arXiv subjects

Jiro Kitagawa

Publications and source records attributed to Jiro Kitagawa.

At least 19 recordsLinked to original sources

arXiv2026

Experimental investigation of magnetic properties of MnFeCo$_{4}$Si$_{2}$ discovered by GNoME

AI-driven inorganic materials research has garnered significant attention due to its ability to reduce the time, labor, and cost associated with experiments. An AI model known as GNoME, recently developed by Google DeepMind, is particularly fascinating because it is integrated with the Materials Project open database. The experimental verification of compounds identified by GNoME is a crucial process for advancing AI-driven materials research. Here, we focus on the magnetic compound MnFeCo$_{4}$Si$_{2}$ (Materials ID: mp-3203253), which possesses a layered-like structure. Consistent with the GNoME prediction, MnFeCo$_{4}$Si$_{2}$ crystallizes in a rhombohedral structure with a single-phase nature. We have characterized its magnetic properties and determined that MnFeCo$_{4}$Si$_{2}$ is a soft ferromagnet with a Curie temperature of 1039 K.

cond-mat.mtrl-sci↗

arXiv2026

Enhanced $T_\mathrm{c}$ in eutectic high-entropy alloy superconductors Hf-Nb-Sc-Ti-Zr

The present investigation into the superconducting properties of eutectic high-entropy alloy (HEA) Hf-Nb-Sc-Ti-Zr systems reveals an enhanced superconducting critical temperature ($T_\mathrm{c}$) in body-centered cubic (bcc) phases compared to typical quinary bcc HEAs. In Hf$_{10}$Nb$_{25}$Sc$_{25}$Ti$_{20}$Zr$_{20}$, Hf$_{5}$Nb$_{45}$Sc$_{20}$Ti$_{15}$Zr$_{15}$, and Hf$_{5}$Nb$_{45}$Sc$_{10}$Ti$_{5}$Zr$_{35}$ systems, which span a broad range of valence electron concentration per atom, lattice strain and the presence of partial or absent eutectic phases are characteristic features at lower annealing temperatures. The eutectic regions expand rapidly following annealing at 600$^{\circ}$C in all systems. The $T_\mathrm{c}$ of each system increases markedly with rising annealing temperatures from 400$^{\circ}$C to 600$^{\circ}$C, reaching a maximum value of 9.93 K in the Hf$_{5}$Nb$_{45}$Sc$_{10}$Ti$_{5}$Zr$_{35}$ sample annealed at 800$^{\circ}$C. Nearly all samples can be classified as strong-coupling superconductors. The sample annealed at 500$^{\circ}$C in the Hf$_{5}$Nb$_{45}$Sc$_{10}$Ti$_{5}$Zr$_{35}$ system exhibits a critical current density ($J_\mathrm{c}$) exceeding the practical threshold of 10$^{5}$ A/cm$^{2}$ up to approximately 4 T at 4.2 K and 6 T at 2 K. The elevated $J_\mathrm{c}$ is attributed to significant lattice strain and phase instability. The underlying mechanism for the enhanced $T_\mathrm{c}$ in Hf-Nb-Sc-Ti-Zr systems is examined through specific heat data analysis, suggesting that the expansion of the eutectic regions induced by thermal annealing plays a pivotal role.

