Disorder and magnetic excitations in CaCrxFe2−xO4(x=0,0.5)

Polycrystalline $\mathrm{CaF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{CaC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{1.5}{\mathrm{O}}_{4}$ have been investigated by elastic and inelastic neutron scattering. In agreement with previous reports, $\mathrm{CaF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ undergoes two magnetic transitions, first to a $B$ phase, below ${T}_{\mathrm{NB}}=200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, then to an $A$ phase, below ${T}_{\mathrm{NA}}=175\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, while substituted $\mathrm{CaC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{1.5}{\mathrm{O}}_{4}$ undergoes a magnetic transition to the $B$ phase only, at ${T}_{\mathrm{N}}=125\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Each phase corresponds to staggered antiferromagnetic chains coupled either ferromagnetically $(A$ phase) or antiferromagnetically $(B$ phase). In the $A$ phase of $\mathrm{CaF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$, inelastic scattering measurements show clearly defined gapped spin waves, which can be modeled with classical calculations, based on a simple exchange Hamiltonian following the topology of the crystal structure. In contrast, in the $B$ phase of both compounds, the interpretation of the excitation spectrum evades completely the classical approach, even at low temperature. These results are interpreted based on an interchain exchange close to the threshold between ferromagnetic and antiferromagnetic bonding geometry. This induces random interchain coupling, thus creating magnetic exchange disorder whose dominating effect is to blur out the magnetic excitation spectrum of the $B$ phase. A magnetoelastic effect, through which the interchain coupling becomes sizably ferromagnetic, and which is not observed in $\mathrm{CaC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{1.5}{\mathrm{O}}_{4}$, stabilizes the $A$ phase at low temperature in $\mathrm{CaF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$.