Showing 1–50 of 54 results for author: Kikkawa, A

Bridging ambient- and high-pressure superconductivity in La$_2$LnNi$_2$O$_7$ films

Authors: Motoki Osada, Chieko Terakura, Shusaku Imajo, Jean-Baptiste Morée, Akiko Kikkawa, Masamichi Nakajima, Hsiao-Yi Chen, Yusuke Nomura, Koichi Kindo, Ryotaro Arita, Yoshinori Tokura, Atsushi Tsukazaki

Abstract: The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity near 80 K in bilayer nickelates under high pressure has sparked extensive studies. While superconductivity exceeding 40 K was subsequently discovered at ambient pressure in compressively strained films, the relationship between ambient- and high-pressure regimes remains an open question. Here we present a systematic inve… ▽ More The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity near 80 K in bilayer nickelates under high pressure has sparked extensive studies. While superconductivity exceeding 40 K was subsequently discovered at ambient pressure in compressively strained films, the relationship between ambient- and high-pressure regimes remains an open question. Here we present a systematic investigation of superconductivity in compressively strained La$_2$LnNi$_2$O$_7$ films (Ln = lanthanides) at ambient and high pressures. The normal-state resistivity at ambient pressure, revealed by suppressing superconductivity with magnetic fields of 59 T, tends toward $T^2$ behaviour. Under high pressure in a cubic-anvil cell, $T_{\mathrm{c}}$ was enhanced from 41-42 K at ambient pressure to 67-73 K at 16 GPa. On the other hand, lattice compression induced by Ln substitution, which may mimic effects of pressure, lowers $T_{\mathrm{c}}$. In both cases, $T_{\mathrm{c}}$ correlates with the evolution of normal-state transport between $T^2$ and $T$-linear behaviour, offering insight into the interplay between lattice structure and superconductivity in bilayer nickelates. △ Less

Submitted 18 August, 2026; originally announced August 2026.

Comments: 31 pages, 4 figures

Journal ref: Nature Materials (2026)

Uniaxial stress effects on magnetic and electric properties of the ground state in a centrosymmetric magnetic skyrmion host Gd$_2$PdSi$_3$

Authors: Hiraku Saito, Naoki Kobayashi, Takuro Kawasaki, Tatsuya Nakamura, Akiko Kikkawa, Yasujiro Taguchi, Yoshinori Tokura, Taro Nakajima

Abstract: We investigate effects of uniaxial stress to magnetic orders and electrical resistivity of the centrosymmetric magnetic skyrmion compound Gd$_2$PdSi$_3$, which has a hexagonal crystal structure composed of triangular lattice layers of magnetic Gd$^{3+}$ ions. This compound is known to exhibit the triple-$q$ magnetic skyrmion lattice phase with a giant topological Hall effect in the first field ind… ▽ More We investigate effects of uniaxial stress to magnetic orders and electrical resistivity of the centrosymmetric magnetic skyrmion compound Gd$_2$PdSi$_3$, which has a hexagonal crystal structure composed of triangular lattice layers of magnetic Gd$^{3+}$ ions. This compound is known to exhibit the triple-$q$ magnetic skyrmion lattice phase with a giant topological Hall effect in the first field induced phase [T. Kurumaji $\textit{et al}$. Science $\textbf{365}$, 914-918 (2019)]. In contrast to the established picture of the field-induced phase, the ground state of this system still remains to be studied. Although previous studies reported the existence of the incommensurate magnetic modulations described by a magnetic modulation wave vector ${\bf q}=(q,0,0)$ where $q\sim 0.14$ and its equivalents, it is still unclear whether the magnetic structure is a single-$q$ structure or a multiple-$q$ structure. In the present study, we performed magnetization, resistivity and neutron diffraction measurements with a compressive uniaxial stress applied perpendicular to the $c$ axis. The observed data revealed that the system did not exhibit anisotropic magnetic and electric properties expected from a single-$q$ magnetic order, suggesting that the magnetic ground state of this system is a multi-$q$ magnetic order. △ Less

Submitted 10 August, 2026; originally announced August 2026.

Comments: 7 pages, 4 figures

Density-wave phases, anisotropic transport, and Planckian dissipation in single crystals of the superconductor La3Ni2O7

Authors: Zhehong Liu, Masamichi Nakajima, Markus Kriener, Shunsuke Kitou, Xiaowei Lyu, Chieko Terakura, Kosuke Karube, Ka Man Yip, Sorin Lazar, Nobuto Nakanishi, Keiko Shimada, Akiko Kikkawa, Yukako Fujishiro, Xiuzhen Yu, Taka-hisa Arima, Yoshinori Tokura, Yasujiro Taguchi

Abstract: Pressure-induced superconductivity in bilayer nickelates provides a platform for investigating intertwined roles of charge/spin orders and electric transport in unconventional superconductivity. However, important quantitative information on the transport, such as the absolute value of the resistivity, the anisotropy, and the scattering rate of carriers, remains insufficient due to the lack of acc… ▽ More Pressure-induced superconductivity in bilayer nickelates provides a platform for investigating intertwined roles of charge/spin orders and electric transport in unconventional superconductivity. However, important quantitative information on the transport, such as the absolute value of the resistivity, the anisotropy, and the scattering rate of carriers, remains insufficient due to the lack of accurate measurements using large single crystals. Here we establish a high-precision pressure-temperature phase diagram of high-quality La3Ni2O7 single crystals, by measuring the in-plane and out-of-plane resistivities. We resolve two distinct anomalies associated with density-wave formation with contrasting pressure dependences. The pressure-induced structural transition enhances not only the resistivity values for both directions, but also its anisotropy at low temperatures, demonstrating a pronounced effect of density-wave order on the charge dynamics. Superconductivity with zero-resistance emerges near the boundary where the density-wave phases are fully suppressed, and above Tc, the resistivity exhibits a temperature-linear dependence over a wide temperature range while the scattering rate falls within a regime of the Planckian limit. Our results show that pressure dramatically changes the anisotropic charge transport via modifying density-wave orders, and eventually produces a pronounced strange-metal state with strong scatterings, from which superconductivity develops. This establishes robust density-wave correlations and Planckian dissipation as remarkable features of La3Ni2O7. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Comments: 44 pages, 4 figures, 9 extended data, 5 supplementary data, 4 supplementary tables

Emergent toroidal induction in a polar Weyl ferromagnet

Authors: Yuuri Suzuki, Yukako Fujishiro, Masataka Mogi, Juba Bouaziz, Takahiro Anan, Akiko Kikkawa, Daiki Yamaguchi, Max T. Birch, Yuto Kiyonaga, Minoru Kawamura, Yasujiro Taguchi, Takahiro Morimoto, Naoto Nagaosa, Ryotaro Arita, Yoshinori Tokura

Abstract: Spin-orbit coupling (SOC) underpins modern spintronics by enabling the electrical generation of spin torques. Its reciprocal counterpart, in which magnetization dynamics produce electromotive forces through a spin-dependent Berry phase, is known as emergent electromagnetic induction (EEMI). However, this effect has previously been observed only in magnetic textures with spatial gradients, such as… ▽ More Spin-orbit coupling (SOC) underpins modern spintronics by enabling the electrical generation of spin torques. Its reciprocal counterpart, in which magnetization dynamics produce electromotive forces through a spin-dependent Berry phase, is known as emergent electromagnetic induction (EEMI). However, this effect has previously been observed only in magnetic textures with spatial gradients, such as domain walls, helices, and skyrmions. Here, we demonstrate that even a spatially uniform ferromagnet can host EEMI through a previously unrecognized Berry-phase mechanism inherent to noncentrosymmetric conductors. In the polar Weyl ferromagnet PrAlGe, an applied alternating current generates spin-orbit torques that drive collective magnetization dynamics. The resulting emergent toroidal moment (T = P \times M), where (P) is the crystal's polar axis and (M) is the net magnetization, acts as a gauge potential whose time derivative (dT/dt) induces a Hall voltage. This contribution appears specifically in the out-of-phase component of the AC Hall response and scales linearly with frequency, providing direct evidence for EEMI. First-principles calculations further reveal that this toroidal vector encodes the collective motion of Weyl nodes in momentum space. These findings establish "emergent toroidal induction" as a new manifestation of spin-orbit entanglement, unifying Berry phase, topology, and spin dynamics while opening a pathway toward intrinsic and energy-efficient spin-charge interconversion. △ Less

Submitted 14 July, 2026; originally announced July 2026.

Comments: 35 pages, 4 figures, 2 extended figures

Pressure-induced superconductivity in epitaxially-stabilized Pr$_3$Ni$_2$O$_7$ films

Authors: Motoki Osada, Chieko Terakura, Hsiao-Yi Chen, Akiko Kikkawa, Masamichi Nakajima, Ryoma Asai, Jean-Baptiste Morée, Yusuke Nomura, Ryotaro Arita, Yoshinori Tokura, Atsushi Tsukazaki

Abstract: The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity in La$_3$Ni$_2$O$_7$ under high pressure has led to a rapid expansion of the $T_{\mathrm{c}}$ range through lanthanide $Ln$ substitution, and to ambient-pressure superconductivity in strained thin films, yet the exploration of new bilayer nickelates remains strongly constrained by thermodynamic stability. Beyond the diff… ▽ More The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity in La$_3$Ni$_2$O$_7$ under high pressure has led to a rapid expansion of the $T_{\mathrm{c}}$ range through lanthanide $Ln$ substitution, and to ambient-pressure superconductivity in strained thin films, yet the exploration of new bilayer nickelates remains strongly constrained by thermodynamic stability. Beyond the difficulty of synthesis of bulk single-crystals, here we report on the pressure-induced high-$T_{\mathrm{c}}$ superconductivity in epitaxially-stabilized Pr$_3$Ni$_2$O$_7$ thin films. While the Pr$_3$Ni$_2$O$_7$ films exhibit insulating behaviour at ambient pressure regardless of ozone-annealing treatment, they show $T$-linear metallic transport and superconductivity reaching an onset $T_{\mathrm{c}}$ of 66 K and zero-resistance at nearly 40 K at 22 GPa. Furthermore, Nd$_3$Ni$_2$O$_7$, with the smaller rare-earth ion Nd, can also be stabilized, however, superconductivity is not observed in the measured pressure range. Epitaxial stabilization enables us to examine the dependence of $T_{\mathrm{c}}$ and the critical pressure $P_{\mathrm{c}}$ for superconductivity on the $Ln$ ion in $Ln_3$Ni$_2$O$_7$ ($Ln$ = La, Pr, Nd). These results suggest that a higher $P_{\mathrm{c}}$ is required for smaller $Ln$ ions, consistent with trends observed in bulk studies of $Ln$ substitution. This study demonstrates that epitaxial stabilization is a powerful technique to further expand the family of superconducting bilayer nickelates. △ Less

Submitted 19 May, 2026; originally announced May 2026.

