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

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We perform ultrasound velocity measurements on a single crystal of nearly metallic spinel Co1.21V1.79O₄ which exhibits a ferrimagnetic phase transition at TC~165 K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above TC but also the ferrimagnetic phase below TC, which should be driven by the nearly itinerant character of the orbitally degenerate V $3d$ electrons. In the paramagnetic phase above TC, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above TC, 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 TC, the elastic moduli exhibit distinct anomalies at T₁~95 K and T₂~50 K, with a sign change of the magnetoresistance at T₁ (positive below T₁) and an enhancement of the positive magnetoresistance below T₂, respectively. These observations below TC suggest the successive occurrence of an orbital glassy order at T₁ and a structural phase transition at T₂, where the rather localized character of the V $3d$ electrons evolves below T₁ and is further enhanced below T₂.