cond-mat.supr-con↗

arXiv2026

Insight into high-entropy effect in body-centered cubic superconducting alloys

We have characterized the superconducting critical temperature ($T_\mathrm{c}$), the Debye temperature ($\theta_\mathrm{D}$), the electronic specific heat coefficient, and the Vickers microhardness of HfNbTiVZr, NbTiZr, HfNbTi, HfNbZr, and HfNbTa, all possessing a body-centered cubic (bcc) structure. By compiling a comparable dataset for other equiatomic quinary bcc high-entropy alloy (HEA) superconductors, we have examined the validity of the hypothesis regarding the high-entropy effect in bcc HEA superconductors, as proposed in our previous work. This hypothesis attributes the observed negative correlation between the electron-phonon coupling constant ($\lambda_\mathrm{e-p}$) and $\theta_\mathrm{D}$ to a reduced phonon lifetime at higher $\theta_\mathrm{D}$, arising from the uncertainty principle in highly disordered quinary alloys. However, a pronounced change in this negative correlation is not evident in equiatomic ternary alloys with a lower degree of atomic disorder, thereby providing limited support for the hypothesis. Alternatively, by assembling the full dataset of bcc alloys spanning binary through senary systems, we have identified a universal negative correlation between $\lambda_\mathrm{e-p}$ and $\theta_{D}$. This result would be useful for the materials design of bcc superconducting alloys. We further propose that the Vickers microhardness offers an alternative means to evaluate $\theta_{D}$ and may serve as a rapid screening metric for identifying bcc alloys with desired properties.

cond-mat.supr-con↗

arXiv2025

High critical current densities of body-centered cubic high-entropy alloy superconductors: recent research progress

High-entropy alloy (HEA) superconductors have garnered significant attention due to their unique characteristics, such as robust superconductivity under extremely high pressure and irradiation, the cocktail effect, and the enhancement of the upper critical field. A high critical current density is another noteworthy feature observed in HEAs. Several body-centered cubic (bcc) HEAs have exhibited critical current densities comparable to those of Nb-Ti superconducting alloys. Such HEAs hold potential for applications as multifunctional superconducting wires, a capability rarely achieved in conventional alloys. In this context, we review recent advancements in research on critical current densities in bcc HEA superconductors, including Ta$_{1/6}$Nb$_{2/6}$Hf$_{1/6}$Zr$_{1/6}$Ti$_{1/6}$, (TaNb)$_{0.7}$(HfZrTi)$_{0.5}$, NbScTiZr, and others. Comparative analyses among these HEAs reveal that both eutectic microstructures, which accompany lattice strain, and nanosized precipitates play pivotal roles in achieving elevated critical current densities across wide magnetic field ranges. Furthermore, we propose several future directions for research. These include elucidating the origin of lattice strain, exploring more fine eutectic microstructures, artificially introducing nanoscale pinning sites, improving the superconducting critical temperature, and investigating the mechanical properties of these materials.

cond-mat.supr-con↗

arXiv2024

Superconducting properties of eutectic high-entropy alloy superconductor NbScTiZr

The influence of annealing on the superconducting properties of eutectic high-entropy alloy NbScTiZr was examined by measuring magnetization, electrical resistivity, and zero-field specific heat. Additionally, the extent of lattice strain was assessed via the analysis of lattice parameters and Vickers microhardness. The greatest lattice strain was inferred in the sample annealed at 400 $^{\circ}$C. Field-dependent magnetization datasets indicate enhanced flux pinning in the as-cast, 400 $^{\circ}$C annealed, and 600 $^{\circ}$C annealed samples. The superconducting parameters such as upper critical field, Ginzburg-Landau coherence length, and magnetic penetration depth do not strongly depend on the annealing temperature. However, the Maki parameter shows a peak at 400 $^{\circ}$C annealing temperature and correlates with the magnitude of lattice strain. This suggests that greater lattice strain may enhance the Maki parameter by altering the orbital-limited field through the modification of flux pinning strength. Although the influence of lattice strain is less discernible in zero-field specific heat measurements, the superconducting parameters deduced from specific heat data suggest a potential for strong-coupled superconductivity in samples heat-treated above 600 $^{\circ}$C. Our investigation underscores the substantial impact of lattice strain on the Maki parameter of eutectic HEA superconductors, primarily through the modulation of flux pinning strength.

