Magnetic ground state and multi-magnons in the spin chain chalcogenide RbFeS2

Stefano Agrestini^1*, Sahil Tippireddi¹, Miriam Garcia-Fernandez¹, Ke-Jin Zhou¹

¹Physical Science, Diamond Light Source, Didcot, UK

* Presenter:Stefano Agrestini, email:stefano.agrestini@diamond.ac.uk

Ternary metal chalcogenides like RbFeS₂ are quasi-one-dimensional antiferromagnets with a high T_N (~250 K), very unusual, and poorly understood magnetic properties: including an “anti-Curie–Weiss” susceptibility, an almost vanishing magnetic contribution to the specific heat, drastically reduced magnetic moment, etc. The spin state of the Fe3+ ion is highly controversial with all three spin states S=1/2, 3/2, and 5/2 being proposed in literature. Here, we report a RIXS investigation of single crystal RbFeS₂. The comparison of the dd excitations with full-multiplet configuration-interaction calculations allowed us to obtain a full characterization (S=5/2 spin state, crystal field, orbital occupation, and hybridization strength) of the electronic and magnetic ground state of RbFeS₂. A RIXS scan along the L-direction of the reciprocal space show single-magnons highly dispersive with a spin wave-like behaviour. Most interestingly, we also observe the presence of collective quadrupolar magnons. Detecting higher-rank magnons is challenging and the present results shows the unique capability of RIXS technique. In RbFeS2 the quadrupolar magnons exhibits a characteristic dispersion in antiphase with the single-magnons. A discussion of the localized or itinerant nature of these exotic excitations will be presented.

Keywords: spin state, magnon, multi-magnon, RIXS, chalcogenide