arXiv subjects

Takanori Kida

Publications and source records attributed to Takanori Kida.

At least 19 recordsLinked to original sources

arXiv2026

Emergence of Kondo-assisted Néel order in a Kondo necklace model

The interplay between Kondo screening and magnetic order has long been a central issue in the physics of strongly correlated systems. While the Kondo effect has traditionally been understood to suppress magnetism through the formation of local singlets, recent studies suggest that Kondo interactions may enhance magnetic order under certain conditions. However, these scenarios often rely on complex electronic structures, including orbital and charge degrees of freedom, making the essential mechanisms difficult to isolate. Here we report the realization of a spin-(1/2,1) Kondo necklace model in a Ni-based complex-a minimal spin-only analog of the Kondo lattice that isolates quantum spin correlations by eliminating charge degrees of freedom. Thermodynamic measurements identify a magnetic phase transition and a field-induced quantum phase transition. Perturbative analysis reveals that the Kondo coupling mediates effective antiferromagnetic interactions between the spin-1 sites, stabilizing the Néel order across the entire chain. Our results establish a universal boundary in Kondo physics, where coupling to spin-1/2 moments yields singlets, but to spin-1 and higher stabilizes magnetic order.

cond-mat.str-el↗

arXiv2026

Realization of a triangular spin necklace in a verdazyl-based Ni complex

We successfully synthesized a verdazyl-based complex, ($m$-Py-V)$_3$[Ni(NO$_3$)$_2$], in which Ni$^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl and Ni$^{2+}$ spins form frustrated triangular units, creating a distinctive spin network. Magnetic susceptibility and specific heat measurements identify a phase transition to an AF order. The application of magnetic fields suppresses the phase transition signal, suggesting field-induced decoupling of the spin-1 moments. Electron spin resonance measurements are used to evaluate the easy-axis anisotropy of spin-1, which may promote the AF order. This work provides a rare example of a geometrically frustrated quantum spin chain realized via molecular design, thereby offering a platform for exploring frustration-driven quantum phases in low-dimensional materials.

cond-mat.str-el↗

arXiv2025

Effect of crystallinity on spin-orbit torque in 5$\textit{d}$ iridium oxide IrO$_{2}$

The 5$\textit{d}$ transition-metal oxides provide an intriguing platform for generating an efficient spin current due to a unique electronic structure dominated by 5d electrons with strong spin-orbit coupling. Here, we report on the effect of crystallinity on current-driven spin-orbit torque (SOT) in binary 5$\textit{d}$ iridium oxide IrO$_{2}$ thin films by controlling amorphous, polycrystalline, and epitaxial states. By conducting harmonic Hall measurement in bilayers composed of ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ and IrO$_{2}$, we find that dampinglike (DL) SOT is larger than fieldlike SOT for all the samples. We also demonstrate that both electrical resistivity and the DL SOT efficiency increase in order of epitaxial, polycrystalline, and amorphous IrO$_{2}$. Despite their different electrical conductivities, spin Hall conductivities of the three states of the IrO$_{2}$ layer are found to be nearly constant, which is consistent with the intrinsic regime of the spin Hall effect scaling relation. Our results highlight the important role that crystallinity plays in the spin-current generation, leading to the potential technological development of spintronic devices based on the 5$\textit{d}$ transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2025

Gapless behavior in a two-leg spin ladder with bond randomness

We successfully synthesized [Cu$_2$(AcO)$_4$($p$-Py-V-$p$-F)$_2$]$\cdot$4CHCl$_3$, a verdazyl-based complex with a paddlewheel structure comprising two Cu atoms, which induces strong antiferromagnetic (AF) exchange interactions between Cu spins, generating a nonmagnetic singlet state at low temperatures. Two primary exchange interactions between radical spins generate a spin-1/2 AF two-leg ladder. In addition, two possible positional configurations of the F atom in the complex create four different overlap patterns of molecular orbitals, introducing bond randomness in the spin ladder. The observed experimental behaviors, such as the Curie tail in the magnetic susceptibility and the gapless gradual increase in the magnetization curve, are attributed to a broad distribution of excitation energies and a few orphan spins in the random-singlet (RS) state that are stabilized by bond randomness. The low-temperature specific heat exhibits a temperature dependence with $\propto 1/|{\rm{ln}}T|^3$, demonstrating the formation of the RS state in unfrustrated systems. We also consider the effect of restricted patterns of exchange interactions and one-dimensional nature of the system on the RS state.

