arXiv subjects

Munehisa Matsumoto

Publications and source records attributed to Munehisa Matsumoto.

At least 19 recordsLinked to original sources

arXiv2020

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

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.

cond-mat.mtrl-sci↗

arXiv2020

(Sm,Zr)Fe$_{12-x}$M$_x$ (M=Zr,Ti,Co) for permanent-magnet applications: Ab initio material design integrated with experimental characterization

In rare-earth permanent magnets (REPM's), trade-off's between intrinsic magnetic properties are often encountered. A recent example is SmFe$_{12}$ where excellent magnetic properties can be achieved at the sacrifice of bulk structure stability. Bulk structure stability is sustained by the presence of the third substitute element as is the case with SmFe$_{11}$Ti, where Ti degrades magnetic properties. It is now in high demand to find out with which chemical composition a good compromise in the trade-off between structure stability and strong ferromagnetism is reached. We inspect the effects of representative substitute elements, Zr, Ti, and Co in SmFe$_{12}$ by combining ab initio data with experimental data from neutron diffraction. The trend in the intrinsic properties with respect to the concentration of substitute elements are monitored and a systematic way to search the best compromise is constructed. A certain minimum amount of Ti is identified with respect to the added amount of Co and Zr. It is found that Zr brings about a positive effect on magnetization, in line with recent experimental developments, and we argue that this can be understood as an effective doping of extra electrons.

cond-mat.mtrl-sci↗

arXiv2020

Magnetism trends in doped Ce-Cu intermetallics in the vicinity of quantum criticality: realistic Kondo lattice models based on dynamical mean-field theory

The quantum critical point (QCP) in the archetypical heavy-fermion compound CeCu$_6$ doped by Au is described, accounting for the localized $4f$-electron of Ce, using realistic electronic structure calculations combined with dynamical mean-field theory (DMFT). Magnetism trends in Ce(Cu$_{1-ε}$Au$_ε$)$_6$ are compared with those in Co-doped CeCu$_{5}$, which resides on the non-ferromagnetic side of the composition space of one of the earliest rare-earth permanent magnet compounds, Ce(Co,Cu)$_5$. The construction of a realistic Doniach phase diagram shows that the system crosses over a magnetic quantum critical point in the Kondo lattice in $0.2<x<0.4$ of Ce(Cu$_{1-x}$Co$_x$)$_5$. Comparison between Au-doped CeCu$_6$ and Co-doped CeCu$_5$ reveals that the swept region in the vicinity of QCP for the latter thoroughly covers that of the former. The implications of these trends on the coercivity of the bulk rare-earth permanent magnets are discussed.

cond-mat.mtrl-sci↗

arXiv2020

Calculated Curie temperatures for rare-earth permanent magnets: ab initio inspection on localized magnetic moments in d-electron ferromagnetism

We present a data set of calculated Curie temperatures for the main-phase compounds of rare-earth permanent magnets. We employ ab initio electronic structure calculations for the itinerant ferromagnetism and an effective spin model for the finite-temperature magnetism. Curie temperatures are derived on the basis of a classical Heisenberg model mapped via Liechtenstein's formula for atomic-pair-wise exchange couplings. Relative trends with respect to the species of rare-earth elements in calculated Curie temperatures for R2Fe14B are in agreement with experimental trends. Quantitative comparison between calculation and experimental data found in the literature point to an effective range of the exchange couplings imposing a limit on the validity range of the effective spin model.

cond-mat.mtrl-sci↗

arXiv2020

Navigating the trends in the proximity to magnetic quantum critical point and superconductivity for Ce-based heavy-fermion compounds

Pressure-induced magnetic quantum phase transition of CeRhIn$_5$ is described from first principles based on a realistic Kondo lattice model. Simulations of CeRhIn$_5$ above ambient pressure up to P=5 [GPa] show that it is driven from an antiferromagnetic phase into the paramagnetic phase crossing the possible QCP around P=2 [GPa], being consistent with the experimental trends. Our calculations suggest a possibility for a coexistence phase around $P\stackrel{<}{\sim} 3$ [GPa]. On the basis of the data from our realistic simulations, a comprehensive materials navigator is compiled including pressure dependent CeRhIn5 to possibly provide a guideline for further discovery of Ce-based heavy fermion superconductors.

cond-mat.mtrl-sci↗

arXiv2019

Efficient algorithm based on Liechtenstein method for computing exchange coupling constants using localized basis set

