arXiv subjects

Tetsuya Hajiri

Publications and source records attributed to Tetsuya Hajiri.

7 recordsLinked to original sources

arXiv2026

Higher-order Hall response arises from octupole order and scalar spin chirality in a noncollinear antiferromagnet

Noncollinear antiferromagnets can generate a transverse electrical response known as the anomalous Hall effect, even though they possess almost no net magnetization. The microscopic origin of this behaviour, however, has remained unclear because conventional measurement geometries mix different contributions to the measured response. Here, we show that applying magnetic fields in selected in-plane directions allows us to disentangle the mechanisms underlying the Hall effect in a representative noncollinear antiferromagnet. By suppressing any dipole-related signal, we isolate a purely octupole-driven Hall response that exhibits a characteristic three-fold angular symmetry. At low magnetic fields, we further observe an additional Hall-like contribution that arises from the scalar spin chirality associated with noncoplanar spin textures. Combining symmetry analysis, first-principles calculations, and transport measurements, we reveal that octupole order, dipole moments, and chirality coexist and contribute in distinct field regimes. These findings establish a framework for identifying and controlling complex magnetic order parameters for spintronic applications.

cond-mat.mes-hall↗

arXiv2024

Fluctuation-mediated spin-orbit torque enhancement in the noncollinear antiferromagnet Mn3Ni0.35Cu0.65N

The role of spin fluctuations near magnetic phase transitions is crucial for generating various exotic phenomena, including anomalies in the extraordinary Hall effect, excess spin-current generation through the spin-Hall effect (SHE), and enhanced spin-pumping, amongst others. In this study, we experimentally investigate the temperature dependence of spin-orbit torques (SOTs) generated by Mn3Ni0.35Cu0.65N (MNCN), a member of the noncollinear antiferromagnetic family that exhibits unconventional magnetotransport properties. Our work uncovers a strong and nontrivial temperature dependence of SOTs, peaking near the Néel temperature of MNCN, which cannot be explained by conventional intrinsic and extrinsic scattering mechanisms of the SHE. Notably, we measure a maximum SOT efficiency of 30%, which is substantially larger than that of commonly studied nonmagnetic materials such as Pt. Theoretical calculations confirm a negligible SHE and a strong orbital Hall effect that can explain the observed SOTs. We propose a previously unidentified mechanism wherein fluctuating antiferromagnetic moments trigger the generation of substantial orbital currents near the Néel temperature due to the emergence of scalar spin chirality. Our findings present an approach for enhancing SOTs, which holds promise for magnetic memory applications by leveraging antiferromagnetic spin fluctuations to amplify both orbital and spin currents.

cond-mat.mes-hall↗

arXiv2018

Electromagnetic effects induced by time-dependent axion field

We studied the dynamics of the so-called $θ$-term, which exists in topological materials and is related to a hypothetical field predicted by Peccei-Quinn in particle physics, in a magnetic superlattice constructed using a topological insulator and two ferromagnetic insulators, where the ferromagnetic insulators had perpendicular magnetic anisotropies and different magnetic coercive fields. We examined a way to drive the dynamics of the $θ$-term in the magnetic superlattice through changing the inversion symmetry (from an anti-parallel to a parallel magnetic configuration) using an external magnetic field. As a result, we found that unconventional electromagnetic fields, which are magnetic field-induced charge currents and vice versa, are generated by the nonzero dynamics of the $θ$-term.

cond-mat.mes-hall↗

arXiv2017

Large-gap magnetic topological heterostructure formed by subsurface incorporation of a ferromagnetic layer

Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4 /Bi2Se3 heterostructure which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the co-deposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4 /Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac-cone gap opening of ~100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future " topotronics" devices.

cond-mat.mtrl-sci↗

arXiv2013

Polarization-Dependent Three-Dimensional Angle-Resolved Photoemission Spectroscopy of BaFe$_{1.8}$Co$_{0.2}$As$_{2}$

We performed polarization- and photon-energy-dependent angle-resolved photoemission spectroscopy of a slightly overdoped iron pnictide superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the three-dimensional electronic structure including its orbital characters at the Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and $d_{xy/x^2-y^2}$ orbitals were observed but $d_{z^2}$ hole FS, which has nodes according to a theory of the spin-fluctuation superconductivity mechanism, did not appear. These results suggest that no node will appear at hole FSs at the zone center.

cond-mat.supr-con↗

arXiv2012

Momentum-Dependent Hybridization Gap and dispersive in-gap state of The Kondo Semiconductor SmB6

We report the temperature-dependent three-dimensional angle-resolved photoemission spectra of the Kondo semiconductor SmB$_6$. We found a difference in the temperature dependence of the peaks at the X and $Γ$ points, due to hybridization between the Sm 5d conduction band and the nearly localized Sm 4f state. The peak intensity at the X point has the same temperature dependence as the valence transition below 120 K, while that at the $Γ$ point is consistent with the magnetic excitation at Q=(0.5,0.5,0.5) below 30 K. This suggests that the hybridization with the valence transition mainly occurs at the X point, and the initial state of the magnetic excitation is located at the $Γ$ point.

cond-mat.str-el↗

arXiv2011

Optical study of charge instability in CeRu2Al10 in comparison with CeOs2Al10 and CeFe2Al10

The anisotropic electronic structure responsible for the antiferromagnetic transition in CeRu$_2$Al$_{10}$ at the unusually high temperature of $T_0$ = 28 K was studied using optical conductivity spectra, Ce 3d X-ray photoemission spectra, and band calculation. It was found that the electronic structure in the $ac$ plane is that of a Kondo semiconductor, whereas that along the b axis has a nesting below 32 K (slightly higher than $T_0$). These characteristics are the same as those of CeOs$_2$Al$_{10}$ [S. Kimura {\it et al.}, Phys. Rev. Lett. 106, 056404 (2011).]. The $c$-$f$ hybridization intensities between the conduction and $4f$ electrons of CeRu$_2$Al$_{10}$ and CeOs$_2$Al$_{10}$ are weaker than that of CeFe$_2$Al$_{10}$, showing no magnetic ordering. These results suggest that the electronic structure with one-dimensional weak $c$-$f$ hybridization along the b axis combined with two-dimensional strong hybridization in the $ac$ plane causes charge-density wave (CDW) instability, and the CDW state then induces magnetic ordering.

cond-mat.str-el↗