Spin wave damping by valence fluctuations in the double layer perovskite YBaFe₂O₅

Sung Chang (Ames Laboratory)

One of the first instances in which strong electron correlations were invoked was to explain the Verwey transition (a metal-insulator transition to a charge ordered ground state), first observed in magnetite. A new class of Verwey compounds RBaM₂O₅ (R = rare-earth, M = transition metal) are based on the perovskite structure with a doubled unit cell and a layer of oxygen vacancies. The structure consists of pyramids of five-coordinated M-sites, and more importantly the M-site is mixed valent in the stoichiometric formula unit (with an average valence of +2.5). I will present results of inelastic neutron scattering experiments on polycrystalline YBaFe₂O₅. In the charge ordered insulating ground state, spin dynamics in YBaFe₂O₅ can be well understood in terms of eg superexchange interactions. However, a dramatic shift in the spin wave spectrum at the Verwey transition with a softening and severe damping of zone-boundary spin waves in the valence-mixed but still antiferromagnetically ordered intermediate temperature phase indicates strong interactions between spin waves and charge fluctuations.

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