Origin of magnetovolume effect investigated by visualization of Medium-Range Atomic arrangement

2023 Fiscal Year Final Research Report

• Project/Area Number: 21H01043
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 13030:Magnetism, superconductivity and strongly correlated systems-related
• Research Institution: Hiroshima University
• Principal Investigator: Ishimatsu Naoki 広島大学, 先進理工系科学研究科(理), 特定准教授 (70343291)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: インバー合金 / 磁気体積効果 / X線吸収分光法 / 逆モンテカルロ法

Outline of Final Research Achievements

Fcc-based Fe alloys exhibit anomalous thermal expansion due to the large magnetovolume effect. In this study, atomic-scale origin of the Invar effect, which is nearly-zero thermal expansion observed, was investigated by reverse Monte Carlo analysis using complementary data sets of EXAFS and XRD. The inter atomic distances of Fe-Fe atomic pairs were ~0.02 A longer than those of the Fe-Ni and Ni-Ni pairs. The elongation in the Fe-Fe pairs was suppressed with increasing pressure, and the distances of the three pairs were comparable under pressures above the magnetic transition from ferromagnetic to paramagnetic phase. Therefore, the Fe-Fe pairs dominantly contribute to the volume expansion due to the magnetovolume effect. Because a similar magnitude of elongation was observed in the Fe-Fe pairs of a non-Invar Fe-Ni alloy, we conclude that the Invar effect originates from the delicate balance between the number of Fe-Fe pairs and their elongation depending on the magnetization.

Free Research Field

高圧下の構造物性研究

Academic Significance and Societal Importance of the Research Achievements

インバー合金のほぼゼロの熱膨張は1897年の発見から120年以上が経過したが原子レベルでのその原因が分かっていなかった．X線吸収分光法と逆モンテカルロ法を用いた合金構造の可視化より，今回初めてインバー合金の特異な磁気体積効果がFe-Fe間の原子間距離の伸長・収縮によって生じていることを見出した．一般的に面心立方格子中のFeの磁気状態には大きな磁気体積効果が期待されるが，Fe-Fe間の伸長という特長がインバー合金に限らず組成の異なる合金中においても実現していることも見出しており，意義ある結果といえる．