The ${\mathrm{Fe}}^{3+}$ lattice in the ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ compound is found to materialize the first analogue of a magnetic pentagonal lattice. Because of its odd number of bonds per elemental brick, this lattice, subject to first neighbor antiferromagnetic interactions, is prone to geometric frustration. The ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ magnetic properties have been investigated by macroscopic magnetic measurements and neutron diffraction. The observed noncollinear magnetic arrangement is related to the one stabilized on a perfect tiling as obtained from a mean field analysis with direct space magnetic configuration calculations. The peculiarity of this structure arises from the complex connectivity of the pentagonal lattice, a novel feature compared to the well-known case of triangle-based lattices.

• Received 27 August 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.267204