Fourier Transform Infrared

The Infrared part of the electromagnetic spectrum lies above the visible wavelengths and just below the Terahertz.

The family of the four Ni-based tellurates, Ni₃TeO₆, Ni₂MnTeO₆, Ni₂CoTeO₆ and NiCo₂TeO₆, crystalize in the polar R3 space group, at least from RT down to 4 K and it is known that Ni₃TeO₆ keeps the same rhombohedral structure up to 1000 K [1]. As expected, all compounds present almost identical vibrational spectra. All 18 vibrational modes predicted by factor group analysis (9 E(x,y,x²-y²,xy,xz,yz) and 9 A(x²+y²,z²,z)), both IR and Raman active, are present in the IR reflectivity spectra of both Ni₂CoTeO₆ and NiCo₂TeO₆ compounds (see figure on the right) [2,3]. In analogy to the mode at 310 cm^-1 in Ni₃TeO₆, the modes seen in Ni₂CoTeO₆ and NiCo₂TeO₆ at 224 and 211 cm^-1_, respectively, exhibit a conspicuous frequency hardening, accompanied by a decrease in damping. The observed phenomenon of mode intensity loss below T_N might be related to a coupling between the phonon and a lower energy magnon. As we show below, both compounds evidence several spin excitations in the THz range, thus a transfer of dielectric strength from the phonon to a spin excitation may take place when the magnetic ordering occurs and the magnon becomes electrically active (i.e., it contributes to the permittivity), which is a typical signature of electromagnons.