Inelastic neutron scattering study of the Verwey transition in YBaCo$_2$O$_5$

A new class of oxides with the chemical formula $R$Ba$M_2$O$_5$ ($R$~=~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). Here, we present results of inelastic neutron scattering experiments on polycrystalline YBaCo$_2$O$_5$, which orders antiferromagnetically at $T_{\mathrm{N}}$~=~330~K and is accompanied by a tetragonal to orthorhombic distortion. Below $T_{\mathrm{V}}$~=~220~K, orbital/charge ordering occurs as well as a change in the Co$^{2+}$ spin state from low to high spin. A dramatic change in the spin wave spectrum at $T_{\mathrm{V}}$, as well as a large damping of zone-boundary spin waves above $T_{\mathrm{V}}$ are similar to those observed in YBaFe$_2$O$_5$ and are discussed in terms of a coupling of magnetism and valence fluctuations.