https://scholar.dgist.ac.kr/handle/20.500.11750/810 2026-11-15T15:28:07Z https://scholar.dgist.ac.kr/handle/20.500.11750/61012 Title: Comprehensive Raman spectroscopy study of MBE-grown off-stoichiometric van der Waals ferromagnet Fe5-xGeTe2 thin film Author(s): Gayen, Anabil; Nguyen, Van Quang; Kim, Jungwon; Mustaghfiroh, Qoimatul; Das, Priyanka; Rahmani, Fathiya; Lee, June Hyuk; You, Chun-Yeol; Lee, Hyun Seok; Phan, The-Long; Kim, Dong-Hyun Abstract: Off-stoichiometric Fe5-xGeTe2 (F5GT) has recently emerged as a promising two-dimensional (2D) magnetic material within the van der Waals (vdW) family, owing to its tunable magnetic properties and high Curie temperature, attractive for potential spintronic applications. Here, we report a comprehensive Raman spectroscopic investigation of a high-quality epitaxial F5GT thin film (similar to 50 nm) grown by molecular beam epitaxy on c-cut Al2O3. The epitaxial nature and structural quality of the film are confirmed by in-situ reflection high-energy electron diffraction (RHEED) and out-of-plane X-ray diffraction (XRD), revealing strong c-axis orientation. For the first time, polarization-resolved Raman measurements using both linearly and circularly polarized light are performed to identify and assign phonon symmetries. We also performed temperature-dependent polarized Raman studies to explore the thermal stability and evolution of the phonon modes as well as the crystal symmetry. Polarization-resolved Raman spectroscopy results were compared and corroborated with calculated optical selection rules based on crystal symmetry-dependent Raman tensors. This comparison indicates the breaking of intrinsic three-fold crystal symmetry, resulting in mixing of in-plane and out of plane vibrational modes, possibly due to Fe deficiency in off-stoichiometric Fe5-xGeTe2. Additionally, to explore temperature-driven anharmonicity in phonon modes, we investigated the temperature-dependent evolution of the primary phonon modes, excited with unpolarized light, both in the absence and presence of applied magnetic field. The observed temperature-induced frequency and linewidth variations are correlated with the temperature-dependent magnetization behavior, providing possible insights into the interplay between lattice dynamics and the magnetic ordering. A high Curie temperature of similar to 307 K and a weak spin-phonon coupling are observed in this off-stoichiometric F5GT film. The present study reveals, based on the first extensive Raman spectroscopy investigation, (i) off-stoichiometry-driven phonon symmetry modification and (ii) existence of magnetic ordering near room temperature in F5GT, validating its potential as a 2D vdW magnet and establishing Raman spectroscopy as a powerful tool for probing 2D vdW magnets. 2026-09-30T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/60978 Title: Néel-Vector-Dependent Unconventional Spin-Orbit Torque for Deterministic Field-Free Switching in NiO (110)-Based Trilayers Author(s): Seo, Hyeong-Joo; Kim, Seok-Jong; Van, Phuoc Cao; Truong, Hong-Cuong; Han, Dong-Hyeon; Kim, Geunwoo; Jo, Younghun; Jung, Jong Hoon; Kim, Soojung; You, Chun-Yeol; Jeong, Jong-Ryul; Lee, Kyung-Jin; Park, Byong-Guk Abstract: Spin-orbit torque (SOT) provides an efficient route for ultrafast and energy-efficient magnetization control in spintronic devices. However, deterministic switching of perpendicular magnetization in conventional non-magnet/ferromagnet bilayers is fundamentally limited by symmetry, typically necessitating an external magnetic field. In this work, we demonstrate unconventional spin currents and associated SOTs in antiferromagnet (AFM)-based NiO(110)/Ta/CoFeB/MgO trilayers, enabling nearly complete field-free switching of perpendicular magnetization. We find that the switching efficiency is maximized when the applied current is oriented transverse to the AFM easy axis, indicating that the generated spin currents are governed by the relative orientation between the N & eacute;el vector and the current direction. Second-harmonic measurements reveal angle-dependent spin currents with both in-plane and out-of-plane spin polarizations, which is consistent with micromagnetic simulations incorporating the N & eacute;el-vector dynamics in the AFM layer. Our findings suggest that AFM-based trilayers offer a versatile platform for efficient field-free switching and highlight the potential of N & eacute;el-vector-dependent spin currents for SOT-based spintronic applications. 