Magnetic and Structural Properties of Rapidly Quenched Tetragonal Mn $_{3 - {\rm x}}$ Ga Nanostructures

Nanostructured Mn_3-x Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D0₁₉ and cubic L2₁ Heusler crystal structures based on the concentration of Mn in Mn_3-x Ga. Upon vacuum-annealing the samples at 450 °C for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D0₂₂ structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn_1.9 Ga and Mn_2.1 Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn_2.6 Ga and Mn_3.0 Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn_3-x Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn_2.6 Ga and Mn_3.0 Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn_3-x Ga. The maximum magnetization of 57 emu/g (300 emu/cm³) and the coercivity of 6.5 kOe were measured in the Mn_1.9 Ga and Mn_3.0 Ga ribbons, respectively.