Ferromagnetic Tetragonal ${\rm L}1_{0}$ -Type MnGa Isotropic Nanocrystalline Microparticles

This paper reports the structure and magnetic properties of ferromagnetic face-centered tetragonal L₁₀-type MnGa isotropic nanocrystalline microparticles prepared by high energy ball milling and subsequent annealing. The L₁₀-type microparticles with a nominal composition of Mn₅₆Ga₄₄ size mainly in the range of 10-50 μm, an average crystallite size in the range of 40-62 nm and a Curie temperature of 356°C. Post-annealing had little effect on the particle size and morphology but dramatically increased the hard magnetic properties. As the annealing temperature increased, coercivity ⁱH_c first increased from 4.8 kOe after annealing at 400°C for 20 min, to a maximum value of 6.2 kOe after annealing at 600°C for 20 min, and then decreased after annealing at 650°C. The increase in ⁱH_c is attributed to an improvement of crystallinity and chemical ordering of fct δ-MnGa phase while the decrease may be due to a too coarse crystallite size. Remanence M_r remained almost the same, in the range of 41-42 emu/g, for the samples annealed from 400 to 650°C for 20 min. These magnetic properties are comparable to the highest ones reported so far for ferromagnetic fct MnGa films grown by molecular beam epitaxy. After annealing at 700°C for 20 min, ⁱH_c decreased to 2.3 kOe and M_r to 10.9 emu/g due to both, an excessive grain growth, and re-precipitation of a nonmagnetic ζ - Mn₈Ga₅ phase. Annealing time increase from 10 to 90 min has little effect (within ~10%) on the magnetic properties for the samples annealed at 550°C.