Magnetism on the stretched diamond lattice in lanthanide orthotantalates

The magnetic lanthanide $({\mathrm{Ln}}^{3+})$ ions in the fergusonite and scheelite crystal structures form a distorted or stretched diamond lattice which is predicted to host exotic magnetic ground states. In this study, polycrystalline samples of the fergusonite orthotantalates $M\text{\ensuremath{-}}{\mathrm{LnTaO}}_{4}$ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) were synthesized and then characterized using powder diffraction and bulk magnetometry and heat capacity. ${\mathrm{TbTaO}}_{4}$ orders antiferromagnetically at 2.25 K into a commensurate magnetic cell with $\stackrel{P\vec}{k}=0$, magnetic space group 14.77 $(P{2}_{1}^{\ensuremath{'}}/c)$, and Tb moments parallel to the $a$ axis. No magnetic order was observed in the other materials studied, leaving open the possibility of exotic magnetic states at $T<2$ K.