Showing 1–50 of 84 results for author: Miyake, T

Crystal-structure design by agentic AI in a language of motifs

Authors: Dinh-Khiet Le, Minh-Quyet Ha, Hong-Phuc Vu-Dinh, Takashi Miyake, Hiori Kino, Hieu-Chi Dam

Abstract: Data-driven materials discovery interpolates more reliably than it extrapolates and seldom reaches new structure types. We present MatEvolve, an agentic-AI framework designing crystals, proposing each candidate with a stated rationale and testing it. The agent reasons in an interpretable \emph{language of motifs}, writing each crystal as a \emph{motif profile} that describes the recurring geometri… ▽ More Data-driven materials discovery interpolates more reliably than it extrapolates and seldom reaches new structure types. We present MatEvolve, an agentic-AI framework designing crystals, proposing each candidate with a stated rationale and testing it. The agent reasons in an interpretable \emph{language of motifs}, writing each crystal as a \emph{motif profile} that describes the recurring geometric patterns---the \emph{motifs}---composing it. The motif profile serves not merely as a description of a material but as the medium for material design: the agent edits the profile and constructs a crystal from the modified one, and the most promising candidates are validated by first-principles calculation. Applied to the design of rare-earth-lean permanent magnets, MatEvolve---built on the state-of-the-art language model Claude Fable~5 without fine-tuning---reaches new structural prototypes more than three times as often as generative models under an equal validation budget, at a comparable on-target-magnet rate. Beyond design, analysing the discovered crystals' human-readable profiles reveals structure--property relationships. △ Less

Submitted 16 August, 2026; originally announced August 2026.

Impact of Lattice Distortions on Magnetocrystalline Anisotropy and Magnetization in (Nd$_{1-x}$Pr$_x$)$_2$Fe$_{14}$B Alloys

Authors: Haruki Okumura, Takashi Miyake, Taro Fukazawa, Noritsugu Sakuma, Yuta Suzuki, Tetsuya Shoji, Hisazumi Akai, Masako Ogura, Tetsuya Fukushima

Abstract: Nd$_{2}$Fe$_{14}$B -- a widely used permanent magnet -- has magnetocrystalline anisotropy constants that differ between the bulk and interface regions. This study explores the effects of lattice distortion on the magnetocrystalline anisotropy ($K_{\rm u}$) and magnetization of (Nd$_{1-x}$Pr$_x$)$_2$Fe$_{14}$B. Nd$_2$Fe$_{14}$B alloys were fabricated; scanning transmission electron microscopy revea… ▽ More Nd$_{2}$Fe$_{14}$B -- a widely used permanent magnet -- has magnetocrystalline anisotropy constants that differ between the bulk and interface regions. This study explores the effects of lattice distortion on the magnetocrystalline anisotropy ($K_{\rm u}$) and magnetization of (Nd$_{1-x}$Pr$_x$)$_2$Fe$_{14}$B. Nd$_2$Fe$_{14}$B alloys were fabricated; scanning transmission electron microscopy revealed a compressive strain of up to 25% near grain boundaries. Using the full-potential Korringa--Kohn--Rostoker method, we calculated the strain dependence of $K_{\rm u}$, showing that although $K_{\rm u}$ is 4.2 MJ/m$^3$ under strain-free conditions at 0 K, it becomes negative in regions with 25% compressive strain. Additionally, Pr$_{2}$Fe$_{14}$B exhibits a larger $K_{\rm u}$ than Pr$_{2}$Fe$_{14}$B under undistorted conditions, whereas Pr-rich alloys exhibit a more pronounced reduction in $K_{\rm u}$ under strain. These findings highlight the critical influence of lattice distortions on magnetic properties. The calculated strain-dependent magnetic anisotropy parameters provide valuable inputs for future micromagnetic simulations, aiding the design of advanced magnetic materials. △ Less

Submitted 8 December, 2025; originally announced December 2025.

Comments: 11 pages, 8 figures, includes experimental and first-principles calculations

Efficient method for magnetic structure exploration based on first-principles calculations: application to MnO and hexagonal ferrites SrFe$_{12}$O$_{19}$

Authors: Taro Fukazawa, Haruki Okumura, Tetsuya Fukushima, Hisazumi Akai, Takashi Miyake

Abstract: We propose an approach for exploring magnetic structures by using Liechtenstein's method for exchange couplings from the results of first-principles calculations. Our method enables efficient and accurate exploration of stable magnetic structures by greatly reducing the number of firstprinciples calculations required. We apply our method to the magnetic structures of MnO and hexagonal ferrite SrFe… ▽ More We propose an approach for exploring magnetic structures by using Liechtenstein's method for exchange couplings from the results of first-principles calculations. Our method enables efficient and accurate exploration of stable magnetic structures by greatly reducing the number of firstprinciples calculations required. We apply our method to the magnetic structures of MnO and hexagonal ferrite SrFe12O19. Our method correctly identifies the ground-state magnetic structure with a small number of first-principles calculations in these systems. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Comments: 13 pages, 7 figures

Covariance Linkage Assimilation method for Unobserved Data Exploration

Authors: Yosuke Harashima, Takashi Miyake, Ryuto Baba, Tomoaki Takayama, Shogo Takasuka, Yasuteru Shigeta, Yuichi Yamaguchi, Akihiko Kudo, Mikiya Fujii

Abstract: This study proposes a materials search method combining a data assimilation technique based on a multivariate Gaussian distribution with Bayesian optimization. The efficiency of the search using this method was demonstrated using a pair of example functions. By combining Bayesian optimization with the data assimilation technique, the maximum value of the example function was found more efficiently… ▽ More This study proposes a materials search method combining a data assimilation technique based on a multivariate Gaussian distribution with Bayesian optimization. The efficiency of the search using this method was demonstrated using a pair of example functions. By combining Bayesian optimization with the data assimilation technique, the maximum value of the example function was found more efficiently compared to ordinary Bayesian optimization without the data assimilation. A practical demonstration was also conducted by constructing a data assimilation model for the bandgap of (Sr$_{1-x_{1}-x_{2}}$La$_{x_{1}}$Na$_{x_{2}}$)(Ti$_{1-x_{1}-x_{2}}$Ga$_{x_{1}}$Ta$_{x_{2}}$)O$_{3}$. The concentration dependence of the bandgap was analyzed, and synthesis was performed with chemical compositions in the sparse region of the training data points to validate the predictions. △ Less

Submitted 15 July, 2025; v1 submitted 16 August, 2024; originally announced August 2024.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. Materials 9, 093803 (2025)

Designing single and degenerate flat bands in the kagome lattice with long-range hopping

Authors: Yuta Taguchi, Motoaki Hirayama, Takashi Miyake

Abstract: We investigate the electronic structure of the kagome lattice model with first, second, and two kinds of third nearest-neighbor hoppings. We reveal that by tuning the third nearest-neighbor hoppings, not only single flat band but also degenerate flat band can be created on the Γ- M line. We provide the detailed conditions to realize them. The coexistence of these bands can be almost realized near… ▽ More We investigate the electronic structure of the kagome lattice model with first, second, and two kinds of third nearest-neighbor hoppings. We reveal that by tuning the third nearest-neighbor hoppings, not only single flat band but also degenerate flat band can be created on the Γ- M line. We provide the detailed conditions to realize them. The coexistence of these bands can be almost realized near the fundamental band gap in graphene with triangular defects in a superhoneycomb arrangement. Furthermore, due to these flat bands, several sharp peaks appear in the optical conductivity. Our results strongly indicate that long-range electron hopping has a new possibility for designing electronic structures. △ Less

Submitted 4 March, 2025; v1 submitted 23 July, 2024; originally announced July 2024.

Comments: 19 pages, 11 figures: Published Version

Journal ref: Physical Review B 111, 115105 (2025)

Pareto front analysis and multi-objective Bayesian optimization for (R, Z)(Fe,Co,Ti)12 (R = Y, Nd, Sm; Z = Zr, Dy)

Authors: Taro Fukazawa, Takashi Miyake

Abstract: We propose a scheme for investigating the correlation and trade-off among target variables using a multi-objective Bayesian optimization (MBO). We discuss the features of the Pareto front (PF) of ThMn12-type compounds, (R, Z)(Fe,Co,Ti)12 (R = Y, Nd, Sm; Z = Zr, Dy) in terms of magne- tization, Curie temperature, and a price index by using data from first-principles calculations, and we extract the… ▽ More We propose a scheme for investigating the correlation and trade-off among target variables using a multi-objective Bayesian optimization (MBO). We discuss the features of the Pareto front (PF) of ThMn12-type compounds, (R, Z)(Fe,Co,Ti)12 (R = Y, Nd, Sm; Z = Zr, Dy) in terms of magne- tization, Curie temperature, and a price index by using data from first-principles calculations, and we extract the trade-off relations from the analysis. We show that the trade-off relationships can be used to determine changes in the controllable variables by using partial least squares regression. For example, the tendency toward low cost and high Curie temperature is related to the reduction in Dy and increase in Co. We also discuss the efficiency of MBO as a practical scheme to obtain the features of the PF. We show that MBO can offer an approximated set for the PF even when obtaining the true PF is difficult. △ Less

Submitted 25 August, 2022; originally announced August 2022.

