학술연구정보서비스(KERIS)
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Title Page
Abstract
Contents
List of Abbreviations 16
Chapter 1. Introduction 18
1.1. Spin Caloritronic 18
1.1.1. History and Origin of Spin Caloritronic 18
1.1.2. Phenomena of Group Dynamics in the Spin Caloritronic 20
1.1.3. Spin Seebeck Effect in Devices 23
1.2. Magnetic Materials 25
1.2.1. Historical development of magnetism 25
1.2.2. Magnetism and classification of magnetics 26
1.2.3. Yttrium Iron Garnet 32
1.3. Research Overview and Objective 36
1.4. References 38
Chapter 2. Experimental Synthesis and Characterization Techniques 45
2.1. Sol-Gel Synthesis 45
2.1.1. Process and characteristics 45
2.1.2. Mechanism and variations 48
2.1.3. Synthesis Technique 50
2.2. Characterization Technique 54
2.2.1. X-ray diffraction (XRD) 54
2.2.2. Raman spectroscopy 56
2.2.3. X-ray photoelectron spectroscopy (XPS) 58
2.2.4. Field emission scanning electron microscopy (FESEM) 60
2.2.5. Vibration sample magnetometer (VSM) 62
2.2.6. Ferromagnetic resonance spectroscopy (FMR) 65
2.3. References 66
Chapter 3. Dramatic Enhancement of the Saturation Magnetization of a Sol-Gel Synthesized Y3Fe5O12 by a Mechanical Pressing Process[이미지참조] 69
3.1. Research Background 69
3.2. Experimental Producers 69
3.3. Measurement Techniques 71
3.4. Results and Discussion 71
3.5. Conclusion 76
3.6. References 77
Chapter 4. Optimizing sol-gel synthesis process for a crystalline and amorphous Y3Fe5O12 with high saturation magnetization[이미지참조] 80
4.1. Research Background 80
4.2. Experimental Producers 80
4.3. Measurement Techniques 81
4.4. Result and Discussion 81
4.5. Conclusion 87
4.6. References 87
Chapter 5. Grain Size Effect on the Spin Seebeck Effect in a Polycrystalline Yttrium Iron Garnet 89
5.1. Research Background 89
5.2. Experimental Producers 90
5.3. Measurement Techniques 90
5.4. Result and discussion 91
5.5. Conclusion 102
5.6. Reference 103
Chapter 6. Fabrication of Flexible YIG Sheet and Improving the Performance of Flexible Spin Devices 106
6.1. Research Background 106
6.2. Experimental 107
6.3. Measurement techniques 109
6.4. Results and discussion 110
6.5. Conclusion 117
6.6. References 118
Chapter 7. Summary 120
CURRICULUM VITAE 124
Figure 1. Overview of various thermal properties in the spin caloritronic. 18
Figure 2. Relationship among the spin, heat, and charge in the spin caloritronic. 19
Figure 3. Different mechanism of spin motion and resulting in phenomena. 20
Figure 4. Geometry of SSE. (a) longitudinal and (b) transverse geometry, respectively. 24
Figure 5. Magnetic curves for ferro-, ferri-, antiferro-, and para-magnet. 26
Figure 6. Schematic illustration of ferromagnetic materials having an overall magnetic moment. 27
Figure 7. Schematic illustration of antiferromagnetic materials having a magnetic moment of zero. 30
Figure 8. Schematic illustration of paramagnetic materials with unpaired randomly arranged electrons. 30
Figure 9. Schematic illustration of ferrimagnetic materials. The electron spins are orientated antiparallel... 31
Figure 10. Schematic of the crystal structure of YIG. 33
Figure 11. Schematic illustrate of the octahedra and tetrahedral site in the YIG crystal structure. 33
Figure 12. Schematic illustrate of origin of YIG magnetic characteristic. (a) the entire magnetization of... 34
Figure 13. Schematic illustration of the progress of sol-gel synthesis. From solid particles are dispersed... 46
Figure 14. Schematic illustration of the variety of resulting materials from the sol-gel synthesis. 47
Figure 15. Schematic descriptions of the process of forming a solid mass of material by heat. 51
Figure 16. Flow chart of the total progress of sol-gel synthesis which included an external mechanical... 53
Figure 17. Deriving Bragg's Law using the reflection geometry and applying trigonometry. d is the... 55
Figure 18. Schematic diagrams of energy-level show the involved states in Raman spectra. 56
Figure 19. Raman shift. 57
Figure 20. The Electromagnetic Spectrum. 58
Figure 21. Basic principles of various soft X-ray spectroscopies. 59
Figure 22. Schematic drawing of the electron and x-ray optics of a combined SEM. 61
Figure 23. Schematic drawing of the vibrating sample magnetometer. 63
Figure 24. Hysteresis loop of magnetic, relates to the magnetization properties of a material. 64
Figure 25. Schematic illustration of the FMR. (a) sample stage (b) experimental set-up. 65
Figure 26. Schematic illustration of the experimental process of sol-gel synthesis. 70
Figure 27. (a) XRD patterns of the YIG powders obtained after calcination of the precursors at... 72
Figure 28. FE-SEM images of YIG powders (a) after calcination at 850 °C and (b) after both calcination... 74
Figure 29. Magnetic hysteresis loop for the different types of YIG measured at room temperature. The... 75
Figure 30. Magnetic hysteresis loops measured at room temperature for the different types of YIG in... 75
Figure 31. XPS spectra recorded for each YIG sample: (a) Y 3d (b) C 1s (c) O 1s and (d) Fe 2p. The... 76
Figure 32. (a) Schematic of the enhanced sol-gel synthesis process. Pc denotes YIG powder that was...[이미지참조] 83
