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

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 similar or equal to 5 K marks the onset of the antiferromagnetic order with the propagation vector and the ordered moment of 4.33 +/- 0.08 mu(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.