Structural and magnetic properties of the mixed fluorides KMn1-xCoxFeF6

Magnetic substitution in KMnFeF6 with a tetragonal tungsten bronze-type structure has been performed. Structural characterization of the series of compounds has been carried out from x-ray powder patterns. It is seen that the lattice parameter a decreases linearly with the increase in x, whereas the parameter c remains almost unaltered up to x=0.5 and then increases continuously till x=1. The unit-cell volume decreases in the range 0 less than or equal to x less than or equal to 0.6 and then increases continuously in the range 0.6 less than or equal to x less than or equal to 1. The bulk magnetic properties have been investigated in high magnetic fields as a function of cobalt concentration and temperature. These results show that the ferrimagnetic transition temperature T-N in KMnFeF6 decreases linearly with the increase in Co2+ concentration (x) at the Mn2+ sites. However, the magnitude of the Curie-Weiss temperature which indicates the strength of the exchange interaction does not follow this behavior throughout the whole concentration range. The results suggest that the extent of magnetic frustration in the triangular cycles of the magnetic ions increases above x=0.6. Furthermore, the nature of chi(M)(-1) vs T curves clearly shows the coexistence of antifer romagnetic (AF) and ferromagnetic (F) short-range correlations above T-N in the samples with the concentration of the Co2+ ions lying in the range with 0.6 less than or equal to x less than or equal to 1. The M-H curves below T-N in all the samples are analyzed by using the law of approach to saturation. These results show that the saturation magnetization decreases continuously with the increase in x. Furthermore, these results also indicate the existence of noncollinear magnetic structure in the mixed systems, below the magnetic ordering temperature.