Pentagonal All-in-All-out Antiferromagnetic Chains in NaMn₆Bi₅

Quasi-one-dimensional systems have garnered significant attention owing to the exotic properties they can host including superconductivity, charge density waves, topological spin excitations and more. Pressure-induced superconductivity has been realized in a new family of Mn-based Q1D materials, AMn₆Bi₅ (A = K, Rb, Cs, Na), with unique [Mn₆Bi₅]⁻¹ double-walled columns. The smallest countercation Na⁺ yields the highest chemical pressure experienced in this family, reducing the Mn interatomic bond lengths and enhancing the metallicity and magnetic frustration within the Mn pentagonal antiprisms, thus, driving NaMn₆Bi₅ closer to the high-pressure superconducting phase. Distinct from the single magnetic transition in other family members, NaMn₆Bi₅ goes through multiple magnetic transitions at T_N1 ~88 K, T_N2 ~52 K and T_N3 ~48 K. In this talk I will present the findings of the unique low temperature, below T_N3 ~48 K, noncolinear “all-in-all-out” pentagonal antiferromagnetic order and high temperature in-plane moment dispersed pentagon phase in NaMn₆Bi₅ determined from single crystal neutron diffraction. The low temperature “all-in-all-out” state exhibits spins pointing all towards or away from the center of the pentagon and alternating down the Mn pentagonal antiprism columns along the b axis. The innermost central Mn-site continuously shows no/negligible ordered moment, resulting from the magnetic frustration within the Mn pentagonal antiprisms and nearly metallic bond distances. High pressure X-ray diffraction up to 18.5 GPa revealed no additional lattice transition, indicating the magnetic variation under pressure is highly relevant to the high-pressure superconducting phase found in this family. This investigation has, therefore, shed new light on the rare one-dimensional Mn-based superconductors.