Symmetry breaking and magnetic structural transformations in hexagonal Mn₃Ga

Mn₃X (X=Sn or Ge) crystallizing in hexagonal structure with space group P63/mmc have recently been identified as a new class of magnetic Weyl semimetals. In this talk, I will present the structural and magnetic transitions on isostructural Mn₃Ga compound using neutron powder diffraction technology and density functional theory (DFT). A triangular antiferromagnetic order with moment perpendicular to the crystalline axes was found below T_N≈480 K. Upon further cooling to T_d≈290 K, we found a lattice symmetry breaking to a monoclinic structure with space group P21/m. This is distinct from Mn₃X (X=Sn or Ge) compounds, which retain their hexagonal structure down to zero K. Accompanied with the symmetry breaking at T_d, a magnetic structure undergoes a dramatic transformation, evolving into a highly distorted triangular antiferromagnetic order, with a noticeable net ferromagnetic component. This reveals the nature of the magneto-structural transition at T_d. Notably, the T_d temperature in our sample is much higher than the previously reported value of 140 K. We ascribe this to the nonstoichiometric nature of our sample, with a refined composition of Mn_2.934Ga. Given that the topological Hall effect is observed only for temperatures below T_d, our findings suggest the possibility of achieving this intriguing phenomenon near room temperature in our sample. Using DFT calculations, we examined the stable magnetic structure for both the high temperature undistorted phase and the low temperature distorted phase. This study unveils an intimate interplay between lattice, spins and Hall effect in Mn₃Ga.