Magnetic and specific heat studies of the cation-ordered pyrochloreNH4CoAlF6

The cation-ordered form of the pyrochlore related fluoride ${\mathrm{NH}}_4{\mathrm{CoAlF}}_6$ 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 $5.70\left(1\right)\mathrm{}{\mu }_B/{\mathrm{Co}}^{2+}$ and ${\theta }_P=-62\left(1\right)\mathrm{}\mathrm{K}.$ 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 $0.4{\mu }_B/{\mathrm{Co}}^{2+}$ 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 $S=\frac{1}{2}$ XY model which is consistent with the chain structure of the ${\mathrm{Co}}^{2+}$ sublattice in ordered ${\mathrm{NH}}_4{\mathrm{CoAlF}}_6.$