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

Abstract

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.

Softening of breathing elastic mode and trigonal elastic mode in disordered pyrochlore magnet NaCaCo$_2$F$_7$

“…It is noted that, similar to NaCaCo 2 F 7 , the softening with minimum in C Γ (T ) is also observed in the frustrated spinel oxides, the origin of which is considered to be the coupling of the lattice to the spin-cluster excitations via the exchange striction mechanism [ 20,21,[23][24][25] ]. This spincluster-driven elastic softening is generally explained as the presence of a finite gap for the excitations, which is sensitive to strain [ 21 ].…”

“…Furthermore, as the ultrasound velocity can be measured with a high precision of ∼ppm, its measurements can sensitively probe elastic anomalies driven by phase transition, fluctuations, and excitations [ 18 ]. For the frustrated magnets, the ultrasound velocity measurements have proven to be a useful tool for studying not only the ground state but also the excited states [ [19][20][21][22][23][24][25][26] ]. In the present study on NaCaCo 2 F 7 , we find two different types of elastic anomalies at low temperatures above T f , which reveal, respec-tively, an isostructural lattice instability toward the spin freezing at T f and a trigonal lattice instability arising from the emergent dynamical spin-cluster state.…”

Magnetic and electric properties of spinel oxide CoV2O4 (001) films

“…As the temperature further decreases, another magnetic transition from C-FM to noncollinear ferrimagnetic (NC-FM) occurs at T* ∼ 95 K, which is characterized by the spin reorientation. In addition, a second structural phase transition from I4 1 /amd to I4 1 /a occurs at ¢ T 1 ∼ 60 K. [11][12][13][14][15][16] This material is also known to exhibit spin canting, 17) and the saturation magnetization of the material is 1.5 μ B f.u −1 . 18,19) CVO has an extremely low electrical resistivity (ρ) of 0.065 Ωcm at RT; 20) therefore, it has been intensively investigated as a candidate electrode for secondary batteries.…”

Symmetry-resolved elastic anomalies in spin-crossover cobaltite LaCoO$_3$

“…The elastic modulus of a crystal is a thermodynamic tensor quantity, and the ultrasound velocity measurements of the symmetrically independent elastic moduli of a crystal thus provide symmetry-resolved thermodynamic information. In magnets, the modified sound dispersions caused by magnetoelastic coupling allow the extraction of detailed information on the interplay of the lattice and electronic degrees of freedom [ [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] ]. For LaCoO 3 , it has been reported that the longitudinal sound velocity along the cubic [111] direction has an unusual temperature dependence, which should arise from the spin crossover [ 48 ].…”