Comments: 23 pages, 9 figures

Frustration of harmonic and solitonic helimagnetism on the body-centered tetragonal lattice of GdAlSi

Authors: Ryota Nakano, Rinsuke Yamada, Oleg I. Utesov, Masaki Gen, Akiko Kikkawa, Hajime Sagayama, Hironori Nakao, Masashi Tokunaga, Taka-hisa Arima, Yoshinori Tokura, Se Kwon Kim, Max Hirschberger

Abstract: The triangular lattice antiferromagnet (TLAF) with nearest-neighbor exchange interaction is a model platform in the field of frustrated magnetism. Here, anharmonic (`up-up-down') and harmonic (`120 degree') magnetic states compete, because the fundamental helimagnetic wave and its higher harmonic are degenerate in energy. We show that a body-centered tetragonal lattice (BCTL) can realize a similar… ▽ More The triangular lattice antiferromagnet (TLAF) with nearest-neighbor exchange interaction is a model platform in the field of frustrated magnetism. Here, anharmonic (`up-up-down') and harmonic (`120 degree') magnetic states compete, because the fundamental helimagnetic wave and its higher harmonic are degenerate in energy. We show that a body-centered tetragonal lattice (BCTL) can realize a similar frustration of harmonic and anharmonic helimagnetic states, and that the tetragonal magnetic Weyl semimetal GdAlSi realizes this scenario. In an applied magnetic field, resonant elastic X-ray scattering reveals a competition of harmonic cycloidal and solitonic double-Q states, well consistent with mean-field calculations. Our work provides a new paradigm for frustration physics in BCTL materials. △ Less

Submitted 20 May, 2026; v1 submitted 8 May, 2026; originally announced May 2026.

Collinear ferromagnetism with reduced moment length in kagome magnet Nd3Ru4Al12

Authors: Yuki Ishihara, Ryota Nakano, Rinsuke Yamada, Takuya Nomoto, Priya R. Baral, Moritz M. Hirschmann, Kamini Gautam, Kamil K. Kolincio, Akiko Kikkawa, Seno Aji, Hiraku Saitoh, Masaaki Matsuda, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura, Taro Nakajima, Max Hirschberger

Abstract: We determine the magnetic ground state of the kagome lattice magnet Nd3Ru4Al12 by single-crystal neutron diffraction, supported by experiments with polarized neutrons. We identify this material as a collinear ferromagnet ("hex-FM") with uniform moment length mc = 2.1 μB/Nd and ordering vector Q = 0, in contrast to a previous, seminal report that proposed unequal moment lengths on two Nd sites, her… ▽ More We determine the magnetic ground state of the kagome lattice magnet Nd3Ru4Al12 by single-crystal neutron diffraction, supported by experiments with polarized neutrons. We identify this material as a collinear ferromagnet ("hex-FM") with uniform moment length mc = 2.1 μB/Nd and ordering vector Q = 0, in contrast to a previous, seminal report that proposed unequal moment lengths on two Nd sites, here called the "ortho-FM" state. Our analysis of the flipping ratio in polarized neutron scattering is consistent with the hex-FM state. The results provide a microscopic basis for understanding the large fluctuation-induced Hall and Nernst responses near TC = 41 K, as previously reported for Nd3Ru4Al12. △ Less

Submitted 2 June, 2026; v1 submitted 2 May, 2026; originally announced May 2026.

Comments: 15 pages, 6 figures

Journal ref: Phys. Rev. B 113, 214410 (2026)

Controlling the Band Filling and the Band Width in Nickelate Superconductors

Authors: M. Kriener, C. Terakura, A. Kikkawa, Z. Liu, H. Murayama, M. Nakajima, Y. Fujishiro, S. Sasano, R. Ishikawa, N. Shibata, Y. Tokura, Y. Taguchi

Abstract: The new family of superconducting nickelates centered around La$_{3}$Ni$_{2}$O$_{7}$ possesses attractive features, such as the high transition temperature and the presence of an antiferromagnetic ground state at ambient pressure, suggesting an unconventional pairing mechanism. In the nonsuperconducting state, the possibility of different density-wave orders with opposite pressure dependencies is… ▽ More The new family of superconducting nickelates centered around La$_{3}$Ni$_{2}$O$_{7}$ possesses attractive features, such as the high transition temperature and the presence of an antiferromagnetic ground state at ambient pressure, suggesting an unconventional pairing mechanism. In the nonsuperconducting state, the possibility of different density-wave orders with opposite pressure dependencies is discussed, whose relationships and microscopic origins are largely unknown. However, sample-quality issues, such as impurity-phase formation or oxygen vacancies, impede the progress in the field. Here, we employ high-pressure synthesis and hydrostatic high-pressure transport techniques to investigate bilayer nickelates with controlled band width and filling, and perform a systematic study on their impact on the superconductivity and other characteristic properties. While increasing the tilting of the NiO$_6$ octahedra shifts the superconducting phase to higher pressure, simultaneous hole doping reverts this trend. We also observe up to three distinct anomalies in the nonsuperconducting state which are possibly related to density-wave formation. △ Less

Submitted 15 April, 2026; originally announced April 2026.

Comments: 20 pages, 4 Figures (main) and 19 pages, 12 Figures (Supplement)

Emergent electric field induced by dissipative sliding dynamics of domain walls in a Weyl magnet

Authors: Rinsuke Yamada, Daichi Kurebayashi, Yukako Fujishiro, Shun Okumura, Daisuke Nakamura, Fehmi S. Yasin, Taro Nakajima, Tomoyuki Yokouchi, Akiko Kikkawa, Yasujiro Taguchi, Yoshinori Tokura, Oleg A. Tretiakov, Max Hirschberger

Abstract: The dynamic motion of topological defects in magnets induces an emergent electric field, as exemplified by the continuous flow of skyrmion vortices. However, the electrodynamics underlying this emergent field remains poorly understood. In this context, magnetic domain walls - one dimensional topological defects with two collective modes, sliding and spin tilt - offer a promising platform for explo… ▽ More The dynamic motion of topological defects in magnets induces an emergent electric field, as exemplified by the continuous flow of skyrmion vortices. However, the electrodynamics underlying this emergent field remains poorly understood. In this context, magnetic domain walls - one dimensional topological defects with two collective modes, sliding and spin tilt - offer a promising platform for exploration. Here, we demonstrate that the dissipative motion of domain walls under oscillatory current excitation generates an emergent electric field. We image domain patterns and quantify domain wall length under applied magnetic fields in mesoscopic devices based on the magnetic Weyl semimetal NdAlSi. These devices exhibit exceptionally strong domain wall scattering and a pronounced emergent electric field, observed in the imaginary component of the complex impedance. Spin dynamics simulations reveal that domain wall sliding dominates over spin tilting, where the phase delay of the domain wall motion with respect to the driving force impacts the emergent electric field. Our findings establish domain-wall dynamics as a platform for studying emergent electromagnetic fields and motivate further investigations on the coupled motion of magnetic solitons and conduction electrons. △ Less

Submitted 15 January, 2026; originally announced January 2026.

Skyrmion and Meron Crystals in Intermetallic Gd$_3$Ru$_4$Al$_{12}$: Microscopic Model Insights into Chiral Phases

Authors: Jiajun Mo, Leandro M. Chinellato, Fletcher Williams, Akiko Kikkawa, Joseph A. M. Paddison, Matthias D. Frontzek, Gabriele Sala, Chris Pasco, Kipton Barros, Taro Nakajima, Taka-hisa Arima, Yasujiro Taguchi, Yoshinori Tokura, Matthew B. Stone, Andrew D. Christianson, Cristian D. Batista, Shang Gao

Abstract: Topological spin textures in frustrated intermetallics hold great promise for spintronics applications. However, understanding their origin and properties remains a significant challenge due to competing and often long-range interactions mediated by conduction electrons. Here, by combining neutron scattering experiments with theoretical modeling via unprecedented multi-target fits that further inc… ▽ More Topological spin textures in frustrated intermetallics hold great promise for spintronics applications. However, understanding their origin and properties remains a significant challenge due to competing and often long-range interactions mediated by conduction electrons. Here, by combining neutron scattering experiments with theoretical modeling via unprecedented multi-target fits that further incorporate the ferromagnentic resonance data and magnetization curve, we construct a realistic microscopic model for the prototypical intermetallic skyrmion host \text{Gd}$_3$\text{Ru}$_4$\text{Al}$_{12}$. Beyond magnetic frustration, we identify the competition between dipolar interactions and easy-plane single-ion anisotropy as a key ingredient for stabilizing the rich chiral magnetic phases observed in this compound -- including a hexagonal skyrmion crystal and two distinct meron crystals. Remarkably, the meron crystal in lower field is revealed to be commensurate with the underlying lattice, and its unique three-meron-one-antimeron spin texture is verified by the polarized x-ray diffraction data. At elevated temperatures, the short-range spin correlations in \text{Gd}$_3$\text{Ru}$_4$\text{Al}$_{12}$ are well described by a codimension-two spiral spin-liquid. Perturbations from staggered Dzyaloshinskii-Moriya interactions give rise to chiral fluctuations that account for the temperature and field dependence of the anomalous Hall response. Our results highlight the unique power of neutron scattering, especially when combined with complementary experimental techniques, to unravel complex magnetic phase transitions and provide new insights into the rich variety of topological spin textures in frustrated systems. △ Less

Submitted 30 December, 2025; originally announced December 2025.

