Structural, magnetic, and spin dynamical properties of the polar antiferromagnets Ni3-xCoxTeO6 (x=1, 2)

Abstract

We present results of a multimethod investigation of the polar antiferromagnets Ni2CoTeO6 and NiCo2TeO6, inspired by the colossal magnetoelectric effect present in Ni3TeO6. Both compounds crystalize in the same polar space group R3 as Ni3TeO6, preserving the crystal symmetry at least from room temperature down to 2 K. Ni2CoTeO6 and NiCo2TeO6 undergo antiferromagnetic phase transitions at T-N = 55 and 52 K, and spin-flop transitions at an external magnetic field of approximately 8 and 4 T, respectively. Both compounds present an incommensurate antiferromagnetic helical structure with spins lying in the ab plane, in contrast to the collinear one along the c axis in Ni3TeO6. Moreover, dielectric anomalies are observed at their antiferromagnetic phase transitions, suggesting a magnetoelectric behavior. Spin and lattice dynamics studies by a combination of infrared, Raman, and terahertz spectroscopies were performed. Below T-N, in both Ni2CoTeO6 and NiCo2TeO6, low-frequency spin excitations extremely sensitive to external magnetic field were observed. At least one of these magnons was simultaneously seen in Raman and THz spectra of both compounds, therefore we propose to assign them to electromagnons.