Comments: 6 pages, 8 figures

Function Decomposition Tree with Causality-First Perspective and Systematic Description of Problems in Materials Informatics

Authors: Hiori Kino, Hieu-Chi Dam, Takashi Miyake, Riichiro Mizoguchi

Abstract: As interdisciplinary science is flourishing because of materials informatics and additional factors; a systematic way is required for expressing knowledge and facilitating communication between scientists in various fields. A function decomposition tree is such a representation, but domain scientists face difficulty in constructing it. Thus, this study cites the general problems encountered by beg… ▽ More As interdisciplinary science is flourishing because of materials informatics and additional factors; a systematic way is required for expressing knowledge and facilitating communication between scientists in various fields. A function decomposition tree is such a representation, but domain scientists face difficulty in constructing it. Thus, this study cites the general problems encountered by beginners in generating function decomposition trees and proposes a new function decomposition representation method based on a causality-first perspective for resolution of these problems. The causality-first decomposition tree was obtained from a workflow expressed according to the processing sequence. Moreover, we developed a program that performed automatic conversion using the features of the causality-first decomposition trees. The proposed method was applied to materials informatics to demonstrate the systematic representation of expert knowledge and its usefullness. △ Less

Submitted 26 April, 2022; originally announced May 2022.

Comments: 41 page, 13 figures

First-principles study on the stability of ($R$, Zr)(Fe, Co, Ti)$_{12}$ against 2-17 and unary phases ($R$ = Y, Nd, Sm)

Authors: Taro Fukazawa, Yosuke Harashima, Takashi Miyake

Abstract: The stability of ($R$, Zr)(Fe, Co, Ti)$_{12}$ with a ThMn$_{12}$ structure is investigated using first-principles calculations. We consider energetic competition with multiple phases that have the Th$_2$Zn$_{17}$ structure and the unary phases of $R$, Zr, Fe, Co, and Ti simultaneously by constructing a quinary energy convex hull. From the analysis, we list the stable phases at zero temperature, an… ▽ More The stability of ($R$, Zr)(Fe, Co, Ti)$_{12}$ with a ThMn$_{12}$ structure is investigated using first-principles calculations. We consider energetic competition with multiple phases that have the Th$_2$Zn$_{17}$ structure and the unary phases of $R$, Zr, Fe, Co, and Ti simultaneously by constructing a quinary energy convex hull. From the analysis, we list the stable phases at zero temperature, and show possible stable and metastable ThMn$_{12}$ phases. △ Less

Submitted 28 March, 2022; v1 submitted 16 December, 2021; originally announced December 2021.

Comments: 7 pages, 8 figures

First-principles investigation of Nd(Fe,M)12 (M = K--Br) and Nd(Fe,Cr,Co,Ni,Ge,As)12: Possible enhancers of Curie temperature for NdFe12 magnetic compounds

Authors: Taro Fukazawa, Hisazumi Akai, Yosuke Harashima, Takashi Miyake

Abstract: We investigate the effects of various dopants (M = K--Br) on the Curie temperature of the magnetic compound NdFe12 through first-principles calculations. Analysis by the Korringa--Kohn--Rostoker method with the coherent potential approximation reveals that doping the Fe sites with optimal concentrations of Ge and As is a promising strategy for increasing the Curie temperature. To search over a wid… ▽ More We investigate the effects of various dopants (M = K--Br) on the Curie temperature of the magnetic compound NdFe12 through first-principles calculations. Analysis by the Korringa--Kohn--Rostoker method with the coherent potential approximation reveals that doping the Fe sites with optimal concentrations of Ge and As is a promising strategy for increasing the Curie temperature. To search over a wider space, we also perform Bayesian optimization. Out of over 180,000 candidate compositions, co-doped systems with Co, Ge, and As are found to have the highest Curie temperatures. △ Less

Submitted 17 August, 2021; originally announced August 2021.

Comments: 9 pages, 13 figures

Lattice dynamics effects on finite-temperature stability of $R_{1-x}$Fe$_{x}$ ($R$ = Y, Ce, Nd, Sm, and Dy) alloys from first principles

Authors: Guangzong Xing, Takahiro Ishikawa, Yoshio Miura, Takashi Miyake, Terumasa Tadano

Abstract: We report the effects of lattice dynamics on thermodynamic stability of binary $R_{1-x}$Fe$_x$ $(0<x<1)$ compounds ($R$: rare-earth elements, Y, Ce, Nd, Sm, and Dy) at finite temperature predicted by first-principles calculation based on density functional theory (DFT). We first demonstrate that the thermodynamic stability of $R_{1-x}$Fe$_x$ $(0<x<1)$ alloys cannot be predicted accurately by the c… ▽ More We report the effects of lattice dynamics on thermodynamic stability of binary $R_{1-x}$Fe$_x$ $(0<x<1)$ compounds ($R$: rare-earth elements, Y, Ce, Nd, Sm, and Dy) at finite temperature predicted by first-principles calculation based on density functional theory (DFT). We first demonstrate that the thermodynamic stability of $R_{1-x}$Fe$_x$ $(0<x<1)$ alloys cannot be predicted accurately by the conventional approach, where only the static DFT energy at $T = 0$ K is used. This issue can be overcome by considering the entropy contribution, including electronic and vibrational free energies, and we obtained convex hull plots at finite temperatures that successfully explain the thermodynamic stability of various known compounds. Our systematic calculation indicates that vibrational entropy helps stabilize various $R_{1-x}$Fe$_x$ compounds with increasing temperature. In particular, experimentally reported $R_2$Fe$_{17}$ compounds are predicted to become thermodynamically stable above $\sim$800 K. We also show that thermodynamic stability is rare-earth dependent and discuss its origin. Besides the experimentally reported structures, the stability of two new monoclinic $R$Fe$_{12}$ structures found by Ishikawa \textit{et al.} [Phys. Rev. Mater.~\textbf{4}, 104408 (2020)] based on a genetic algorithm are investigated. These monoclinic phases are found to be dynamically stable and have larger magnetization than the ThMn$_{12}$-type $R$Fe$_{12}$. Although they are thermodynamically unstable, the formation energies decrease significantly with increasing temperature, indicating the possibility of synthesizing these compounds at high temperatures. △ Less

Submitted 4 February, 2021; originally announced February 2021.

Comments: 12 pages, 6 figures

Evolutionary search for cobalt-rich compounds in the yttrium-cobalt-boron system

Authors: Takahiro Ishikawa, Taro Fukazawa, Guangzong Xing, Terumasa Tadano, Takashi Miyake

Abstract: Modern high-performance permanent magnets are made from alloys of rare earth and transition metal elements, and large magnetization is achieved in the alloys with high concentration of transition metals. We applied evolutionary search scheme based on first-principles calculations to the Y-Co-B system and predicted 37 cobalt-rich compounds with high probability of being stable. Focusing on remarkab… ▽ More Modern high-performance permanent magnets are made from alloys of rare earth and transition metal elements, and large magnetization is achieved in the alloys with high concentration of transition metals. We applied evolutionary search scheme based on first-principles calculations to the Y-Co-B system and predicted 37 cobalt-rich compounds with high probability of being stable. Focusing on remarkably cobalt-rich compounds, YCo$_{16}$ and YCo$_{20}$, we found that, although they are metastable phases, the phase stability is increased with increase of temperature due to the contribution of vibrational entropy. The magnetization and Curie temperature are higher by 0.22 T and 204 K in YCo$_{16}$ and by 0.29 T and 204 K in YCo$_{20}$ than those of Y$_{2}$Co$_{17}$ which has been well studied as strong magnetic compounds. △ Less

Submitted 3 February, 2021; originally announced February 2021.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Materials 5, 054408 (2021)

Spin-wave dispersion and exchange stiffness in Nd$_2$Fe$_{14}$B and $R$Fe$_{11}$Ti ($R$=Y, Nd, Sm) from first-principles calculations

Authors: Taro Fukazawa, Hisazumi Akai, Yosuke Harashima, Takashi Miyake

Abstract: We theoretically investigate spin-wave dispersion in rare-earth magnet compounds by using first-principles calculations and a method we call the reciprocal-space algorithm (RSA). The value of the calculated exchange stiffness for Nd$_2$Fe$_{14}$B is within the range of reported experimental values. We find that the exchange stiffness is considerably anisotropic when only short-range exchange coupl… ▽ More We theoretically investigate spin-wave dispersion in rare-earth magnet compounds by using first-principles calculations and a method we call the reciprocal-space algorithm (RSA). The value of the calculated exchange stiffness for Nd$_2$Fe$_{14}$B is within the range of reported experimental values. We find that the exchange stiffness is considerably anisotropic when only short-range exchange couplings are considered, whereas inclusion of long-range couplings weakens the anisotropy. In contrast, $R$Fe$_{11}$Ti ($R$=Y, Nd, Sm) shows large anisotropy in the exchange stiffness. △ Less

Submitted 20 December, 2020; v1 submitted 29 October, 2020; originally announced October 2020.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 103, 024418 (2021)

Ensemble learning reveals dissimilarity between rare-earth transition metal binary alloys with respect to the Curie temperature

Authors: Duong-Nguyen Nguyen, Tien-Lam Pham, Viet-Cuong Nguyen, Hiori Kino, Takashi Miyake, Hieu-Chi Dam

Abstract: We propose a data-driven method to extract dissimilarity between materials, with respect to a given target physical property. The technique is based on an ensemble method with Kernel ridge regression as the predicting model; multiple random subset sampling of the materials is done to generate prediction models and the corresponding contributions of the reference training materials in detail. The d… ▽ More We propose a data-driven method to extract dissimilarity between materials, with respect to a given target physical property. The technique is based on an ensemble method with Kernel ridge regression as the predicting model; multiple random subset sampling of the materials is done to generate prediction models and the corresponding contributions of the reference training materials in detail. The distribution of the predicted values for each material can be approximated by a Gaussian mixture model. The reference training materials contributed to the prediction model that accurately predicts the physical property value of a specific material, are considered to be similar to that material, or vice versa. Evaluations using synthesized data demonstrate that the proposed method can effectively measure the dissimilarity between data instances. An application of the analysis method on the data of Curie temperature (TC) of binary 3d transition metal 4f rare earth binary alloys also reveals meaningful results on the relations between the materials. The proposed method can be considered as a potential tool for obtaining a deeper understanding of the structure of data, with respect to a target property, in particular. △ Less

Submitted 20 August, 2020; originally announced August 2020.

