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

TL;DR

This study investigates the magnetic properties, structure, and phase diagram of the frustrated zigzag spin-5/2 antiferromagnet Ba$_2$Mn(PO$_4)_2$ using neutron diffraction, calculations, and magnetization measurements, revealing complex magnetic behavior influenced by frustration and anisotropy.

Contribution

First comprehensive analysis of the magnetic structure and phase diagram of Ba$_2$Mn(PO$_4)_2$, highlighting the effects of geometrical frustration and weak single-ion anisotropy.

Findings

Antiferromagnetic transition at T_N ≈ 5 K with a specific propagation vector.

Observation of a spin-flop transition around 2.5 T.

Detailed magnetic phase diagram in the H-T plane.

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 ${\mathbf k} = (\frac12\, 0\, \frac12)$ and ordered moment of $4.33\pm0.08~\mu_B$/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.