Neutron diffraction study and theoretical analysis of the antiferromagnetic order and diffuse scattering in the layered Kagome system CaBaCo$_2$Fe$_2$O$_7$

TL;DR

This study combines neutron diffraction experiments and theoretical modeling to investigate the antiferromagnetic order, diffuse scattering, and spin correlations in the layered Kagome system CaBaCo$_2$Fe$_2$O$_7$, revealing complex magnetic behavior and degeneracy.

Contribution

It provides the first detailed experimental and theoretical analysis of magnetic order, diffuse scattering, and spin correlations in CaBaCo$_2$Fe$_2$O$_7$, highlighting the coexistence of order and disorder.

Findings

Long-range $\,\sqrt{3} imes \,\sqrt{3}$ antiferromagnetic order below 160 K.

Presence of diffuse scattering indicating partial spin order.

Detection of long-period cycloidal spin correlations due to broken inversion symmetry.

Abstract

The hexagonal swedenborgite, CaBaCo$_2$Fe$_2$O$_7$, is a chiral frustrated antiferromagnet, in which magnetic ions form alternating Kagome and triangular layers. We observe a long range $\sqrt{3} \times \sqrt{3}$ antiferromagnetic order setting in below $T_N = 160$ K by neutron diffraction on single crystals of CaBaCo$_2$Fe$_2$O$_7$. Both magnetization and polarized neutron single crystal diffraction measurements show that close to $T_N$ spins lie predominantly in the $ab$-plane, while upon cooling the spin structure becomes increasingly canted due to Dzyaloshinskii-Moriya interactions. The ordered structure can be described and refined within the magnetic space group $P31m^\prime$. Diffuse scattering between the magnetic peaks reveals that the spin order is partial. Monte Carlo simulations based on a Heisenberg model with two nearest-neighbor exchange interactions show a similar diffuse scattering and coexistence of the $\sqrt{3} \times \sqrt{3}$ order with disorder. The coexistence can be explained by the freedom to vary spins without affecting the long range order, which gives rise to ground-state degeneracy. Polarization analysis of the magnetic peaks indicates the presence of long-period cycloidal spin correlations resulting from the broken inversion symmetry of the lattice, in agreement with our symmetry analysis.