Comments: 28 pages, 24 figures

X-ray imaging of antiferromagnetic octupole domains in Mn$_3$Sn

Authors: Max T. Birch, Sebastian Wintz, Yuhan Sun, Akiko Kikkawa, Markus Weigand, Takahisa Arima, Yoshinori Tokura

Abstract: Novel antiferromagnets with broken time reversal symmetry (TRS) have launched a new direction in spintronics research, combining the advantageous dynamical properties of conventional antiferromagnets with the controllability typically associated with ferromagnets. However, antiferromagnetic domains are notoriously challenging to image in real-space. X-ray magnetic circular dichroism (XMCD) offers… ▽ More Novel antiferromagnets with broken time reversal symmetry (TRS) have launched a new direction in spintronics research, combining the advantageous dynamical properties of conventional antiferromagnets with the controllability typically associated with ferromagnets. However, antiferromagnetic domains are notoriously challenging to image in real-space. X-ray magnetic circular dichroism (XMCD) offers a route to overcome this difficulty: XMCD contrast may be finite in TRS-breaking antiferromagnets with an appropriate magnetic space group. Here, we exploit this to image the octupole domains in a focused ion beam-fabricated device of the non-collinear antiferromagnet Mn$_3$Sn. Using scanning transmission x-ray microscopy, we spatially resolve the weak pre-edge XMCD contrast (of 0.2%) that is sensitive to $T_z$, achieving a contrast resolution better than 0.02%. We observe hysteretic switching of the octupole order through both the XMCD contrast and the corresponding anomalous Hall effect within the same device. These results confirm the bulk nature of this contrast, and establish XMCD-based microscopy as a powerful real space imaging method for TRS-breaking antiferromagnets, including altermagnets, enabling future studies of their dynamics, switching, and symmetry-tunable phenomena. △ Less

Submitted 13 October, 2025; v1 submitted 10 October, 2025; originally announced October 2025.

Journal ref: Phys. Rev. B 114, 094409 (2026)

Ultralow-Temperature Cryogenic Transmission Electron Microscopy Using a New Helium Flow Cryostat Stage

Authors: Young-Hoon Kim, Fehmi Sami Yasin, Na Yeon Kim, Max Birch, Xiuzhen Yu, Akiko Kikkawa, Yasujiro Taguchi, Jiaqiang Yan, Miaofang Chi

Abstract: Advances in cryogenic electron microscopy have opened new avenues for probing quantum phenomena in correlated materials. This study reports the installation and performance of a new side-entry condenZero cryogenic cooling system for JEOL (Scanning) Transmission Electron Microscopes (S/TEM), utilizing compressed liquid helium (LHe) and designed for imaging and spectroscopy at ultra-low temperatures… ▽ More Advances in cryogenic electron microscopy have opened new avenues for probing quantum phenomena in correlated materials. This study reports the installation and performance of a new side-entry condenZero cryogenic cooling system for JEOL (Scanning) Transmission Electron Microscopes (S/TEM), utilizing compressed liquid helium (LHe) and designed for imaging and spectroscopy at ultra-low temperatures. The system includes an external dewar mounted on a vibration-damping stage and a pressurized, low-noise helium transfer line with a remotely controllable needle valve, ensuring stable and efficient LHe flow with minimal thermal and mechanical noise. Performance evaluation demonstrates a stable base temperature of 6.58 K measured using a Cernox bare chip sensor on the holder with temperature fluctuations within 0.04 K. Complementary in-situ electron energy-loss spectroscopy (EELS) via aluminum bulk plasmon analysis was used to measure the local specimen temperature and validate cryogenic operation during experiments. The integration of cryogenic cooling with other microscopy techniques, including electron diffraction and Lorentz TEM, was demonstrated by resolving charge density wave (CDW) transitions in NbSe2 using electron diffraction, and imaging nanometric magnetic skyrmions in MnSi via Lorentz TEM. This platform provides reliable cryogenic operation below 7 K, establishing a low-drift route for direct visualization of electronic and magnetic phase transformations in quantum materials. △ Less

Submitted 26 September, 2025; originally announced September 2025.

Comments: 23 pages for main text, 7 figures, 8 supporting figures

Strain-tuning for superconductivity in La$_3$Ni$_2$O$_7$ thin films

Authors: Motoki Osada, Chieko Terakura, Akiko Kikkawa, Masamichi Nakajima, Hsiao-Yi Chen, Yusuke Nomura, Yoshinori Tokura, Atsushi Tsukazaki

Abstract: The recent discovery of high-transition temperature ($T_\mathrm{c}$) superconductivity in pressurized La$_{3}$Ni$_{2}$O$_{7}$ bulk crystals has attracted keen attention due to its characteristic energy diagram of $e_{g}$ orbitals, containing nearly half-filled $d_{3z^2 - r^2}$ and quarter-filled $d_{x^2 - y^2}$ orbitals. This finding provides valuable insights into the orbital contributions and in… ▽ More The recent discovery of high-transition temperature ($T_\mathrm{c}$) superconductivity in pressurized La$_{3}$Ni$_{2}$O$_{7}$ bulk crystals has attracted keen attention due to its characteristic energy diagram of $e_{g}$ orbitals, containing nearly half-filled $d_{3z^2 - r^2}$ and quarter-filled $d_{x^2 - y^2}$ orbitals. This finding provides valuable insights into the orbital contributions and interlayer interactions in double NiO$_{6}$ octahedra, offering opportunities to control the electronic structure via ligand field variations. Here, we demonstrate strain-tuning of $T_\mathrm{c}$ over a range of 50 K in La$_{3}$Ni$_{2}$O$_{7}$ films grown on different oxide substrates under 20 GPa. As the $c/a$ ratio increases, the onset $T_\mathrm{c}$ systematically rises from 10 K in the tensile-strained film on SrTiO$_{3}$ to a maximum of about 60 K in the compressively strained film on LaAlO$_{3}$. These systematic variations suggest that strain engineering is a promising strategy for expanding superconductivity in bilayer nickelates by tuning the orbital energy landscape toward high-$T_\mathrm{c}$ superconductivity. △ Less

Submitted 18 June, 2025; originally announced June 2025.

Comments: 18 pages, 4 figures

Journal ref: Commun. Phys. 8, 251 (2025)

Emergent reactance induced by the deformation of a current-driven skyrmion lattice

Authors: Matthew T. Littlehales, Max T. Birch, Akiko Kikkawa, Yasujiro Taguchi, Diego Alba Venero, Peter D. Hatton, Naoto Nagaosa, Yoshinori Tokura, Tomoyuki Yokouchi

Abstract: The interaction between conduction electrons and spin textures gives rise to remarkable phenomena associated with the Berry phase. The Berry phase acquired by conduction electrons acts as an emergent electromagnetic field, facilitating phenomena analogous to classical electromagnetism, such as the Lorentz force and electromagnetic induction. Magnetic skyrmions, spin vortices with non-trivial topol… ▽ More The interaction between conduction electrons and spin textures gives rise to remarkable phenomena associated with the Berry phase. The Berry phase acquired by conduction electrons acts as an emergent electromagnetic field, facilitating phenomena analogous to classical electromagnetism, such as the Lorentz force and electromagnetic induction. Magnetic skyrmions, spin vortices with non-trivial topology, serve as a key platform for such studies. For example, non-trivial transport responses are recognized as being induced by the emergent Lorentz force and the emergent electromagnetic induction. Despite remarkable progress in skyrmion physics, emergent reactance, in which the phase of an applied AC current is modified by emergent electromagnetism, has not been thoroughly investigated. Here, we report emergent reactance in the prototypical skyrmion-hosting material, MnSi. We observe longitudinal and Hall reactance signals as the skyrmion lattice undergoes creep motion, in which the skyrmions deform while moving. The Hall reactance is attributed to the emergent electric field associated with the inertial translational motion arising from the skyrmion effective mass. In contrast, the longitudinal reactance results from the emergent electric fields generated by the phason and spin-tilting modes excited by their deformation. Our findings shed light on the internal deformation degrees of freedom in skyrmions as a important factor for efficient generation of the emergent electric field. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Entropy-assisted, long-period stacking of honeycomb layers in an AlB2-type silicide

Authors: Leonie Spitz, Takuya Nomoto, Shunsuke Kitou, Hironori Nakao, Akiko Kikkawa, Sonia Francoual, Yasujiro Taguchi, Ryotaro Arita, Yoshinori Tokura, Taka-hisa Arima, Max Hirschberger

Abstract: Configurational entropy can impact crystallization processes, tipping the scales between structures of nearly equal internal energy. Using alloyed single crystals of Gd2PdSi3 in the AlB2-type structure, we explore the formation of complex layer sequences made from alternating, two-dimensional triangular and honeycomb slabs. A four-period and an eight-period stacking sequence are found to be very c… ▽ More Configurational entropy can impact crystallization processes, tipping the scales between structures of nearly equal internal energy. Using alloyed single crystals of Gd2PdSi3 in the AlB2-type structure, we explore the formation of complex layer sequences made from alternating, two-dimensional triangular and honeycomb slabs. A four-period and an eight-period stacking sequence are found to be very close in internal energy, the latter being favored by entropy associated with covering the full configuration space of interlayer bonds. Possible consequences of polytype formation on magnetism in Gd2PdSi3 are discussed. △ Less

Submitted 24 March, 2025; originally announced March 2025.

Comments: 16 pages, 4 figures

Journal ref: J. Am. Chem. Soc. 144, 37, 16866-16871 (2022)

Perfectly harmonic spin cycloid and multi-$Q$ textures in the Weyl semimetal GdAlSi

Authors: Ryota Nakano, Rinsuke Yamada, Juba Bouaziz, Maurice Colling, Masaki Gen, Kentaro Shoriki, Yoshihiro Okamura, Akiko Kikkawa, Hiroyuki Ohsumi, Yoshikazu Tanaka, Hajime Sagayama, Hironori Nakao, Yasujiro Taguchi, Youtarou Takahashi, Masashi Tokunaga, Taka-hisa Arima, Yoshinori Tokura, Ryotaro Arita, Jan Masell, Satoru Hayami, Max Hirschberger

Abstract: A fundamental question concerns how topological electronic states are influenced by many-body correlations, and magnetic Weyl semimetals represent an important material platform to address this problem. However, the magnetic structures realized in these materials are limited, and in particular, no clear example of an undistorted helimagnetic state has been definitively identified. Here, we report… ▽ More A fundamental question concerns how topological electronic states are influenced by many-body correlations, and magnetic Weyl semimetals represent an important material platform to address this problem. However, the magnetic structures realized in these materials are limited, and in particular, no clear example of an undistorted helimagnetic state has been definitively identified. Here, we report clear evidence of a harmonic helimagnetic cycloid with an incommensurate magnetic propagation vector in the Weyl semimetal GdAlSi via resonant elastic X-ray scattering, including rigorous polarization analysis. This cycloidal structure is consistent with the Dzyaloshinskii-Moriya (DM) interaction prescribed by the polar crystal structure of GdAlSi. Upon applying a magnetic field, the cycloid undergoes a transition to a novel multi-$Q$ state. This field-induced, noncoplanar texture is consistent with our numerical spin model, which incorporates the DM interaction and, crucially, anisotropic exchange. The perfectly harmonic Weyl helimagnet GdAlSi serves as a prototypical platform to study electronic correlation effects in periodically modulated Weyl semimetals. △ Less

Submitted 18 March, 2025; originally announced March 2025.

