研究

Calculation of the Biquadratic Spin Interactions Based on the Spin Cluster Expansion for Ab initio Tight-binding Models

Journal of the Physical Society of Japan 94, 124709 (2025) [Editors' Choice] 

磁性体の性質を理解・予測するためには、電子スピン間にはたらく相互作用を実物質に即して評価することが重要です。従来の有効スピン模型では、ハイゼンベルク型の相互作用がよく用いられてきましたが、近年注目される複雑な磁気構造や量子物性を記述するには、より高次のスピン間相互作用が本質的な役割を果たす場合があります。本研究では、スピンクラスター展開と無秩序局所モーメント法を組み合わせた枠組みを第一原理計算と組み合わせ、高次スピン相互作用を評価する計算手法を開発しました。これにより、複雑な磁気秩序を支配する高次スピン相互作用の電子論的起源を明らかにし、実物質に基づいた解析を行う計算基盤を提示しました。

We develop a calculation scheme using ab initio tight-binding Hamiltonians to evaluate biquadratic magnetic interactions. This approach relies on the spin cluster expansion combined with the disordered local moment (DLM) method, originally developed within the multiple scattering Korringa–Kohn–Rostoker method. Applying it to a single-orbital Hubbard model with two sublattices, we show that the evaluated DLM biquadratic interactions are in good agreement with those obtained from the strongly correlated limit, demonstrating the wide applicability of the method to various magnetic systems with large local moments. We then apply it to the ab initio tight-binding models for elemental magnetic metals; the resulting magnetic interactions align well with previous literature. Finally, we explore its performance in more complex compounds, such as transition metal dichalcogenides with intercalation of 3d transition metals and potassium electrosodalite. The obtained results for both compounds show good agreement with experiments. The present approach offers a convenient ab initio path for evaluating biquadratic interactions and understanding the electronic mechanisms controlling them.

Symmetry analysis with spin crystallographic groups: 

Disentangling effects free of spin-orbit coupling in emergent electromagnetism

Physical Review B 109, 094438 (2024) [Editors' Suggestion] 

対称性は物質を特徴付ける最も基本的な量の一つですが、最近、物質の磁気構造を特徴付ける対称性として、従来の磁気空間群とは異なるスピン空間群という対称性が注目を集めています。スピン空間群は、物理的にはスピン軌道相互作用の影響を無視できるような系の対称性であり、スピン軌道相互作用フリーな応答現象を特徴付けます。最近、我々は、与えられた磁気構造と結晶構造に対してスピン空間群を同定するコードを開発しました。本研究では、開発したコードを用いてデータベース上の広域な物質群に対する解析を行い、スピン空間群で記述される新奇現象の予測や物質提案を行なっています。

Recent studies identified spin-order-driven phenomena such as spin-charge interconversion without relying on the relativistic spin-orbit interaction. Those physical properties can be prominent in systems containing light magnetic atoms due to sizable exchange splitting and may pave the way for realization of giant responses correlated with the spin degree of freedom. In this paper, we present a systematic symmetry analysis based on the spin crystallographic groups and identify the physical property of a vast number of magnetic materials up to 1500 in total. By decoupling the spin and orbital degrees of freedom, our analysis enables us to take a closer look into the relation between the dimensionality of spin structures and the resultant physical properties and to identify the spin and orbital contributions separately. In stark contrast to the established analysis with magnetic space groups, the spin crystallographic group manifests richer symmetry including spin-translation symmetry and leads to emergent responses. For representative examples, we discuss the geometrical nature of the anomalous Hall effect and magnetoelectric effect and classify the spin Hall effect arising from the nonrelativistic spin-charge coupling. Using the power of computational analysis, we apply our symmetry analysis to a wide range of magnets, encompassing complex magnets such as those with noncoplanar spin structures as well as collinear and coplanar magnets. We identify emergent multipoles relevant to physical responses and argue that our method provides a systematic tool for exploring sizable electromagnetic responses driven by spin order.