Title: A variety of elastic anomalies in orbital-active nearly-itinerant cobalt vanadate spinel,
Abstract: We perform ultrasound velocity measurements on a single crystal of
nearly-metallic spinel Co$_{1.21}$V$_{1.79}$O$_4$ which exhibits a
ferrimagnetic phase transition at $T_C \sim$ 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 3$d$ 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 3$d$ electrons emerges from the spin-cluster state. This should be
triggered by the inter-V-site interactions acting on the orbitally-degenerate
3$d$ electrons. In the ferrimagnetic phase below $T_C$, the elastic moduli
exhibit distinct anomalies at $T_1\sim$ 95 K and $T_2\sim$ 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 3$d$ electrons evolves below $T_1$ and is further
enhanced below $T_2$.