Lattice and spin dynamics in a low-symmetry antiferromagnet NiWO4

Lattice and magnetic dynamics of NiWO4 single crystals were studied with the use of polarized Raman spectroscopy in a wide temperature range of 10-300 K including the antiferromagnetic ordering temperature T-N = 62 K. Static magnetic measurements were used for characterizing the single crystals. All Raman-active phonons predicted by the group theory were observed and characterized. Magnetic symmetry analysis was used to determine possible magnetic space groups for NiWO4 which can be also applied to any other isostructural crystal with the same magnetic propagation vector k = (1/2,0,0). Although the magnetic structure of NiWO4 is relatively simple, a rich set of narrow and broad magnetic excitations with different polarization properties and temperature behavior in the very broad frequency range of 10-200 cm(-1) was observed, with some modes surviving at temperatures much higher than TN up to 220 K. Part of the magnetic excitations were identified as acoustic and optical spin-wave branches which allowed us to construct exchange structure and estimate exchange and anisotropy constants with the use of linear spin-wave theory.