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날짜 2023-01-11 11:00 
장소 E6-2 #1323 
연사 Sangjun Kang 
물리학과 양용수 교수님 연구실에서 한국과학기술연구원(KIST) 강상준 박사님을 초청하여,
로렌츠-주사투과전자현미경을 이용한 유연한 강자성체의 자기-탄성 결합에 대한 직접 관측에 대한 세미나를 아래와 같이 개최하고자 합니다.
구성원 여러분들의 많은 관심과 참여 부탁 드립니다.
* Title: Directly proving magnetoelastic coupling in a soft ferromagnet using Lorentz 4D-STEM

* Speaker: Sangjun Kang (KIST)   강상준 박사

* Date: 11am, 11th January 2023 (Wednesday)

* Place: E6-2 1323 (no zoom broadcasting)

Soft ferromagnetic materials, e.g. silicon ferrites and Fe-based amorphous alloys, play a major role in the conversion of energy owing to their high energy efficiency and power density [1]. Their magnetic structure consists of domains, where the magnetic dipoles are aligned to minimize the magnetostatic energy. The resulting magnetic structure is highly sensitive to local variation in the atomic spacing, i.e., atomic strain, of the materials due to magnetoelastic coupling through magnetocrystalline anisotropy (K_c) and stress anisotropy (K_σ) [2]. The anisotropy contributions raise coercivity (H_c) by restricting domain wall motions. In particular, for Fe-based amorphous alloys, which originally possess an isotropic atomic structure and extremely low H_c, the magnetic properties are extremely sensitive and usually deteriorated to the imposed stress [3]. This can be critical for their application in magnetoelectric machines, e.g. induction motors, which can be mechanically stressed during usage. To understand fundamental magnetism, e.g. magnetoelastic coupling, as a basis to design new materials, correlative measurements of the magnetic and atomic structure of soft ferromagnetic materials are desired.
We have developed Lorentz 4-dimensional scanning transmission electron microscopy (Ltz-4D-STEM) for correlative mapping of the magnetic structure, strain fields, and relative packing density and applied this approach to deformed Fe-based metallic glasses as illustrated in Figure 1. Our approach considers the momentum transfer of the electron beam due to the local magnetic field, the elliptic distortion of the amorphous diffraction ring under strain, and the area encompassed by the ring to quantify the relative atomic density and reveal their spatial-correlative variance [4]. This enables a direct pixel-level correlation of the magnetic and atomic structure and thus experimentally maps the magnetoelastic energy of soft ferromagnets. This method opens a new door to studying magnetic materials.

[1] Li et al., Progress in Materials Science 103, 235-318 (2019)

[2] Silveyra et al., Science 362, 418 (2018)

[3] Shen et al., Nat. Commun. 9, (4414), 2018

[4] Kang et al, Nat. Commun, Under review. Currently available at Nature portfolio (2022)

Attached: C.V


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