Implementation of Field-Free Switching SOT Device and Material Investigation for Efficient Operation of SOT-MRAM

본 연구는 차세대 메모리 소자인 Spin orbit torque 자기 메모리(SOT-MRAM)의 성능을 향상하고, 상용화의 주요 과제를 해결하기 위해 두 가지 실험을 수행하였다. 첫 번째 실험은 외부 자기장이 필요 없는 무자장 SOT 스위칭 기술을 구현하는 것을 목표로, 헬륨 이온 조사법을 활용하여 수직 자기이방성을 가진 [Ni/Co]₆ 다중층 소자의 측면 대칭성을 깨뜨리는 방법을 적용하였다. 헬륨 이온 조사를 통해 횡방향 대칭성이 파괴되어 무자장 SOT 수직자화반전이 유도되었다. 또한, SRIM 시뮬레이션을 통해 헬륨 이온이 생성한 결함 구조와 자기적 특성 변화의 상관관계를 규명하였으며, 결과적으로 자화 스위칭 비율이 약 4 배 향상되었음을 확인하였다. 두 번째 실험은 효율적인 스핀 전류 생성 물질의 후보이라고 알려진 Altermagnetic MnTe 의 GaAs 기판에서의 성장가능성을 조사하기 위해 GaAs(111) 기판 위에 분자빔 에피택시(MBE) 기법으로 MnTe 박막을 성장시키고, 이의 구조적 및 자기적 특성을 분석하였다. X-ray 회절(XRD)과 투과전자현미경(TEM) 분석 결과, MnTe 박막은 육방정계(hexagonal) 구조를 가지며 결정 c 축이 기판에 수직으로 성장된 고결정성 에피택셜 박막임을 확인하였다. 그러나 SQUID 자기 측정에서는 반강자성체로 알려진 α-MnTe 가 강자성체 또는 페리자성체와 유사한 자성 신호를 나타냈다. TEM 과 FFT 분석을 통해 이러한 신호가 박막과 기판 간 intermixing 으로 형성된 (Mn, Ga)As 또는 MnGa 물질에서 기인함을 규명하였으며, 이는 기판과 박막 간 계면 제어의 필요성을 확인하였다. 본 연구는 헬륨 이온 조사법을 통한 다중층 구조의 강자성체의 무자장 SOT 스위칭 구현과 더 효율적인 스핀 전류 생성 물질의 탐구를 통해 SOT-MRAM 및 스핀트로닉스 소자의 에너지 효율적 구현과 상용화를 위한 새로운 가능성을 제시하였다.|This study aims to enhance the performance of Spin Orbit Torque Magnetic Random Access Memory (SOT-MRAM), a next-generation memory device, and address critical challenges for its commercialization through two experiments. The first experiment focused on implementing a field-free SOT switching technique by disrupting the lateral symmetry of [Ni/Co]₆ multilayer structures with perpendicular magnetic anisotropy (PMA) using helium ion irradiation. The helium ion irradiation induced lateral symmetry breaking, enabling field-free SOT-induced perpendicular magnetization switching. Additionally, SRIM simulations were employed to analyze the correlation between helium-ion-induced defect structures and changes in magnetic properties. The results demonstrated an approximately fourfold improvement in magnetization switching efficiency. The second experiment investigated the growth potential of MnTe, an altermagnetic material candidate for efficient spin current generation, on GaAs substrates. MnTe thin films were grown on GaAs(111) substrates using molecular beam epitaxy (MBE), and their structural and magnetic properties were analyzed. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses confirmed that the MnTe thin films exhibited a hexagonal structure with high crystalline quality and that their c-axis was oriented perpendicular to the substrate surface. However, SQUID magnetic measurements revealed that α-MnTe, known as an antiferromagnet, displayed ferromagnetic or ferrimagnetic-like magnetic signals. TEM and FFT analyses indicated that these signals originated from intermixing between the MnTe film and the GaAs substrate, forming ferromagnetic materials such as (Mn, Ga)As or MnGa. These findings underscore the necessity of interface control between the film and the substrate. This study demonstrates the potential of helium ion irradiation for achieving field-free SOT switching in ferromagnetic multilayer structures and explores the growth of efficient spin current generation materials. These results provide new insights into the energy-efficient implementation and commercialization of SOT-MRAM and spintronic devices.Maste

