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

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.

10 pages, 9 figures

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• Mantid - Data Analysis and Visualization Package for Neutron Scattering and $\mu SR$ Experiments
• Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3