Comments: 32 pages, 3 main text figures, 5 supplementary figures

Observation of the nonlinear chiral thermoelectric Hall effect in tellurium

Authors: Tetsuya Nomoto, Akiko Kikkawa, Kazuki Nakazawa, Terufumi Yamaguchi, Fumitaka Kagawa

Abstract: The nonlinear thermoelectric effect is a key factor for realising unconventional thermoelectric phenomena, such as heat rectification and power generation using thermal fluctuations. Recent theoretical advances have indicated that chiral materials can host a variety of exotic nonlinear thermoelectric transport arising from inversion-symmetry breaking. However, experimental demonstration has yet to… ▽ More The nonlinear thermoelectric effect is a key factor for realising unconventional thermoelectric phenomena, such as heat rectification and power generation using thermal fluctuations. Recent theoretical advances have indicated that chiral materials can host a variety of exotic nonlinear thermoelectric transport arising from inversion-symmetry breaking. However, experimental demonstration has yet to be achieved. Here, we report the observation of the nonlinear chiral thermoelectric Hall effect in chiral tellurium at room temperature, where a voltage is generated as a cross product of the temperature gradient and electric field. The resulting thermoelectric Hall voltage is on the order of μV, consistent with the prediction from the ab initio calculation. Furthermore, the sign of the thermoelectric Hall voltage is reversed depending on the crystal chirality, demonstrating a novel functionality of sign control of the thermoelectric effect by the chirality degrees of freedom. Our findings reveal the potential of chiral systems as nonlinear thermoelectric materials for advanced thermal management and energy harvesting. △ Less

Submitted 14 March, 2025; v1 submitted 11 March, 2025; originally announced March 2025.

Comments: 12 pages, 7 figures

Dynamic transition and Galilean relativity of current-driven skyrmions

Authors: Max T. Birch, Ilya Belopolski, Yukako Fujishiro, Minoru Kawamura, Akiko Kikkawa, Yasujiro Taguchi, Max Hirschberger, Naoto Nagaosa, Yoshinori Tokura

Abstract: The coupling of conduction electrons and magnetic textures leads to quantum transport phenomena described by the language of emergent electromagnetic fields [1-3]. For magnetic skyrmions, spin-swirling particle-like objects, an emergent magnetic field is produced by their topological winding [4-6], resulting in the conduction electrons exhibiting the topological Hall effect (THE) [7]. When the sky… ▽ More The coupling of conduction electrons and magnetic textures leads to quantum transport phenomena described by the language of emergent electromagnetic fields [1-3]. For magnetic skyrmions, spin-swirling particle-like objects, an emergent magnetic field is produced by their topological winding [4-6], resulting in the conduction electrons exhibiting the topological Hall effect (THE) [7]. When the skyrmion lattice (SkL) acquires a drift velocity under conduction electron flow, an emergent electric field is also generated [8,9]. The resulting emergent electrodynamics dictate the magnitude of the THE via the relative motion of SkL and conduction electrons. Here, we report the emergent electrodynamics induced by SkL motion in Gd$_2$PdSi$_3$, facilitated by its giant THE [10,11]. With increasing current excitation, we observe the dynamic transition of the SkL motion from the pinned to creep regime and finally to the flow regime, where the THE is totally suppressed. We argue that the Galilean relativity required for the total cancellation of the THE can be generically recovered in the flow regime, even in complex multiband systems such as the present compound. Moreover, the observed THE voltages are large enough to enable real-time measurement of the SkL velocity-current profile, which reveals the inertial-like motion of the SkL in the creep regime, appearing as current-hysteretic behavior of the skyrmion velocity. △ Less

Submitted 3 June, 2024; originally announced June 2024.

Journal ref: Nature 633, 554-559 (2024)

Detailed dynamics of a moving magnetic skyrmion lattice in MnSi observed using a small-angle neutron scattering under an alternating electric current flow

Authors: D. Okuyama, M. Bleuel, Q. Ye, J. Krzywon, N. Nagaosa, A. Kikkawa, Y. Taguchi, Y. Tokura, J. D. Reim, Y. Nambu, T. J. Sato

Abstract: Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory. H… ▽ More Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory. Here, we report in detail on the transient state of a moving magnetic skyrmion lattice in bulk single-crystalline MnSi under alternating current (AC) using small-angle neutron scattering. A rotation and concomitant broadening of the spot width in the azimuthal direction of the magnetic skyrmion reflections originating from the plastic deformation of the magnetic skyrmion lattice were found only at low AC frequencies, whereas above the threshold AC frequency (ft ~ 0.12 Hz) the rotation was not observed, and the spot width becomes sharper. The observed complex response of the magnetic skyrmion reflections can be explained by the change in dislocation density in the magnetic skyrmion lattice. At frequencies higher than ft, the magnetic skyrmions oscillate removing the dislocations, indicating that the dislocation density is controlled by the AC frequency. △ Less

Submitted 26 April, 2024; originally announced April 2024.

Comments: 13 pages, 10 figures

Nernst effect of high-mobility Weyl electrons in NdAlSi enhanced by a Fermi surface nesting instability

Authors: Rinsuke Yamada, Takuya Nomoto, Atsushi Miyake, Toshihiro Terakawa, Akiko Kikkawa, Ryotaro Arita, Masashi Tokunaga, Yasujiro Taguchi, Yoshinori Tokura, Max Hirschberger

Abstract: The thermoelectric Nernst effect of solids converts heat flow to beneficial electronic voltages. Here, using a correlated topological semimetal with high carrier mobility $μ$ in presence of magnetic fluctuations, we demonstrate an enhancement of the Nernst effect close to a magnetic phase transition. A magnetic instability in NdAlSi modifies the carrier relaxation time on 'hotspots' in momentum sp… ▽ More The thermoelectric Nernst effect of solids converts heat flow to beneficial electronic voltages. Here, using a correlated topological semimetal with high carrier mobility $μ$ in presence of magnetic fluctuations, we demonstrate an enhancement of the Nernst effect close to a magnetic phase transition. A magnetic instability in NdAlSi modifies the carrier relaxation time on 'hotspots' in momentum space, causing a strong band filling dependence of $μ$. We quantitatively derive electronic band parameters from a novel two-band analysis of the Nernst effect $S_{xy}$, in good agreement with quantum oscillation measurements and band calculations. While the Nernst response of NdAlSi behaves much like conventional semimetals at high temperatures, an additional contribution $ΔS_{xy}$ from electronic correlations appears just above the magnetic transition. Our work demonstrates the engineering of the relaxation time, or the momentum-dependent self energy, to generate a large Nernst response independent of a material's carrier density, i.e. for metals, semimetals, and semiconductors with large $μ$. △ Less

Submitted 4 April, 2024; originally announced April 2024.

Journal ref: Physical Review x 14, 021012 (2024)

Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet

Authors: T. Nomura, X. -X. Zhang, R. Takagi, K. Karube, A. Kikkawa, Y. Taguchi, Y. Tokura, S. Zherlitsyn, Y. Kohama, S. Seki

Abstract: The phonon magnetochiral effect (MChE) is the nonreciprocal acoustic and thermal transports of phonons caused by the simultaneous breaking of the mirror and time-reversal symmetries. So far, the phonon MChE has been observed only in a ferrimagnetic insulator Cu2OSeO3, where the nonreciprocal response disappears above the Curie temperature of 58 K. Here, we study the nonreciprocal acoustic properti… ▽ More The phonon magnetochiral effect (MChE) is the nonreciprocal acoustic and thermal transports of phonons caused by the simultaneous breaking of the mirror and time-reversal symmetries. So far, the phonon MChE has been observed only in a ferrimagnetic insulator Cu2OSeO3, where the nonreciprocal response disappears above the Curie temperature of 58 K. Here, we study the nonreciprocal acoustic properties of a room-temperature ferromagnet Co9Zn9Mn2 for unveiling the phonon MChE close to the room temperature. Surprisingly, the nonreciprocity in this metallic compound is enhanced at higher temperatures and observed up to 250 K. This clear contrast between insulating Cu2OSeO3 and metallic Co9Zn9Mn2 suggests that metallic magnets have a mechanism to enhance the nonreciprocity at higher temperatures. From the ultrasound and microwave-spectroscopy experiments, we conclude that the magnitude of the phonon MChE of Co9Zn9Mn2 mostly depends on the magnon bandwidth, which increases at low temperatures and hinders the magnon-phonon hybridization. Our results suggest that the phonon nonreciprocity could be further enhanced by engineering the magnon band of materials. △ Less

Submitted 8 April, 2023; v1 submitted 30 November, 2022; originally announced December 2022.

Comments: 6 pages, 4 figures, 1 table

Journal ref: Phys. Rev. Lett. 130, 176301 (2023)

Topological melting of the metastable skyrmion lattice in the chiral magnet Co$_9$Zn$_9$Mn$_2$

Authors: Victor Ukleev, Daisuke Morikawa, Kosuke Karube, Akiko Kikkawa, Kiyou Shibata, Yasujiro Taguchi, Yoshinori Tokura, Taka-hisa Arima, Jonathan S. White

Abstract: In a $β$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, we demonstrate that the magnetic field-driven collapse of a room temperature metastable topological skyrmion lattice passes through a regime described by a partial topological charge inversion. Using Lorentz transmission electron microscopy, the magnetization distribution was observed directly as the magnetic field was swept antiparallel to the or… ▽ More In a $β$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, we demonstrate that the magnetic field-driven collapse of a room temperature metastable topological skyrmion lattice passes through a regime described by a partial topological charge inversion. Using Lorentz transmission electron microscopy, the magnetization distribution was observed directly as the magnetic field was swept antiparallel to the original skyrmion core magnetization, i.e. negative magnetic fields. Due to the topological stability of skyrmions, a direct transition of the metastable skyrmion lattice to the equilibrium helical state is avoided for increasingly negative fields. Instead, the metastable skyrmion lattice gradually transforms into giant magnetic bubbles separated by $2π$ domain walls. Eventually these large structures give way to form a near-homogeneously magnetized medium that unexpectedly hosts a low density of isolated skyrmions with inverted core magnetization, and thus a total topological charge of reduced size and opposite sign compared with the initial state. A similar phenomenon has been observed previously in systems hosting ordered lattices of magnetic bubbles stabilized by the dipolar interaction and called "topological melting". With support from numerical calculations, we argue that the observed regime of partial topological charge inversion has its origin in the topological protection of the starting metastable skyrmion state. △ Less

Submitted 22 August, 2022; originally announced August 2022.

