response of the energy harvester.- 140
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`-
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`Fig. 5.17 Elastic potential energy, magnetic potential energy and total potential
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`energy of the oscillation system as functions of free end displancement of the
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`beam. ........................................................................................................... - 141 -
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`Fig. 5.18 The schematic design and working mechanism of the high power vibration
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`energy harvester. (a) The magnet pair moves to the top. (b) The magnet pair
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`moves to the bottom. ................................................................................... - 146 -
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`Fig. 5.19 Structure of the vibration energy harvester. Dimension of each component
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`is: 2 × 2.5 × 1 cm3 for the solenoids, 1.25 × 2.2 × 1.5 cm3 for the magnetic pair,
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`including the gap in between. ..................................................................... - 149 -
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`Fig. 5.20 Measured results of the open circuit voltage for the energy harvesting
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`device with three different springs at respective resonance frequencies: spring
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`#1 at 27 Hz; spring #2 at 33 Hz and spring #3 at 42 Hz. ............................ - 150 -
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`Fig. 5.21 Measured maximum output power of the harvester with three different
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`springs, at resonance frequency of each. .................................................... - 151 -
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`Fig. 5.22 Measured output power spectrum of the harvester with spring #3.
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`Maximum output is 610.62 mW, obtained at 42 Hz, corresponding to a volume
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`density of 20.84 mW/cm3. This curve shows a half-peak working bandwidth of
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`6 Hz. ............................................................................................................ - 152 -
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`1 Introduction
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`Magnetic materials are traditionally classified by their volume magnetic
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`susceptibility, c which represents the relationship between the magnetization
`
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`M and the
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`magnetic field H strength, M=c H. The first type is diamagnetic, for which c is small and
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`negative c ≈ -10 -5 and its magnetic response opposes the applied magnetic field.
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`Examples of diamagnetics are copper, silver, gold, and etc. Superconductors form
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`another special group of diamagnetics whose c ≈-1. T he second group has small and
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`positive susceptibility, c ≈10-3 ~10 -5
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`
`
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`called paramagnets. It has weak magnetization but
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`aligned parallel with the bias magnetic field. Typical paramagnetic materials are
`
`aluminum, platinum and manganese. The most widely recognized magnetic materials are
`
`the ferromagnetic materials, whose susceptibility is positive and much greater than 1,
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`c ≈50 to 10000. Examples of ferromagnetic material are iron, cobalt, nickel and several
`
`rare earth metals. [1.1]
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`Based on the coercivity, ferromagnetic materials are divided in to two groups.
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`One is called “hard” magnetic material, with coercivity above 10 kA m-1; the other group
`
`is call “soft” magnetic material, whose coercivity is below 1 kA m-1.
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`Ferromagnetic materials are widely applied in different aspects of everyday life
`
`and work, such as permanent magnets, electrical motors, magnetic recording, power
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`generation, energy harvesting, and inductors.
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`1.1 Basic Magnetic Characteristics
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`The mostly desired essential characteristics for all soft magnetic materials are high
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`permeability, low coercivity, high saturation magnetization and low magnetic loss.
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`1.2 Hysteresis Loop and Coercivity
`
`In an external magnetic field H, the magnetic material gets magnetized and shows a
`
`finite spontaneous magnetization M. In the MKS unit, total magnetic flux B=m 0H +M,
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`Figure 1.1 shows a typical H vs. M magnetization hysteresis curve of a magnetic material.
`
`For a
`
`Fig. 1.1 A typical magnetization hysteresis loop of magnetic materials.
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`soft magnetic material, M follows H readily; with a high relative permeability m=B/m
`
`0H.
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`Soft magnetic materials differ from hard magnetic materials for their much smaller
`
`coercivity values.
`
`1.3 Anisotropy
`
`The dependence of magnetic properties on a preferred direction is called magnetic
`
`anisotropy. Different types of anisotropy include: magnetocrystalline anisotropy, shape
`
`anisotropy and magnetoelastic anisotropy.
`
`Magnetocrystalline anisotropy depends on the crystallographic orientation of the
`
`sample in the magnetic field. The magnetization reaches saturation in different fields.
`
`Shape anisotropy is due to the shape of a mineral grain. A magnetized body produces
`
`demagnetizing field which acts in opposition to the applied magnetic field.
