Interaction of spin-orbital-lattice degrees of freedom: Vibronic state of the corner-sharing-tetrahedral frustrated spin system HoBaFe4O7 by dynamical Jahn-Teller effect

Abstract

A powder inelastic neutron-scattering study of ${\mathrm{HoBaFe}}_{4}{\mathrm{O}}_{7}$ (HBFO) revealed characteristic magnetic excitations associated with geometrical spin frustration. Dispersionless excitations in energy ($\ensuremath{\omega}$) and wave-vector ($Q$) space are observed at several discrete energies. Some of them can be attributed to crystal-field excitations of ${\mathrm{Ho}}^{3+}$ in octahedral symmetry, but the others are explained instead by the vibronic state of the ${\mathrm{Fe}}^{2+}$ dynamical Jahn-Teller effect with orbit-spin coupling, indicating interaction among the spin, the orbital, and the lattice degree of freedom. The antiferromagnetic HBFO lattice has cubic symmetry, and both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ reside on corner-sharing tetrahedra with a number ratio of 3:1. Even though ${\mathrm{Fe}}^{2+}$ is a Jahn-Teller active ion in tetrahedral symmetry, the system does not exhibit any static lattice distortion to the lowest temperature studied (4 K). The observed excitations can be understood by considering the dynamical interaction among spin-orbital-lattice degrees of freedom, indicating that spin fluctuation due to the frustration effect induces the dynamical Jahn-Teller effect, although in most cases a Jahn-Teller active ion ${\mathrm{Fe}}^{2+}$ takes the static Jahn-Teller effect in a magnetic oxide system.