Masterthesis: Ab Initio Magnetic Properties of Rare-Earth Lean Nd-based Hard Magnets

TL;DR

This study uses density functional theory to explore Nd-based magnets with reduced rare-earth content, focusing on magnetic properties and stability, aiming to develop RE-lean permanent magnets with competitive performance.

Contribution

It systematically investigates the magnetic properties of Nd-lean (Nd,X)Fe$_{11}$Ti compounds, including effects of Ti concentration and nitrogenation, providing new insights into RE-lean magnet design.

Findings

(Nd,Y)Fe$_{11}$Ti has $|BH|_{max}$ of 384 kJ/m$^3$ and $T_C$ of 595 K.

(Nd,Ce)Fe$_{11}$Ti has $|BH|_{max}$ of 365 kJ/m$^3$ and $T_C$ of 593 K.

Both Nd-lean magnets outperform Sm$_2$Co$_{17}$ and Nd$_2$Fe$_{14}$B in certain magnetic properties.

Abstract

Due to the resource criticality of rare-earths (RE), an alternative to the well-known Nd$_2$Fe$_{14}$B magnets with a lower amount of critical elements is required. In this work, density functional theory (DFT) calculations to investigate the influence of partial Nd substitution with more abundant elements (X: Y and Ce) in ThMn$_{12}$-type (Nd,X)Fe$_{11}$Ti compounds were performed. In order to have a systematic understanding, the intrinsic magnetic properties such as the saturation magnetization $M_S$, Curie temperature $T_C$ and magnetocrystalline anisotropy energy, are screened starting from binaries RFe$_{12}$ (R: Y, Ce and Nd), and only considering the spin magnetic contribution in case of the $f$ electrons. Ti is considered for the thermodynamic stabilization and different concentrations of Ti are taken into account for ternaries RFe$_{12-y}$Ti$_y$ and quaternaries (Nd,X)Fe$_{12-y}$Ti$_y$ (0.5$\le$ $y$ $\le$1). In addition, the effect of nitrogenation is examined for each considered compound. In case of (Nd,Y)Fe$_{11}$Ti, $|BH|$$_{max}$ is found to be 384 kJ/m$^3$ and $T_C$ is calculated to be 595 K. Similarly, $|BH|$$_{max}$ and $T_C$ are calculated to be 365 kJ/m$^3$ and 593 K for (Nd,Ce)Fe$_{11}$Ti magnet, respectively. Both 50 % Nd-lean magnets exhibit higher $|BH|$$_{max}$ compared to Sm$_2$Co$_{17}$ and $T_C$ than Nd$_2$Fe$_{14}$B. For both cases, the theoretical magnetic hardness factor $\kappa$ is calculated to be 1.20, which qualifies them as good candidates for RE-lean permanent magnets.