Composition Design And Structure Optimization For High-Performance ThMn₁₂-Type Rare-Earth Permanent Magnet

The heavy rare-earth free ThMn₁₂-type RFe₁₂-based permanent magnet exhibits a great potential to be an alternative for Nd-Fe-B magnets in the advanced magnet motors,because of its excellent intrinsic magnetic properties and thermal stability.However,due to the poor phase stability of RFe₁₂phase,a high content of non-magnetic elements are required to stabilize the phase structure,which leads to a significant decrease in the intrinsic magnetic properties,thus limiting its further industrialization.In this paper,the phase stability and magnetic properties of the RFe₁₂and RFe₁₁Ti alloys were theoretically analyzed by the first principles calculation method.In addition,the effects of substitution elements Y,Co and Ti on the phase stability and magnetic properties of Sm Fe₁₂phase were systematically investigated.What’s more,a spinodal decomposition is realized in the（Sm,Y）Fe₁₂alloy to construct a core-shell structure,which results in the enhancement of magnetic properties.The findings are as follows:（1）Phase stability,crystal structure,density of states,and intrinsic magnetic properties of RFe₁₂and RFe₁₁Ti（R=La,Ce,Pr,Nd,Sm,Y and Zr）were calculated by the first principles calculation method.The results show that the Sm Fe₁₂phase has the largest uniaxial anisotropy and Curie temperature,while YFe₁₂and Zr Fe₁₂have relatively good structural stability and high magnetic moment.The introduction of Ti atom may weaken the interaction between Fe-Fe and Fe-Ti atoms,thus resulting in the decreased total magnetic moments and Curie temperature.（2）The simultaneous enhancement of phase stability and saturation magnetization were achieved attributed to the synergistic effects of the Y,Co,and Ti substitutions.Firstly,the mixing enthalpyΔH_mixof the(Sm_1-nY_n)(Fe_0.8Co_0.2)₁₁Ti was calculated.The largest driving force for spinodal decomposition is obtained in n=0.25 alloy with the highest value ofΔH_mix.Furthermore,the formation energy and intrinsic magnetic properties of Sm_0.75Y_0.25(Fe_0.8Co_0.2)_12-lTi_lphases were calculated.The results depict that the saturation magnetization increases with the decreasing Ti content.The close values of formation energy are got in the l=0.65 and Sm Fe₁₁Ti phase,but a relatively high saturation magnetization of 1.52 T was obtained for l=0.65phase,by comparing with that of 1.39 T of Sm Fe₁₁Ti phase.（3）When l≥0.65,the spinodal decomposition takes place in Sm_0.75Y_0.25(Fe_0.8Co_0.2)_12-lTi_lalloys prepared by arc-melting followed by solution treatment.The spontaneous formed core–shell structure（Sm-rich shell and Y-rich core）is observed in the l=0.75 alloy,thus resulting in the optimized anisotropy field from the theoretical value of 8.24 T to the experimental value of 9.24 T.Furthermore,the magnetic reversal processes were characterized by the in-suit Lorentz transmission electron microscopy.The strong domain wall pinning by the magnetic-hardening Sm-rich shell contributes to the coercivity enhancement,which is reconstructed by micromagnetic simulation.This study provides a guidance for the preparation of ThMn₁₂-type permanent magnet with high phase stability and magnetic properties.