Universality between Domain-Wall Motions Driven by Two Distinct Forces :Magnetic Field and Electric Current : 자구벽 이동의 보편성을 결정하는 두 가지 구동력 : 자기장과 전류

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자연과학대학 물리·천문학부(물리학전공)
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서울대학교 대학원
driving forcemagnetic domaindomain wallfacetcreepspin transfer torqueperpendicular magnetic anisotropy
학위논문 (박사)-- 서울대학교 대학원 : 물리·천문학부(물리학전공), 2012. 8. 최석봉.
Despite of so many diversities of natural phenomena, they are classified by a few universality classes because the phenomena have something in common. These common is stemmed from that the almost natural phenomena occurs on interfaces and the interface properties determine the natural phenomenon. Among the interface properties, the shape of the interface is the static property and depends on the symmetry and dimension of the system. Another property of interface is dynamic property that is a speed. The speed depends on driving force and the speed exhibits characteristic force-speed relation determines the universality class of the interface. Thus so many researches concentrated on the intensity of the driving force from old times.
Here, we researched a new possibility which determines the universality class of the interface. That is kinds of driving force. In nature, there are many driving forces and the forces exert a force to same interface, respectively. So, the study concentrated on the type of the driving forces should be done. For this study, it is very useful to observe the same interface which has two kinds of driving forces which are clearly distinguishable respectively.
A magnetic domain wall (DW) has two distinct driving forces such as the magnetic field and the electric current. So, the DW owns suitability for studying the interface motions driven by different forces. The DW motion driven by the magnetic field has been well known in the last century. According to the researches, the field driven DW motion belongs to the universality class of the creep scaling. Recently, the current is known as another driving force of the DW. Despite of the short time to study the current driven DW motions, novel applications based on a wire geometry such as race track memory devices were suggested and then explosive studies are has being done. Among the properties of the current driven DW motion, the DW speed induced by the current determine the device operating speed thus, many researcher studied the speed of the DW. Recently, it was known that the current driven DW motion also belongs to the creep scaling just as the field driven DW motion.
If this result is true, the shape of the DW induced by the field and the current should be the same. However, there are no researches on the DW shape of the current driven case due to Joule heating problem. To solve this problem, we fabricate ultra clean perpendicular magnetic anisotropy (PMA) films. This films show clear circular domains expansion by the field, implying that there are a few disorders and the film appropriate for studying the current driven DW motions.
Using these fabricated films, we investigate the uniaxial anisotropy constant of the PMA films with respect to the repetitions of multilayer structure. When the anisotropy constant is higher, the DW exhibits more clear morphology. We develop extraordinary Hall effect (EHE) probe and rotation stage for measuring the anisotropy constants then find out that the anisotropy constant has the maximum value at the one repetition of the PMA structure.
Next, we investigate the field driven DW dynamics. By using magneto-optical Kerr effect (MOKE) microscope, we measure the expansion and shrinking speed of the radius of the clear circular domains. We find out that there exists a long-range tension effect which is induced by the DW energy density.
We also investigate the electric current driven DW motions with circular and linear DWs. Quite interestingly, the DW speed depends on the DW tilting angle with respect to the current directions. From the angle dependence of the DW speeds, fully two dimensional DW driving equation with field and current was established.
Finally, we measure the DW motion in long-time range. Surprisingly, the current driven DW forms facet-like geometry and the speed of the DW drastically reduce after facet formation. But, the field driven DW shows a smooth shape with a constant speed. This discordance comes from the sign of the nonlinear coefficient of the motions.
All of our findings enhance the fundamental understanding of the universality class with different driving force and also, provide technological as well as theoretical foundation to the novel devices based on DW motions.
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College of Natural Sciences (자연과학대학)Dept. of Physics and Astronomy (물리·천문학부)Physics (물리학전공)Theses (Ph.D. / Sc.D._물리학전공)
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