The Dugganites: A new, frustrated, and potentially multiferroic class of compounds that exhibit rich magnetic behavior

Ba$_{\mathrm{3}}$NbFe$_{\mathrm{3}}$Si$_{\mathrm{2}}$O$_{\mathrm{14}}$ is a multiferroic langasite (s.g. $P$321) wherein the Fe$^{\mathrm{3+}}$ atoms (S$=$5/2) occupy isolated trimers that stack along the $c$-axis. The spins uniquely order below T$_{\mathrm{N}}=$26 K, where single domain helicity simultaneously exists with triangular chirality. Preparations of other langasites of this type are possible, so long as Fe$^{\mathrm{3+}}$ remains in the trimer site leaving the magnetism relatively unchanged. This is because Fe$^{\mathrm{3+}}$ occupies a tetrahedral site, where most other transition metal ions prefer the octahedral site occupied by Nb$^{\mathrm{5+}}$. Building on previous research, we have circumvented this problem by replacing Nb$^{\mathrm{5+}}$ with Te$^{\mathrm{6+}}$, which is found exclusively in octahedral coordination. Isostructural compounds Pb$_{\mathrm{3}}$TeCo$_{\mathrm{3}}$A$_{\mathrm{2}}$O$_{\mathrm{14}}$ (A$=$V$^{\mathrm{5+}}$, P$^{\mathrm{5+}})$ and Pb$_{\mathrm{3}}$TeMn$_{\mathrm{3}}$P$_{\mathrm{2}}$O$_{\mathrm{14}}$ (where the only magnetic ions are Co$^{\mathrm{2+}}$ and Mn$^{\mathrm{2+}}$ respectively) have been prepared and studied. Despite being isostructural to Ba$_{\mathrm{3}}$NbFe$_{\mathrm{3}}$Si$_{\mathrm{2}}$O$_{\mathrm{14}}$, the dugganites exhibit a rich variety of magnetic behavior, including evidence for multi-k magnetic structural arrangements, long-range coexistence of static and dynamic spins, and spin-spin interactions that potentially exist over 150 unit cells. In at least one dugganite, magnetoelectric coupling was observed at T$_{\mathrm{N}}$ entertaining the possibility that these compounds may also be multiferroic.