Quasi- and approximant-crystalline Al-Mn-Ge alloys with high coercive force

The compositional dependences of structure and magnetization for melt-spun Al-Mn-Ge alloys were examined. Metastable icosahedral(i-) and decagonal(d-) phases were found in the concentration ranges of 20 to 25 at%Mn, 0 to 20 at%Ge and 25 to 35 at%Mn, 15 to 20 at%Ge, respectively. A decagonal approximant (ap-) phase with orthorhombic (a=1.48 nn, b=1.24 nm, c=1.25 nm) structure was also formed in the range of 35 to 45 at%Mn and 15 to 30 at%Ge in as-quenched state. The ap-phase was identified as an equilibrium phase in the vicinity of 35 at%Mn and 25 at%Ge. The ap-phase appears by the peritectic reaction of liquid (L) + gamma(Al8Mn5) --> ap-phase. A large single ap-phase was synthesized by the Czochralski method and had a large magnetic anisotropy with an easy magnetization axis along the quasiperiodic direction and a hard magnetization axis along the periodic direction. By the grain size refinement of the ap-and d-phases with a large magnetization anisotropy, the intrinsic coercive force (iH(c)) increased significantly. High iH(c) of 504 kA/m (6.3 kOe), residual magnetization (I-i) of 15.1 mu Wbm/kg (12 emu/g) and magnetization of 21.4 mu Wbm/kg (17 emu/g) at 1430 A/m (I-1430) were obtained for the finely mixed structure consisting of ap-and d-phases with a size of about 200 nm which was prepared by annealing the decagonal melt-spun ribbon.