Comments: 10 pages, 3 figures

Explainable Machine Learning for Materials Discovery: Predicting the Potentially Formable Nd-Fe-B Crystal Structures and Extracting Structure-Stability Relationship

Authors: Tien-Lam Pham, Duong-Nguyen Nguyen, Minh-Quyet Ha, Hiori Kino, Takashi Miyake, Hieu-Chi Dam

Abstract: New Nd-Fe-B crystal structures can be formed via the elemental substitution of LATX host structures, including lanthanides LA, transition metals T, and light elements X as B, C, N, and O. The 5967 samples of ternary LATX materials that are collected are then used as the host structures. For each host crystal structure, a substituted crystal structure is created by substituting all lanthanide sites… ▽ More New Nd-Fe-B crystal structures can be formed via the elemental substitution of LATX host structures, including lanthanides LA, transition metals T, and light elements X as B, C, N, and O. The 5967 samples of ternary LATX materials that are collected are then used as the host structures. For each host crystal structure, a substituted crystal structure is created by substituting all lanthanide sites with Nd, all transition metal sites with Fe, and all light element sites with B. High throughput first-principles calculations are applied to evaluate the phase stability of the newly created crystal structures, and 20 of them are found to be potentially formable. A data driven approach based on supervised and unsupervised learning techniques is applied to estimate the stability and analyze the structure stability relationship of the newly created NdFeB crystal structures. For predicting the stability for the newly created NdFeB structures, three supervised learning models, kernel ridge regression, logistic classification, and decision tree model, are learned from the LATX host crystal structures; the models achieve the maximum accuracy and recall scores of 70.4 and 68.7 percent, respectively. On the other hand, our proposed unsupervised learning model based on the integration of descriptor-relevance analysis and a Gaussian mixture model achieves accuracy and recall score of 72.9 and 82.1 percent, respectively, which are significantly better than those of the supervised models. While capturing and interpreting the structure stability relationship of the NdFeB crystal structures, the unsupervised learning model indicates that the average atomic coordination number and coordination number of the Fe sites are the most important factors in determining the phase stability of the new substituted NdFeB crystal structures. △ Less

Submitted 20 August, 2020; originally announced August 2020.

Comments: 14 pages, 7 figures

Boron cage effects on Nd-Fe-B crystal structure's stability

Authors: Duong-Nguyen Nguyen, Duc-Anh Dao, Takashi Miyake, Hieu-Chi Dam

Abstract: In this study, we investigate the structure-stability relationship of hypothetical Nd-Fe-B crystal structures using descriptor-relevance analysis and the t-SNE dimensionality reduction method. 149 hypothetical Nd-Fe-B crystal structures are generated from 5967 LA-T-X host structures in Open Quantum Materials Database by using the elemental substitution method, with LA denoting lanthanides, T denot… ▽ More In this study, we investigate the structure-stability relationship of hypothetical Nd-Fe-B crystal structures using descriptor-relevance analysis and the t-SNE dimensionality reduction method. 149 hypothetical Nd-Fe-B crystal structures are generated from 5967 LA-T-X host structures in Open Quantum Materials Database by using the elemental substitution method, with LA denoting lanthanides, T denoting transition metals, and X denoting light elements such as B, C, N and O. A hypothetical crystal structure is created by substituting all lanthanide sites with Nd, all transition metal sites with Fe, and all light element sites with B. High-throughput first-principle calculations are applied to evaluate the phase stability of these structures. Twenty of them are found to be potentially formable. The descriptor-relevance analysis on the orbital field matrix (OFM) materials' descriptor reveals the average atomic coordination number as the essential factor in determining the structure stability of these substituted Nd-Fe-B crystal structures. 19 among 20 hypothetical structures that are found potentially formable have an average coordination number larger than 6.5. In addition, all the local structures represented by the OFM descriptors are integrated into a visible space to study the detailed correlation between their characteristics and the stability of the crystal structure to which they belong. We discover that unstable substituted structures frequently carry Nd and Fe local structures with two prominent points: low average coordination numbers and fully occupied B neighboring atoms. Moreover, there are only three popular forms of B local structures appearing on all potentially formable substituted structures: cage networks, planar networks, and interstitial sites. The discovered relationships are promising to speed up the screening process for the new formable crystal structures. △ Less

Submitted 20 August, 2020; originally announced August 2020.

Comments: 12 pages, 8 figures

Monoclinic YFe$_{12}$ phases predicted from first principles

Authors: Takahiro Ishikawa, Taro Fukazawa, Takashi Miyake

Abstract: We searched for stable crystal structures of YFe$_{12}$ using a crystal structure prediction technique based on a genetic algorithm and first-principles calculations. We obtained two monoclinic $C2/m$ structures as metastable phases that are different from the well-known ThMn$_{12}$ structure. These two phases have advantages in their magnetism over the ThMn$_{12}$ structure: The total magnetizati… ▽ More We searched for stable crystal structures of YFe$_{12}$ using a crystal structure prediction technique based on a genetic algorithm and first-principles calculations. We obtained two monoclinic $C2/m$ structures as metastable phases that are different from the well-known ThMn$_{12}$ structure. These two phases have advantages in their magnetism over the ThMn$_{12}$ structure: The total magnetization $M$ is increased from 25.6 $μ_{\text{B}}$/f.u. up to 26.8 $μ_{\text{B}}$/f.u. by the transformations. We also calculated Curie temperature $T_{\text{C}}$ for these structures within the mean-field approximation and predicted the increase of $T_{\text{C}}$ from 792 K up to 940 K, which is mainly caused by the increase of intersite magnetic couplings within the distance of 2.3--3.1Å. The similar enhancements of $M$ and $T_{\text{C}}$ are also obtained in the pseudo-binary system Y(Fe$_{1-x}$Co$_{x}$)$_{12}$ with $x$ of 0--0.7. △ Less

Submitted 19 August, 2020; originally announced August 2020.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. Materials 4, 104408 (2020)

Data Assimilation Method for Experimental and First-Principles Data: Finite-Temperature Magnetization of (Nd,Pr,La,Ce)$_{2}$(Fe,Co,Ni)$_{14}$B

Authors: Yosuke Harashima, Keiichi Tamai, Shotaro Doi, Munehisa Matsumoto, Hisazumi Akai, Naoki Kawashima, Masaaki Ito, Noritsugu Sakuma, Akira Kato, Tetsuya Shoji, Takashi Miyake

Abstract: We propose a data-assimilation method for evaluating the finite-temperature magnetization of a permanent magnet over a high-dimensional composition space. Based on a general framework for constructing a predictor from two data sets including missing values, a practical scheme for magnetic materials is formulated in which a small number of experimental data in limited composition space are integrat… ▽ More We propose a data-assimilation method for evaluating the finite-temperature magnetization of a permanent magnet over a high-dimensional composition space. Based on a general framework for constructing a predictor from two data sets including missing values, a practical scheme for magnetic materials is formulated in which a small number of experimental data in limited composition space are integrated with a larger number of first-principles calculation data. We apply the scheme to (Nd$_{1-α-β-γ}$Pr$_α$La$_β$Ce$_γ$)$_{2}$(Fe$_{1-δ-ζ}$Co$_δ$Ni$_ζ$)$_{14}$B. The magnetization in the whole $(α, β, γ, δ, ζ)$ space at arbitrary temperature is obtained. It is shown that the Co doping does not enhance the magnetization at low temperatures, whereas the magnetization increases with increasing $δ$ above 320 K. △ Less

Submitted 28 July, 2020; originally announced July 2020.

Comments: 11 pages, 7 figures

Journal ref: Phys. Rev. Materials 5, 013806 (2021)

Evolutionary construction of formation energy convex hull: Practical scheme and application to carbon-hydrogen binary system

Authors: Takahiro Ishikawa, Takashi Miyake

Abstract: We present an evolutionary construction technique of formation energy convex hull to search for thermodynamically stable compounds. In this technique, candidates with a wide variety of chemical compositions and crystal structures are created by systematically applying evolutionary operators, "mating", "mutation", and "adaptive mutation", to two target compounds, and the convex hull is directly upd… ▽ More We present an evolutionary construction technique of formation energy convex hull to search for thermodynamically stable compounds. In this technique, candidates with a wide variety of chemical compositions and crystal structures are created by systematically applying evolutionary operators, "mating", "mutation", and "adaptive mutation", to two target compounds, and the convex hull is directly updated through the evolution. We applied the technique to carbon-hydrogen binary system at 10 GPa and obtained 15 hydrocarbons within the convex hull distance less than 0.5 mRy/atom: graphane, polybutadiene, polyethylene, butane, ethane, methane, three molecular compounds of ethane and methane, and six molecular compounds of methane and hydrogen. These results suggest that our evolutionary construction technique is useful for the exploration of stable phases under extreme conditions and the synthesis of new compounds. △ Less

Submitted 13 March, 2020; originally announced March 2020.

Journal ref: Phys. Rev. B 101, 214106 (2020)

Pressure-induced topological phase transition in noncentrosymmetric elemental Tellurium

Authors: Toshiya Ideue, Motoaki Hirayama, Hiroaki Taiko, Takanari Takahashi, Masayuki Murase, Takashi Miyake, Shuichi Murakami, Takao Sasagawa, Yoshihiro Iwasa

Abstract: Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological view point. Here we report the pressure-induced topological phase transition from a… ▽ More Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological view point. Here we report the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov-de Haas oscillations, as well as oscillations phases, shows an anomaly around the pressure theoretically predicted for topological phase transition. This behavior can be well understood by the pressure-induced band deformation and resultant band crossing effect. Moreover, effective cyclotron mass is reduced toward the critical pressure, potentially reflecting the emergence of massless linear dispersion. The present result paves the way for studying the electronic band topology in well-known compounds and topological phase transition by the external field. △ Less

Submitted 5 September, 2019; originally announced September 2019.

