The Magnetic and Crystal Structure of Mn_xGa (1.15 ≤ x ≤ 1.8) Alloys

Neutron powder diffraction patterns measured above T _C have been used to determine the location of the excess Mn in Mn_xGa (1.15 ≤ x ≤ 1.8). This information has then been used to constrain the fits to neutron powder diffraction patterns measured at ambient temperature and so determine unambiguously the Mn moments in this system. We find that Mn randomly occupies the two Ga sites (2a and 2b) in the I4/mmm structure and propose that it is more appropriate to use a simpler structure based on the P4/mmm space group with a reduced unit cell. In this structure the two Ga sites are formally equivalent (they occupy the 1a site while Mn occupies the 1d site). Our experimental observations are supported by DFT calculations. Below T _C we find that the Mn(1d) moment is constant at 2.45(3) μ _B , while Mn on the 1a site carries a slightly larger moment (~3 μ _B ) that is coupled antiparallel to the Mn(1d) moments, leading to the observed drop in magnetisation with increasing Mn content in Mn_xGa.