Experimental determination of the magnetic interactions of frustrated Cairo pentagon lattice materials

TL;DR

This study uses neutron scattering and magnetisation measurements to analyze magnetic interactions in frustrated Cairo pentagon lattice materials, revealing strong antiferromagnetic couplings and significant single-ion anisotropy.

Contribution

It provides the first detailed determination of exchange interactions and anisotropy in Cairo lattice magnets using combined experimental and theoretical methods.

Findings

Identification of a strongly antiferromagnetic, highly coupled dimer interaction.

Detection of a non-negligible single-ion anisotropy causing a spin gap.

Observation of a spin-flop transition in high field magnetisation.

Abstract

We present inelastic neutron scattering measurements of the Cairo pentagon lattice magnets Bi$_2$Fe$_4$O$_9$ and Bi$_4$Fe$_5$O$_{13}$F, supported by high field magnetisation measurements of Bi$_2$Fe$_4$O$_9$. Using linear spin wave theory and mean field analyses we determine the spin exchange interactions and single-ion anisotropy in these materials. The Cairo lattice is geometrically frustrated and consists of two inequivalent magnetic sites, both occupied by Fe$^{3+}$ ions and connected by two competing nearest neighbour interactions. We found that one of these interactions, coupling nearest neighbour spins on the three-fold symmetric sites, is extremely strong and antiferromagnetic. These strongly coupled dimers are then weakly coupled to a framework formed from spins occupying the other inequivalent site. In addition we found that the Fe$^{3+}$ $S=5/2$ spins have a non-negligible single-ion anisotropy, which manifests as a spin anisotropy gap in the neutron spectrum and a spin-flop transition in high field magnetisation measurements.