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

Abstract

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 iHc 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 iHc 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 Mr 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, iHc decreased to 2.3 kOe and Mr 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.