• Journal of Magnetics
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• 제17권2호
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• pp.73-77
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• 2012

We investigate spin transfer torque (STT) in magnetic multilayer structures using micromagnetic simulations. We implement the STT contribution for magnetic multilayer structures in addition to the Landau-Lifshitz-Gilbert (LLG) micromagnetic simulators. In addition to the Sloncewski STT term, the zero, first, and second order field-like terms are also considered as well as the effects of the Oersted field due to the running current are addressed. We determine the switching current densities of the free layer with the exchange biased synthetic ferrimagnetic reference layers for various cases.

• JSTS:Journal of Semiconductor Technology and Science
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• 제14권6호
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• pp.728-732
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• 2014

Macro-model of magnetic tunnel junction (MTJ) for spin transfer torque magnetic random access memory (STT-MRAM) has been developed. The macro-model can describe the dynamic behavior such as the state change of MTJ as a function of the pulse width of driving current and voltage. The statistical behavior has been included in the model to represent the variation of the MTJ characteristic due to process variation. The macro-model has been developed in Verilog-A.

• 한국자기학회지
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• 제19권1호
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• pp.22-27
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• 2009

자기터널접합 기반의 MRAM(Magnetoresistive Random Access Memory)의 상용화를 위해서 가장 중요한 이슈는 쓰기 과정(writing operation)에서의 자화반전에 필요한 자화반전전류를 감소시키는 것이다. 본고에서는 나노자기소자 기술의 중요한 분야인 MRAM의 기술발전방향과 특히 스핀전달토크(Spin Transfer Torque, STT)를 이용한 자화반전전류의 저감기술 개발동향을 재료기술, 구조기술 등으로 살펴보았다.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2007년도 The 1st International Symposium on Advanced Magnetic Materials
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• pp.62.1-62.1
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• 2007

• Journal of Magnetics
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• 제15권4호
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• pp.149-154
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• 2010

Synthetic antiferromagnets (SAFs) are used as free layer structures for various magnetic devices utilizing spintransfer torque (STT). Therefore, it is important to understand current-induced excitation of SAFs. By means of drift-diffusion and macrospin models, we studied the current-induced excitation of a SAF-free layer structure (NiFe/Ru/NiFe). The simulation results were compared with the previous experimental results [N. Smith et al., Phys. Rev. Lett. 101, 247205 (2008)]. We confirmed that a nonzero STT through the Ru layer is essential for explaining the experimental results.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제42회 동계 정기 학술대회 초록집
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• pp.385-385
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• 2012

A two terminal N+/P/N+ junction device to replace the conventional selective transistor was studied as a bilateral switching device for spin transfer torque (STT) MRAM based on 3D device simulation. An N+/P/N+ junction structure with $30{\times}30nm$ area requires bi-directional current flow enough to write a data by a drain induced barrier lowering (DIBL) under a reverse bias at N+/P (or P/N+ junction), and high current on/off ratio of 106. The SiGe materials are widely used in hetero-junction bipolar transistors, bipolar compensation metal-oxide semiconductors (BiCMOS) since the band gap of SiGe materials can be controlled by changing the fraction and the strain epilayers, and the drift mobility is increased with the increasing Ge content. In this work, N+/P/N+ SiGe material based junction provides that drive current is increased from 40 to $130{\mu}A$ by increased Ge content from 10~80%. When Ge content is about 20%, the drive current density of SiGe device substantially increased to 2~3 times better than Si-based junction device in case of 28 nm P length, which is sufficient current to operation of STT-MRAM.

