Short range order in the quantum XXZ honeycomb lattice material BaCo$_2$(PO$_4$)$_2$

TL;DR

This study investigates the highly frustrated quasi-2D magnetic correlations in the honeycomb lattice compound $ ext{BaCo}_2( ext{PO}_4)_2$, revealing short-range order, competing magnetic phases, and persistent high-energy excitations indicative of strong in-plane correlations.

Contribution

The paper provides detailed experimental characterization of magnetic correlations and dynamics in $ ext{BaCo}_2( ext{PO}_4)_2$, identifying multiple short-range orders and constraining theoretical models of its magnetic interactions.

Findings

Short-range quasi-2D magnetic order established below 6 K and 3.5 K.

Two incompatible short-range magnetic orders with distinct correlation lengths.

High-energy magnetic excitations persist up to 40 K, indicating strong in-plane correlations.

Abstract

We present observations of highly frustrated quasi two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the S$_{eff}$ = 1/2 compound $\gamma$-BaCo$_2$(PO$_4$)$_2$ ($\gamma$-BCPO). Specific heat shows a broad peak comprised of two weak kink features at $T_{N1} \sim$ 6 K and $T_{N2} \sim$ 3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below $T_{N1}$ and $T_{N2}$, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length $\xi_c = 60\pm2$ \AA \ ($T_{N1}$) and in quasi-2D helical domains with $\xi_h = 350 \pm 11$ \AA \ ($T_{N2}$). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on $\gamma$-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ $J_1$-$J_2$-$J_3$ model with ferromagnetic nearest-neighbor exchange $J_1$ are favored, both near regions of high classical degeneracy. High energy coherent excitations ($\sim 10$ meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above $T_N$. These data show that $\gamma$-BCPO is a rare highly frustrated, quasi-2D S$_{eff}$ = 1/2 honeycomb lattice material which resists long range magnetic order and spin freezing.