Complex magnetic behavior of the sawtooth Fe chains in Rb ₂ Fe ₂ O(AsO ₄) ₂

Results of magnetic field and temperature-dependent neutron-diffraction and magnetization measurements on oxy-arsenate Rb₂Fe ₂O(AsO₄)2 are reported. The crystal structure of this compound contains pseudo-one-dimensional [Fe₂O₆]∞ sawtoothlike chains, formed by corner-sharing isosceles triangles of Fe3+ ions occupying two nonequivalent crystallographic sites. The chains extend infinitely along the crystallographic b axis and are structurally confined from one another via diamagnetic (AsO₄)3- units along the a axis and Rb+ cations along the c-axis direction. Neutron-diffraction measurements indicate the onset of a long-range antiferromagnetic order below approximately 25 K. The magnetic structure consists of ferrimagnetic chains which are antiferromagnetically coupled with each other. Within each chain, one of the two Fe sites carries a moment which lies along the b axis, while the second site bears a canted moment in the opposite direction. Externally applied magnetic field induces a transition to a ferrimagnetic state, in which the coupling between the sawtooth chains becomes ferromagnetic. Magnetization measurements performed on optically aligned single crystals reveal evidence for an uncompensated magnetization at low magnetic fields that could emerge from a phase-segregated state with ferrimagnetic inclusions or from antiferromagnetic domain walls. The observed magnetic states and the competition between them are expected to arise from strongly frustrated interactions within the sawtooth chains and relatively weak coupling between them.