Radial spin texture in elemental tellurium with chiral crystal structure

Authors: M. Sakano, M. Hirayama, T. Takahashi, S. Akebi, M. Nakayama, K. Kuroda, K. Taguchi, T. Yoshikawa, K. Miyamoto, T. Okuda, K. Ono, H. Kumigashira, T. Ideue, Y. Iwasa, N. Mitsuishi, K. Ishizaka, S. Shin, T. Miyake, S. Murakami, T. Sasagawa, Takeshi Kondo

Abstract: The chiral crystal is characterized by a lack of mirror symmetry and an inversion center, resulting in the inequivalent right- and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, here we investigate the spin and elec… ▽ More The chiral crystal is characterized by a lack of mirror symmetry and an inversion center, resulting in the inequivalent right- and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, here we investigate the spin and electronic structure in p-type semiconductor elemental tellurium with a chiral crystal structure by using spin- and angle-resolved photoemission spectroscopy. Our data demonstrate that the highest valence band crossing the Fermi level has a spin component parallel to the electron momentum around the BZ corners. Significantly, we have also confirmed that the spin polarization is reversed in the crystal with the opposite chirality. The results indicate that the spin textures of the right- and left-handed chiral crystals are hedgehog-like, leading to unconventional magnetoelectric effects and nonreciprocal phenomena. △ Less

Submitted 26 August, 2019; originally announced August 2019.

Comments: 15 pages, 4 figures

Journal ref: Phys. Rev. Lett. 124, 136404 (2020)

Cerium as a possible stabilizer of ThMn$_{12}$-type iron-based compounds: A first-principles study

Authors: Yosuke Harashima, Taro Fukazawa, Takashi Miyake

Abstract: The structural stability of CeFe$_{12}$ is investigated by using first-principles calculation. The formation energies of CeFe$_{12}$ relative to the Ce$_{2}$Fe$_{17}$ + bcc-Fe phase and to the CeFe$_{2}$ + bcc-Fe phase are calculated with the assumptions of trivalency and tetravalency for Ce. Those values are compared with corresponding results in $R$Fe$_{12}$ for $R=$ Nd, Sm, and Zr. Our results… ▽ More The structural stability of CeFe$_{12}$ is investigated by using first-principles calculation. The formation energies of CeFe$_{12}$ relative to the Ce$_{2}$Fe$_{17}$ + bcc-Fe phase and to the CeFe$_{2}$ + bcc-Fe phase are calculated with the assumptions of trivalency and tetravalency for Ce. Those values are compared with corresponding results in $R$Fe$_{12}$ for $R=$ Nd, Sm, and Zr. Our results suggest that the tetravalent Ce is a promising stabilizer of the ThMn$_{12}$ structure. We also show that the stabilizing effect of an element depends as much on the valency as on the size of the $R$ element by investigating $R$Fe$_{12}$ where $R$ is assumed to have a hypothetical valency on the basis of first-principles calculation. △ Less

Submitted 8 August, 2019; originally announced August 2019.

Comments: 5pages, 4figures

Journal ref: Scripta Mater. 179, 12, 2020

Materials informatics based on evolutionary algorithms: Application to search for superconducting hydrogen compounds

Authors: Takahiro Ishikawa, Takashi Miyake, Katsuya Shimizu

Abstract: We present materials informatics approach to search for superconducting hydrogen compounds, which is based on a genetic algorithm and a genetic programming. This method consists of four stages: (i) search for stable crystal structures of materials by a genetic algorithm, (ii) collection of physical and chemical property data by first-principles calculations, (iii) development of superconductivity… ▽ More We present materials informatics approach to search for superconducting hydrogen compounds, which is based on a genetic algorithm and a genetic programming. This method consists of four stages: (i) search for stable crystal structures of materials by a genetic algorithm, (ii) collection of physical and chemical property data by first-principles calculations, (iii) development of superconductivity predictor based on the database by a genetic programming, and (iv) discovery of potential candidates by regression analysis. By repeatedly performing the process as (i) $\rightarrow$ (ii) $\rightarrow$ (iii) $\rightarrow$ (iv) $\rightarrow$ (i) $\rightarrow$ $\dots$, the superconductivity of the discovered candidates is validated by first-principles calculations, and the database and predictor are further improved, which leads to an efficient search for superconducting materials. We applied this method to hypothetical ternary hydrogen compounds and predicted KScH$_{12}$ with a modulated hydrogen cage showing the superconducting critical temperature of 122 K at 300 GPa and GaAsH$_{6}$ showing 98 K at 180 GPa. △ Less

Submitted 2 August, 2019; originally announced August 2019.

Journal ref: Phys. Rev. B 100, 174506 (2019)

Bayesian optimization of chemical composition: a comprehensive framework and its application to $R$Fe$_{12}$-type magnet compounds

Authors: Taro Fukazawa, Yosuke Harashima, Zhufeng Hou, Takashi Miyake

Abstract: We propose a framework for optimization of the chemical composition of multinary compounds with the aid of machine learning. The scheme is based on first-principles calculation using the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA). We introduce a method for integrating datasets to reduce systematic errors in a dataset, where the data are corrected using a small… ▽ More We propose a framework for optimization of the chemical composition of multinary compounds with the aid of machine learning. The scheme is based on first-principles calculation using the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA). We introduce a method for integrating datasets to reduce systematic errors in a dataset, where the data are corrected using a smaller and more accurate dataset. We apply this method to values of the formation energy calculated by KKR-CPA for nonstoichiometric systems to improve them using a small dataset for stoichiometric systems obtained by the projector-augmented-wave (PAW) method. We apply our framework to optimization of $R$Fe$_{12}$-type magnet compounds (R$_{1-α}$Z$_α$)(Fe$_{1-β}$Co$_β$)$_{12-γ}$Ti$_γ$, and benchmark the efficiency in determination of the optimal choice of elements (R and Z) and ratio ($α$, $β$ and $γ$) with respect to magnetization, Curie temperature and formation energy. We find that the optimization efficiency depends on descriptors significantly. The variable $β$, $γ$ and the number of electrons from the R and Z elements per cell are important in improving the efficiency. When the descriptor is appropriately chosen, the Bayesian optimization becomes much more efficient than random sampling. △ Less

Submitted 8 May, 2019; v1 submitted 22 March, 2019; originally announced March 2019.

Comments: 16 pages, 13 figures

Journal ref: Phys. Rev. Materials 3, 053807 (2019)

Evolution of Magnetic Double Helix and Quantum Criticality near a Dome of Superconductivity in CrAs

Authors: M. Matsuda, F. K. Lin, R. Yu, J. -G. Cheng, W. Wu, J. P. Sun, J. H. Zhang, P. J. Sun, K. Matsubayashi, T. Miyake, T. Kato, J. -Q. Yan, M. B. Stone, Qimiao Si, J. L. Luo, Y. Uwatoko

Abstract: At ambient pressure CrAs undergoes a first-order transition into a double-helical magnetic state at TN = 265 K, which is accompanied by a structural transition. The recent discovery of pressure-induced superconductivity in CrAs makes it important to clarify the nature of quantum phase transitions out of its coupled structural/helimagnetic order. Here we show, via neutron diffraction on the single-… ▽ More At ambient pressure CrAs undergoes a first-order transition into a double-helical magnetic state at TN = 265 K, which is accompanied by a structural transition. The recent discovery of pressure-induced superconductivity in CrAs makes it important to clarify the nature of quantum phase transitions out of its coupled structural/helimagnetic order. Here we show, via neutron diffraction on the single-crystal CrAs under hydrostatic pressure (P), that the combined order is suppressed at Pc ~ 10 kbar, near which bulk superconductivity develops with a maximal transition temperature Tc ~ 2 K. We further show that the coupled order is also completely suppressed by phosphorus doping in CrAs1-xPx at a critical xc ~ 0.05, above which inelastic neutron scattering evidenced persistent antiferromagnetic correlations, providing a possible link between magnetism and superconductivity. In line with the presence of antiferromagnetic fluctuations near Pc (xc), the A coefficient of the quadratic temperature dependence of resistivity exhibits a dramatic enhancement as P (x) approaches Pc (xc), around which Res(T) has a non-Fermi-liquid form. Accordingly, the electronic specific-heat coefficient of CrAs1-xPx peaks out around xc. These properties provide clear evidences for quantum criticality, which we interpret as originating from a nearly second-order helimagnetic quantum phase transition that is concomitant with a first-order structural transition. Our findings in CrAs highlight the distinct characteristics of quantum criticality in bad metals, thereby bringing out new insights into the physics of unconventional superconductivity such as occurring in the high-Tc iron pnictides. △ Less

Submitted 8 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. X 8, 031017 (2018)

Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B: a first-principles study

Authors: Taro Fukazawa, Hisazumi Akai, Yosuke Harashima, Takashi Miyake

Abstract: We calculate intersite magnetic couplings for Sm$_2$Fe$_{17}$, Nd$_2$Fe$_{14}$ and Nd$_2$Fe$_{14}$X (X = B, C, N, O, F) using Liechtenstein's formula on the basis of first-principles calculation, and analyze them to investigate the Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B. We find that the magnetic coupling in the dumbbell bond is strongly ferromagnetic in our calculation, which i… ▽ More We calculate intersite magnetic couplings for Sm$_2$Fe$_{17}$, Nd$_2$Fe$_{14}$ and Nd$_2$Fe$_{14}$X (X = B, C, N, O, F) using Liechtenstein's formula on the basis of first-principles calculation, and analyze them to investigate the Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B. We find that the magnetic coupling in the dumbbell bond is strongly ferromagnetic in our calculation, which is against a previous conjecture explaining the low Curie temperature for Sm$_2$Fe$_{17}$. The calculated values of the couplings explain the experimentally observed difference in the Curie temperature of Sm$_2$Fe$_{17}$ and Nd$_2$Fe$_{14}$B. We also address boron's effects on the Curie temperature of Nd$_2$Fe$_{14}$B, especially in connection with Kanamori's theory of cobaltization. △ Less

Submitted 8 January, 2019; v1 submitted 1 November, 2018; originally announced November 2018.