cond-mat.supr-con↗

arXiv2023

Effect of annealing in eutectic high-entropy alloy superconductor NbScTiZr

We investigated the impact of annealing on the structural characteristics and superconducting critical temperature ($T_\mathrm{c}$) of the eutectic high-entropy alloy (HEA) superconductor NbScTiZr. The HEA manifests an eutectic microstructure composed of body-centered cubic (bcc) and hexagonal close-packed phases. Both the lattice parameters of the bcc phase and grain size of the eutectic structure exhibited pronounced sensitivity to variations in annealing temperature. The observed dependence of the lattice parameter on annealing temperature supports the possibility that lattice strain occurs at lower annealing temperatures. The as-cast sample demonstrated superconductivity at $T_\mathrm{c}$ of 7.9 K, which increased to 9 K after annealing at 800 $^{\circ}$C. However, when subjected to annealing at 1000 $^{\circ}$C, $T_\mathrm{c}$ diminishes to 8.7 K. The annealing-temperature dependence of $T_\mathrm{c}$ cannot be comprehensively elucidated based solely on the electronic density of states at the Fermi level. It is plausible that the lattice strain may influence the annealing temperature dependence of $T_\mathrm{c}$. Our results for the critical current density $J_{c}$ reveal that the self-field $J_{c}$ of the as-cast NbScTiZr at 2 K exceeds 10$^{6}$ A/cm$^{2}$.

cond-mat.supr-con↗

arXiv2023

Low-temperature giant coercivity in Co$_{6.2}$Ga$_{3.8-x}$Ge$_{x}$ ($x$=2.4 to 3.2)

The observation of giant coercivity exceeding 20 kOe at low temperatures in several transition-metal-based compounds has attracted significant attention from a fundamental perspective. This research is also relevant to developing rare-earth-free permanent magnets, wherein cobalt is one of the primary elements used. To facilitate easy fabrication, rare-earth-free and Co-based inorganic bulk magnets that exhibit giant coercivity are highly demanded but rarely reported. Herein, we report the observation of low-temperature giant coercivity in polycrystalline metallic Co$_{6.2}$Ga$_{3.8-x}$Ge$_{x}$ ($x$=2.4 to 3.2) with the hexagonal Fe$_{13}$Ge$_{8}$-type structure composed of Kagome and triangular lattices. As the Ge content $x$ decreases from 3.2, the magnetic ground state changes from ferrimagnetism to ferromagnetism at $x$=2.6. In the ferrimagnetic state, we observed a signature of spin frustration arising from the Kagome and/or triangular lattices of Co atoms. The ferromagnetic ordering temperatures for the $x$=2.6 and 2.4 samples are 46 K and 60 K, respectively. The coercive fields rapidly increase upon cooling and reach values of 26 kOe and 44 kOe in the $x$=2.6 and 2.4 samples, respectively, at 2 K.

cond-mat.mtrl-sci↗

arXiv2023

High-entropy effect at rare-earth site in DyNi

We report the structural and magnetic properties of RNi (R=Dy, Tb$_{1/3}$Dy$_{1/3}$Ho$_{1/3}$, and Gd$_{1/5}$Tb$_{1/5}$Dy$_{1/5}$Ho$_{1/5}$Er$_{1/5}$) to investigate the high-entropy effect at the rare-earth site. The lattice parameters are almost unchanged by the increase of configurational entropy, which is due to the successive partial substitution of Dy by pair of rare earth elements located on both sides of Dy in the periodic table. All compounds exhibit ferromagnetic ground states. The replacement of Dy with Tb+Ho, which does not have magnetic interactions in competition with Dy, does not affect the magnetic ordering temperature. Although (Gd$_{1/5}$Tb$_{1/5}$Dy$_{1/5}$Ho$_{1/5}$Er$_{1/5}$)Ni shows the Curie temperature close to that of DyNi, an additional magnetic anomaly, which would be a spin reorientation, is observed probably due to the introduction of competing magnetic interactions between R=Gd and Er compounds and R=Tb, Dy, and Ho ones. We have also assessed the magnetocaloric effect, and the configurational entropy dependence of the magnetic entropy change reflects that of the temperature derivative of the magnetic susceptibility. Our analysis suggests the possibility of enhancing magnetocaloric properties by designing the anisotropy of rare-earth magnetic moments in the high-entropy state.

