# Ba8MnNb6O24: a model two-dimensional spin-5/2 triangular lattice antiferromagnet

## TL;DR

This study synthesizes and characterizes Ba8MnNb6O24, a two-dimensional spin-5/2 triangular lattice antiferromagnet, revealing a 120-degree magnetic order driven by weak easy-plane anisotropy, with detailed experimental and theoretical analysis.

## Contribution

It provides the first detailed experimental and theoretical characterization of Ba8MnNb6O24 as a model 2D spin-5/2 triangular antiferromagnet with specific magnetic ordering properties.

## Key findings

- Exhibits 120-degree magnetic order below 1.45 K.
- Weak easy-plane anisotropy influences magnetic ordering.
- Inter-layer interactions are negligible due to large separation.

## Abstract

We successfully synthesized and characterized the triangular lattice anitferromagnet Ba$_8$MnNb$_6$O$_{24}$, which comprises equilateral spin-5/2 Mn$^{2+}$ triangular layers separated by six non-magnetic Nb$^{5+}$ layers. The detailed susceptibility, specific heat, elastic and inelastic neutron scattering measurements, and spin wave theory simulation on this system reveal that it has a 120 degree ordering ground state below T$_N$ = 1.45 K with in-plane nearest-neighbor exchange interaction ~0.11 meV. While the large separation 18.9 A between magnetic layers makes the inter-layer exchange interaction virtually zero, our results suggest that a weak easy-plane anisotropy is the driving force for the k$_m$ = (1/3 1/3 0) magnetic ordering. The magnetic properties of Ba$_8$MnNb$_6$O$_{24}$, along with its classical excitation spectra, contrast with the related triple perovskite Ba$_3$MnNb$_2$O$_9$, which shows easy-axis anisotropy, and the iso-structural compound Ba$_8$CoNb$_6$O$_{24}$, in which the effective spin-1/2 Co$^{2+}$ spins do not order down to 60 mK and in which the spin dynamics shows sign of strong quantum effects.