Variety of elastic anomalies in an orbital-active nearly itinerant cobalt vanadate spinel

We perform ultrasound velocity measurements on a single crystal of nearly metallic spinel ${\mathrm{Co}}_1.21{\mathrm{V}}_1.79{\mathrm{O}}₄$ which exhibits a ferrimagnetic phase transition at ${T}_C\ensuremath{\sim}165$ K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above ${T}_C$ but also the ferrimagnetic phase below ${T}_C$, which should be driven by the nearly itinerant character of the orbitally degenerate V $3d$ electrons. In the paramagnetic phase above ${T}_C$, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above ${T}_C$, the sensitive magnetic-field response of the elastic moduli suggests that, with the negative magnetoresistance, the magnetic-field-enhanced nearly itinerant character of the V $3d$ electrons emerges from the spin-cluster state. This should be triggered by the inter-V-site interactions acting on the orbitally degenerate $3d$ electrons. In the ferrimagnetic phase below ${T}_C$, the elastic moduli exhibit distinct anomalies at ${T}₁\ensuremath{\sim}95$ K and ${T}₂\ensuremath{\sim}50$ K, with a sign change of the magnetoresistance at ${T}₁$ (positive below ${T}₁$) and an enhancement of the positive magnetoresistance below ${T}₂$, respectively. These observations below ${T}_C$ suggest the successive occurrence of an orbital glassy order at ${T}₁$ and a structural phase transition at ${T}₂$, where the rather localized character of the V $3d$ electrons evolves below ${T}₁$ and is further enhanced below ${T}₂$.