F-19 NMR study of KMnFeF6: A frustrated magnetic system

F-19 NMR of polycrystalline KMnFeF6 at 300 K has been studied. The experimental spectrum has been fitted satisfactorily by superposition of six constituent lines having different values of shift parameters and intrinsic linewidth. By comparing these parameters with the calculated values of the principal components of the dipolar fields at different F-19 sites, an attempt has been made to assign a particular type of fluorine to a definite constituent line. The results clearly reveal that among the six types of fluorines those [F(1), F(2), F(3), F(6), and F(7)] which are linked with the magnetic ions belonging to the square platelets, experience nearly axially symmetric internal fields. Whereas, the fluorines, viz., F(4), F(5), and F(8), which are linked with the magnetic ions belonging to the triangles, experience a comparatively large amount of nonaxial field. The variation of the isotropic shift (K-150) among the different types of fluorines are consistent with the M-F bond lengths. The values of K-150, and the anisotropic part of the hyperfine coupling tensors, A(s) and A(p) are found to be close to those reported for other transition-metal fluorides. The fractional unpaired spin transfers (f(s) and f(p)) in the 2s and 2p orbitals of the different F- ions indicate an antiferromagnetic-type exchange dominating the magnetic properties of such compounds, which is in agreement with the susceptibility result.