Comments: 9 pages, 4 figures

Journal ref: Advanced Quantum Technologies, 2200066, 2022

Polarized neutron scattering study on the centrosymmetric skyrmion host material Gd2PdSi3

Authors: Jiwon Ju, Hiraku Saito, Takashi Kurumaji, Max Hirschberger, Akiko Kikkawa, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura, Taro Nakajima

Abstract: We have investigated magnetic structures of the centrosymmetrric skyrmion material Gd2PdSi3 by means of polarized neutron scattering near zero field with an isotope-160Gd-enriched single crystal. In a previous study, magnetic structures in Gd2PdSi3 at low temperatures were studied by resonant X-ray scattering measurements [T. Kurumaji et al. Science 365, 914 (2019)]. The present polarized neutron… ▽ More We have investigated magnetic structures of the centrosymmetrric skyrmion material Gd2PdSi3 by means of polarized neutron scattering near zero field with an isotope-160Gd-enriched single crystal. In a previous study, magnetic structures in Gd2PdSi3 at low temperatures were studied by resonant X-ray scattering measurements [T. Kurumaji et al. Science 365, 914 (2019)]. The present polarized neutron results confirm that the magnetic structure in zero field has elliptic screw-type magnetic modulation with a propagation vector of (q, 0, 0) with q ~ 0.14 and its equivalents. As the temperature increases, the system undergoes a magnetic phase transition while keeping the incommensurate q-vector of (q,0,0). We found that the thermally-induced phase has sinusoidal magnetic modulations with magnetic moments perpendicular both to the c axis and to the q-vector. We also investigate the spin-helicity degree of freedom in the ground state by polarized neutrons, revealing that the system contains equal fractions of the left-handed and right-handed screw-type orders as expected from the centrosymmetric crystal structure. △ Less

Submitted 17 August, 2022; originally announced August 2022.

Comments: 6 pages, 4 figures

Vital role of anisotropy in cubic chiral skyrmion hosts

Authors: M. Preißinger, K. Karube, D. Ehlers, B. Szigeti, H. -A. Krug von Nidda, J. S. White, V. Ukleev, H. M. Rønnow, Y. Tokunaga, A. Kikkawa, Y. Tokura, Y. Taguchi, I. Kézsmárki

Abstract: The impact of magnetic anisotropy on the skyrmion lattice (SkL) state in cubic chiral magnets has been overlooked for long, partly because a semi-quantitative description of the thermodynamically stable SkL phase pocket forming near the Curie temperature could be achieved without invoking anisotropy effects. However, there has been a range of phenomena reported recently in these materials, such as… ▽ More The impact of magnetic anisotropy on the skyrmion lattice (SkL) state in cubic chiral magnets has been overlooked for long, partly because a semi-quantitative description of the thermodynamically stable SkL phase pocket forming near the Curie temperature could be achieved without invoking anisotropy effects. However, there has been a range of phenomena reported recently in these materials, such as the formation of low-temperature tilted conical and SkL states as well as temperature-induced transformations of lattice geometry in metastable SkL states, where anisotropy was suspected to play a key role. To settle this issue on experimental basis, we quantified the cubic anisotropy in a series of CoZnMn-type cubic chiral magnets. We found that the strength of anisotropy is highly enhanced towards low temperatures in all the compounds, moreover, not only the magnitude but also the character of cubic anisotropy drastically varies upon changing the Co/Mn ratio. We correlate these changes with temperature- and composition-induced variations of the helical modulation vectors, the anharmonicity and structural rearrangements of the metastable SkLs and the spin relaxation rates. Similar systematic studies on magnetic anisotropy may not only pave the way for a quantitative and unified description of the stable and metastable modulated spin textures in cubic chiral magnets but would also help exploring further topological spin textures in this large class of skyrmion hosts. △ Less

Submitted 11 November, 2020; originally announced November 2020.

Metastable skyrmion lattices governed by magnetic disorder and anisotropy in $β$-Mn-type chiral magnets

Authors: K. Karube, J. S. White, V. Ukleev, C. D. Dewhurst, R. Cubitt, A. Kikkawa, Y. Tokunaga, H. M. Rønnow, Y. Tokura, Y. Taguchi

Abstract: Magnetic skyrmions are vortex-like topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a $β$-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state,… ▽ More Magnetic skyrmions are vortex-like topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a $β$-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state, and that the skyrmion lattice transforms from a triangular lattice to a square one. To obtain perspective on chiral magnetism in Co-Zn-Mn alloys and clarify how various properties related to the skyrmion vary with the composition, we performed systematic studies on Co$_{10}$Zn$_{10}$, Co$_9$Zn$_9$Mn$_2$, Co$_8$Zn$_8$Mn$_4$ and Co$_7$Zn$_7$Mn$_6$ in terms of magnetic susceptibility and small-angle neutron scattering measurements. The robust metastable skyrmions with extremely long lifetime are commonly observed in all the compounds. On the other hand, preferred orientation of a helimagnetic propagation vector and its temperature dependence dramatically change upon varying the Mn concentration. The robustness of the metastable skyrmions in these materials is attributed to topological nature of the skyrmions as affected by structural and magnetic disorder. Magnetocrystalline anisotropy as well as magnetic disorder due to the frustrated Mn spins play crucial roles in giving rise to the observed change in helical states and corresponding skyrmion lattice form. △ Less

Submitted 11 August, 2020; originally announced August 2020.

Comments: 70 pages, 19 figures

Journal ref: Physical Review B 102, 064408 (2020)

Slow steady flow of a skyrmion lattice in a confined geometry probed by resistance narrow-band noise

Authors: Takuro Sato, Wataru Koshibae, Akiko Kikkawa, Tomoyuki Yokouchi, Hiroshi Oike, Yasujiro Taguchi, Naoto Nagaosa, Yoshinori Tokura, Fumitaka Kagawa

Abstract: Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implyin… ▽ More Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $μ$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen. △ Less

Submitted 15 April, 2020; originally announced April 2020.

Comments: 5 pages, 4 figures

Journal ref: Phys.Rev.B 100, 094410 (2019)

High-field depinned phase and planar Hall effect in skyrmion-host Gd$_2$PdSi$_3$

Authors: Max Hirschberger, Taro Nakajima, Markus Kriener, Takashi Kurumaji, Leonie Spitz, Shang Gao, Akiko Kikkawa, Yuichi Yamasaki, Hajime Sagayama, Hironori Nakao, Seiko Ohira-Kawamura, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura

Abstract: For the skyrmion-hosting intermetallic Gd$_2$PdSi$_3$ with centrosymmetric hexagonal lattice and triangular net of rare earth sites, we report a thorough investigation of the magnetic phase diagram. Our work reveals a new magnetic phase with isotropic value of the critical field for all orientations, where the magnetic ordering vector $\mathbf{q}$ is depinned from its preferred directions in the b… ▽ More For the skyrmion-hosting intermetallic Gd$_2$PdSi$_3$ with centrosymmetric hexagonal lattice and triangular net of rare earth sites, we report a thorough investigation of the magnetic phase diagram. Our work reveals a new magnetic phase with isotropic value of the critical field for all orientations, where the magnetic ordering vector $\mathbf{q}$ is depinned from its preferred directions in the basal plane. This is in contrast to the highly anisotropic behavior of the low field phases, such as the skyrmion lattice (SkL), which are easily destroyed by in-plane magnetic field. The bulk nature of the SkL and of other magnetic phases was evidenced by specific-heat measurements. Resistivity anisotropy, likely originating from partial gapping of the density of states along $\mathbf{q}$ in this RKKY magnet, is picked up via the planar Hall effect (PHE). The PHE confirms the single-$\mathbf{q}$ nature of the magnetic order when the field is in the hexagonal plane, and allows to detect the preferred directions of $\mathbf{q}$. For field aligned perpendicular to the basal plane, several scenarios for the depinned phase (DP), such as tilted conical order, are discussed on the basis of the data. △ Less

Submitted 11 April, 2020; originally announced April 2020.

Comments: 27 pages, 12 figures (including SI)

Journal ref: Phys. Rev. B 101, 220401 (2020)

Topological Nernst effect of the two-dimensional skyrmion lattice

Authors: Max Hirschberger, Leonie Spitz, Takuya Nomoto, Takashi Kurumaji, Shang Gao, Jan Masell, Taro Nakajima, Akiko Kikkawa, Yuichi Yamasaki, Hajime Sagayama, Hironori Nakao, Yasujiro Taguchi, Ryotaro Arita, Taka-hisa Arima, Yoshinori Tokura

Abstract: The topological Hall effect (THE) and its thermoelectric counterpart, the topological Nernst effect (TNE), are hallmarks of the skyrmion lattice phase (SkL). We observed the giant TNE of the SkL in centrosymmetric Gd$_2$PdSi$_3$, comparable in magnitude to the largest anomalous Nernst signals in ferromagnets. Significant enhancement (suppression) of the THE occurs when doping electrons (holes) to… ▽ More The topological Hall effect (THE) and its thermoelectric counterpart, the topological Nernst effect (TNE), are hallmarks of the skyrmion lattice phase (SkL). We observed the giant TNE of the SkL in centrosymmetric Gd$_2$PdSi$_3$, comparable in magnitude to the largest anomalous Nernst signals in ferromagnets. Significant enhancement (suppression) of the THE occurs when doping electrons (holes) to Gd$_2$PdSi$_3$. On the electron-doped side, the topological Hall conductivity approaches the characteristic threshold $\sim 1000\,\left(\mathrm{Ωcm}\right)^{-1}$ for the intrinsic regime. We use the filling-controlled samples to confirm Mott's relation between TNE and THE and discuss the importance of Gd-5d orbitals for transport in this compound. △ Less

Submitted 16 August, 2020; v1 submitted 14 October, 2019; originally announced October 2019.