• 한국자기학회지
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• 제26권6호
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• pp.179-184
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• 2016

스핀전달토크(Spin-Transfer Torque: STT) MRAM의 상용화를 위해서는 낮은 반전전류와 높은 열적 안정성을 동시에 만족해야 하고, 이를 위해서는 큰 스핀 분극, 강한 수직자기이방성 에너지을 가지는 물질이 요구된다. 본 연구에서는 STT-MRAM에 적합한 물질로 알려진 B2 CoFe 면심에 X(O, F, S, Cl) 원자가 위치한 $CoFeX_3$ 합금의 전자구조와 자기결정이방성(Magnetocrystalline anisotropy: MCA) 에너지를 계산하였다. X 원자가 F나 Cl일 때는 페르미 준위에서의 스핀 분극율이 각각 97 %, 96 %로, 반쪽 금속에 근접한 전자구조를 가짐을 확인할 수 있었다. 뿐만 아니라 표면이 Co 원자로 끝나는 5층 박막은 모든 X에 대해 수직 자기이방성를 가졌으며, 특히 $CoFeCl_3$의 자기이방성 에너지는 약 1.0 meV/cell로 상당히 컸다. 따라서 6, 7 족 원소를 잘 활용하면 높은 스핀 분극율과 강한 수직 자기이방성를 동시에 가지는 물질을 제조할 수 있게 되어 STT-MRAM의 상용화에 기여를 할 수 있을 것으로 기대한다.

• 대한임베디드공학회논문지
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• 제11권2호
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• pp.67-78
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• 2016

The emerging Spin-Transfer Torque RAM (STT-RAM) is a promising component that can be used to improve the efficiency as a result of its high storage density and low leakage power. However, the state-of-the-art STT-RAM is not ready to replace SRAM technology due to the negative effect of its write operations. The write operations require longer latency and more power than the same operations in SRAM. Therefore, a hybrid cache with SRAM and STT-RAM technologies is proposed to obtain the benefits of STT-RAM while minimizing its negative effects by using SRAM. To efficiently use of the hybrid cache, it is important to place write intensive data onto the cache. Such data should be placed on SRAM to minimize the negative effect. Thus, we propose a technique that optimizes placement of data in main memory. It drives the proper combination of advantages and disadvantages for SRAM and STT-RAM in the hybrid cache. As a result of the proposed technique, write intensive data are loaded to SRAM and read intensive data are loaded to STT-RAM. In addition, our technique also optimizes temporal locality to minimize conflict misses. Therefore, it improves performance and energy consumption of the hybrid cache architecture in a certain range.

• IEIE Transactions on Smart Processing and Computing
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• 제5권4호
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• pp.274-282
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• 2016

This paper proposes code optimization techniques to reduce energy consumption of complex multimedia applications in a hybrid memory system with volatile dynamic random access memory (DRAM) and non-volatile spin-transfer torque magnetoresistive RAM (STT-MRAM). The proposed approach analyzes read/write operations for variables in an application. Based on the profile, variables with a high read operation are allocated to STT-MRAM, and variables with a high write operation are allocated to DRAM to reduce energy consumption. In this paper, to optimize code for real-life complicated applications, we develop a profiler, a code modifier, and compiler/link scripts. The proposed techniques are applied to a Fast Forward Motion Picture Experts Group (FFmpeg) application. The experiment reduces energy consumption by up to 22%.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제42회 동계 정기 학술대회 초록집
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• pp.386-386
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• 2012

We studied a bilateral switching device for spin transfer torque (STT-MRAM) based on 3D device simulation. Metal/P/N+or Metal/N/P+ junction device with $30{\times}30nm2$ area which is composed of one side schottky junction at Metal/P/N+ and Metal/N/P+ provides sufficient bidirectional current flow to write data by a drain induced barrier lowering (DIBL). In this work, Junction device confirmed that write current is more than 30 uA at 2 V, It is also has high on-off ratio over 105 under read operation. Junction device has good process feasibility because metal material of junction device could have been replaced by bottom layer of MTJ. Therefore, additional process to fabricate two outer terminals is not need. so, it provides simple fabrication procedures. it is expected that Metal/P/N+ or Metal/N/P+ structure with one side schottky junction will be a promising switch device for beyond 30 nm STT-MRAM.