Magnetic properties of natural and synthetic wolframites Fe x Mn1?x WO4

We report in the present paper the main results concerning the evolution of the structural, magnetic and hyperfine properties of natural and synthetic wolframites Fe _x Mn_1?x WO₄ in the composition range 0.16<x<1. The iron and the manganese content of the samples are obtained by atomic absorption spectrophotometry. The lattice parameters are determined from least-squares fits of the X-ray diffraction data. Mössbauer experiments have been carried out between 200 and 4.2 K. Their parameters can be attributed to a high spin ferrous ion in a quite distorted octahedral symmetry. From the magnetic spectra, we deduce a very weak value for the magnetic hyperfine field (?50 kG), a negative sign for the quadrupole interaction and an asymmetry parameter η different from zero for x>0.3 ( \(\bar \eta\) ?0.7). In addition we discuss the variation of the magnitude of H _hf and of the angle ? between the H _hf direction and the principal axis of the electric field gradient (EFG) V _zz with the iron content. A single crystal Mössbauer study has also been performed to determine the orientation of the principal axis of the EFG and of the H _hf direction relative to the crystallographic axes. Mössbauer spectroscopy and magnetic susceptibility techniques have been used to obtain the magnetic ordering temperature T _N. One finds a linear variation of T _N with x for 0.4<x<1 and a deviation from the linear behavior for 0.16<x<0.3. These results are discussed in terms of the contributions of the different Fe-Fe, Fe-Mn and Mn-Mn exchange interactions.