Speaker
Description
Normal spinels AM$_2$O$_4$ crystallize in the cubic space group Fd-3m at room temperature. The magnetic ions M form a pyrochlore sublattice, consisting of a network of corner-sharing tetrahedra prone to magnetic frustration. Complex magnetic ground states arise in these spinel compounds from a strong competition between magnetic interactions beyond the third neighbor [1-5]. In this talk, we focus on GeCo$_2$O$_4$, which orders at TN = 23 K into a complex antiferromagnetic structure characterized by the propagation vector k = (½ ½ ½) [3-5]. The magnetic ordering is accompanied by a cubic to tetragonal structural transition [3,4], which partly releases the magnetic frustration through magneto-structural effects. Moreover, elastic constant studies using ultrasound velocity measurements reported acoustic anomalies (in particular, in the C44 elastic constant related to the transverse acoustic phonons) at the magnetic transition, possibly associated to the structural distortion, thus suggesting a coupling between the magnetic excitations and the acoustic phonons [6].
In this context, we have studied in detail the dynamics in GeCo2O4 and particularly focused on the coupling between spin and lattice degrees of freedom in the dynamical regime. We have performed inelastic neutron scattering on IN5 [10], which shows an interesting excitation spectrum around the Brillouin zone center, with acoustic phonons crossing gapped spin waves. Such crossing may hide the presence of hybrid excitations as previously evidenced in the hexagonal multiferroic YMnO3 [7-9]. We have tested this hypothesis and used longitudinal polarization analysis on THALES and IN20 in order to separate the nuclear and magnetic contributions to the inelastic neutron scattering cross section in GeCo2O4. The presence of a novel mode was evidenced in the nuclear scattering. It follows the dispersion of a magnon and disappears above TN, suggesting a spin-lattice hybrid excitation. However, the intrinsic nature of this mode remains unclear and will be discussed in this talk.
