Zigzag spin chains in the spin-5/2 antiferromagnet Ba2Mn(PO4)2

Abstract

Magnetic properties and magnetic structure of the Ba₂Mn(PO₄)₂ antiferromagnet featuring frustrated zigzag chains of Mn²⁺ ions are reported based on neutron diffraction, density-functional band-structure calculations, as well as temperature- and field-dependent measurements of magnetization and specific heat. A magnetic transition at T_N ≃ 5 K marks the onset of the antiferromagnetic order with the propagation vector and the ordered moment of 4.33 ± 0.08μ_B/Mn²⁺ at 1.5 K, pointing along the c direction. The direction of the magnetic moment is chosen by single-ion anisotropy, which is relatively weak compared to the isostructural Ni²⁺ compound. Geometrical frustration has a strong impact on the thermodynamic properties of Ba₂Mn(PO₄)₂, but manifestations of the frustration are different from those in Ba₂Ni(PO₄)₂, where frustration by isotropic exchange couplings is minor, yet strong and competing single-ion anisotropies are present. A spin-flop transition is observed around 2.5 T. The evaluation of the magnetic structure from the ground state via the spin-flop state to the field-polarized ferromagnetic state has been revealed by a comprehensive neutron diffraction study as a function of magnetic field below T_N. Finally, a magnetic phase diagram in the H–T plane is obtained.