For large-scale computation of the exchange coupling constants $J_{ij}$, we reconstruct the Liechtenstein formula for localized orbital representation and simplify the energy integrations by adopting the finite pole approximation of the Fermi function proposed by Ozaki [Phys. Rev. B 75, 035123 (2007)]. We calculate the exchange coupling constant $J_{\mathrm{1NN}}$ of the first-nearest-neighbor sites in body-centered-cubic Fe systems of various sizes to estimate the optimal computational parameters that yield appropriate values at the lowest computational cost. It is shown that the number of poles needed for a computational accuracy of 0.05 meV is determined as $\sim$ 60, whereas the number of necessary Matsubara poles needed to obtain similar accuracy, which was determined in previous studies, is on the order of 1000. Finally, we show $J_{ij}$ as a function of atomic distance, and compared it with one derived from Korringa-Kohn-Rostoker Green's function formalism. The distance profile of $J_{ij}$ derived by KKR formalism agrees well with that derived by our study, and this agreement supports the reliability of our newly derived formalism.

cond-mat.mtrl-sci↗

arXiv2019

Electronic Structure Calculations of CeRh$_{3}$B$_{2}$

The electronic structure of the ferromagnetic material CeRh$_{3}$B$_{2}$ is studied by means of first-principles calculations with an emphasis on the treatment of localized $4f$ states around Ce. Via the construction of an effective spin model from electronic structure calculations, we estimated the Curie temperature $T_{\rm C}$ of CeRh$_{3}$B$_{2}$ and found a specific configuration of the electronic structure which explains the exceptionally high measured value of $T_{\rm C} \sim 120$ K as well as the small saturation magnetization and the topology of the Fermi surface. Present advance in the understanding of the subtle nature of the $4f$-electron state in CeRh$_{3}$B$_{2}$ that brings about the exceptionally high $T_{\rm C}$ should be also of technological relevance to exploit the utility of Ce in rare-earth permanent magnets.

cond-mat.mtrl-sci↗

arXiv2019

Anisotropic ferromagnetism and structure stability in $4f$-$3d$ intermetallics: ab initio structure optimization and magnetic anisotropy for RCo$_5$ (R=Ce, La, and Y)

Electronic mechanism in the interplay between ferromagnetism and structure stability of $4f$-$3d$ intermetallics in the main phase of rare-earth permanent magnets is investigated from first principles. We present a case study with an archetypical materials family RCo$_5$ (R=Ce, La, Y), which was a part of the earliest rare-earth permanent magnets and from which other representative main-phase compounds can be regarded as a derived type. Comparison with the champion magnet materials family R$_2$T$_{14}$B and recently revisited materials family RT$_{12}$ (T=Co and Fe) points to a direction leading to a mid-class magnet for the possible next generation materials.

cond-mat.mtrl-sci↗

arXiv2019

Optimal uni-axial ferromagnetism in (La,Ce)$_2$Fe$_{14}$B for permanent magnets

Prospects for light-rare-earth-based permanent magnet compound R$_{2}$Fe$_{14}$B (R=La$_{1-x}$Ce$_{x}$ with $0 \le x\le 1$) are inspected from first principles referring to the latest experimental data. Ce-rich 2:14:1 compounds come with good structure stability, reasonably good combination of magnetization and magnetic anisotropy, while a drawback lies in the low Curie temperature that is only 120~K above the room temperature at the Ce$_2$Fe$_{14}$B limit. Best compromise is inspected on the basis of ab initio data for (La$_{1-x}$Ce$_{x}$)$_2$Fe$_{14}$B referring to the magnetic properties of the champion magnet compound Nd$_{2}$Fe$_{14}$B and prerequisite conditions imposed by practical utility.

cond-mat.mtrl-sci↗

arXiv2019

First-principles calculations of the magnetocrystalline anisotropy of the prototype 2:17 cell boundary phase Y(Co$_{1-x-y}$Fe$_x$Cu$_y$)$_5$

