研究

最近の研究から

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.

High-throughput calculations of antiferromagnets hosting anomalous transport phenomena

Physical Review B 109, 094435 (2024)

ハードディスクなどの磁気デバイスに使われるのは、通常強磁性体ですが、最近、反強磁性体を用いた磁気デバイスの研究、すなわちスピントロニクスが注目を集めています。実際、反強磁性体を使うことでさまざまなメリットがあるのですが、磁気デバイスに使用可能な反強磁性体はただの反強磁性体とは少し異なり、その数も限られています。本研究は、第一原理計算とクラスター多極子法と呼ばれる磁気構造の生成手法を用いてハイスループット計算を行い、スピントロニクスに利用可能な少し"変わった"反強磁性体の網羅的な探索を行なったものです。

We develop a high-throughput computational scheme based on cluster multipole theory to identify new functional antiferromagnets (AFMs). This approach is applied to 228 magnetic compounds listed in the AtomWork-Adv database, known for their elevated Néel temperatures. We conduct systematic investigations of both stable and metastable magnetic configurations of these materials. Our findings reveal that 34 of these compounds exhibit AFM structures with zero propagation vectors and magnetic symmetries identical to conventional ferromagnets, rendering them potentially invaluable for spintronics applications. By cross-referencing our predictions with the existing MAGNDATA database and published literature, we verify the reliability of our findings for 26 out of 28 compounds with partially or fully elucidated magnetic structures in the experiments. These results not only affirm the reliability of our scheme but also point to its potential for broader applicability in the ongoing quest for the discovery of functional magnets.

High-performance descriptor for magnetic materials: Accurate discrimination of magnetic structure

Physical Review B 108, 014403 (2023)

The magnetic structure is crucial in determining the physical properties inherent in magnetic compounds. We present an adequate descriptor for magnetic structure with proper magnetic symmetry and high discrimination performance, which does not depend on artificial choices for coordinate origin, axis, and magnetic unit cell in crystal. We extend the formalism called “smooth overlap of atomic positions” (SOAP), providing a numerical representation of atomic configurations to that of magnetic moment configurations. We introduce the descriptor in terms of the vector spherical harmonics to describe a magnetic moment configuration and partial spectra from the expansion coefficients. We discuss that the lowest-order partial spectrum is insufficient to discriminate the magnetic structures with different magnetic anisotropy, and a higher-order partial spectrum is required in general to differentiate detailed magnetic structures on the same atomic configuration. We then introduce the fourth-order partial spectrum and evaluate the discrimination performance for different magnetic structures, mainly focusing on the difference in magnetic symmetry. The modified partial spectra that are defined not to reflect the difference of magnetic anisotropy are also useful in evaluating magnetic structures obtained from the first-principles calculations performed without spin-orbit coupling. We apply the present method to the symmetry-classified magnetic structures for the crystals of Mn3r and Mn3Sn, which are known to exhibit anomalous transport under the antiferromagnetic order, and examine the discrimination performance of the descriptor for different magnetic structures on the same crystal.

Local density of states as a probe for tunneling magnetoresistance effect: Application to ferrimagnetic tunnel junctions

Physical Review B 107, 214442 (2023)

We investigate the tunneling magnetoresistance (TMR) effect using the lattice models which describe the magnetic tunnel junctions (MTJ). First, taking a conventional ferromagnetic MTJ as an example, we show that the product of the local density of states (LDOS) at the center of the barrier traces the TMR effect qualitatively. The LDOS inside the barrier has the information on the electrodes and the electron tunneling through the barrier, which enables us to easily evaluate the tunneling conductance more precisely than the conventional Julliere's picture. We then apply this method to the MTJs with collinear ferrimagnets including antiferromagnets. The TMR effect in the ferrimagnetic MTJs changes depending on the interfacial magnetic structures originating from the sublattice structure, which can also be captured by the LDOS. Our findings will reduce the computational cost for the qualitative evaluation of the TMR effect and be useful for a broader search for the materials which work as the TMR devices showing high performance.

Importance of self-consistency in first-principles Eliashberg calculation for superconducting transition temperature

Journal of Physics and Chemistry of Solids 178, 111348 (2023)

We have performed first-principles Eliashberg calculations for several conventional superconductors. Using our recently developed scheme of solving the anisotropic Eliashberg gap equation considering the retardation effect and mass renormalization effect, we investigate the importance of the self-consistency in the calculation. We compare the estimated 𝑇c given by the self-consistent calculation and the one-shot calculation for the self-energy of electrons. We show that the difference of the one-shot results and self-consistent results will become larger for systems for which the density of states changes rapidly around the Fermi level, such as the presence of the van Hove singularities in pressurized hydrides.