cond-mat.str-el↗

arXiv2024

Magnetic properties of a spin-1/2 octagonal lattice

We successfully synthesized a verdazyl-based complex, ($p$-Py-V-$p$-CN)$_2$[Cu(hfac)$_2$]. Molecular orbital calculations reveal that three types of antiferromagnetic (AF) interactions are involved in the formation of a spin-1/2 distorted octagonal lattice composed of the verdazyl radical and Cu spins. The magnetic properties of the compound exhibited contributions from AF correlations and a phase transition to an AF ordered state at approximately $T_{\rm{N}}$ = 2.5 K. Below $T_{\rm{N}}$, we observed a $T^2$ dependence of the specific heat, indicating dominant two-dimensional AF correlations within the octagonal lattice. The magnetization curve in the low-temperature region exhibited a low-field linear increase, subsequent bending at 1/3 magnetization, and high-field nonlinear increase. We reproduced the observed unique magnetic behavior through the numerical analysis based on the octagonal lattice. These results demonstrate that the present compound exhibits magnetic properties characteristic of octagonal lattice topology.

cond-mat.str-el↗

arXiv2023

Effect of interface quality on spin Hall magnetoresistance in Pt/MgFe$_{2}$O$_{4}$ bilayers

We report on spin Hall magnetoresistance (SMR) in bilayers composed of Pt and magnetic insulator MgFe$_{2}$O$_{4}$ (MFO) with spinel structure. The Pt thickness dependence of the SMR reveals that annealing of the MFO surface before depositing the Pt layer is crucial for a large SMR with better interface quality. We also found that oxygen pressure during the MFO growth hardly affects the SMR while it influences on magnetic property of the MFO film. Our findings provide important clues to further understanding the spin transport at interfaces containing magnetic insulators, facilitating development of low power consumption devices.

cond-mat.mtrl-sci↗

arXiv2023

NiCrAl piston-cylinder cell for magnetic susceptibility measurements under high pressures in pulsed high magnetic fields

We developed a metallic pressure cell made of nickel-chromium-aluminum (NiCrAl) for use with a non-destructive pulse magnet and a magnetic susceptibility measurement apparatus with a proximity detector oscillator (PDO) in pulsed magnetic fields of up to 51 T under pressures of up to 2.1 GPa. Both the sample and sensor coil of the PDO were placed in the cell so that the magnetic signal from NiCrAl would not overlay the intrinsic magnetic susceptibility of the sample. A systematic investigation of the Joule heating originating from metallic parts of the pressure cell revealed that the temperature at the sample position remains at almost 1.4 K until approximately 80 $\%$ of the maximum applied magnetic field ($H_{\rm max}$) in the field-ascending process (e.g., 40 T for $H_{\rm max}$ of 51 T). The effectiveness of our apparatus was demonstrated, by investigating the pressure dependence of the magnetization process of the triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$.

cond-mat.mtrl-sci↗

arXiv2023

Quantum gapped state in a spin-1/2 distorted honeycomb-based lattice with frustration

We successfully synthesized ($p$-Py-V)[Cu(hfac)$_2$], a verdazyl-based complex. Molecular orbital calculations revealed five types of intermolecular interactions between the radical spins and two types of intramolecular interactions between the radical and the Cu spins, resulting in a spin-1/2 distorted honeycomb-based lattice. Additionally, competing ferromagnetic and antiferromagnetic (AF) interactions induce frustration. The magnetization curve displayed a multistage increase, including a zero-field energy gap. Considering the stronger AF interactions that form dimers and tetramers, the magnetic susceptibility and magnetization curves were qualitatively explained. These findings demonstrated that the quantum state, based on the dominant AF interactions, was stabilized due to the effects of frustration in the lattice. Hence, the exchange interactions forming two-dimensional couplings decoupled, reducing energy loss caused by frustration and leading to frustration-induced dimensional reduction.

cond-mat.str-el↗

arXiv2023

Field-induced quantum phase in a frustrated zigzag-square lattice

This study presents the experimental realization of a spin-1/2 zigzag-square lattice in a verdazyl-based complex, namely ($m$-Py-V-2,6-F$_2$)$[$Cu(hfac)$_2]$. Molecular orbital calculations suggest the presence of five types of frustrated exchange couplings. Our observations reveal an incremental increase in the magnetization curve beyond a critical field, signifying a phase transition from the antiferromagnetic ordered state to a quantum state characterized by a 1/2 plateau. This intriguing behavior arises from the effective stabilization of a zigzag chain by the external fields. These results provide evidence for field-induced dimensional reduction in a zigzag-square lattice attributed to the effects of frustration.