2026-08-31T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/60968 Title: Electrical Tuning of Ferrimagnetic Spin Alignment Angle and Compensation Temperature in Pt/IrMn3/CoGd Multilayers Author(s): Choi, Won-Chang; 심태보; Park, Kwon-Jin; Kim, Tae-Hwan; Lee, Soobeom; Tran, Bao Xuan; Kim, Soojung; You, Chun- Yeol; Hong, Jung- Il Abstract: Compensation temperature (T M) is a characteristic parameter for a ferrimagnetic (FIM) material that requires careful consideration in the functional applications of FIM spintronics. Attempts at precise and localized control of T M have remained rare and challenging, but apparently a worthy issue for the expansion of spintronics to include FIMs. Conventional approaches such as thermal annealing or ion injection suffer from limited spatial selectivity and switching speed. Here, we show a novel route to control the T M of FIM by manipulating the alignments of the constituent spins through spin-orbit torque (SOT)-controlled exchange couplings with adjacent antiferromagnets. At the interface between antiferromagnetic IrMn3 and FIM CoGd multilayers, it was found that the antiferromagnet established firm exchange couplings with Co atoms, but negligible couplings with Gd. Such selective coupling modified the traditionally known collinear antiparallel arrangement of CoGd spins into a noncollinear spiral spin configuration, and the T M was found to correspondingly change significantly. Additionally, precise tuning of FIM properties through the modulation of SOT amplitude and application time was demonstrated, establishing an efficient electrical pathway for controlling the compensation phenomena in ferrimagnetic systems. 2026-09-30T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/60887 Title: Optimization of MuMax3 by Using Claude Code: A CUDA-Graph-Based Case Study in AI-Assisted Performance Engineering Author(s): You, Chun-Yeol Abstract: MuMax3 is a widely used open-source GPU-accelerated micromagnetic simulator whose computational core - CUDA kernels and cuFFT-based demagnetization convolutions - has changed little since its original release. We report a case study in which an agentic large-language-model coding assistant (Claude Code, Anthropic) was used, under continuous human supervision, to profile and optimize this mature CUDA/Go codebase. Profiling with NVIDIA Nsight Systems revealed that for small and medium grids (64 & times;64 & times;1 to 256 & times;256 & times;1 cells), 75-79 % of step time is spent in CPU-side cuLaunchKernel driver calls rather than in GPU computation, because every solver step launches roughly 27 kernels sequentially on a single CUDA stream. Building on this finding, we implemented a "split-graph" execution strategy: the time-step-independent torque-evaluation kernel sequences of each solver stage are captured once as CUDA Graphs and replayed via cudaGraphLaunch, while the time-step-dependent update, error estimate, and adaptive-step-size logic remain ordinary stream-ordered calls. The optimization is exposed transparently through Run()/Steps(), guarded by a compatibility check (constant excitation, zero thermal field, no custom field terms, time-independent material parameters, mesh size below a tunable threshold) that falls back silently to the original code path when violated. Across five solvers (Heun, RK23, RK45DP, RK56, Backward Euler), the optimization yields up to 5.4 & times; throughput for a 64 & times;64 & times;1 grid with a fixed time step, 2.4-3.6 & times; for adaptive time-stepping, decreasing smoothly to approximate to 1.0 & times; near 10(6) cells, essentially independent of which physical field terms (exchange, anisotropy, DMI) are active. All results were verified bit-for-bit identical (ndiff=0) against the unmodified code, and the full 176-script mumax3 regression suite passes with zero failures. We discuss the workflow itself - including three episodes in which the assistant autonomously diagnosed and corrected its own defects - as a template for AI-assisted optimization of legacy scientific HPC codes. 2026-05-31T15:00:00Z