Comments: 5 pages; 7 figures

First-principles studies of spin-orbital physics in pyrochlore oxides

Authors: Hiroshi Shinaoka, Yukitoshi Motome, Takashi Miyake, Shoji Ishibashi, Philipp Werner

Abstract: The pyrochlore oxides $A_2B_2$O$_7$ exhibit a complex interplay between geometrical frustration, electronic correlations, and spin-orbit coupling, due to the lattice structure and active charge, spin, and orbital degrees of freedom. Understanding the properties of these materials is a theoretical chalenge, because their intricate nature depends on material-specific details and quantum many-body ef… ▽ More The pyrochlore oxides $A_2B_2$O$_7$ exhibit a complex interplay between geometrical frustration, electronic correlations, and spin-orbit coupling, due to the lattice structure and active charge, spin, and orbital degrees of freedom. Understanding the properties of these materials is a theoretical chalenge, because their intricate nature depends on material-specific details and quantum many-body effects. Here we review our recent studies based on first-principles calculations and quantum many-body theories for 4$d$ and 5$d$ pyrochlore oxides with $B$=Mo, Os, and Ir. In these studies, the spin-orbit coupling and local electron correlations are treated within the LDA+$U$ and LDA+dynamical mean-field theory formalisms. We also discuss the technical aspects of these calculations. △ Less

Submitted 5 June, 2019; v1 submitted 22 October, 2018; originally announced October 2018.

Comments: Published version

Journal ref: Journal of Physics: Condensed Matter, Volume 31, Number 32 (2019)

Important descriptors and descriptor groups of Curie temperatures of rare-earth transition-metal binary alloys

Authors: Hieu Chi Dam, Viet Cuong Nguyen, Tien Lam Pham, Anh Tuan Nguyen, Kiyoyuki Terakura, Takashi Miyake, Hiori Kino

Abstract: We analyze Curie temperatures of rare-earth transition metal binary alloys with machine learning method. In order to select important descriptors and descriptor groups, we introduce newly developed subgroup relevance analysis and adopt the hierarchical clustering in the representation. We execute the exhaustive search and successfully illustrate the importance of descriptors and descriptor groups.… ▽ More We analyze Curie temperatures of rare-earth transition metal binary alloys with machine learning method. In order to select important descriptors and descriptor groups, we introduce newly developed subgroup relevance analysis and adopt the hierarchical clustering in the representation. We execute the exhaustive search and successfully illustrate the importance of descriptors and descriptor groups. We execute the exhaustive search and illustrate that our approach indeed leads to the successful selection of important descriptors and descriptor groups. It helps us to choose the combination of the descriptors and to understand the meaning of the selected combination of descriptors. △ Less

Submitted 15 October, 2018; v1 submitted 12 September, 2018; originally announced September 2018.

Comments: 16 pages, 8 figures

Effect of $R$-site substitution and the pressure on stability of $R$Fe$_{12}$: A first-principles study

Authors: Yosuke Harashima, Taro Fukazawa, Hiori Kino, Takashi Miyake

Abstract: We theoretically study the structural stability of $R$Fe$_{12}$ with the ThMn$_{12}$ structure ($R$: rare-earth elements, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, Lu, Y, or Sc, or group-IV elements, Zr or Hf) based on density functional theory. The formation energy has a strong correlation with the atomic radius of $R$. The formation energy relative to simple substances decreases as the atomic radius d… ▽ More We theoretically study the structural stability of $R$Fe$_{12}$ with the ThMn$_{12}$ structure ($R$: rare-earth elements, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, Lu, Y, or Sc, or group-IV elements, Zr or Hf) based on density functional theory. The formation energy has a strong correlation with the atomic radius of $R$. The formation energy relative to simple substances decreases as the atomic radius decreases, except for $R=$ Sc and Hf, while that relative to $R_{2}$Fe$_{17}$ and bcc Fe has a minimum for $R=$ Dy. The present results are consistent with recent experimental reports in which the partial substitution of Zr at $R$ sites stabilizes $R$Fe$_{12}$-type compounds with $R=$ Nd or Sm. Our results also suggest that the partial substitution of Y, Dy, Ho, Er, or Tm for Nd or Sm is a possible way to enhance the stability of the ThMn$_{12}$ structure. Under hydrostatic pressure, the formation enthalpy decreases up to $\approx$ 6 GPa and then starts to increase at higher pressures. △ Less

Submitted 25 September, 2018; v1 submitted 30 May, 2018; originally announced May 2018.

Comments: 6 pages, 6 figures

Journal ref: J. Appl. Phys. 124, 163902 (2018)

Anisotropy of exchange stiffness based on atomic-scale magnetic properties in rare-earth permanent magnet Nd$_2$Fe$_{14}$B

Authors: Yuta Toga, Masamichi Nishino, Seiji Miyashita, Takashi Miyake, Akimasa Sakuma

Abstract: We examine the anisotropic properties of the exchange stiffness constant, $\mathcal{A}$, for rare-earth permanent magnet, Nd$_2$Fe$_{14}$B, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of… ▽ More We examine the anisotropic properties of the exchange stiffness constant, $\mathcal{A}$, for rare-earth permanent magnet, Nd$_2$Fe$_{14}$B, by connecting analyses with two different scales of length, i.e., Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG) equation with a continuous magnetic model. The atomistic MC simulations are performed on the spin model of Nd$_2$Fe$_{14}$B constructed from ab-initio calculations, and the LLG micromagnetics simulations are performed with the parameters obtained by the MC simulations. We clarify that the amplitude and the thermal property of $\mathcal{A}$ depend on the orientation in the crystal, which are attributed to the layered structure of Nd atoms and weak exchange couplings between Nd and Fe atoms. We also confirm that the anisotropy of $\mathcal{A}$ significantly affects the threshold field for the magnetization reversal (coercivity) given by the depinning process. △ Less

Submitted 24 July, 2018; v1 submitted 16 April, 2018; originally announced April 2018.

Comments: 11 pages, 10 figures

Journal ref: Phys. Rev. B 98, 054418 (2018)

First-principles study of spin-wave dispersion in Sm(Fe$_{1-x}$Co$_{x}$)$_{12}$

Authors: Taro Fukazawa, Hisazumi Akai, Yosuke Harashima, Takashi Miyake

Abstract: We present spin-wave dispersion in Sm(Fe$_{1-x}$Co$_x$)$_{12}$ calculated based on first-principles. Anisotropy in the lowest branch of the spin-wave dispersion around the $Γ$ point is discussed. Spin-waves propagate more easily along $a^*$-axis than along $c^*$-axis, especially in SmFe$_{12}$. We also compare values of the spin-wave stiffness with those obtained from an experiment. The calculated… ▽ More We present spin-wave dispersion in Sm(Fe$_{1-x}$Co$_x$)$_{12}$ calculated based on first-principles. Anisotropy in the lowest branch of the spin-wave dispersion around the $Γ$ point is discussed. Spin-waves propagate more easily along $a^*$-axis than along $c^*$-axis, especially in SmFe$_{12}$. We also compare values of the spin-wave stiffness with those obtained from an experiment. The calculated values are in good agreement with the experimental values. △ Less

Submitted 20 August, 2018; v1 submitted 8 March, 2018; originally announced March 2018.

Comments: 15 pages, 6 figures

Journal ref: Journal of Magnetism and Magnetic Materials, 496 (2019) 296-301

Role of typical elements in Nd$_{2}$Fe$_{14}$$X$ ($X$ = B, C, N, O, F)

Authors: Yasutomi Tatetsu, Yosuke Harashima, Takashi Miyake, Yoshihiro Gohda

Abstract: The magnetic properties and structural stability of Nd$_{2}$Fe$_{14}X$ ($X$ = B, C, N, O, F) are theoretically studied by first-principles calculations focusing on the role of $X$. We find that B reduces the magnetic moment (per formula unit) and magnetization (per volume) in Nd$_{2}$Fe$_{14}$B. The crystal-field parameter $A_2^0 \langle r^2 \rangle$ of Nd is not enhanced either, suggesting that B… ▽ More The magnetic properties and structural stability of Nd$_{2}$Fe$_{14}X$ ($X$ = B, C, N, O, F) are theoretically studied by first-principles calculations focusing on the role of $X$. We find that B reduces the magnetic moment (per formula unit) and magnetization (per volume) in Nd$_{2}$Fe$_{14}$B. The crystal-field parameter $A_2^0 \langle r^2 \rangle$ of Nd is not enhanced either, suggesting that B has minor roles in the uniaxial magnetocrystalline anisotropy of Nd. These findings are in contrast to the long-held belief that B works positively for the magnetic properties of Nd$_{2}$Fe$_{14}$B. As $X$ changes from B to C, N, O and F, both the magnetic properties and stability vary significantly. The formation energies of Nd$_{2}$Fe$_{14}X$ and $α$-Fe relative to that of Nd$_{2}$Fe$_{17}X$ are negative for $X$ = B and C, whereas they are positive when $X$ = N, O and F. This indicates that B plays an important role in stabilizing the Nd$_{2}$Fe$_{14}$B phase. △ Less

