F-19 nuclear magnetic resonance studies of KCoFeF6: a geometrically frustrated system

F-19 nuclear magnetic resonance (NMR) studies are performed on polycrystall ine KCoFeF6 for temperatures in the range T-N (110 K) < T less than or equa l to 400 K. The transferred hyperfine interaction parameters K-iso and K-ax are determined at 294 K for the eight types of fluorine present in the uni t cell. The results give an idea of the variation of the M-F bond lengths f or different F- ions. The fractional spin transfer from the 3d to the 2s or bitals of the different F- ions has also been estimated. The temperature de pendence of K-iso suggests the absence of the development of any short-rang e antiferromagnetic correlations within the magnetic ions bonded on either side of each type of F- ion, even very close to T-N, whereas the temperatur e dependence of K-ax suggests an appreciable enhancement of the anisotropic part of the hyperfine field below 160 K. Moreover, the intrinsic linewidth s of the constituent lines increase appreciably below the same temperature, where the bulk susceptibility does not show such extra enhancement. This f eature resembles the effect of slowing down of electron spin fluctuations o n the F-19 NMR linewidth. A comparison of the behaviour of the shift parame ters with those found for KMnFeF6 indicates the dominance of geometrical fr ustration within the magnetic ions in the triangular lattice over the tende ncy of magnetic ordering of the ions belonging to the square lattice in the paramagnetic phase in KCoFeF6.