Comments: Main text: 9 pages, 4 figures; with SI: 36 pages, 15 figures

Journal ref: Phys. Rev. Lett. 125, 076602 (2020)

Ordering phenomena of spin trimers accompanied by large geometrical Hall effect

Authors: Shang Gao, Max Hirschberger, Oksana Zaharko, Taro Nakajima, Takashi Kurumaji, Akiko Kikkawa, Junichi Shiogai, Atsushi Tsukazaki, Shojiro Kimura, Satoshi Awaji, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura

Abstract: The wavefuntion of conduction electrons moving in the background of a non-coplanar spin structure can gain a quantal phase - Berry phase - as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighbouring spin sites, termed the scalar spin chirality. The… ▽ More The wavefuntion of conduction electrons moving in the background of a non-coplanar spin structure can gain a quantal phase - Berry phase - as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighbouring spin sites, termed the scalar spin chirality. The entire spin chirality of the crystal, unless macroscopically canceled, causes the geometrical Hall effect of real-space Berry-phase origin, whereas the intrinsic anomalous Hall effect (AHE) in a conventional metallic ferromagnet is of the momentum-space Berry-phase origin induced by relativistic spin-orbit coupling (SOC). Here, we report the ordering phenomena of the spin-trimer scalar spin chirality and the consequent large geometrical Hall effect in the breathing kagomé lattice compound Dy$_3$Ru$_4$Al$_{12}$, where the Dy$^{3+}$ moments form non-coplanar spin trimers with local spin chirality. Using neutron diffraction, we show that the local spin chirality of the spin trimers as well as its ferroic/antiferroic orders can be switched by an external magnetic field, accompanying large changes in the geometrical Hall effect. Our finding reveals that systems composed of tunable spin trimers can be a fertile field to explore large emergent electromagnetic responses arising from real-space topological magnetic orders. △ Less

Submitted 21 August, 2019; originally announced August 2019.

Comments: 15 pages, 12 figures

Journal ref: Phys. Rev. B 100, 241115 (2019)

Magnetization-polarization cross-control near room temperature in hexaferrite single crystals

Authors: Vilmos Kocsis, Taro Nakajima, Masaaki Matsuda, Akiko Kikkawa, Yoshio Kaneko, Junya Takashima, Kazuhisa Kakurai, Taka-hisa Arima, Fumitaka Kagawa, Yusuke Tokunaga, Yoshinori Tokura, Yasujiro Taguchi

Abstract: Mutual control of the electricity and magnetism in terms of magnetic (H) and electric (E) fields, the magnetoelectric (ME) effect, offers versatile low power-consumption alternatives to current data storage, logic gate, and spintronic devices. Despite its importance, E-field control over magnetization (M) with significant magnitude was observed only at low temperatures. Here we have successfully s… ▽ More Mutual control of the electricity and magnetism in terms of magnetic (H) and electric (E) fields, the magnetoelectric (ME) effect, offers versatile low power-consumption alternatives to current data storage, logic gate, and spintronic devices. Despite its importance, E-field control over magnetization (M) with significant magnitude was observed only at low temperatures. Here we have successfully stabilized a simultaneously ferrimagnetic and ferroelectric phase in a Y-type hexaferrite single crystal up to T=450K and demonstrated the reversal of large non-volatile M by E field close to room temperature. Manipulation of the magnetic domains by E field is directly visualized at room temperature by using magnetic force microscopy. The present achievement provides an important step towards the application of bulk ME multiferroics. △ Less

Submitted 10 April, 2019; originally announced April 2019.

Comments: 10 + 9 pages, 5 + 10 figures, Supplementary included

Journal ref: Nature Communicationsvolume 10, Article number: 1247 (2019)

Magnetic ordering in pressure-induced phases with giant spin-driven ferroelectricity in multiferroic TbMnO3

Authors: Noriki Terada, Dmitry D. Khalyavin, Pascal Manuel, Toyotaka Osakabe, Akiko Kikkawa, Hideaki Kitazawa

Abstract: In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in TbMnO3, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incomm… ▽ More In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in TbMnO3, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incommensurate magnetic ordering of a cycloid structure was found to be stable below the reported critical pressure of 4.5 GPa. In contrast, a commensurate E-type spin ordering of Mn spins and a noncollinear configuration of Tb spins with k=(0,1/2,0) appeared above 4.5 GPa. The application of a magnetic field along the a axis (H_{||a}) under pressure induces a k=(0,0,0)antiferromagnetic structure in the case of Tb spins above H_{||a}, enhancing the ferroelectric polarization, while the E-type ordering of Mn spins is stable even above the critical field. From the present experimental findings, we conclude that the E-type ordering of Mn spins induces giant ferroelectric polarization through an exchange striction mechanism. The H_{||a}-induced polarization enhancement can be understood by considering that the polarization, reduced by the polar ordering of Tb moments in a zero field, can be recovered through a field-induced change to nonpolar k=(0,0,0) ordering at H_{||a} ~ 2T. △ Less

Submitted 4 March, 2019; originally announced March 2019.

Journal ref: Phys. Rev. B 93, 081104(R) (2016)

Skyrmion phase and competing magnetic orders on a breathing kagome lattice

Authors: Max Hirschberger, Taro Nakajima, Shang Gao, Licong Peng, Akiko Kikkawa, Takashi Kurumaji, Markus Kriener, Yuichi Yamasaki, Hajime Sagayama, Hironori Nakao, Kazuki Ohishi, Kazuhisa Kakurai, Yasujiro Taguchi, Xiuzhen Yu, Taka-hisa Arima, Yoshinori Tokura

Abstract: Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity / vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on the experimental observation of a skyrmion lattice… ▽ More Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity / vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on the experimental observation of a skyrmion lattice (SkL) phase with large topological Hall effect and an incommensurate helical pitch as small as 2.8 nm in metallic Gd3Ru4Al12, which materializes a breathing kagomé lattice of Gadolinium moments. The magnetic structure of several ordered phases, including the SkL, is determined by resonant x-ray diffraction as well as small angle neutron scattering. The SkL and helical phases are also observed directly using Lorentz transmission electron microscopy. Among several competing phases, the SkL is promoted over a low-temperature transverse conical state by thermal fluctuations in an intermediate range of magnetic fields. △ Less

Submitted 4 March, 2020; v1 submitted 6 December, 2018; originally announced December 2018.

Comments: 35 pages, 4 figures

Journal ref: Nature Communications 10, 5831 (2019)

Disordered skyrmion phase stabilized by magnetic frustration in a chiral magnet

Authors: K. Karube, J. S. White, D. Morikawa, C. D. Dewhurst, R. Cubitt, A. Kikkawa, X. Z. Yu, Y. Tokunaga, T. Arima, H. M. Rønnow, Y. Tokura, Y. Taguchi

Abstract: Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Recently $β$-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while $β$-Mn itself is known as hosting an elemental geometrically frustra… ▽ More Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Recently $β$-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while $β$-Mn itself is known as hosting an elemental geometrically frustrated spin liquid. Here we report the intermediate composition system Co$_7$Zn$_7$Mn$_6$ to be a unique host of two disconnected, thermal-equilibrium topological skyrmion phases; one is a conventional skyrmion crystal phase stabilized by thermal fluctuations and restricted to exist just below the magnetic transition temperature $T_\mathrm{c}$, and the other is a novel three-dimensionally disordered skyrmion phase that is stable well below $T_\mathrm{c}$. The stability of this new disordered skyrmion phase is due to a cooperative interplay between the chiral magnetism with Dzyaloshinskii-Moriya interaction and the frustrated magnetism inherent to $β$-Mn. △ Less

Submitted 6 November, 2018; originally announced November 2018.

Comments: 57 pages, 16 figures

Journal ref: Compositio Math. 155 (2019) 2263-2295

Deformation of the Magnetic Skyrmion Lattice in MnSi under Electric Current Flow

Authors: D. Okuyama, M. Bleuel, J. S. White, Q. Ye, J. Krzywon, G. Nagy, Z. Q. Im, I. Zivkovic, M. Bartkowiak, H. M. Ronnow, S. Hoshino, J. Iwasaki, N. Nagaosa, A. Kikkawa, Y. Taguchi, Y. Tokura, D. Higashi, J. D. Reim, Y. Nambu, T. J. Sato

Abstract: Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a… ▽ More Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense observed for opposite sample edges aligned with the direction of current flow. The peak broadening (and the corresponding skyrmion lattice rotations) remain finite even after switching off the electric current. These results indicate that skyrmion lattices under current flow experience significant friction near the sample edges, and plastic deformation due to pinning effects, these being important factors that must be considered for the anticipated skyrmion-based applications in chiral magnets at the nanoscale. △ Less

Submitted 19 July, 2018; originally announced July 2018.

Journal ref: Commun. Phys. 2, 79 (2019)

Skyrmion lattice with a giant topological Hall effect in a frustrated triangular-lattice magnet

Authors: Takashi Kurumaji, Taro Nakajima, Max Hirschberger, Akiko Kikkawa, Yuichi Yamasaki, Hajime Sagayama, Hironori Nakao, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura

Abstract: Geometrically frustrated magnets provide abundant opportunities for discovering complex spin textures, which sometimes yield unconventional electromagnetic responses in correlated electron systems. It is theoretically predicted that magnetic frustration may also promote a topologically nontrivial spin state, i.e., magnetic skyrmions, which are nanometric spin vortices. Empirically, however, skyrmi… ▽ More Geometrically frustrated magnets provide abundant opportunities for discovering complex spin textures, which sometimes yield unconventional electromagnetic responses in correlated electron systems. It is theoretically predicted that magnetic frustration may also promote a topologically nontrivial spin state, i.e., magnetic skyrmions, which are nanometric spin vortices. Empirically, however, skyrmions are essentially concomitant with noncentrosymmetric lattice structures or interfacial-symmetry-breaking heterostructures. Here, we report the emergence of a Bloch-type skyrmion state in the frustrated centrosymmetric triangular-lattice magnet Gd2PdSi3. We identified the field-induced skyrmion phase via a giant topological Hall response, which is further corroborated by the observation of in-plane spin modulation probed by resonant x-ray scattering. Our results exemplify a new gold mine of magnetic frustration for producing topological spin textures endowed with emergent electrodynamics in centrosymmetric magnets. △ Less

Submitted 27 May, 2018; originally announced May 2018.

