Jahn-Teller inactivity and magnetic frustration inGeCo2O4probed by ultrasound velocity measurements

Abstract

Ultrasound velocity measurements of cubic spinel GeCo 2 O 4 in single crystal were performed for the investigation of shear and compression moduli. The shear moduli in the paramagnetic state reveal the absence of Jahn-Teller activity despite the presence of orbital degeneracy in the Co 2+ ions. Such a Jahn-Teller inactivity indicates that the intersite orbital-orbital interaction is much stronger than the Jahn-Teller coupling. The compression moduli in the paramagnetic state near the Néel temperature T N reveal that the most relevant exchange path for the antiferromagnetic transition lies in the ͓111͔ direction. This exchange-path anisotropy is consistent with the antiferromagnetic structure with the wave vector q ʈ ͓111͔, suggesting the presence of bond frustration due to competition among a direct ferromagnetic interaction and several distant-neighbor antiferromagnetic interactions. In the Jahn-Teller-inactive condition, the bond frustration can be induced by geometrical orbital frustration of t 2g -t 2g interaction between the Co 2+ ions, which can be realized in the pyrochlore lattice of the high-spin Co 2+ with t 2g -orbital degeneracy. In GeCo 2 O 4 , the tetragonal elongation below T N releases the orbital frustration by quenching the orbital degeneracy.

Magnetic-Field-Induced Transitions in Spinel GeCo2O4

“…Since the magnetic structure in GeCo 2 O 4 is the same as that in GeNi 2 O 4 below T N2 , it is likely that J 4 , J 0 3 , and J 3 also play important roles in GeCo 2 O 4 . Although the Curie-Weiss temperature in GeCo 2 O 4 was estimated to be 81 K, 14) 57 K, 15) and 41 K, 16) Lashley et al claimed that these large Curie-Weiss temperatures were estimated incorrectly. 11) They analyzed the magnetic susceptibility including the excited crystal-field levels and found that the free ion model describes the observed values reasonably well.…”

“…Watanabe et al suggested that the orbital frustration disturbs the orbital ordering in the cubic phase and the structural transition quenches the orbital degeneracy. 16) The magnetic structure below T N is affected by the t 2g orbitals, which contribute to the direct Co-Co coupling. Since the kagomé and triangular spins order simultaneously, it is expected that the coupling between kagomé and triangular planes is stronger in GeCo 2 O 4 than in GeNi 2 O 4 .…”

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

“…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 .…”

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 .…”