Magnetic order in TbCo₂ Zn₂₀ and TbFe₂ Zn₂₀

We report neutron diffraction studies of TbCo₂ Zn₂₀ and TbFe₂ Zn₂₀, two isostructural compounds which exhibit dramatically different magnetic behavior. In the case of TbCo₂ Zn₂₀, magnetic Bragg peaks corresponding to antiferromagnetic order are observed below T_N ≈2.5 K with a propagation vector of (0.5 0.5 0.5). On the other hand, TbFe₂ Zn₂₀ undergoes a ferromagnetic transition at temperatures as high as 66 K which shows a high sensitivity to sample-to-sample variations. Two samples of TbFe₂ Zn₂₀ with the same nominal compositions but with substantially different magnetic ordering temperatures (T_c ≈51 and 66 K) were measured by single-crystal neutron diffraction. Structural refinements of the neutron diffraction data find no direct signature of atomic site disorder between the two TbFe₂ Zn₂₀ samples except for subtle differences in the anisotropic thermal parameters. The differences in the anisotropic thermal parameters between the two samples are likely due to very small amounts of disorder. This provides further evidence for the extreme sensitivity of the magnetic properties of TbFe₂ Zn₂₀ to small sample variations, even small amounts of disorder.