Phase relations and extrinsic magnetic properties of Sm-(Fe,Co)-Ti-(Ga)-based alloys for ThMn12-type permanent magnets

Realizing a non-ferromagnetic intergranular phase for the ThMn12-type Sm(Fe,Co,Ti)(12) compound is a prerequisite for developing high coercive magnets. In this work, we demonstrate that the addition of Ga into the Sm(Fe,Co)-Ti system results in the formation of non-ferromagnetic Sm-Ga-rich intergranular phases with good wettability on Sm(Fe,Co,Ti)(12) grains. In the Sm-(Fe,Co)-Ti system, when the alloy composition was varied to the Sm-rich region as Sm1+w(Fe0.8Co0.2)(11)Ti (w = 0-0.7), soft ferromagnetic C15-type Sm(Fe,Co)(2) intergranular phase and TbCu7-type Sm(Fe,Co,Ti)(similar to 7) phase formed along with the ThMn12-type main phase in the as-cast alloy. We demonstrated that by introducing Ga, the formation of these secondary soft ferromagnetic phases can be hindered as the excess Sm and Ga formed non-ferromagnetic Ba5Si3-type Sm5Ga3 and TII-type SmGa intergranular phases with an excellent wettability on Sm(Fe,Co,Ti)(12) grains. Fine Sm(Fe,Co,Ti)(12) grains of 1-2 mu m, well isolated by the Sm-Ga-rich intergranular phase, were realized by melt-spinning the Ga-doped alloy and the maximum coercivity obtained was 0.5 T. According to the detailed microstructure characterizations, the weak-links to the coercivity of this novel microstructure of well-isolated and fine Sm(Fe,Co,Ti)(12)-based grains is the presence of defects at the interfaces which could lead to a locally reduced magneto-crystalline anisotropy.