Comments: 23 pages, 3 figures

Journal ref: Science 365, 914 (2019)

Current-induced viscoelastic topological unwinding of metastable skyrmion strings

Authors: F. Kagawa, H. Oike, W. Koshibae, A. Kikkawa, Y. Okamura, Y. Taguchi, N. Nagaosa, Y. Tokura

Abstract: In the MnSi bulk chiral magnet, magnetic skyrmion strings of 17 nm in diameter appear in the form of a lattice, penetrating the sample thickness, 10-1,000 um. Although such a bundle of skyrmion strings may exhibit complex soft-matter-like dynamics when starting to move under the influence of a random pinning potential, the details remain highly elusive. Here, we show that a metastable skyrmion-str… ▽ More In the MnSi bulk chiral magnet, magnetic skyrmion strings of 17 nm in diameter appear in the form of a lattice, penetrating the sample thickness, 10-1,000 um. Although such a bundle of skyrmion strings may exhibit complex soft-matter-like dynamics when starting to move under the influence of a random pinning potential, the details remain highly elusive. Here, we show that a metastable skyrmion-string lattice is subject to topological unwinding under the application of pulsed currents of 3-5x106 A m-2 rather than being transported, as evidenced by measurements of the topological Hall effect. The critical current density above which the topological unwinding occurs is larger for a shorter pulse width, reminiscent of the viscoelastic characteristics accompanying the pinning-creep transition observed in domain-wall motion. Numerical simulations reveal that current-induced depinning of already segmented skyrmion strings initiates the topological unwinding. Thus, the skyrmion-string length is an element to consider when studying current-induced motion. △ Less

Submitted 9 November, 2017; originally announced November 2017.

Comments: 33 pages, 13 figures (including Supplementary Materials)

Journal ref: Nat. Commun. 8: 1332 (2017)

Skyrmion formation in a bulk chiral magnet at zero magnetic field and above room temperature

Authors: K. Karube, J. S. White, D. Morikawa, M. Bartkowiak, A. Kikkawa, Y. Tokunaga, T. Arima, H. M. Rønnow, Y. Tokura, Y. Taguchi

Abstract: We report that in a $β$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field-cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature $T_\mathrm{c}$ $\sim$ 400 K. The once-created metastable skyrmions survive at zero magnetic field both at and above… ▽ More We report that in a $β$-Mn-type chiral magnet Co$_9$Zn$_9$Mn$_2$, skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field-cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature $T_\mathrm{c}$ $\sim$ 400 K. The once-created metastable skyrmions survive at zero magnetic field both at and above room temperature. Such robust skyrmions in a wide temperature and magnetic field region demonstrate the key role of topology, and provide a significant step toward technological applications of skyrmions in bulk chiral magnets. △ Less

Submitted 23 September, 2017; originally announced September 2017.

Journal ref: Phys. Rev. Materials 1, 074405 (2017)

Electronic and magnetic excitations in the "half-stuffed" Cu--O planes of Ba$_2$Cu$_3$O$_4$Cl$_2$ measured by resonant inelastic x-ray scattering

Authors: S. Fatale, C. G. Fatuzzo, P. Babkevich, N. E. Shaik, J. Pelliciari, X. Lu, D. E. McNally, T. Schmitt, A. Kikkawa, Y. Taguchi, Y. Tokura, B. Normand, H. M. Rønnow, M. Grioni

Abstract: We use resonant inelastic x-ray scattering (RIXS) at the Cu L$_3$ edge to measure the charge and spin excitations in the "half-stuffed" Cu--O planes of the cuprate antiferromagnet Ba$_2$Cu$_3$O$_4$Cl$_2$. The RIXS line shape reveals distinct contributions to the $dd$ excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy respons… ▽ More We use resonant inelastic x-ray scattering (RIXS) at the Cu L$_3$ edge to measure the charge and spin excitations in the "half-stuffed" Cu--O planes of the cuprate antiferromagnet Ba$_2$Cu$_3$O$_4$Cl$_2$. The RIXS line shape reveals distinct contributions to the $dd$ excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a CuO$_2$ sublattice, similar to the case of the "fully-stuffed" planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as Sr$_2$CuO$_2$Cl$_2$, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO. △ Less

Submitted 11 September, 2017; originally announced September 2017.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. B 96, 115149 (2017)

Enhanced ferromagnetic transition temperature induced by a microscopic structural rearrangement in the diluted magnetic semiconductor Ge$_{1-x}$Mn$_{x}$Te

Authors: M. Kriener, T. Nakajima, Y. Kaneko, A. Kikkawa, D. Hashizume, K. Kato, M. Takata, T. Arima, Y. Tokura, Y. Taguchi

Abstract: The correlation between magnetic properties and microscopic structural aspects in the diluted magnetic semiconductor Ge$_{1-x}$Mn$_{x}$Te is investigated by x-ray diffraction and magnetization as a function of the Mn concentration $x$. The occurrence of high ferromagnetic-transition temperatures in the rhombohedrally distorted phase of slowly-cooled Ge$_{1-x}$Mn$_{x}$Te is shown to be directly cor… ▽ More The correlation between magnetic properties and microscopic structural aspects in the diluted magnetic semiconductor Ge$_{1-x}$Mn$_{x}$Te is investigated by x-ray diffraction and magnetization as a function of the Mn concentration $x$. The occurrence of high ferromagnetic-transition temperatures in the rhombohedrally distorted phase of slowly-cooled Ge$_{1-x}$Mn$_{x}$Te is shown to be directly correlated with the formation and coexistence of strongly-distorted Mn-poor and weakly-distorted Mn-rich regions. It is demonstrated that the weakly-distorted phase fraction is responsible for the occurrence of high-transition temperatures in Ge$_{1-x}$Mn$_{x}$Te. When the Mn concentration becomes larger, the Mn-rich regions start to switch into the undistorted cubic structure, and the transition temperature is suppressed concurrently. By identifying suitable annealing conditions, we successfully increased the transition temperature to above 200 K for Mn concentrations close to the cubic phase. Structural data indicate that the weakly-distorted phase fraction can be restored at the expense of the cubic regions upon the enhancement of the transition temperature, clearly establishing the direct link between high-transition temperatures and the weakly-distorted Mn-rich phase fraction. △ Less

Submitted 18 June, 2017; originally announced June 2017.

Comments: 12 pages, 5 figures

Journal ref: Phys. Rev. B 95, 224418 (2017)

Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba$_2$Cu$_3$O$_4$Cl$_2$

Authors: P. Babkevich, N. E. Shaik, D. Lançon, A. Kikkawa, M. Enderle, R. A. Ewings, H. C. Walker, D. T. Adroja, P. Manuel, D. D. Khalyavin, Y. Taguchi, Y. Tokura, M. Soda, T. Masuda, H. M. Rønnow

Abstract: We report detailed neutron scattering studies on Ba$_2$Cu$_3$O$_4$Cl$_2$. The compound consists of two interpenetrating sublattices of Cu, labeled as Cu$_{\rm A}$ and Cu$_{\rm B}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields a… ▽ More We report detailed neutron scattering studies on Ba$_2$Cu$_3$O$_4$Cl$_2$. The compound consists of two interpenetrating sublattices of Cu, labeled as Cu$_{\rm A}$ and Cu$_{\rm B}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields an inelastic neutron spectrum of the Cu$_{\rm A}$ sublattice extending up to 300 meV and a much weaker dispersion of Cu$_{\rm B}$ going up to around 20 meV. Using a single-band Hubbard model we derive an effective spin Hamiltonian. From this, we find that linear spin-wave theory gives a good description to the magnetic spectrum. In addition, a magnetic field of 10 T is found to produce effects on the Cu$_{\rm B}$ dispersion that cannot be explained by conventional spin-wave theory. △ Less

Submitted 9 July, 2017; v1 submitted 13 April, 2017; originally announced April 2017.

Comments: 13 pages, 9 figures

Journal ref: Phys. Rev. B 96, 014410 (2017)

Electrical magnetochiral effect induced by chiral spin fluctuations

Authors: T. Yokouchi, N. Kanazawa, A. Kikkawa, D. Morikawa, K. Shibata, T. Arima, Y. Taguchi, F. Kagawa, Y. Tokura

Abstract: Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magne… ▽ More Chirality of matter can produce unique responses in optics, electricity and magnetism. In particular, magnetic crystals transmit their handedness to the magnetism via antisymmetric exchange interaction of relativistic origin, producing helical spin orders as well as their fluctuations. Here we report for a chiral magnet MnSi that chiral spin fluctuations manifest themselves in the electrical magnetochiral effect (eMChE), i.e. the nonreciprocal and nonlinear response characterized by the electrical conductance depending on inner product of electric and magnetic fields $\boldsymbol{E} \cdot \boldsymbol{B}$. Prominent eMChE signals emerge at specific temperature-magnetic field-pressure regions: in the paramagnetic phase just above the helical ordering temperature and in the partially-ordered topological spin state at low temperatures and high pressures, where thermal and quantum spin fluctuations are conspicuous in proximity of classical and quantum phase transitions, respectively. The finding of the asymmetric electron scattering by chiral spin fluctuations may explore new electromagnetic functionality in chiral magnets. △ Less

Submitted 14 March, 2017; originally announced March 2017.

Comments: 25 pages, 9 figures (including Supplementary Information)

Magnon dispersion shift in the induced-ferromagnetic phase of the noncentrosymmetric MnSi

Authors: Taku J Sato, Daisuke Okuyama, Tao Hong, Akiko Kikkawa, Yasujiro Taguchi, Taka-hisa Arima, Yoshinori Tokura

Abstract: Small angle neutron inelastic scattering measurement has been performed to study the magnon dispersion relation in the field-induced-ferromagnetic phase of the noncentrosymmetric binary compound MnSi. For the magnons propagating parallel or anti-parallel to the external magnetic field, we experimentally confirmed that the dispersion relation is asymmetrically shifted along the magnetic field direc… ▽ More Small angle neutron inelastic scattering measurement has been performed to study the magnon dispersion relation in the field-induced-ferromagnetic phase of the noncentrosymmetric binary compound MnSi. For the magnons propagating parallel or anti-parallel to the external magnetic field, we experimentally confirmed that the dispersion relation is asymmetrically shifted along the magnetic field direction. This magnon dispersion shift is attributed to the relativistic Dzyaloshinskii-Moriya interaction, which is finite in noncentrosymmetric magnets, such as MnSi. The shift direction is found to be switchable by reversing the external magnetic field direction. △ Less

Submitted 29 September, 2016; originally announced September 2016.

Comments: 6 pages, 4 figures, accepted for publication in Phys. Rev. B

Robust metastable skyrmions and their triangular-square lattice-structural transition in a high-temperature chiral magnet

Authors: K. Karube, J. S. White, N. Reynolds, J. L. Gavilano, H. Oike, A. Kikkawa, F. Kagawa, Y. Tokunaga, H. M. Rønnow, Y. Tokura, Y. Taguchi

Abstract: Skyrmions, topologically-protected nanometric spin vortices, are being investigated extensively in various magnets. Among them, many of structurally-chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature… ▽ More Skyrmions, topologically-protected nanometric spin vortices, are being investigated extensively in various magnets. Among them, many of structurally-chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature $T_\mathrm{c}$, while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material, $β$-Mn-type Co$_8$Zn$_8$Mn$_4$, a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region, including down to zero temperature and up to the critical magnetic field of the ferromagnetic transition. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications. △ Less

Submitted 23 June, 2016; originally announced June 2016.