cond-mat.str-el↗

arXiv2023

Spin current generation from an epitaxial tungsten dioxide WO$_{2}$

We report on efficient spin current generation at room temperature in rutile type WO$_{2}$ grown on Al$_{2}$O$_{3}$(0001) substrate. The optimal WO$_{2}$ film has (010)-oriented monoclinically distorted rutile structure with metallic conductivity due to 5$\it{d}$$^2$ electrons, as characterized by x-ray diffraction, electronic transport, and x-ray photoelectron spectroscopy. By conducting harmonic Hall measurement in Ni$_{81}$Fe$_{19}$/WO$_{2}$ bilayer, we estimate two symmetries of the spin-orbit torque (SOT), i.e., dampinglike (DL) and fieldlike ones to find that the former is larger than the latter. By comparison with the Ni$_{81}$Fe$_{19}$/W control sample, the observed DL SOT efficiency $ξ$$_{DL}$ of WO$_{2}$ (+0.174) is about two thirds of that of W (-0.281) in magnitude, with a striking difference in their signs. The magnitude of the $ξ$$_{DL}$ of WO$_{2}$ exhibits comparable value to those of widely reported Pt and Ta, and Ir oxide IrO$_{2}$. The positive sign of the $ξ$$_{DL}$ of WO$_{2}$ can be explained by the preceding theoretical study based on the 4$\it{d}$ oxides. These results highlight that the epitaxial WO$_{2}$ offers a great opportunity of rutile oxides with spintronic functionalities, leading to future spin-orbit torque-controlled devices.

cond-mat.mtrl-sci↗

arXiv2023

Spin-orbit torque generation in bilayers composed of CoFeB and epitaxial SrIrO$_{3}$ grown on an orthorhombic DyScO$_{3}$ substrate

We report on the highly efficient spin-orbit torque (SOT) generation in epitaxial SrIrO$_{3}$(SIO), which is grown on an orthorhombic DyScO$_{3}$(110) substrate. By conducting harmonic Hall measurement in Co$_{20}$Fe$_{60}$B$_{20}$ (CoFeB)/SIO bilayers, we characterize two kinds of the SOTs, i.e., dampinglike (DL) and fieldlike ones to find that the former is much larger than the latter. By comparison with the Pt control sample with the same CoFeB thickness, the observed DL SOT efficiency $ξ$$_{DL}$ of SIO ($\sim$0.32) is three times higher than that of Pt ($\sim$0.093). The $ξ$$_{DL}$ is nearly constant as a function of the CoFeB thickness, suggesting that the SIO plays a crucial role in the large SOT generation. These results on the CoFeB/SIO bilayers highlight that the epitaxial SIO is promising for low-current and reliable spin-orbit torque-controlled devices.

cond-mat.mtrl-sci↗

arXiv2023

Spin Hall magnetoresistance in Pt/Y$_{3}$Fe$_{5}$O$_{12}$ bilayers grown on Si and Gd$_{3}$Ga$_{5}$O$_{12}$ substrates

We study spin Hall magnetoresistance (SMR) in Pt/ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) bilayers by focusing on crystallinity, magnetization, and interface roughness by controlling post-annealing temperatures. The SMR in the Pt/YIG grown on Si substrate is comparable to that grown on widely used Gd$_{3}$Ga$_{5}$O$_{12}$ substrate, indicating that the large SMR can be achieved irrespective to the crystallinity. We deduced the spin mixing conductance from the Pt thickness dependence of the SMR to find the high interface quality of the optimized Pt/YIG grown on Si in terms of spin current. We also clarified that the SMR correlates well with the magnetization, the interface roughness, and carrier density. These findings highlight that optimizing YIG properties is a key to control of magnetization by spin current, leading to the development of low power consumption spintronic device based on the magnetic insulator.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic-field-induced Anderson localization in orbital selective antiferromagnet BaMn$_2$Bi$_2$

We report a metal-insulator transition (MIT) in the half-filled multiorbital antiferromagnet (AF) BaMn$_2$Bi$_2$ that is tunable by a magnetic field perpendicular to the AF sublattices. Instead of an Anderson-Mott mechanism usually expected in strongly correlated systems, we find by scaling analyses that the MIT is driven by an Anderson localization. Electrical and thermoelectrical transport measurements in combination with electronic band calculations reveal a strong orbital-dependent correlation effect, where both weakly and strongly correlated $3d$-derived bands coexist with decoupled charge excitations. Weakly correlated holelike carriers in the $d_{xy}$-derived band dominate the transport properties and exhibit the Anderson localization, whereas other $3d$ bands show clear Mott-like behaviors with their spins ordered into AF sublattices. The tuning role played by the perpendicular magnetic field supports a strong spin-spin coupling between itinerant holelike carriers and the AF fluctuations, which is in sharp contrast to their weak charge coupling.

cond-mat.str-el↗

arXiv2021

Stacking-order effect on spin-orbit torque, spin-Hall magnetoresistance, and magnetic anisotropy in Ni$_{81}$Fe$_{19}$-IrO$_2$ bilayers