Figure 33. (a) Schematic of the synthesis process. Pcs1 denotes YIG powder that was calcined and then...[이미지참조] 83
Figure 34. Magnetic hysteresis loop for the different types of YIG measured at 300 K. Pcs1 d...[이미지참조] 84
Figure 35. Magnetic hysteresis loops for amorphous YIG with different calcination temperature of 250,... 86
Figure 36. Average grain size and grain texture analysis of YIG samples after annealing at 1400 ℃ for... 93
Figure 37. (a) Misorientation angle distribution for all samples, and (b) isorientation distribution... 95
Figure 38. XRD peaks depending on the average grain size. 96
Figure 39. Magnetic hysteresis loop for the bulk-YIG with different sintered retention times measured... 98
Figure 40. (a) Schematic illustration of the geometry of the LSSE in a Pt/bulk-YIG structure. The... 99
Figure 41. Differential FMR spectra of polycrystalline bulk-YIG samples in the in-plain mode for 0 to... 101
Figure 42. Spin thermoelectric voltage removed magnetization value at 400 Oe (VLSSE/M400) as function...[이미지참조] 101
Figure 43. XRD pattern for the YIG sheets of a bare PDMS sheet, YIG powder and PDMS mixture... 110
Figure 44. FESEM images of YIG sheets prepared by mixing sintered YIG powder and PDMS with 1:... 112
Figure 45. (a) In-plane hysteresis loops by VSM of samples. (b) graph of saturation magnetization value... 113
Figure 46. An alpha step graph measuring surface changes of YIG sheet with 1:1 mixing ratio before... 115
Figure 47. Photos of the implemented flexible spin thermoelectric device. (a) - (c) show the bending... 115
Figure 48. XPS spectra according to samples of various conditions. The higher YIG ratio of the surface,... 116
Figure 49. A graph showing the variation of the average voltage value according to the sample. The... 116
Magnetic materials and magnetic phenomena continuously have been highlighted as prominent resource for a future green energy because of their amenability to solution based-processing; their superior thermal properties, a wide range of applications, long-lasting own magnetism; and the ability to tune their magnetic responses which indicated a ferro-, ferri-, anti-ferro-, and para-magnetic property through the arrangement of spin in the electron. The ferrimagnet insulator-yttrium iron garnet (Y3Fe5O12, YIG) that is the one of the magnetic materials with metal oxides has been taken center stage owing to its prominent crystal structure based on a garnet and the origin of magnetics that leads to its unique features; extremely low damping and electrically insulating, originate by nontrivial crystal structure of garnet. For these reasons, a representative ferrimagnetic material, YIG has attracted much attention as the next-generation spin caloritronic devices.
One such phenomenon in spin caloritronic is the spin Seebeck effect (SSE) where a spin current is driven by a temperature gradient in magnetic materials under the effect of the external magnetic field. During the past decade, most of the studies on the SSE have been done on well-grown single crystal sample of YIG by pulsed laser deposition method in spite of an expensive, difficult, structural, and technical limitation. Here, we report on the SSE of YIG prepared by a sol-gel synthesis which is well-known a conventional method for metal oxide materials.
Here, I present the temperature of heat treatment and effect of the external mechanical pressure can play a critical role for the magnetic properties of YIG during the sol-gel synthesis. Furthermore, I demonstrate that considerable spin thermoelectric voltage can be generated successfully by the poly crystalline YIG which included a lot of grains on the surface of bulk-YIG. In addition, the important and variety factors for enhanced magnetic property of YIG were investigated by changing the sequence, the number of process step, or changing the temperature of heat treatment. Consequently, a uniform and dense poly crystalline YIG as a spin thermoelectric layer was realized and a platinum (Pt)/YIG or a gold (Au)/YIG structure were developed and measured under the longitudinal spin Seebeck geometry which denoted the parallel relation of spin current and the temperature gradient when applied external magnetic field.
Finally, I give a demonstration that one of the prominent candidates for a flexible thermoelectric generator is SSE devices. To implement flexible spin Seebeck device, a desirable substrate-free longitudinal-SSE (LSSE) device having mechanically flexibility based on magnetic powder was optimized. For fabricating flexible YIG sheet, sol-gel synthesis which enables low-cost mass production was adopted as well as obtain comparable converting spin to charge current through the inverse spin Hall effect in the LSSE configuration of the flexible YIG sheet, a gold electrode was fabricated on the top of surface. Flexible substrate-free SSE device fabricated will open a new way for harnessing spin caloritronic to energy harvesting devices for next-generation smart network devices.*표시는 필수 입력사항입니다.
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