cond-mat.str-el↗

arXiv2023

Metallurgy, superconductivity, and hardness of a new high-entropy alloy superconductor Ti-Hf-Nb-Ta-Re

We explored quinary body-centered cubic (bcc) high-entropy alloy (HEA) superconductors with valence electron concentrations (VECs) ranging from 4.6 to 5.0, a domain that has received limited attention in prior research. Our search has led to the discovery of new bcc Ti-Hf-Nb-Ta-Re superconducting alloys, which exhibit an interesting phenomenon of phase segregation into two bcc phases with slightly different chemical compositions, as the VEC increases. The enthalpy of the formation of each binary compound explains the phase segregation. All the alloys investigated were categorized as type-II superconductors, with superconducting critical temperatures ($T_\mathrm{c}$) ranging from 3.25 K to 4.38 K. We measured the Vickers microhardness, which positively correlated with the Debye temperature, and compared it with the hardness values of other bcc HEA superconductors. Our results indicate that $T_\mathrm{c}$ systematically decreases with an increase in hardness beyond a threshold of approximately 350 HV. Additionally, we plotted $T_\mathrm{c}$ vs. VEC for representative quinary bcc HEAs. The plot revealed the asymmetric VEC dependence. The correlation between the hardness and $T_\mathrm{c}$, as well as the asymmetric dependence of $T_\mathrm{c}$ on VEC can be attributed to the simultaneous effects of the electronic density of states at the Fermi level and electron-phonon coupling under the uncertainty principle, especially in the higher VEC region.

cond-mat.supr-con↗

arXiv2022

Datasets on materials research of hard ferromagnet in TM-Fe-Si (TM=Ti, Zr, Hf, V, Nb, and Ta) ternary systems

The datasets presented in this article are related to materials research on hard ferromagnet in TM-Fe-Si (TM=Ti, Zr, Hf, V, Nb, and Ta) ternary systems. The motivation for data collection is based on the research paper entitled "Novel hard magnetic phase with Zr$_{11.5}$Fe$_{53}$Si$_{35.5}$ composition". The datasets are composed of scanning electron microscope images, X-ray diffraction (XRD) patterns, and magnetization data for TM$_{7}$Fe$_{52}$Si$_{41}$ annealed at 1050 $^{\circ}$C. The chemical compositions of constituent phases were determined by an energy dispersive X-ray spectrometer (EDS). The phase analysis was performed using XRD and EDS results. The Curie temperature of each sample was obtained using magnetization data, and the coercive field was determined for hard ferromagnet samples Zr$_{7}$Fe$_{52}$Si$_{41}$ and Hf$_{7}$Fe$_{52}$Si$_{41}$. The datasets would be useful for developing an Fe-based rare-earth-free permanent magnet, which is one of the central issues of materials science.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic properties, electrical resistivity, and hardness of high-entropy alloys FeCoNiPd and FeCoNiPt

We report the magnetic properties, electrical resistivity, and Vickers microhardness of as-cast and annealed high-entropy alloys (HEAs) FeCoNiPd and FeCoNiPt with the face-centered cubic structure. The heat treatment at 800 $^{\circ}$C does not largely affect the physical properties in each HEA. The values of the Curie temperature and the saturation moment at 50 K are 955 K and 1.458 $μ_\mathrm{B}$/f.u. for the annealed FeCoNiPd, and 851 K and 1.456 $μ_\mathrm{B}$/f.u. for the annealed FeCoNiPt, respectively. Each HEA is a soft ferromagnet and shows metallic resistivity. The electronic structure calculations of both HEAs support the ferromagnetic ground states. The comparisons between experimental and theoretical values are made for the Curie temperature, the saturation moment, and the residual resistivity. The Vickers microhardness of annealed FeCoNiPd and FeCoNiPt are both 188 HV. The hardness vs. valence electron count (VEC) per atom plot of these HEAs does not largely deviate from an expected universal relation forming a broad peak at VEC$\sim$6.8. This study would give some hints on designing a soft ferromagnetic HEA with high hardness.

