Noncollinear 2k antiferromagnetism in the Zintl semiconductor Eu5In2Sb6

Eu5In2Sb6 is an orthorhombic nonsymmorphic small band gap semiconductor with three distinct Eu2+ sites and two low-temperature magnetic phase transitions. The material displays one of the greatest (negative) magnetoresistances of known stoichiometric antiferromagnets and belongs to a family of Zintl materials that may host an axion insulator. Using single crystal neutron diffraction, we show that the TN1 = 14 K second-order phase transition is associated with long-range antiferromagnetic order within the chemical unit cell (k1 = (000)). Upon cooling below TN1, the relative sublattice magnetizations of this structure vary until a second-order phase transition at TN2 = 7 K that doubles the unit cell along the e axis (k2 = (0021)). We show the anisotropic susceptibility and our magnetic neutron diffraction data are consistent with magnetic structures described by the I'3 irreducible representation with the staggered magnetization of the k1 and k2 components polarized along the b and a axis, respectively. As the k2 component develops, the amplitude of the k1 component is reduced, which indicates a 2k noncollinear magnetic structure. Density functional theory is used to calculate the energies of these magnetic structures and to show the k1 phase is a metal so TN1 is a rare example of a unit-cell-preserving second-order phase transition from a paramagnetic semiconductor to an antiferromagnetic metal. DFT indicates the transition at TN2 to a doubled unit cell reduces the carrier density of the metal.