We present a computational study of the compound Y(Co$_{1-x-y}$Fe$_x$Cu$_y$)$_5$ for 0 $\leq x,y \leq 0.2$. This compound was chosen as a prototype for investigating the cell boundary phase believed to play a key role in establishing the high coercivity of commercial Sm-Co 2:17 magnets. Using density-functional theory, we have calculated the magnetization and magnetocrystalline anisotropy at zero temperature for a range of compositions, modeling the doped compounds within the coherent potential approximation. We have also performed finite temperature calculations for YCo$_5$, Y(Co$_{0.838}$Cu$_{0.162}$)$_5$ and Y(Co$_{0.838}$Fe$_{0.081}$Cu$_{0.081}$)$_5$ within the disordered local moment picture. Our calculations find that substituting Co with small amounts of either Fe or Cu boosts the magnetocrystalline anisotropy $K$, but the change in $K$ depends strongly on the location of the dopants. Furthermore, the calculations do not show a particularly large difference between the magnetic properties of Cu-rich Y(Co$_{0.838}$Cu$_{0.162}$)$_5$ and equal Fe-Cu Y(Co$_{0.838}$Fe$_{0.081}$Cu$_{0.081}$)$_5$, despite these two compositions showing different coercivity behavior when found in the cell boundary phase of 2:17 magnets. Our study lays the groundwork for studying the rare earth contribution to the anisotropy of Sm(Co$_{1-x-y}$Fe$_x$Cu$_y$)$_5$, and also shows how a small amount of transition metal substitution can boost the anisotropy field of YCo$_5$.

cond-mat.mtrl-sci↗

arXiv2018

Site preference of dopant elements in Nd2Fe14B

Ab initio calculations of formation energy and mixing energy for (Nd,R)2(Fe,Co)14B [R = rare-earth elements other than Nd] are presented to address the site preference of dopants and the corresponding magnetic properties. Contrasting trends between magnetic calculations and non-magnetic calculations are discussed in conjunction with the nature of localized moments in the metallic ferromagnetism at high temperatures. Implications on the optimal heat treatment temperature to maximize the intrinsic properties are discussed referring to the experimental Curie temperature.

cond-mat.mtrl-sci↗

arXiv2016

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

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.

cond-mat.mtrl-sci↗

arXiv2016

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

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.

cond-mat.mtrl-sci↗

arXiv2015

Robust flat bands in RCo5 (R=rare earth) compounds

The mechanism to realize the peculiar flat bands generally existing in RCo5 (R=rare earth) compounds is clarified by analyzing the first-principles band structures and the tight-binding model. These flat bands are constructed from the localized eigenstates, the existence of which is guaranteed by the partial cancelation between the intersite hopping amplitudes among the Co-3d states at the Kagome sites and those between the Kagome and honeycomb sites. Their relative positions to other bands can be controlled by varying the lattice parameters keeping their dispersion almost flat, which suggests the possibility of flat-band engineering.

cond-mat.mtrl-sci↗

arXiv2014

Ab initio Disordered Local Moment Approach for a Doped Rare-Earth Magnet

Following the finite-temperature ab initio calculation framework based on the relativistic disordered local moments, we computationally demonstrate the possibility of doping-enhanced coercivity at high-temperatures, taking YCo$_5$ as a working material in order to extract the $3d$-electron part of the electronic structure of the rare-earth permanent magnets. Alkaline-earth dopants are shown to be the candidates to realize the proposed phenomenon.

cond-mat.mtrl-sci↗

arXiv2014

Improvement of magnetic hardness at finite temperatures: ab initio disordered local moment approach for YCo$_5$

Temperature dependence of the magnetocrystalline anisotropy energy and magnetization of the prototypical rare-earth magnet YCo$_5$ is calculated from first principles, utilizing the relativistic disordered local moment approach. We discuss a strategy to enhance the finite-temperature anisotropy field by hole doping, paving the way for an improvement of the coercivity near room temperature or higher.

cond-mat.mtrl-sci↗

arXiv2011

Multiple Quantum Phase Transitions of Plutonium compounds

We show by quantum Monte Carlo simulations of realistic Kondo lattice models derived from electronic--structure calculations that multiple quantum critical points can be realized in Plutonium--based materials. We place representative systems including PuCoGa5 on a realistic Doniach phase diagram and identify the regions where the magnetically mediated superconductivity could occur. Solution of an inverse problem to restore the quasiparticle renormalization factor for f-electrons is shown to be sufficiently good to predict the trends among Sommerfeld coefficients and magnetism. Suggestion on the possible experimental verification for this scenario is given for PuAs.

cond-mat.str-el↗

arXiv2010

Dimensional trend in CePt2In7, Ce-115 compounds, and CeIn3

We present realistic Kondo-lattice simulation results for the recently-discovered heavy-fermion antiferromagnet CePt2In7 comparing with its three-dimensional counterpart CeIn3 and the less two-dimensional ones, Ce-115's. We find that the distance to the magnetic quantum critical point is the largest for CeIn3 and the smallest for Ce-115's, and CePt2In7 falls in between. We argue that the trend in quasi-two-dimensional materials stems from the frequency dependence of the hybridization between Cerium 4f-electrons and the conduction bands.

cond-mat.str-el↗