`
`Magnetoelastic anisotropy arises from the strain dependence of the anisotropy
`
`constants. A uniaxial stress can produce a unique uniaxial anisotropy.
`
`The anisotropy constant K is defined as the volume density of anisotropy energy Ea.
`
`The anisotropy field is the magnetic field needed to rotate the magnetization direction in
`
`the hard direction. Hk of a magnetic film can be read on the hysteresis loop along the hard
`
`axis, in Fig. 1.2.
`
`1.4
`
`Saturation Magnetization
`
`The saturation magnetization is defined as the volume density of maximum induced
`
`magnetic moment, which can be shown on the hysteresis loop. The saturation
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`magnetization 4pMs of the magnetic thin film can be read directly from the hysteresis
`
`loop measured along the out-of-plane direction, shown in Fig. 1.2.
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`
`
`Fig. 1.2 A typical hysteresis loop of the magnetic thin film.
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`
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`1.5 Permeability
`
`The magnetic permeability describes the relation between magnetic field and flux,
`
`B=m H, and m =m 0m r, where m r is the relative magnetic permeability. Also, the relative
`
`magnetic permeability is related to the susceptibility by c=m r-1. With the values of Hk
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`and 4pMs, which can be read from the hysteresis loops, the permeability of a magnetic
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`thin film could be evaluated by ࣆ =
`
`࣊ࡹ࢙
`
`ࡴ
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`+ 1.
`
`1.6
`
`Soft Magnetic Material
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`Fig. 1.3 Chronological summary of major developments of soft magnetic materials.
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`Soft magnetic materials are economically and technologically the most important
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`of all magnetic materials, and they have been used to perform a wide variety of magnetic
`
`functions. Some applications demand high permeability; others emphasize low energy
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`loss at high frequencies. [1.2] Figure 1.3 shows a chronological summary of major
`
`developments of soft magnetic materials. [1.3]
`
`From the beginning of the 19th century, research has been focused on the developing
`
`of higher permeability m , saturation magnetization Ms and lower coercivity Hc. The
`
`advent of rapid solidification technology (RST) in the 1970s and 1980s provided
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`metallurgists a route to novel compositions and microstructures.[1.4] Amorphous metals
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`(also called metallic glasses) produced by RST were arguably the most important
`
`development in soft magnetic materials. Four major families of soft magnetic materials
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`are: electrical steels, FeNi and FeCo alloys, ferrites, amorphous metals, and the typical
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`properties are shown in Table 1.1 and 1.2.[1.5]
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`Table 1.1 Major families of soft magnetic materials with typical properties.
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`Table 1.2 Representative commercially available magnetic amorphous metals.
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`1.7
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` Device Applications of Soft Magnetic Materials
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`
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`Reading heads employing soft magnetic materials are widely used in magnetic
`
`recording. The development in soft magnetic material has resulted in reduced size and
`
`improved efficiencies of power-handling electrical devices, such as motors, generators,
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`inductors, transformers and other transducers.
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`REFERENCES
`
`
`1.1 D. Jiles, “Introduction to Magnetism and Magnetic Materials”, 2nd Edition, (1998).
`
`1.2 C. W. Chen, “Magnetism and Metallurgy of Soft Magnetic Materials”, (1986).
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`1.3 Bechtold and Wiener, (1965).
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`1.4 S. K. Das and L. A. Davis, Mater. Sci. Eng., VOL. 98, 1, (1988).
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`1.5 G. E. Fish, Soft Magnetic Material, Proceedings of the IEEE, VOL. 78, NO. 6, June
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`(1990).
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` 2
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` Experimental Methods
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`The self-made soft magnetic films used in this research were deposited by DC or
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`RF magnetron sputtering, which is a physical vapor deposition (PVD) technique.
`
` The magnetostatic properties of these magnetic films were measured with the
`
`vibrating sample magnetometer (VSM).
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`
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`2.1
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` Physical Vapor Deposition (PVD)
`
`Physical vapor deposition denotes the vacuum deposition processes, such as
`
`evaporation, sputtering, ion-plating, and ion-assisted sputtering. During the deposition,
`
`the coating material switches into a vapor transport phase, which does not generally rely
`
`on chemical reactions but by physical mechanism. In the current semiconductor industry,
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`PVD technology is entirely based on physical sputtering.
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`During the magnetron sputtering process, the
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