Short-range magnetic ordering in the highly frustrated pyrochlore systems FeF3 and Mn2Sb2O7

FeF3 and Mn2Sb2O7 exhibit long-range magnetic order at 15.5 and 13 K, respectively. Bulk magnetic susceptibility data for these compounds show a strong dependence on sample history and deviations from a Curie-Weiss-law behavior far above the ordering temperatures. These effects are attributed to short-range order over a wide temperature range. Long-range order is inhibited in these compounds by the highly frustrated lattice of corner-sharing tetrahedra formed by the metal (Fe3+ and Mn2+) sublattices. Neutron-diffraction data given clear evidence for short-range correlations above Tc. Fourier analysis of the data yields an approximation to the radial distribution function, which is interpreted in terms of spin-spin correlations out to the fourth-neighbor coordination shell. For both compounds we observe strong antiferromagnetic first-neighbor, and ferromagnetic second- and third-neighbor, spin-spin correlations.