Magnetic and dielectric response of cobalt-chromium spinel Co[Cr.sub.2][O.sub.4] in the terahertz frequency range

The nature of the phonon and magnon modes in the Co[Cr.sub.2][O.sub.4] multiferroic with a cubic spinel structure has been studied using submillimeter spectroscopy and infrared Fourier spectroscopy. This paper reports on the first measurement of the evolution with temperature of the exchange optical magnon in the ferrimagnetic ([T.sub.C] = 94 K) and two low-symmetry ([T.sub.S] ≅ 26 K, [T.sub.lock-in] = 14.5 K) phases of Co[Cr.sub.2][O.sub.4] down to T = 5 K in zero magnetic field. It has been shown that the detected magnon is not a ferrimagnetic order parameter and originates, most probably, from spin precession in the cobalt sublattices. At the points of the magnetic phase transitions, the oscillator parameters of the two lowest- frequency phonon modes reveal an anomalous temperature behavior, thus evidencing the presence of significant interaction between the magnetic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originating from vibrations of the Co[O.sub.4] tetrahedra during the transition of Co[Cr.sub.2][O.sub.4] to the multiferroic state (T < [T.sub.S]) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.