Multiple lattice instabilities resolved by magnetic-field and disorder sensitivities inMgV2O4

Abstract

Ultrasound velocity measurements of the orbital-degenerate frustrated spinel MgV2O4 are performed in the high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two-types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature, and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of lattice to coexisting orbital fluctuations and orbital-spin-coupled excitations. 3+ ions are characterized by double occupancy of the triply-degenerate t 2g orbitals, and form a sublattice of corner-sharing tetrahedra 1 . These spinels undergo successive structural and magnetic phase transitions: a cubic-to-tetragonal lattice compression at a temperature T s , and an antiferromagnetic (AF) ordering at a lower temperature T N , T s > T N , for nonmagnetic A = Zn 2 , Mg 3 , and Cd 4 (a ferrimagnetic ordering at a higher temperature T c , T s < T c , for magnetic A = Mn 5 ).The possibility of novel orbital and magnetic orderings in AV 2 O 4 has been extensively discussed both theoretically 6-12 and experimentally 2,13-15 which are considered to be driven due to the competition of JahnTeller coupling, Kugel-Khomskii exchange interaction 16 , and relativistic spin-orbit coupling. From the discussion on the orbital and magnetic orders in AV 2 O 4 , it is considered that the absolute and relative magnitude of these three interactions in AV 2 O 4 differs from compound to compound. For instance, it has been pointed out that the effect of the spin-orbit coupling on the orbital-ordered structure is strong in ZnV 2 O 4 while negligibly weak in MnV 2 O 4 9,11 .In this paper, we study the interplay of orbital, spin, and lattice degrees of freedom in the magnesium vanadate spinel MgV 2 O 4 (T s = 65 K and T N = 42 K) by means of ultrasound velocity measurements. The sound velocity or the elastic modulus is a useful probe which can extract symmetry-resolved thermodynamic information in the frustrated magnets 17,18 . In MnV 2 O 4 , recent ultrasound velocity measurements observed a huge elastic softening on cooling in the cubic paramagnetic (PM) phase, which is considered to be driven by the coupling of lattice to orbital-spin-coupled fluctuations 19 .For MgV 2 O 4 , it was suggested from the inelastic neutron scattering experiments that the effect of the spin-orbit coupling on the orbital-ordered structure is stronger than that in MnV 2 O 4 , but weaker than that in ZnV 2 O 4 13 . Thus the orbital and spin states of MgV 2 O 4 are expected to vary from those of MnV 2 O 4 and ZnV 2 O 4 10 . The present study particularly focuses on the orbital and spin states in the orbital-and spindisordered phase (the cubic PM phase) of MgV 2 O 4 .The ultrasound velocity measurements were p...

Transverse acoustic phonon anomalies at intermediate wave vectors in MgV2O4

“…In this work we present results of inelastic neutron scattering experiments above and below the orbitalordering temperature T OO that yield the dispersion of the acoustic phonon branches in MgV 2 O 4 thus complementing the q → 0 results of Watanabe et al 16 Unlike ultrasound measurements which probe acoustic fluctuations in the lim q→0 and Raman spectroscopy which is a strictly q = 0 probe, neutron spectroscopy can investigate excitations associated with a uniform lattice deformation at all wavevectors, and hence wavelengths, over the entire Brillouin zone. We focus our measurements on the acoustic phonons and not the optical phonons previously reported to exist for energies above ∼ 10 meV using optical techniques.…”

“…While the data is suggestive of a softening in energy of the TA2 phonon near the zone centre, the effect is not as large as at inter- mediate wavevectors. The effect at small q close to the zone centre Γ is also less pronounced than the q → 0 behaviour reported by Watanabe et al 16 using ultrasound sensitive to acoustic fluctuations on the MHz timescale.…”

“…The physics of the system is unaffected by this choice.The phonon anomaly in MgV 2 O 4 measured with neutron scattering differs from the response observed with ultrasound43 . At T OO , Watanabe et al16 reported a temperature dependent change of the sound velocity of the TA2 mode ∆v ≈ 25 %. We find no observable change for momentum transfers near the zone centre, however measure a spoon-like anomaly for intermedi-Linewidth Γp of the TA2 phonon branch.…”

Acoustic study of dynamical molecular-spin state without magnetic phase transition in spin-frustratedZnFe2O4

“…The sound velocity or the elastic modulus is a useful probe enabling symmetry-resolved thermodynamic information to be extracted from frustrated magnets. 16,17,[24][25][26][27][28][29][30][31][32] As mentioned earlier, the observed elastic anomalies in the chromite spinels ACr 2 O 4 inferred the coexistence of spin-JT fluctuations and molecularspin excitations in the PM phase. 16,17 The present study finds elastic anomalies in ZnFe 2 O 4 that suggest the evolution of a dynamical molecular-spin state at low temperatures without undergoing precursor spin-lattice fluctuations and spin-lattice ordering, which is a behavior uniquely different from another spin-frustrated molecular-spin system ACr 2 O 4 .…”

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

“…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.…”