arXiv subjects

Masako Ogura

Publications and source records attributed to Masako Ogura.

4 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↗

arXiv2022

First-principles Calculation of Magnetocrystalline Anisotropy of Y(Co,Fe,Ni,Cu)$_5$ Based on Full-potential KKR Green's Function Method

The performance of permanent magnets YCo$_5$ can be improved by replacing cobalt with other elements, such as iron, copper, and nickel. In order to determine its optimum composition, it is necessary to perform systematic theoretical calculations in a consistent framework. In this study, we calculated the magnetocrystalline anisotropy constant $K_{\rm u}$ of Y(Co$_{1-x-y}$Fe$_{x}$Cu$_{y}$)$_3$(Co$_{1-z}$Ni$_{z}$)$_2$ on the basis of the full-potential Korringa-Kohn-Rostoker Green's function method in conjunction with the coherent potential approximation. The calculated $K_{\rm u}$ of YCo$_5$ was smaller than the experimental value because of a missing enhancement due to orbital polarization. Although the value of $K_{\rm u}$ of Y(Co$_{1-x-y}$Fe$_{x}$Cu$_{y}$)$_3$(Co$_{1-z}$Ni$_{z}$)$_2$ was systematically underestimated compared to their experimental counterparts, the doping effect can be analyzed within a consistent framework. The results have shown that YFe$_3$Co$_2$ has much higher $K_{\rm u}=5.00$ MJ/m$^3$ than pristine YCo$_5$ ($K_{\rm u}=1.82$ MJ/m$^3$), and that nickel as a stabilization element decreases $K_{\rm u}$ and magnetization in YFe$_3$(Co$_{1-z}$Ni$_z$)$_2$. However, the anisotropy field of $z\sim0.5$ can compete with the value of YCo$_5$.

cond-mat.mtrl-sci↗

arXiv2020

First principles calculations of steady-state voltage-controlled magnetism: application to x-ray absorption spectroscopy experiment

Recent x-ray absorption experiments have demonstrated the possibility to accurately monitor the magnetism of metallic hetero-structures controlled via a time-independent perturbation caused for example by a static electric field. Using a first-principles, non-equilibrium Green function scheme, we show how the measured dichroic signal for the corresponding steady-state situation can be related to the underlying electronic structure and its response to the external stimulus. The suggested approach works from the infinitesimal limit of linear response to the regime of strong electric field effects, which is realized in present experimental high sensitivity investigations.

cond-mat.mtrl-sci↗

arXiv2011

Enhancement of magnetism of Fe by Cr and V

Enhancement of magnetism of Fe that occurs by alloying with Cr or V is discussed on the basis of first-principles electronic structure calculation. The d states of Fe next to Cr(V) are pushed down to the lower energy side compared to those of pure Fe due to the hybridization with Cr(V) d states, which leads a Co-like electronic structure for the Fe atom. This enhances magnetic moments of other Fe atoms and also the exchange couplings among them. The same mechanism works as well for the magnetism of Fe/Cr heterostructures. The idea is extended to design of a new type of antiferromagnets with high Néel temperature. Such heterostructures could be used to increase a magnetic anisotropy of permanent magnets.

cond-mat.mtrl-sci↗