Paper AC+LS+MI-MoM10
Optimizing the Magnetic Performance of Tetragonal ReFe_12-xM_x Phases by First Principles Computational Simulations

Monday, October 21, 2019, 11:20 am, Room A215

The increase of environmentally friendly energy production is coupled to an increasing demand for new magnetic materials. Especially, new Rare earth (Re) lean permanent magnets are highly sought after as possible replacement for high-performance magnets based on Nd-Fe-B and Dy to limit costs and supply risk. In this context the tetragonal 1:12 phase (TmMn₁₂) which contains 35% less Re than commercial Nd-Fe-B magnets are rediscovered. To stabilize this phase with light Re and Fe instead of Mn a nonmagnetic phase stabilizing element is needed but this degrades the magnetic performance.

To identify new 1:12 phases being suitable for permanent magnet applications materials design based on computational simulations has become an important tool. Here we focus on ReFe_12-xM_x. with Re = Y, Ce, Nd, Sm and M = Ti and V. We use state of the art density functional theory methods (VASP; full potential LMTO (RSPt)). The phase stability and the magnetic properties were calculated depending on the M concentration. Aiming to reduce the Re amount we monitor the performance depending on the Nd/Y ratio.

The key quantities are the magnetocrystalline anisotropy (MAE) and the magnetization. To capture the correct magnetic behavior, it is crucial to describe the localization of the 4f electron properly for each Re. While for Sm-based systems the spin-polarized core approximation is sufficient to describe the localized 4f electrons, it fails for Nd, e.g. the low temperature MAE of NdFe₁₁Ti would be uniaxial instead of conic. Using a DFT+U approach with U = 5 eV, J = 1.1 eV for NdFe₁₁Ti reproduces the experimentally observed behavior. Ce is special since the uniaxial MAE of CeFe₁₁Ti is obtained independent from the treatment of the 4f electron. However, an analysis of the hybridization function analogue to [1] shows that a spin polarized core approximation is more appropriate for Ce-based 1:12 phases. For a deeper insight additional studies are carried out to examine the crystal field splitting.

With SmFe₁₁V system a new phase was found leading to an increase of the magnetization by 17% compared to the commonly used concentrations of V. In view of the MAE a replacement of Nd by Y turned out to be preferable over a reduction of Ti. MAE values of 1.3 MJ/m³ ((NdY)Fe₁₁Ti) and 1.7 MJ/m³ (SmFe₁₁V) are predicted [2]. The latter could already be verified in recent experiments [2].

Supported by the European Research Project NOVAMAG, Swedish Foundation for Strategic Research and STandUP for Energy.

[1] H.C.Herper et al., Phys. Rev. Materials1, 033802 (2017)

[2] A. M. Schönhöbel et al., JALCOM 786, 969 (2019)