arXiv subjects

Taro Fukazawa

Publications and source records attributed to Taro Fukazawa.

16 recordsLinked to original sources

arXiv2025

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

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.

cond-mat.mtrl-sci↗

arXiv2025

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

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.

cond-mat.mtrl-sci↗

arXiv2022

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

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.

cond-mat.mtrl-sci↗

arXiv2022

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

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.

cond-mat.mtrl-sci↗

arXiv2021

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

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.

cond-mat.mtrl-sci↗

arXiv2021

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

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.

cond-mat.mtrl-sci↗

arXiv2020

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

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.

cond-mat.mtrl-sci↗

arXiv2020

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

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.

cond-mat.mtrl-sci↗

arXiv2019

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

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.

cond-mat.mtrl-sci↗

arXiv2019

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

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.

cond-mat.mtrl-sci↗

arXiv2019

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

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.

cond-mat.mtrl-sci↗

arXiv2018

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

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.

cond-mat.mtrl-sci↗

arXiv2018

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

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.

cond-mat.mtrl-sci↗

arXiv2018

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

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.

cond-mat.mtrl-sci↗

arXiv2017

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

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.

cond-mat.mtrl-sci↗

arXiv2016

Optimized effective potential method and application to static RPA correlation

The optimized effective potential (OEP) method is a promising technique for calculating the ground state properties of a system within the density functional theory. However, it is not widely used as its computational cost is rather high and, also, some ambiguity remains in the theoretical framework. In order to overcome these problems, we first introduced a method that accelerates the OEP scheme in a static RPA-level correlation functional. Second, the Krieger-Li-Iafrate (KLI) approximation is exploited to solve the OEP equation. Although seemingly too crude, this approximation did not reduce the accuracy of the description of the magnetic transition metals (Fe, Co, and Ni) examined here, the magnetic properties of which are rather sensitive to correlation effects. Finally, we reformulated the OEP method to render it applicable to the direct RPA correlation functional and other, more precise, functionals. Emphasis is placed on the following three points of the discussion: i) Level-crossing at the Fermi surface is taken into account; ii) eigenvalue variations in a Kohn-Sham functional are correctly treated; and iii) the resultant OEP equation is different from those reported to date.

cond-mat.mtrl-sci↗