Submitted 15 February, 2018; originally announced February 2018.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Materials 2, 074410 (2018)

Quantum Theory of Rare-Earth Magnets

Authors: Takashi Miyake, Hisazumi Akai

Abstract: Strong permanent magnets mainly consist of rare earths ($R$) and transition metals ($T$). The main phase of the neodymium magnet, which is the strongest magnet, is Nd$_2$Fe$_{14}$B. Sm$_{2}$Fe$_{17}$N$_{3}$ is another magnet compound having excellent magnetic properties comparable to those of Nd$_{2}$Fe$_{14}$B. Their large saturation magnetization, strong magnetocrystalline anisotropy, and high C… ▽ More Strong permanent magnets mainly consist of rare earths ($R$) and transition metals ($T$). The main phase of the neodymium magnet, which is the strongest magnet, is Nd$_2$Fe$_{14}$B. Sm$_{2}$Fe$_{17}$N$_{3}$ is another magnet compound having excellent magnetic properties comparable to those of Nd$_{2}$Fe$_{14}$B. Their large saturation magnetization, strong magnetocrystalline anisotropy, and high Curie temperature originate from the interaction between the $T$-3d electrons and $R$-4f electrons. This article discusses the magnetism of rare-earth magnet compounds. The basic theory and first-principles calculation approaches for quantitative description of the magnetic properties are presented, together with applications to typical compounds such as Nd$_2$Fe$_{14}$B, Sm$_{2}$Fe$_{17}$N$_{3}$, and the recently synthesized NdFe$_{12}$N. △ Less

Submitted 10 January, 2018; originally announced January 2018.

Comments: 12 pages, 9 figures

Journal ref: J. Phys. Soc. Jpn. 87, 041009 (2018)

Linear behavior of the optical conductivity and incoherent charge transport in BaCoS2

Authors: D. Santos-Cottin, Y. Klein, Ph. Werner, T. Miyake, L. de' Medici, A. Gauzzi, R. P. S. M. Lobo, M. Casula

Abstract: Optical conductivity measurements on a BaCoS2 single crystal show an unusual linear behavior over a broad spectral range. In the paramagnetic phase above 300 K, the spectrum shows no gap, which contradicts the previously proposed scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field theory calculations including a retarded Hubbard interaction explain the data in terms of an… ▽ More Optical conductivity measurements on a BaCoS2 single crystal show an unusual linear behavior over a broad spectral range. In the paramagnetic phase above 300 K, the spectrum shows no gap, which contradicts the previously proposed scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field theory calculations including a retarded Hubbard interaction explain the data in terms of an incipient opening of a Co(3d)-S(3p) charge-transfer gap concomitant to incoherent charge transport driven by electronic correlations. These results point to a non-Fermi liquid scenario with Hund's metal properties in the paramagnetic state, which arises from an incipient Mott phase destabilized by low-energy charge fluctuations across the vanishing 3d-3p charge-transfer gap. △ Less

Submitted 2 October, 2018; v1 submitted 5 December, 2017; originally announced December 2017.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. Materials 2, 105001 (2018)

First-principles study of inter-site magnetic couplings and Curie temperature in RFe$_{12-x}$Cr$_{x}$ (R = Y, Nd, Sm)

Authors: Taro Fukazawa, Hisazumi Akai, Yosuke Harashima, Takashi Miyake

Abstract: We present a first-principles study of RFe$_{12-x}$Cr$_{x}$ (R = Y, Nd, Sm) crystals with ThMn$_{12}$ structure. We discuss, within the mean field approximation, intersite magnetic couplings calculated using Liechtenstein's formula and convert them into Curie temperatures, $T_{\rm C}$, which are found to become larger when a small amount of Cr ($x \leq 0.5$) is introduced into the system. This enh… ▽ More We present a first-principles study of RFe$_{12-x}$Cr$_{x}$ (R = Y, Nd, Sm) crystals with ThMn$_{12}$ structure. We discuss, within the mean field approximation, intersite magnetic couplings calculated using Liechtenstein's formula and convert them into Curie temperatures, $T_{\rm C}$, which are found to become larger when a small amount of Cr ($x \leq 0.5$) is introduced into the system. This enhancement is larger than that for Co in the dilute limit, $x \rightarrow 0$. In contrast, above $x > 0.5$, the Curie temperature decreases as Cr concentration increases. This behavior is analyzed using an expansion of $T_{\rm C}$ in terms of concentration. △ Less

Submitted 17 January, 2018; v1 submitted 14 November, 2017; originally announced November 2017.

Comments: 14 pages, 5 figures, 1 ancillary file (supplementary material)

Journal ref: J. Phys. Soc. Jpn. 87, 044706 (2018)

Effect of Parallel Magnetic Field on Superconductivity of Ultrathin Metal Films Grown on a Cleaved GaAs Surface

Authors: Takayuki Sekihara, Takahiro Miyake, Ryuichi Masutomi, Tohru Okamoto

Abstract: The parallel-magnetic-field $H_\parallel$ dependence of the superconducting transition temperature $T_c$ is studied for ultrathin films of In, Bi, and Al grown on GaAs(110). In the case of In films in the monolayer regime, $T_c$ exhibits a quadratic-like $H_\parallel$ dependence, which is one order of magnitude stronger than that previously observed in monolayer Pb films by the present authors [Ph… ▽ More The parallel-magnetic-field $H_\parallel$ dependence of the superconducting transition temperature $T_c$ is studied for ultrathin films of In, Bi, and Al grown on GaAs(110). In the case of In films in the monolayer regime, $T_c$ exhibits a quadratic-like $H_\parallel$ dependence, which is one order of magnitude stronger than that previously observed in monolayer Pb films by the present authors [Phys. Rev. Lett. 111, 057005 (2013)]. The results are well reproduced by a model developed for an inhomogeneous two-dimensional superconducting state in the presence of a moderate Rashba spin-orbit interaction. The Rashba spin splitting is estimated to be 0.04 eV, which is much smaller than that expected for monolayer Pb films. In a few-monolayer Bi film, the suppression of $T_c$ with increasing $H_\parallel$ is comparable to that in monolayer Pb films. On the other hand, much stronger suppression, which is attributed to the Pauli paramagnetic effect, was observed for the Al film. △ Less

Submitted 23 October, 2017; originally announced October 2017.

Comments: 5 pages, 7 figures

Journal ref: J. Phys. Soc. Jpn. 84, 064710 (2015)

Electronic band structure of 4d and 5d transition metal trichalcogenides

Authors: Yusuke Sugita, Takashi Miyake, Yukitoshi Motome

Abstract: Transition metal trichalcogenides (TMTs), a family of van der Waals materials, have gained increasing interests from the discovery of magnetism in few-layer forms. Although TMTs with 3d transition metal elements have been studied extensively, much less is explored for the 4d and 5d cases, where the interesting interplay between electron correlations and the relativistic spin-orbit coupling is expe… ▽ More Transition metal trichalcogenides (TMTs), a family of van der Waals materials, have gained increasing interests from the discovery of magnetism in few-layer forms. Although TMTs with 3d transition metal elements have been studied extensively, much less is explored for the 4d and 5d cases, where the interesting interplay between electron correlations and the relativistic spin-orbit coupling is expected. Using ab initio calculations, we here investigate the electronic property of TMTs with 4d and 5d transition metal elements. We show that the band structures exhibit multiple node-like features near the Fermi level. These are the remnant of multiple Dirac cones that were recently discovered in the monolayer cases. Our results indicate that the peculiar two-dimensional multiple Dirac cones are concealed even in the layered bulk systems. △ Less

Submitted 4 July, 2017; originally announced July 2017.

Comments: 4 pages, 3 figures

Journal ref: Physica B: Cond. Matter 536, 48 (2018)

Crystal field splittings in rare earth-based hard magnets: an ab initio approach

Authors: Pascal Delange, Silke Biermann, Takashi Miyake, Leonid Pourovskii

Abstract: We apply the first-principles density functional theory + dynamical mean field theory framework to evaluate the crystal field splitting on rare earth sites in hard magnetic intermetallics. An atomic (Hubbard-I) approximation is employed for local correlations on the rare earth 4$f$ shell and self-consistency in the charge density is implemented. We reduce the density functional theory self-interac… ▽ More We apply the first-principles density functional theory + dynamical mean field theory framework to evaluate the crystal field splitting on rare earth sites in hard magnetic intermetallics. An atomic (Hubbard-I) approximation is employed for local correlations on the rare earth 4$f$ shell and self-consistency in the charge density is implemented. We reduce the density functional theory self-interaction contribution to the crystal field splitting by properly averaging the 4$f$ charge density before recalculating the one-electron Kohn-Sham potential. Our approach is shown to reproduce the experimental crystal field splitting in the prototypical rare earth hard magnet SmCo$_5$. Applying it to $R$Fe$_{12}$ and $R$Fe$_{12}X$ hard magnets ($R=$Nd, Sm and $X=$N, Li), we obtain in particular a large positive value of the crystal field parameter $A_2^0\langle r^2\rangle$ in NdFe$_{12}$N resulting in a strong out-of-plane anisotropy observed experimentally. The sign of $A_2^0\langle r^2\rangle$ is predicted to be reversed by substituting N with Li, leading to a strong out-of-plane anisotropy in SmFe$_{12}$Li. We discuss the origin of this strong impact of N and Li interstitials on the crystal field splitting on rare earth sites. △ Less

Submitted 12 November, 2017; v1 submitted 22 May, 2017; originally announced May 2017.