The 5d transition-metal oxides have been an intriguing platform to demonstrate efficient charge to spin current conversion due to a unique electronic structure dominated by strong spin-orbit coupling. Here, we report on stacking-order effect of spin-orbit torque (SOT), spin-Hall magnetoresistance, and magnetic anisotropy in bilayer Ni$_{81}$Fe$_{19}$-5d iridium oxide, IrO$_2$. While all the IrO$_2$ and Pt control samples exhibit large dampinglike-SOT generation stemming from the efficient charge to spin current conversion, the magnitude of the SOT is larger in the IrO$_2$ (Pt)-bottom sample than in the IrO$_2$ (Pt)-top one. The fieldlike-SOT has even more significant stack order effect, resulting in an opposite sign in the IrO$_2$ samples in contrast to the same sign in the Pt samples. Furthermore, we observe that the magnetic anisotropy energy density and the anomalous Hall effect are increased in the IrO$_2$ (Pt)-bottom sample, suggesting enhanced interfacial perpendicular magnetic anisotropy. Our findings highlight the significant influence of the stack order on spin transport and magnetotransport properties of Ir oxide/ferromagnet systems, providing useful information on design of SOT devices including 5d transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2021

Spin-orbit torque generation in NiFe/IrO2 bilayers

The 5d transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive 5d iridium oxide, IrO2. By measuring second-harmonic Hall resistance of Ni81Fe19/IrO2 bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on IrO2 thickness, which is well described by the drift-diffusion model based on the bulk spin Hall effect. We deduce the effective spin Hall angle of +0.093 +- 0.003 and the spin-diffusion length of 1.7 +- 0.2 nm. By comparison with control samples Pt and Ir, we show that the effective spin Hall angle of IrO2 is comparable to that of Pt and seven times higher than that of Ir. The fieldlike SOT efficiency has a negative sign without appreciable dependence on the thickness, in contrast to the dampinglike SOT. This suggests that the fieldlike SOT likely stems from the interface. These experimental findings suggest that the uniqueness of the electronic structure of 5d transition-metal oxides is crucial for highly efficient charge to spin-current conversion.

cond-mat.mtrl-sci↗

arXiv2021

Large negative magnetoresistance in BaMn$_2$Bi$_2$ antiferromagnet

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. High perpendicular magnetic field eventually suppresses the insulating behavior and allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly unrelated to any field induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multi-orbital electronic structure of BaMn$_2$Bi$_2$.

cond-mat.str-el↗

arXiv2018

Itinerant antiferromagnetic BaMn$_2$Pn$_2$'s showing both negative and positive magnetoresistances

We report the discovery of a novel giant magnetoresistance (GMR) phenomenon in a family of BaMn$_{2}$Pn$_{2}$ antiferromagnets (Pn stands for P, As, Sb, and Bi) with a parity-time symmetry. The resistivities of these materials are reduced by $60$ times in magnetic fields ($\vec{H}$'s), thus yielding the GMR of about $-98\%$. The GMR changes systematically along with the Pn elements, hinting that its origin is the spin orbit coupling (SOC) and/or $d$-$p$ orbital hybridization. A positive MR component emerging on top of the negative GMR at low temperatures suggests an orbital-sensitive magnetotransport as $\vec{H}$ suppresses the conduction of the electron-like carriers in the $d$-like band but enhances those of hole-like ones in the $d$-$p$ hybridized band. The anisotropy of the GMR reveals that the electrical conductivity is extremely sensitive to the minute changes in the direction of the antiferromagnetic moments induced by the parity-time breaking $\vec{H}$, which seems to be associated with a magnetoelectric effect in the dynamic regime of conduction electrons. We attribute the observed GMR to the non-trivial low energy band of BMPn's, which is governed by the parity-time symmetry and an magnetic hexadecapole ordering.

cond-mat.str-el↗

arXiv2018

High-field Magnetism of the Honeycomb-lattice Antiferromagnet Cu$_2$(pymca)$_3$(ClO$_4$)

We report on the experimental results of magnetic susceptibility, specific heat, electron spin resonance (ESR), and high-field magnetization measurements on a polycrystalline sample of the spin-$1/2$ distorted honeycomb-lattice antiferromagnet Cu$_2$(pymca)$_3$(ClO$_4$). Magnetic susceptibility shows a broad peak at about 25~K, which is typical of a low dimensional antiferromagnet, and no long range magnetic order is observed down to 0.6~K in the specific heat measurements. Magnetization curve up to 70~T at 1.4~K shows triple stepwise jumps. Assuming three different exchange bonds $J_{\rm A}$, $J_{\rm B}$ and $J_{\rm C}$ from the structure, the calculated magnetization curve reproduces the observed one when $J_{\rm A}/k_{\rm B} = 43.7~{\rm K}$, $J_{\rm B}/J_{\rm A} = 1$ and $J_{\rm C}/J_{\rm A} = 0.2$ except the magnetization near 70~T, where the observed magnetization indicates another step while the calculated magnetization becomes saturated. The relationship between magnetization plateaus and exchange bonds is discussed based on the numerical calculations.

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