Inelastic neutron scattering study of oxyanion-based compounds with sawtooth-chain lattice*

The sawtooth spin chain with its compelling triangle-based spin structure offers a fascinating playground for competing antiferromagnetic interactions. Theoretical investigations showed that for special relations between exchange interactions such systems can host flat-band magnons that are of great interest for the development of magnonics devices. To date, experimental realization of a magnetic sawtooth lattice has been limited to a handful of compounds. In real materials, such sawtooth topologies imply the presence of magnetic ions in at least two nonequivalent structural positions that creates an interplay between different magnetic order parameters and, consequently, a rich magnetic phase diagram. We recently undertook a systematic investigation of the magnetic properties of several sawtooth systems where magnetic chains are linked by nonmagnetic oxyanion groups such as AsO4 and MoO4. In this presentation, we will discuss the static and dynamic magnetic properties of two transition-metal sawtooth chain systems: Rb2Fe2O(AsO4)2 [1] and CsCo2(MoO4)2(OH) [2]. These compounds exhibit long-range magnetic order that consists of antiferromagnetically coupled ferrimagnetic chains. Within each chain, the magnetic moments located at the tip of the sawtooth are aligned collinearly along the b-direction (the chain direction), while the moments on the spine sites are reversely canted to form a zigzag pattern inside the plane of the triangular chain. For both compounds, applied magnetic fields induce transitions to ferrimagnetic states where the coupling between adjacent sawtooth chains changed from antiferromagnetic to ferromagnetic. Hamiltonian models describing the main magnetic interactions are proposed based on the observed low-energy spin-wave excitations from inelastic neutron scattering data.