Magnetism and electron transport of Mn_yGa (1 < y < 2) nanostructures

Nanostructured Mn_yGa ribbons with varying Mn concentrations including Mn_1.2Ga, Mn_1.4Ga, Mn_1.6Ga, and Mn_1.9Ga were prepared using arc-melting and melt-spinning followed by a heat treatment. Our experimental investigation of the nanostructured ribbons shows that the material with y = 1.2, 1.4, and 1.6 prefers the tetragonal L1₀ structure and that with y = 1.9 prefers the D0₂₂ structure. We have found a maximum saturation magnetization of 621 emu/cm ³ in Mn_1.2Ga which decreases monotonically to 300 emu/cm³ as y reaches 1.9. Although both the L1₀- and D0₂₂-Mn_yGa samples show a high Curie temperature (T _c) well above room temperature, the value of T_c decreases almost linearly from 702 K for Mn_1.9Ga to 551 K for Mn _1.2Ga. All the ribbons are metallic between 2 K and 300 K but the Mn_1.2Ga also shows a resistance minimum near 15 K. The observed magnetic properties of the Mn_yGa ribbons are consistent with the competing ferromagnetic coupling between Mn moments in the regular L1 ₀-MnGa lattice sites and antiferromagnetic coupling with excess Mn moments occupying Ga sites.