Transition-Metal-Induced Uniaxial Anisotropy in YFe$_3$
Achieving uniaxial magnetocrystalline anisotropy while preserving high magnetization and thermal stability remains a central challenge in the development of Fe-rich permanent magnets. YFe$_3$ is a promising candidate owing to its saturation magnetization and Curie temperature, but its intrinsic easy-plane anisotropy limits its potential for permanent-magnet applications. Here, we show that selective transition-metal substitution drives an easy-plane-to-easy-axis transition in YFe$_3$. Systematic first-principles screening identifies V, Nb, and Ta as effective dopants, with YFe$_{2.33}$Ta$_{0.67}$ exhibiting a large uniaxial magnetocrystalline anisotropy energy of 2.3~MJ/m$^{3}$. Calculated exchange interactions and atomistic Monte Carlo simulations show that the enhanced anisotropy is achieved while preserving robust ferromagnetic order and high thermal stability. Micromagnetic simulations further reveal that YFe$_{2.33}$Ta$_{0.67}$ exhibits the highest coercivity and strongest resistance to irreversible demagnetization, whereas YFe$_{2.33}$V$_{0.67}$ achieves a comparable maximum energy product at 300~K. Experimental characterization of V-alloyed YFe$_3$ shows that the YFe$_3$ phase can be retained at low nominal V concentrations (1-3 at.\%) under the employed synthesis and heat-treatment conditions, whereas higher V contents lead to increasingly multiphase samples. Concentration-dependent MAE calculations reveal a sign reversal between approximately 11 and 14~at.\% V, marking the transition from easy-plane to uniaxial easy-axis anisotropy. Consistent with this concentration dependence, electron backscatter diffraction and Kerr microscopy reveal in-plane magnetic domain structures in the experimentally accessible low-V regime. Therefore, our work offers a promising approach to tuning the magnetocrystalline anisotropy of YFe$_3$ while retaining high Curie temperatures.