Electrical Transport in Graphene-Supercoductor Junction

DoctorElectrical properties of graphene have been investigated intensively since it was firstly discovered. One interesting thing in graphene is that a graphene flake exhibits very different electrical properties in accordance with the number of layers. Mono-layer graphene is characterized by a relativistic Dirac fermion of charge carriers due to its linear energy-momentum relation with the conduction and valence band intersecting at p=0. As a consequence of the band structure, a quantum state of mono-layer graphene carriers has an additional quantum number called a pseudospin which is coupled to the momentum direction. On the other hand, bi-layer graphene consisting of two graphene mono-layers exhibits totally different energy-momentum relation because two mono-layer graphenes are weakly coupled by an interlayer hopping of electrons. Although non-Dirac-like and parabolic dispersion relation are intermediate properties between mono-layer graphene and bulk graphite, bi-layer graphene system is still chiral due to the sublattice symmetry. In addition, electrical field-induced band-gap opening makes bi-layer graphene unique by itself compared with mono-layer graphene and bulk graphite. Quantum nature of charge carriers becomes significant as the length scale of a device is comparable with the phase-coherent length. The Berry phase associated with chiral nature of mono-layer graphene, π, is revealed in the half-integer quantum-Hall effect, anti weak-localization and Fabry-perot type interference experiments. Superconductivity, characterized by a superconducting order parameter, can enhance quantum interference effects of a mesocscopic device. When a non-superconducting material is placed between two superconductors, a Josephson junction will be formed. In this thesis, we have investigated electrical transport properties of mono- and bi-layer graphene Josephson junctions. The chiral nature of mono- and bi-layer graphene in a Josephson junction is not observed because our devices are in the diffusive region, thus the momentum coupled pseudospin is not maintained in the whole device. However, two-dimensional structure of graphene layer and relative low carrier density enable one to modulated carrier density of graphene by an external electric field. In the first part, we demonstrate realization and measurement of Al and Pb0.93In0.07 mono-layer graphene Josephson junction. The often-adopted superconducting electrode material, Al, shows unsatisfactorily low superconducting transition temperature (1 K) and energy gap (125 µeV). To overcome the disadvantage, we have fabricated and measured proximity-coupled superconducting junctions consisting of a mono-layer graphene sheet in a contact with Pb0.93In0.07 electrodes. A much higher superconducting transition temperature (TC ~ 7.0 K) and a large superconducting energy gap (∆PbIn ~ 1.1 meV) of PbIn alloy allow the observation of the Josephson supercurrent for temperature as high as 4.8 K with a large value of the ICRN product of ~255 µeV, an order of magnitude higher than that for Al-based graphene Josephson junction. Magnetic-field and microwave responses of the junction yield direct evidences for genuine Josephson coupling through mono-layer graphene. It is also revealed that the subgap structure of differential conductance (dI/dV) induced by the multiple Andreev reflection. Moreover, gate-dependence of IC and IC/IR is well explained by theory for diffusive and long Josephson junction. In the second part, we report electronic transport measurement on superconducting proximity effect in dual-gated bilayer graphene Josephson junction with PbIn electrodes. The effect of band gap opening is confirmed by increase of resistance as we modulated top and back gate voltage away from zero-gap charge neutral point. The resistive states near the charge neutral point shows insulating behavior in R-T curve regardless of superconducting transition of PbIn electrode below TC. However, highly doped regime shows metallic R-T behavior and junction becomes superconducting at T < TC. Moreover, magnetic-field-induced Fraunhofer-pattern like supercurrent modulation and microwave irradiation indicate formation of genuine Josephson junction at the superconducting state. The transition from superconducting state to insulating state is related with normal state conductance of the junction and the transition occurs at Gsq,c ~ 7e^2/h

열린충남 1호-[정책제언]충남의 문화특성 -백제문화의 고정된 틀을 벗어나자-

충남의 문화특성을 모든 분야에 걸쳐 조감해 보는 일은 상당히 어려운 일이며, 글쓴이의 능력을 벗어나는 일이어서 여기에서는 글쓴이가 주된 관심을 가지고 있는 고고·민속분야로 한정하여 충남의 문화를 보는 시각에 대한 몇가지 제언을 하고자 한다. 충남하면 백제, 백제하면 공주, 부여를 생각하게 되며 아직까지 백제라는 시대적 틀 속에서 튀어나오지 못하고 있는 것이 충남문화에 대한 인식의 틀이라 할 수 있다. -이후 생략N/

Gate-tunable Supercurrent in Graphene-based Josephson Junction

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Superconductor-insulator transition in dual-gated bilayer-graphene proximity Josephson junction

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Continuous and reversible tuning of the disorder-driven superconductor–insulator transition in bilayer graphene

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