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Direct Observation of Fe-Ge ordering in Fe<SUB>5-x</SUB>GeTe₂ Crystals and Resultant Helimagnetism
Trinh Thi Ly,Jungmin Park,Kyoo Kim,Hyo-Bin Ahn,Nyun Jong Lee,Kwangsu Kim,Tae-Eon Park,Ganbat Duvjir,Nguyen Huu Lam,Kyuha Jang,Chun-Yeol You,Younghun Jo,Se Kwon Kim,Changgu Lee,Sanghoon Kim,Jungdae Kim 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
The van der Waals (vdW) ferromagnet compared to conventional ferromagnetic materials provides a unique configuration of magnetic interactions due to its quasi two-dimensional (2D) nature. The pair-exchange interaction is mainly established within the 2D plane, while magnetic coupling between interlayers is weak. Therefore, the vdW ferromagnets are ideal for 2D spintronic devices that show various emergent spin-orbit coupled phenomena with time reversal and inversion symmetry breaking. As a promising vdW ferromagnet, the Fe<sub>n</sub>GeTe<sub>2</sub> family (n = 3, 4, 5) was proposed based on its high T<sub>C</sub> (260 ~ 310 K) and large saturation magnetization. Recently, the unconventional magnetic properties of Fe<sub>5</sub>GeTe<sub>2</sub> have been explained with magnetic anisotropy or/and spin reorientation by the Fe atoms in Fe<sub>5</sub>GeTe<sub>2</sub>. However, the exact mechanism of such non-trivial magnetic behaviors is still unclear because it is also involved with the structural complexity in Fe<sub>5</sub>GeTe<sub>2</sub>. Therefore, systematic study for finding the correlation between such non-trivial magnetic behaviors and the atomistic structure of Fe<sub>5</sub>GeTe<sub>2</sub> is needed to realize room temperature spintronic devices with this new vdW ferromagnet. In this study, we explore atomistic structures of an Fe<sub>5-x</sub>GeTe<sub>2</sub> single crystal by low temperature scanning tunneling microscopy (STM) and its temperature dependent magnetic behaviors. Subset Fe layers in Fe<sub>5</sub>GeTe<sub>2</sub> are expected to play a major role in magnetic ordering. STM topography reveals √3×√3 superstructures on the cleavage surface of Fe<sub>5-x</sub>GeTe<sub>2</sub>, which are attributed to the ordering of Fe(1) layer. Intriguingly, observed √3×√3 ordering of Fe(1)-Ge pair breaks the inversion symmetry, which is an important microscopic origin of the antisymmetric exchange interaction, known as Dzyaloshinskii-Moriya interaction (DMI). The temperature dependent magnetization of Fe<sub>5-x</sub>GeTe<sub>2</sub> clearly shows commensurate-incommensurate transition, a typical helimagnetic behavior arising from the DMI, just below T<sub>C</sub> = 310 K. We also confirm that spin reorientation behaviors are observed within the ranges of 120 ~ 260 K and 20 ~ 100 K due to competition between helimagnetism and other collinear properties. Our findings suggest that the Fe<sub>5</sub>GeTe<sub>2</sub> crystal has helical magnetism with non-centrosymmetric ordering of Fe(1)-Ge pairs, which can be a source of topologically protected spin solitons to develop new types of topology-based spin devices.
Effects of Instability Tools on Muscles Activities in Lunge Exercise in Healthy Adult Males
Kim, KwangSu,Lee, JaeHong,Lee, JinHwan,Lee, JaeKwang The Korean Society of Physical Therapy 2019 대한물리치료학회지 Vol.31 No.6
Purpose: This study was to investigate the effect of instability on the activation of lower limb muscles during lunge exercise. Methods: The study subjects were healthy twenties and were selected to have no orthopedic or neurological diseases. 1) Flat lunge exercise (Lunge 1, 2) Lunge exercise on TOGU (Lunge 2, 3) Holding the olympic bar and moving the lunge on TOGU (Lunge 3, 4) Holding the Surge and moving the lunge on TOGU (Lunge 4). Through the above four actions, we can see how the top-down instability influences the lower limb unlike the Bottom-up instability. EMG attachment sites were gluteus medius, vastus medialis, gastrocnemius, tibialis anterior. Results: These result suggest that exercise using Lunge 4 activity was the highest in muscle activity compared to other exercises, but vastus medialis showed the highest muscle activity in Lunge 2 exercise(p<0.05). Conclusion: This study showed the muscular activity of the lower extremities according to the lunge exercise using the instability tools.
Manufacturing of size controlled a-Si:H nanoparticles in plasma using pulsed hydrogen gas.
Kim, Kwangsu,Park, Jinhwan,Doo, Seokgwang,Nam, Jaedo,Kim, Taesung American Scientific Publishers 2009 Journal of nanoscience and nanotechnology Vol.9 No.12
<P>We synthesized size controlled a-Si:H nanoparticles in plasma using a hydrogen gas pulse. An inductively-coupled plasma chamber with RF power (13.56 MHz) was designed for this study. Experiments were performed with various hydrogen pulse ON-times ranging between 0.1 to 0.9 seconds for 1 second periods. The size of the synthesized nanoparticles was measured and analyzed using electron microscopy, Scanning Mobility Particle Sizer (SMPS) and Image J software. We observed that the size of a-Si:H nanoparticles decreased when the hydrogen pulse ON-time was increased. The a-Si:H nanoparticles were found to be spherical in shape and uniform. For hydrogen pulse ON-time greater than 0.3 second, we observed agglomerated particles due to an increase in their concentration. We hope that these particles can be used as an alternative anode material for lithium ion secondary batteries.</P>