Journal ref: Nature Materials 15, 1237 (2016)

Heat-Treatment-Induced Switching of Magnetic States in the Doped Polar Semiconductor Ge$_{1-x}$Mn$_x$Te

Authors: M. Kriener, T. Nakajima, Y. Kaneko, A. Kikkawa, X. Z. Yu, N. Endo, K. Kato, M. Takata, T. Arima, Y. Tokura, Y. Taguchi

Abstract: Cross-control of a material property - manipulation of a physical quantity (e.g., magnetisation) by a nonconjugate field (e.g., electrical field) - is a challenge in fundamental science and also important for technological device applications. It has been demonstrated that magnetic properties can be controlled by electrical and optical stimuli in various magnets. Here we find that heat-treatment a… ▽ More Cross-control of a material property - manipulation of a physical quantity (e.g., magnetisation) by a nonconjugate field (e.g., electrical field) - is a challenge in fundamental science and also important for technological device applications. It has been demonstrated that magnetic properties can be controlled by electrical and optical stimuli in various magnets. Here we find that heat-treatment allows the control over two competing magnetic phases in the Mn-doped polar semiconductor GeTe. The onset temperatures $T_{\rm c}$ of ferromagnetism vary at low Mn concentrations by a factor of five to six with a maximum $T_{\rm c} \approx 180$ K, depending on the selected phase. Analyses in terms of synchrotron x-ray diffraction and energy dispersive x-ray spectroscopy indicate a possible segregation of the Mn ions, which is responsible for the high-$T_{\rm c}$ phase. More importantly, we demonstrate that the two states can be switched back and forth repeatedly from either phase by changing the heat-treatment of a sample, thereby confirming magnetic phase-change- memory functionality. △ Less

Submitted 18 May, 2016; originally announced May 2016.

Comments: 18 pages (preprint style), 5 figures

Journal ref: Sci. Rep. 6, 25748 (2016)

Formation of In-plane Skyrmions in Epitaxial MnSi Thin Films as Revealed by Planar Hall Effect

Authors: T. Yokouchi, N. Kanazawa, A. Tsukazaki, Y. Kozuka, A. Kikkawa, Y. Taguchi, M. Kawasaki, M. Ichikawa, F. Kagawa, Y. Tokura

Abstract: We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect t… ▽ More We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the in-plane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial magnetic anisotropy plays an important role in stabilizing the in-plane skyrmions in the MnSi thin film. △ Less

Submitted 15 June, 2015; originally announced June 2015.

Comments: 12 pages, 6 figures

Topological stability versus thermal agitation in a metastable magnetic skyrmion lattice

Authors: H. Oike, A. Kikkawa, N. Kanazawa, Y. Taguchi, M. Kawasaki, Y. Tokura, F. Kagawa

Abstract: Topologically stable matters can have a long lifetime, even if thermodynamically costly, when the thermal agitation is sufficiently low. A magnetic skyrmion lattice (SkL) represents a unique form of long-range magnetic order that is topologically stable, and therefore, a long-lived, metastable SkL can form. Experimental observations of the SkL in bulk crystals, however, have mostly been limited to… ▽ More Topologically stable matters can have a long lifetime, even if thermodynamically costly, when the thermal agitation is sufficiently low. A magnetic skyrmion lattice (SkL) represents a unique form of long-range magnetic order that is topologically stable, and therefore, a long-lived, metastable SkL can form. Experimental observations of the SkL in bulk crystals, however, have mostly been limited to a finite and narrow temperature region in which the SkL is thermodynamically stable; thus, the benefits of the topological stability remain unclear. Here, we report a metastable SkL created by quenching a thermodynamically stable SkL. Hall-resistivity measurements of MnSi reveal that, although the metastable SkL is short-lived at high temperatures, the lifetime becomes prolonged (>> 1 week) at low temperatures. The manipulation of a delicate balance between thermal agitation and the topological stability enables a deterministic creation/annihilation of the metastable SkL by exploiting electric heating and subsequent rapid cooling, thus establishing a facile method to control the formation of a SkL. △ Less

Submitted 3 October, 2015; v1 submitted 1 June, 2015; originally announced June 2015.

Comments: 23 pages, 8 figures (including Supplementary Information), revised version is accepted in Nat. Phys

Journal ref: Nat. Phys. 12, 62-66 (2016)

Modification of electronic structure and thermoelectric properties of hole-doped tungsten dichalcogenides

Authors: M. Kriener, A. Kikkawa, T. Suzuki, R. Akashi, R. Arita, Y. Tokura, Y. Taguchi

Abstract: We present a study on the modification of the electronic structure and hole-doping effect for the layered dichalcogenide WSe_2 with a multi-valley band structure, where Ta is doped on the W site along with a partial substitution of Te for its lighter counterpart Se. By means of band-structure calculations and specific-heat measurements, the introduction of Te is theoretically and experimentally fo… ▽ More We present a study on the modification of the electronic structure and hole-doping effect for the layered dichalcogenide WSe_2 with a multi-valley band structure, where Ta is doped on the W site along with a partial substitution of Te for its lighter counterpart Se. By means of band-structure calculations and specific-heat measurements, the introduction of Te is theoretically and experimentally found to change the electronic states in WSe_2. While in WSe_2 the valence-band maximum is located at the Gamma point, the introduction of Te raises the bands at the K point with respect to the Gamma point. In addition, thermal-transport measurements reveal a smaller thermal conductivity at room temperature of W_1-xTa_xSe_1.6Te_0.4 than reported for W_1-xTa_xSe_2. However, when approaching 900 K, the thermal conductivities of both systems converge while the resistivity in W_1-xTa_xSe_1.6Te_0.4 is larger than in W_1-xTa_xSe_2, leading to comparable but slightly smaller values of the figure of merit in W_1-xTa_xSe_1.6Te_0.4. △ Less

Submitted 19 February, 2015; originally announced February 2015.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. B 91, 075205 (2015)

Common Origin of the Circular-dichroism Pattern in ARPES of SrTiO3 and CuxBi2Se3

Authors: Y. Ishida, H. Kanto, A. Kikkawa, Y. Taguchi, Y. Ito, Y. Ota, K. Okazaki, W. Malaeb, M. Mulazzi, M. Okawa, S. Watanabe, C. -T. Chen, M. Kim, C. Bell, Y. Kozuka, H. Y. Hwang, Y. Tokura, S. Shin

Abstract: Circular dichroism in the angular distribution (CDAD) of photoelectrons from SrTiO3:Nb and CuxBi2Se3 is investigated by 7-eV laser ARPES. In addition to the well-known node that occurs in CDAD when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is… ▽ More Circular dichroism in the angular distribution (CDAD) of photoelectrons from SrTiO3:Nb and CuxBi2Se3 is investigated by 7-eV laser ARPES. In addition to the well-known node that occurs in CDAD when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two dimensional. The flower-shaped CDAD's occurring around the Fermi level of SrTiO3:Nb and around the Dirac point of CuxBi2Se3 are explained on equal footings. We point out that the penetration depth of the topological states of CuxBi2Se3 depends on momentum. △ Less

Submitted 22 August, 2011; originally announced August 2011.

Journal ref: Phys. Rev. Lett. 107, 077601 (2011)

Triakontadipole and high-order dysprosium multipoles in the antiferromagnetic phase of DyB$_2$C$_2$

Authors: A. J. Princep, A. M. Mulders, U. Staub, V. Scagnoli, T. Nakamura, A. Kikkawa, S. W. Lovesey, E. Balcar

Abstract: Resonant soft x-ray Bragg diffraction at the Dy M$_{4,5}$ edges has been used to study Dy multipoles in the combined magnetic and orbitally ordered phase of DyB$_2$C$_2$. The analysis incorporates both the intra-atomic magnetic and quadrupolar interactions between the 3d core and 4f valence shells. Additionally, we introduce to the formalism the interference of magnetic and nonmagnetic oscillators… ▽ More Resonant soft x-ray Bragg diffraction at the Dy M$_{4,5}$ edges has been used to study Dy multipoles in the combined magnetic and orbitally ordered phase of DyB$_2$C$_2$. The analysis incorporates both the intra-atomic magnetic and quadrupolar interactions between the 3d core and 4f valence shells. Additionally, we introduce to the formalism the interference of magnetic and nonmagnetic oscillators. This allows a determination of the higher order multipole moments of rank 1 (dipole) to 6 (hexacontatetrapole). The strength of the Dy 4f multipole moments have been estimated at being between 7 and 78 % of the quadrupolar moment. △ Less

Submitted 12 April, 2011; originally announced April 2011.

Comments: 11 pages, 2 figures

Journal ref: 2011 J. Phys.: Condens. Matter 23 266002

Magnetotransport studies of EuFe$_2$As$_2$: the influence of the Eu$^{2+}$ magnetic moments

Authors: Taichi Terashima, Nobuyuki Kurita, Akiko Kikkawa, Hiroyuki S. Suzuki, Takehiko Matsumoto, Keizo Murata, Shinya Uji

Abstract: We report resistivity $ρ$ and Hall effect measurements on EuFe$_2$As$_2$ at ambient pressure and 28 kbar and magnetization measurements at ambient pressure. We analyze the temperature and magnetic-field dependence of $ρ$ and the Hall effect using a molecular-field theory for magnetoresistance and an empirical formula for the anomalous Hall effect and find that electron scattering due to the Eu… ▽ More We report resistivity $ρ$ and Hall effect measurements on EuFe$_2$As$_2$ at ambient pressure and 28 kbar and magnetization measurements at ambient pressure. We analyze the temperature and magnetic-field dependence of $ρ$ and the Hall effect using a molecular-field theory for magnetoresistance and an empirical formula for the anomalous Hall effect and find that electron scattering due to the Eu$^{2+}$ local moments plays only a minor role in determining electronic transport properties of EuFe$_2$As$_2$. △ Less

Submitted 27 September, 2010; v1 submitted 11 August, 2010; originally announced August 2010.

Comments: Published in J. Phys. Soc. Jpn

Journal ref: J. Phys. Soc. Jpn. 79, 103706 (2010)