Magnetic properties of frustrated pyrochlore fluorides Cd2M2F6S (M=Fe, Co, and Ni)

Abstract

Magnetic properties of pyrochlore fluorides ${\mathrm{Cd}}_{2}{M}_{2}{\mathrm{F}}_{6}{\mathrm{S}}_{}$ (${M}_{}=\text{Fe}$, Co, and Ni) were investigated. We found that the $3d$ transition metal ${M}_{}$ is partially substituted by Cd, and the chemical formula is represented by ${\mathrm{Cd}}_{2+2x}{M}_{2\ensuremath{-}2x}{\mathrm{F}}_{6}{\mathrm{S}}_{}$. For all three systems, negative Curie-Weiss temperatures ${\ensuremath{\Theta}}_{\mathrm{CW}}$ indicate dominant antiferromagnetic interactions between the nearest-neighbor magnetic ions. Magnetic orderings are observed much below $|{\ensuremath{\Theta}}_{\mathrm{CW}}|$ for the ${\mathrm{Fe}}_{}$ and ${\mathrm{Co}}_{}$ compounds, and the ordering temperatures are reduced by increasing $x$. The frustration index of the ${\mathrm{Fe}}_{}$ compound is about 10, and that of the Co compound is about 45, indicating strong frustration. No explicit ordering is observed in the ${\mathrm{Ni}}_{}$ compound, which is likely due to large Cd substitution $x$. In addition, field-induced magnetic transitions of the ${\mathrm{Fe}}_{}$ compound are observed. These transitions are likely to correspond to the boundary of a half-magnetization-plateau state found in chromium spinels, which suggests the important role of orbital degrees of freedom to exhibit the field-induced transitions.