cond-mat.mtrl-sci↗

arXiv2022

Superconductivity and hardness of the equiatomic high-entropy alloy HfMoNbTiZr

We have found that body-centered cubic (bcc) HfMoNbTiZr is a type-II BCS high-entropy alloy (HEA) superconductor with a superconducting critical temperature of $T_\mathrm{c}$=4.1 K. By employing a Debye temperature $θ_\mathrm{D}$ of 263 K and $T_\mathrm{c}$, the electron-phonon coupling constant $λ_\mathrm{e-p}$ is calculated to be 0.63. The electronic structure calculation revealed band broadening with energy uncertainties due to atomic disorders. The superconducting properties are compared among equiatomic quinary bcc HEA superconductors. $T_\mathrm{c}$ decreases with decreasing $λ_\mathrm{e-p}$, which is negatively correlated with $θ_{D}$. The negative correlation between $λ_\mathrm{e-p}$ and $θ_{D}$ would be caused by a shorter phonon lifetime at a higher $θ_{D}$, which is based on phonon broadening due to atomic disorder and the uncertainty principle. The Vickers microhardness was measured for several bcc HEA superconductors. $T_\mathrm{c}$ is exceptionally low in a superconductor with relatively high hardness, because the hardness generally increases with increasing $θ_\mathrm{D}$, and $θ_\mathrm{D}$ is negatively correlated with $T_\mathrm{c}$ in the high-entropy state.

cond-mat.supr-con↗

arXiv2021

Superconductivity in Al-Nb-Ti-V-Zr multicomponent alloy

The superconducting high-entropy alloys (HEAs) recently attract considerable attention due to their exciting properties, such as the robustness of superconductivity against atomic disorder and extremely high-pressure. The well-studied crystal structure of superconducting HEAs is body-centered-cubic (bcc) containing Nb, Ti, and Zr atoms. The same elements are contained in Al5Nb24Ti40V5Zr26, which is a recently discovered bcc HEA and shows a gum-metal-like behavior after cold rolling. The gum metal is also an interesting system, exhibiting superelasticity and low Young's modulus. If gum metals show superconductivity and can be used as a superconducting wire, the gum-metal HEA superconductors might be the next-generation superconducting wire materials. Aiming at a fundamental assessment of as-cast Al-Nb-Ti-V-Zr multicomponent alloys including Al5Nb24Ti40V5Zr26, we have investigated the structural and superconducting properties of the alloys. All alloys investigated show the superconductivity, and the valence electron concentration dependence of the superconducting critical temperature is very close to those of typical superconducting bcc HEAs.

cond-mat.supr-con↗

arXiv2020

Cutting edge of high-entropy alloy superconductors from the perspective of materials research

High-entropy alloys (HEAs) are a new class of materials which are being energetically studied around the world. HEAs are characterized by a multi-component alloy in which five or more elements randomly occupy a crystallographic site. The conventional HEA concept has developed into simple crystal structures such as face-centered-cubic (fcc), body-centered-cubic (bcc) and hexagonal-closed packing (hcp) structures. The highly atomic-disordered state produces many superior mechanical or thermal properties. Superconductivity has been one of the topics of focus in the field of HEAs since the discovery of the bcc HEA superconductor in 2014. A characteristic of superconductivity is robustness against atomic disorder or extremely high pressure. The materials research on HEA superconductors has just begun, and there are open possibilities for unexpectedly finding new phenomena. The present review updates the research status of HEA superconductors. We survey bcc and hcp HEA superconductors and discuss the simple material design. The concept of HEA is extended to materials possessing multiple crystallographic sites; thus, we also introduce multi-site HEA superconductors with the CsCl-type, α-Mn-type, A15, NaCl-type, σ-phase and layered structures and discuss the materials research on multi-site HEA superconductors. Finally, we present the new perspectives of eutectic HEA superconductors and gum metal HEA superconductors.

