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홋카이도 남부 Warm Core Ring의 탄성파 반사법 영상화
( Mikiya Yamashita ),( Kanako Yokota ),( Yoshio Fukao ),( Shuichi Kodaira1 ),( Seiichi Miura1 ),( Katsuro Katsumata ) 한국지구물리·물리탐사학회 2011 지구물리와 물리탐사 Vol.14 No.1
홋카이도 남쪽 태평양 판의 심부 지각 구조를 규명하기 위해 다중채널 탄성파 반사법 탐사가 2009년에 수행되었다. 탐사 측선은 250km 넓이의 WCR을 가로지르며, 쿠로시오 속류에 의해 생성된 난류가 흐르는 지역에 위치한다. 본 논문에서는 다중채널 탄성파 반사법 자료를 사용하여 WCR의 세부 구조를 규명하고자 하였다. 탐사 측선은 2개의 프로파일로 구성되는데, 그 중 하나는 송신원 간격이 200미터이고, 다른 하나는 50미터 간격이다. 밀집된 송신원을 갖는 측선의 기록자료가 성긴 송신원 측선의 기록자료보다 배경 잡음이 훨씬 많은 것을 관찰할 수 있다. 이 잡음의 발생원은 이전 송신원으로부터 발생한 해수면과 해저면, 그리고 지하 불연속면 사이의 음향 다중반향음으로 확인되었다. 음파 속도 정보가 동시에 수행하는 온도 측정으로부터 구해질 수 있다면 중합전 구조보정 기술을 통해 배경잡음에 묻혀 있는 신호를 효과적으로 강조할 수 있음을 알 수 있었다. WCR은 음향학적으로 볼 때 해양쪽으로 급경사(~2°)이고 해변쪽으로 완경사(~1°)인 오목한 반사면들의 집합체라고 할 수 있다. WCR 내부에서 30km 넓이의 반사면들로 둘러싸인 렌즈 형태의 구조를 확인할 수 있었다. A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200mand the other with a shot interval of 50m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (~2°) on the ocean side and gentler slopes (~1°) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.
Design of MgAl₂O₄ Spinel-Oxide-Based Tunnel Barriers for Advanced Spintronics Devices
Kenji Nawa,Keisuke Masuda,Shinto Ichikawa,Hiroaki Sukegawa,Tsuyoshi Suzuki,Katsuyuki Nakada,Seiji Mitani,Yoshio Miura 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Tunnel magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) is one of the central properties to develop high-performance spintronics devices. MgO(001) with bcc-Fe or CoFe electrodes has been mostly studied as a tunnel barrier of MTJs for the last decades because of its giant TMR ratio originating from the spin-filtering effect of Δ₁-symmetric evanescent states (s, pz, d3z²-r² orbitals), as predicted in 2001. However, the TMR ratio of MgO-MTJs reduces significantly by bias voltage applications, indicating a limitation of the output voltage of the MTJ. Recent experiments show that the use of a (001)-oriented spinel-type oxide, MgAl₂O₄, as a tunnel barrier improves the robustness of the TMR ratio under bias applications, but a theoretical TMR limit in Fe/MgAl₂O₄/Fe(001) MTJs is very small compared to the MgO-MTJs. This is because the in-plane lattice periodicity of Fe electrode is half of that of MgAl₂O₄ and a band-folding effect is induced in the two-dimensional Brillouin zone of the in-plane wave vector in the Fe electrode. This effect provides additional conductive states at the Δ line in MgAl₂O₄-MTJ, contributing to the reduction of the TMR ratio. In this talk, we propose a combined trilayer tunnel barrier, MgO/MgAl₂O₄/MgO, to overcome the above issue of the small TMR limit of MgAl₂O₄-MTJs on the basis of the first-principles calculations. We performed ballistic-conductance calculations in an Fe/MgO(n)/MgAl₂O₄/MgO(n)/Fe(001) MTJ using the non-equilibrium Green’s functions method to clarify the TMR ratio under bias voltage application. Here, number of MgO layers (n-ML) is changed as n = 1, 2, 3. In the case of n = 1, a large TMR ratio of 1184% is obtained at a zero-bias voltage and this large value is almost maintained up to V = 1.2 V (see blue in Fig. 1), leading to a large voltage output. In contrast, a single barrier MgAl₂O₄ shows only a small TMR ratio (~125%), which is constant below V = 1.6 V (see orange in Fig. 1). These results indicate that both the models have a similar tendency in bias voltage dependence of TMR, except for the magnitude of a TMR ratio. Moreover, we clarified that the presence of an MgO interlayer between Fe and MgAl₂O₄ plays an important role in retaining (blocking) the Δ1 evanescent state for majority (minority) spin. The former leads to the robustness of the TMR ratio against bias voltage as observed in single MgAl₂O₄ MTJs, while the latter does to the large TMR ratio as in single MgO MTJs. 〈그림 본문참조〉
Seismic reflection imaging of a Warm Core Ring south of Hokkaido
야먀스타 미키야,요코다 카나코,푸카시오 요시오,고다이라 슈이치,미우라 세이치,가츠마타 카츠로,Yamashita, Mikiya,Yokota, Kanako,Fukao, Yoshio,Kodaira, Shuichi,Miura, Seiichi,Katsumata, Katsuro Korean Society of Earth and Exploration Geophysici 2011 지구물리와 물리탐사 Vol.14 No.1
A multi-channel seismic reflection (MCS) survey was conducted in 2009 to explore the deep crustal structure of the Pacific Plate south of Hokkaido. The survey line happened to traverse a 250-km-wide Warm Core Ring (WCR), a current eddy that had been generated by the Kuroshio Extension. We attempted to use these MCS data to delineate the WCR fine structure. The survey line consists of two profiles: one with a shot interval of 200m and the other with a shot interval of 50 m. Records from the denser shot point line show much higher background noise than the records from the sparser shot point line. We identified the origin of this noise as acoustic reverberations between the sea surface, seafloor and subsurface discontinuities, from previous shots. Results showed that a prestack migration technique could enhance the signal buried in this background noise efficiently, if the sound speed information acquired from concurrent temperature measurements is available. The WCR is acoustically an assemblage of concave reflectors dipping inward, with steeper slopes (${\sim}2^{\circ}$) on th ocean side and gentler slopes (${\sim}1^{\circ}$) on the coastal side. Within the WCR, we recognised a 30-km-wide lens-shaped structure with reflectors on the perimeter.