Comments: 17 pages, 11 figures

Journal ref: Phys. Rev. B 96, 155132 (2017)

Machine learning reveals orbital interaction in crystalline materials

Authors: Tien Lam Pham, Hiori Kino, Kiyoyuki Terakura, Takashi Miyake, Ichigaku Takigawa, Koji Tsuda, Hieu Chi Dam

Abstract: We propose a novel representation of crystalline materials named orbital-field matrix (OFM) based on the distribution of valence shell electrons. We demonstrate that this new representation can be highly useful in mining material data. Our experiment shows that the formation energies of crystalline materials, the atomization energies of molecular materials, and the local magnetic moments of the co… ▽ More We propose a novel representation of crystalline materials named orbital-field matrix (OFM) based on the distribution of valence shell electrons. We demonstrate that this new representation can be highly useful in mining material data. Our experiment shows that the formation energies of crystalline materials, the atomization energies of molecular materials, and the local magnetic moments of the constituent atoms in transition metal--rare-earth metal bimetal alloys can be predicted with high accuracy using the OFM. Knowledge regarding the role of coordination numbers of transition-metal and rare-earth metal elements in determining the local magnetic moment of transition metal sites can be acquired directly from decision tree regression analyses using the OFM. △ Less

Submitted 3 May, 2017; v1 submitted 2 May, 2017; originally announced May 2017.

Comments: 10 pages

A regression-based feature selection study of the Curie temperature of transition-metal rare-earth compounds: prediction and understanding

Authors: Hieu Chi Dam, Viet Cuong Nguyen, Tien Lam Pham, Anh Tuan Nguyen, Hiori Kino, Kiyoyuki Terakura, Takashi Miyake

Abstract: The Curie temperature ($T_C$) of binary alloy compounds consisting of 3$d$ transition-metal and 4$f$ rare-earth elements is analyzed by a machine learning technique. We first demonstrate that nonlinear regression can accurately reproduce $T_C$ of the compounds. The prediction accuracy for $T_C$ is maximized when five to ten descriptors are selected, with the rare-earth concentration being the most… ▽ More The Curie temperature ($T_C$) of binary alloy compounds consisting of 3$d$ transition-metal and 4$f$ rare-earth elements is analyzed by a machine learning technique. We first demonstrate that nonlinear regression can accurately reproduce $T_C$ of the compounds. The prediction accuracy for $T_C$ is maximized when five to ten descriptors are selected, with the rare-earth concentration being the most relevant. We then discuss an attempt to utilize a regression-based model selection technique to learn the relation between the descriptors and the actuation mechanism of the corresponding physical phenomenon, i.e., $T_C$ in the present case. △ Less

Submitted 2 May, 2017; v1 submitted 2 May, 2017; originally announced May 2017.

Comments: 4 pages

Multiple Dirac Cones and Topological Magnetism in Honeycomb-Monolayer Transition Metal Trichalcogenides

Authors: Yusuke Sugita, Takashi Miyake, Yukitoshi Motome

Abstract: The discovery of monolayer graphene has initiated two fertile fields in modern condensed matter physics, Dirac semimetals and atomically-thin layered materials. When these trends meet again in transition metal compounds, which possess spin and orbital degrees of freedom and strong electron correlations, more exotic phenomena are expected to emerge in the cross section of topological states of matt… ▽ More The discovery of monolayer graphene has initiated two fertile fields in modern condensed matter physics, Dirac semimetals and atomically-thin layered materials. When these trends meet again in transition metal compounds, which possess spin and orbital degrees of freedom and strong electron correlations, more exotic phenomena are expected to emerge in the cross section of topological states of matter and Mott physics. Here, we show by using ab initio calculations that a monolayer form of transition metal trichalcogenides (TMTs), which has a honeycomb network of transition metal cations, may exhibit multiple Dirac cones with tunable gaps in the electronic structure. Furthermore, we elucidate that electron correlations and carrier doping turn the multiple-Dirac semimetal into a topological ferromagnet with high Chern number. Our findings raise the honeycomb-monolayer TMTs to a new paradigm to explore correlated Dirac electrons and topologically-nontrivial magnetism. In turn, the unique wide-ranging properties of the materials will deliver new building blocks for atomically thin heterostructures. △ Less

Submitted 18 May, 2018; v1 submitted 2 April, 2017; originally announced April 2017.

Comments: 8 pages, 8 figures, 4 tables

Journal ref: Phys. Rev. B 97, 035125 (2018)

First-principles study of intersite magnetic couplings in NdFe$_{12}$ and NdFe$_{12}$X (X = B, C, N, O, F)

Authors: Taro Fukazawa, Hisazumi Akai, Yosuke Harashima, Takashi Miyake

Abstract: We present a first-principles investigation of NdFe$_{12}$ and NdFe$_{12}$X (X = B, C, N, O, F) crystals with the ThMn$_{12}$ structure. Intersite magnetic couplings in these compounds, so-called exchange couplings, are estimated by Liechtenstein's method. It is found that the Nd--Fe couplings are sensitive to the interstitial dopant X, with the Nd--Fe(8j) coupling in particular reduced significan… ▽ More We present a first-principles investigation of NdFe$_{12}$ and NdFe$_{12}$X (X = B, C, N, O, F) crystals with the ThMn$_{12}$ structure. Intersite magnetic couplings in these compounds, so-called exchange couplings, are estimated by Liechtenstein's method. It is found that the Nd--Fe couplings are sensitive to the interstitial dopant X, with the Nd--Fe(8j) coupling in particular reduced significantly for X = N. This suggests that the magnetocrystalline anisotropy decays quickly with rising temperature in the X = N system although nitrogenation has advantages over the other dopants in terms of enhancing low-temperature magnetic properties. The Curie temperature is also calculated from the magnetic couplings by using the mean field approximation. Introduction of X enhances the Curie temperature, with both structural changes and chemical effects found to play important roles in this enhancement. △ Less

Submitted 18 July, 2017; v1 submitted 13 December, 2016; originally announced December 2016.

Comments: 18 pages, 11 figures; data added to Appendix sections

Journal ref: Journal of Applied Physics 122, 053901 (2017)

Emergence of topological semimetals in gap closing in semiconductors without inversion symmetry

Authors: Shuichi Murakami, Motoaki Hirayama, Ryo Okugawa, Takashi Miyake

Abstract: A band gap for electronic states in crystals governs various properties of solids, such as transport, optical and magnetic properties. Its estimation and control have been an important issue in solid state physics. The band gap can be controlled externally by various parameters, such as pressure, atomic compositions and external field. Sometimes, the gap even collapses by tuning some parameter. In… ▽ More A band gap for electronic states in crystals governs various properties of solids, such as transport, optical and magnetic properties. Its estimation and control have been an important issue in solid state physics. The band gap can be controlled externally by various parameters, such as pressure, atomic compositions and external field. Sometimes, the gap even collapses by tuning some parameter. In the field of topological insulators, such closing of the gap at a time-reversal invariant momentum indicates a band inversion, i.e. it leads to a topological phase transition from a normal insulator to a topological insulator. Here we show that the gap losing in inversion-asymmetric crystals is universal, in the sense that the gap closing always leads either to a Weyl semimetal or a nodal-line semimetal, from an exhaustive study on possible space groups. We here consider three-dimensional spinful systems with time-reversal symmetry. The space group of the system and the wavevector at the gap closing uniquely determine which possibility occurs and where the gap-closing points or lines lie in the wavevector space after closing of the gap. In particular, we show that an insulator-to-insulator transition never happens, which is in sharp contrast with inversion-symmetric systems. △ Less

Submitted 23 October, 2016; originally announced October 2016.

Comments: 13 pages, 8 figures

Journal ref: Science Advances 3, e1602680 (2017)

First-principles study on stability and magnetism of NdFe11M and NdFe11MN for M=Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

Authors: Yosuke Harashima, Kiyoyuki Terakura, Hiori Kino, Shoji Ishibashi, Takashi Miyake

Abstract: Recently synthesized NdFe12N has excellent magnetic properties, while it is thermodynamically unstable. Using first-principles method, we study the effect of substitutional 3d transition metal elements to the mother compound NdFe12. We find that Co has positive effect on the stability of the ThMn12 structure. In contrast with Ti substitution, Co substitution does not reduce the magnetization signi… ▽ More Recently synthesized NdFe12N has excellent magnetic properties, while it is thermodynamically unstable. Using first-principles method, we study the effect of substitutional 3d transition metal elements to the mother compound NdFe12. We find that Co has positive effect on the stability of the ThMn12 structure. In contrast with Ti substitution, Co substitution does not reduce the magnetization significantly. The crystal field parameter <r^{2}>A_{0}^{2} is nearly unchanged by Co substitution, and nitrogenation to NdFe11Co greatly enhances <r^{2}>A_{0}^{2}. This suggests that Co is a good candidate as a substitutional element for NdFe12N. △ Less

Submitted 14 November, 2016; v1 submitted 23 September, 2016; originally announced September 2016.

