遠くまで届く磁気の力が相転移を支配―絶縁性フェリ磁性体で「平均場臨界性」を初めて実証―

2026-08-19 京都大学

フェリ磁性相転移の性質を表す臨界指数β（赤）、γ（緑）、δ（青）を、中性子粉末回折（NPD）、Kouvel-Fisher（KF）解析、および磁化の磁場依存性M(μ0H) から求め、平均場理論と代表的な理論モデルの値と比較した。（作成：南部雄亮）

＜関連情報＞

• https://www.kyoto-u.ac.jp/ja/research-news/2026-08-19-1
• https://journals.aps.org/prl/abstract/10.1103/5ftk-7qyg

フェリ磁性体 Eu2MnSi2O7における双極⼦相互作⽤による平均場臨界性 Dipolar-Driven Mean-Field Criticality in the Ferrimagnet Eu2⁢MnSi2⁢O7

Masahiro Kawamata, Maxim Avdeev, and Yusuke Nambu
Physical Review Letters  Published: 12 August, 2026
DOI: https://doi.org/10.1103/5ftk-7qyg

Abstract

We report mean-field critical behavior in Eu₂⁢MnSi₂⁢O₇, a melilite-type ferrimagnet with spin-only Eu²⁺ and Mn²⁺ moments and negligible orbital contributions. Magnetization measurements combined with neutron powder diffraction reveal critical exponents close to the mean-field values, indicating that long-range dipolar interactions govern the asymptotic critical behavior in this insulating ferrimagnet. The refined magnetic structure, described by the magnetic space group ⁢2₁⁢2′₁⁢2′, exhibits a tilted ferrimagnetic configuration driven by the Dzyaloshinskii-Moriya interaction, reflecting the noncentrosymmetric nature of the lattice. These results extend the applicability of mean-field theory to complex insulating magnets and establish Eu₂⁢MnSi₂⁢O₇ as a platform for exploring ferrimagnetism and long-range interactions. To our knowledge, this is the first insulating ferrimagnet in which dipolar interactions drive mean-field criticality.