Zigzag spin chains in the spin-5/2 antiferromagnet Ba${}_2$Mn(PO${}_4{)}_2$

Abstract

Magnetic properties and magnetic structure of the Ba${}_2$Mn(PO${}_4$)${}_2$ antiferromagnet featuring frustrated zigzag chains of $S=\frac{5}{2}$ Mn${}^{2+}$ ions are reported based on neutron diffraction, density-functional band-structure calculations, as well as temperature- and field-dependent measurements of the magnetization and specific heat. A magnetic transition at $T_N\simeq 5$\,K marks the onset of the antiferromagnetic order with the propagation vector $\mathbf{k}=(\frac{1}{2}0\frac{1}{2})$ and ordered moment of $4.33\pm 0.08{\mu }_B$/Mn${}^{2+}$ at 1.5\,K, pointing along the $c$ direction. Direction of the magnetic moment is chosen by the single-ion anisotropy, which is relatively weak compared to the isostructural Ni${}^{2+}$ compound. Geometrical frustration has strong impact on thermodynamic properties of Ba${}_2$Mn(PO${}_4{)}_2$, but manifestations of the frustration are different from those in Ba${}_2$Ni(PO${}_4{)}_2$, 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.

12 pages, 12 figures