Comments: 6 pages, 7 figures

Journal ref: J. Appl. Phys. 120, 203904 (2016)

Monte Carlo analysis for finite temperature magnetism of Nd$_2$Fe$_{14}$B permanent magnet

Authors: Yuta Toga, Munehisa Matsumoto, Seiji Miyashita, Hisazumi Akai, Shotaro Doi, Takashi Miyake, Akimasa Sakuma

Abstract: We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare earth intermetallic compound, Nd$_2$Fe$_{14}$B. The constrained Monte Carlo method is applied to a Nd$_2$Fe$_{14}$B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants $K^{\rm A}_m (m=1, 2, 4)$ at finite temperatures. % Subsequentl… ▽ More We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare earth intermetallic compound, Nd$_2$Fe$_{14}$B. The constrained Monte Carlo method is applied to a Nd$_2$Fe$_{14}$B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants $K^{\rm A}_m (m=1, 2, 4)$ at finite temperatures. % Subsequently, it is found that the temperature dependence of $K^{\rm A}_1$ deviates from the Callen--Callen law, $K^{\rm A}_1(T) \propto M(T)^3$, even above room temperature, $T_{\rm R}\sim 300\rm\, K$, when the Fe (Nd) anisotropy terms are removed to leave only the Nd (Fe) anisotropy terms. This is because the exchange couplings between Nd moments and Fe spins are much smaller than those between Fe spins. % It is also found that the exponent $n$ in the external magnetic field $H_{\rm ext}$ response of barrier height $\mathcal{F}_{\rm B}=\mathcal{F}_{\rm B}^0(1-H_{\rm ext}/H_0)^n$ is less than $2$ in the low-temperature region below $T_{\rm R}$, whereas $n$ approaches $2$ when $T>T_{\rm R}$, indicating the presence of Stoner--Wohlfarth-type magnetization rotation. This reflects the fact that the magnetic anisotropy is mainly governed by the $K^{\rm A}_1$ term in the $T>T_{\rm R}$ region. △ Less

Submitted 5 October, 2016; v1 submitted 1 June, 2016; originally announced June 2016.

Comments: 10 pages, 14 figures

Journal ref: Phys. Rev. B 94, 174433 (2016)

Topological Dirac Nodal Lines and Surface Charges in fcc Alkaline Earth Metals

Authors: Motoaki Hirayama, Ryo Okugawa, Takashi Miyake, Shuichi Murakami

Abstract: In nodal-line semimetals, the gaps close along loops in ${\bf k}$ space, which are not at high-symmetry points. Typical mechanisms for the emergence of nodal lines involve mirror symmetry and the $π$ Berry phase. Here, we show via ab initio calculations that fcc calcium (Ca), strontium (Sr) and ytterbium (Yb) have topological nodal lines with the $π$ Berry phase near the Fermi level, when spin-orb… ▽ More In nodal-line semimetals, the gaps close along loops in ${\bf k}$ space, which are not at high-symmetry points. Typical mechanisms for the emergence of nodal lines involve mirror symmetry and the $π$ Berry phase. Here, we show via ab initio calculations that fcc calcium (Ca), strontium (Sr) and ytterbium (Yb) have topological nodal lines with the $π$ Berry phase near the Fermi level, when spin-orbit interaction is neglected. In particular, Ca becomes a nodal-line semimetal at high pressure. Owing to nodal lines, the Zak phase becomes either $π$ or 0 depending on the wavavector ${\bf k}$, and the $π$ Zak phase leads to surface polarization charge. Carriers eventually screen it, leaving behind large surface dipoles. In materials with nodal lines, both the large surface polarization charge and the emergent drumhead surface states enhance Rashba splitting when heavy adatoms are present, as we have shown to occur in Bi/Sr(111) and in Bi/Ag(111). △ Less

Submitted 8 February, 2017; v1 submitted 21 February, 2016; originally announced February 2016.

Comments: 14 pages, 11 figures

Journal ref: Nature Communications 8, 14022 (2017)

Green function theory of orbital magnetic moment of interacting electrons in solids

Authors: F. Aryasetiawan, K. Karlsson, T. Miyake

Abstract: A general formula for the orbital magnetic moment of interacting electrons in solids is derived using the many-electron Green function method. The formula factorizes into two parts, a part that contains the information about the one-particle band structure of the system and a part that contains the effects of exchange and correlations carried by the Green function. The derived formula provides a c… ▽ More A general formula for the orbital magnetic moment of interacting electrons in solids is derived using the many-electron Green function method. The formula factorizes into two parts, a part that contains the information about the one-particle band structure of the system and a part that contains the effects of exchange and correlations carried by the Green function. The derived formula provides a convenient means of including the effects of exchange and correlations beyond the commonly used local density approximation of density functional theory. △ Less

Submitted 21 December, 2015; originally announced December 2015.

Comments: 4 pages, no figures

Journal ref: Phys. Rev. B 93, 161104 (2016)

Low-energy effective Hamiltonians for correlated electron systems beyond density functional theory

Authors: Motoaki Hirayama, Takashi Miyake, Masatoshi Imada, Silke Biermann

Abstract: We propose a refined scheme of deriving an effective low-energy Hamiltonian for materials with strong electronic Coulomb correlations beyond density functional theory (DFT). By tracing out the electronic states away from the target degrees of freedom in a controlled way by a perturbative scheme we construct an effective model for a restricted low-energy target space incorporating the effects of hi… ▽ More We propose a refined scheme of deriving an effective low-energy Hamiltonian for materials with strong electronic Coulomb correlations beyond density functional theory (DFT). By tracing out the electronic states away from the target degrees of freedom in a controlled way by a perturbative scheme we construct an effective model for a restricted low-energy target space incorporating the effects of high-energy degrees of freedom in an effective manner. The resulting effective model can afterwards be solved by accurate many-body solvers. We improve this "multi-scale ab initio scheme for correlated electrons" (MACE) primarily in two directions: (1) Double counting of electronic correlations between the DFT and the low-energy solver is avoided by using the constrained GW scheme. (2) The frequency dependence of the interaction emerging from the partial trace summation is taken into account as a renormalization to the low-energy dispersion. The scheme is successfully tested on the example of SrVO3. Our work opens unexplored ways to understanding the electronic structure of strongly correlated systems beyond current DFT methods. △ Less

Submitted 11 November, 2015; originally announced November 2015.

Comments: 20 pages, 17 figures

Journal ref: Phys. Rev. B 96, 075102 (2017)

Nitrogen as the best interstitial dopant among $X$=B, C, N, O and F for strong permanent magnet NdFe$_{11}$Ti$X$: First-principles study

Authors: Yosuke Harashima, Kiyoyuki Terakura, Hiori Kino, Shoji Ishibashi, Takashi Miyake

Abstract: We study magnetic properties of NdFe$_{11}$Ti$X$, where $X$=B, C, N, O, and F, by using the first-principles calculation based on the density functional theory. Its parent compound NdFe$_{11}$Ti has the ThMn$_{12}$ structure, which has the symmetry of space group $I4/mmm$, No. 139. The magnetization increases by doping B, C, N, O, and F at the $2b$ site of the ThMn$_{12}$ structure. The amount of… ▽ More We study magnetic properties of NdFe$_{11}$Ti$X$, where $X$=B, C, N, O, and F, by using the first-principles calculation based on the density functional theory. Its parent compound NdFe$_{11}$Ti has the ThMn$_{12}$ structure, which has the symmetry of space group $I4/mmm$, No. 139. The magnetization increases by doping B, C, N, O, and F at the $2b$ site of the ThMn$_{12}$ structure. The amount of the increase is larger for $X$=N, O, F than for $X$=B, C. On the other hand, the crystal field parameter $\langle r^{2} \rangle A_{0}^{2}$, which controls the axial magnetic anisotropy of the Nd $4f$ magnetic moment, depends differently on the dopant. With increase of the atomic number from $X$=B, $\langle r^{2} \rangle A_{0}^{2}$ increases, takes a maximum value for $X$=N, and then turns to decrease. This suggests that in NdFe$_{11}$Ti$X$, nitrogen is the most appropriate dopant among B, C, N, O, and F for permanent magnets in terms of magnetization and anisotropy. The above calculated properties are explained based on the detailed analysis of the electronic structures of NdFe$_{11}$Ti$X$. △ Less

Submitted 4 December, 2015; v1 submitted 14 July, 2015; originally announced July 2015.

Comments: 14 pages, 19 figures

Journal ref: Phys. Rev. B 92, 184426 (2015)

Ab initio Studies of Magnetism in the Iron Chalcogenides FeTe and FeSe

Authors: Motoaki Hirayama, Takahiro Misawa, Takashi Miyake, Masatoshi Imada

Abstract: The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating f… ▽ More The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure. △ Less

Submitted 15 August, 2015; v1 submitted 14 May, 2015; originally announced May 2015.

Comments: 10 pages, 7 figures, 5 tables

Journal ref: J. Phys. Soc. Jpn. 84, 093703 (2015)

Relevance of $4f$-$3d$ exchange to finite-temperature magnetism of rare-earth permanent magnets: an ab-initio-based spin model approach for NdFe$_{12}$N

Authors: Munehisa Matsumoto, Hisazumi Akai, Yosuke Harashima, Shotaro Doi, Takashi Miyake

Abstract: A classical spin model derived ab initio for rare-earth-based permanent magnet compounds is presented. Our target compound, NdFe$_{12}$N, is a material that goes beyond today's champion magnet compound Nd$_{2}$Fe$_{14}$B in its intrinsic magnetic properties with a simpler crystal structure. Calculated temperature dependence of the magnetization and the anisotropy field agree with the latest experi… ▽ More A classical spin model derived ab initio for rare-earth-based permanent magnet compounds is presented. Our target compound, NdFe$_{12}$N, is a material that goes beyond today's champion magnet compound Nd$_{2}$Fe$_{14}$B in its intrinsic magnetic properties with a simpler crystal structure. Calculated temperature dependence of the magnetization and the anisotropy field agree with the latest experimental results in the leading order. Having put the realistic observables under our numerical control, we propose that engineering $5d$-electron-mediated indirect exchange coupling between $4f$-electrons in Nd and $3d$-electrons from Fe would most critically help to enhance the material's utility over the operation-temperature range. △ Less

Submitted 18 May, 2016; v1 submitted 25 April, 2015; originally announced April 2015.

Comments: 7 pages, 7 figures, ab initio parameters re-defined sustaining the same conclusion, accepted for publication on J. Appl. Phys

Journal ref: J. Appl. Phys. 119, 213901 (2016)