Jahn-Teller Inactivity and Magnetic Frustration in GeCo${}_2$O${}_4$ Probed 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 an 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 N$\acute{e}$el temperature $T_N$ reveal that the most relevant exchange path for the antiferromagnetic transition lies in the [111] direction. This exchange-path anisotropy is consistent with the antiferromagnetic structure with the wave vector $q\parallel$ [111], suggesting the presence of bond frustration due to competition among a direct ferromagnetic and several distant-neighbors antiferromagnetic interactions. In the JT-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.

7 pages, 7figures, to appear in Phys. Rev. B

References in corpus (2)

• Orbital order in ZnV${}_2$O${}_4$
• Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4

Cited by in corpus (1)

• Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCo${}_2$O${}_4$