Ga: Crossover between cubic and tetragonal phase upon Co-substitution*

Structural and magnetic properties of Co substituted Mn_3-xCo_xGa (x = 0~1) is investigated using ab initio calculations. Ferrimagnetic Mn₃Ga possesses two inequivalent Mn sites, octahedral Mn-I and tetrahedral Mn-II, where Co prefers Mn-II site, which leads to intriguing physics. Notably, structural transition, from tetragonal to cubic phase, occurs around x=0.6. Unlike other Co compounds, local moment of Co nearly vanishes in tetragonal phase, which results in decreasing total moment with Co concentration. In contrast, in cubic phase Co retains usual moment: Magnetic moment increases linearly with x obeying Slater-Pauling rule of cubic Heusler, M = N_V - 24, where M is the magnetization, N_V is numbers of valance electrons. In particular, the integral M, a signature of half-metallicity, emerges in cubic phase for x=0.5 and 1.0. Furthermore, the vanishing moment of Co for tetragonal phase is analyzed phenomenologically using Heisenberg model, where the exchange coefficients of two Mn sites tends to compensate.