cond-mat.supr-con↗

arXiv2020

Trial of a search for a face-centered-cubic high-entropy alloy superconductor

With the aim of the discovery of face-centered-cubic (fcc) high-entropy alloy (HEA) superconductor, we have carried out materials research on Nb or Pb-containing multi-component alloys. Although the X-ray diffraction (XRD) patterns of some Nb-containing samples exhibited the dominant fcc phases, no superconducting signals were observed down to 3 K. Examination with an energy dispersive X-ray spectrometer revealed that all samples were multi-phase, but the existence of several new Nb-containing HEA phases was found in them. It was confirmed that the synthesis of Pb-containing an HEA or quaternary alloy would be difficult, probably due to the large differences in the crystal structure and atomic radius among constituent elements, low reaction temperature and the lack of a rapid cooling process in the synthesis. Despite the negative results in this research, some hints for an improved strategy for the search for an fcc HEA superconductor are provided. Moreover, our results are useful as fundamental data for future HEA predictions or for studies of phase relations in Nb or Pb-containing multi-component alloys based on the CALPHAD (calculation of phase diagram) method.

cond-mat.mtrl-sci↗

arXiv2020

Competition between ferromagnetic and antiferromagnetic states in Al8.5-xFe23Ge12.5+x (0<=x<=3)

Polycrystalline Al7+xFe23Ge14-x-type orthorhombic Al8.5-xFe23Ge12.5+x (0<=x<=3) compounds were studied by X-ray diffraction measurement, metallographic examination, and magnetization and electrical resistivity measurements. The lattice parameters show anisotropic x dependences, reflecting a lower symmetry of rather complex crystal structure. In the x=0 sample, a ferromagnetic (FM) transition occurs at the Curie temperature TC of 255 K, which is systematically reduced with increasing x and disappears at x=2.5 where an antiferromagnetic (AFM) ground state is realized. The Neel temperature TN seems to appear at x>=1, and grows to 143 K at x=3 with the increment of x. In the intermediate range of x (1<=x<=2), a FM to AFM phase transition occurs below TC. In the AFM phase, the electrical resistivity rho shows a Fermi surface instability characterized by the upturn of rho below approximately TN. The competition between the FM and AFM states would be ascribed to the anisotropic x dependences of Fe-Fe bond lengths.

cond-mat.mtrl-sci↗

arXiv2019

Superconductivity in Nb5Ir3-xPtxO

We have investigated the superconducting critical temperature Tc of solid solution Nb5Ir3-xPtxO with the Ti5Ga4-type structure. The both end-members of Nb5Ir3O and Nb5Pt3O are reported to be superconductors with Tc of 10.5 K and 3.8 K, respectively. Particularly Nb5Ir3O is considered as a two-gap superconductor. The entire series of alloy hold the Ti5Ga4-type structure and show the linear x dependence of lattice parameters. On the other hand, a nonlinear Tc vs x plot was obtained, suggesting that Tc is not determined by only lattice parameters. The experimental result is discussed based on the Matthias rule and compared with those of the other two-gap superconductors such as MgB2, Mo8Ga41 and Nb3Sn.

cond-mat.supr-con↗

arXiv2019

New high-entropy alloy superconductor Hf$_{21}$Nb$_{25}$Ti$_{15}$V$_{15}$Zr$_{24}$

High-entropy alloys are a new class of alloys, and attract much attention due to their unique properties. Since the discovery of a superconducting high-entropy alloy (HEA) in 2014, the materials research on HEA superconductor is a hot topic. We have found that Hf$_{21}$Nb$_{25}$Ti$_{15}$V$_{15}$Zr$_{24}$ body-centered-cubic (BCC) HEA is a new superconductor with the superconducting critical temperature Tc of 5.3 K. We briefly discussed the comparison of cocktail effect of Tc among BCC HEA superconductors.

cond-mat.supr-con↗