Magnetic and specific heat studies of the cation-ordered pyrochlore NH4CoAlF6

The cation-ordered form of the pyrochlore related fluoride NH4CoAlF6 has been studied with respect to the magnetic susceptibility and specific heat. Susceptibility and magnetization studies indicate Curie-Weiss behavior above about 60 K with an effective moment of 5.70(1)μB/Co2+ and θP=-62(1) K. Evidence for magnetic ordering is seen below about 6 K, and the field-cooled–zero-field-cooled susceptibilities, measured at an applied field of 100 G, deviate below 5 K. A substantial ferromagnetic moment of 0.4μB/Co2+ is found at 2 K in the magnetization. The specific heat data indicate a remarkably complex behavior with three transitions at 0.8, 2.5, and 5.8 K, the highest and lowest being also observed in the magnetic measurements. Furthermore, short-range correlations are also evident from the presence of a broad peak centered at about 9 K and a considerable excess specific heat contribution extending up to higher temperatures. In fact, 50% of the total observed entropy removal occurs between 25 and 5.8 K. The magnetic specific heat data above 6 K are in good agreement with the one-dimensional S=12 XY model which is consistent with the chain structure of the Co2+ sublattice in ordered NH4CoAlF6.