Magnetic Structures of DyAu2 and DyAg2

Neutron–powder‐diffraction study has shown that, upon cooling, DyAu₂ (tetragonal, isostructural to MnAu₂) exhibits the onset of antiferromagnetism at $T_N\text{(α)\hspace{0.17em}=\hspace{0.17em}33.8°}\mathrm{K}$ and a first‐order‐like magnetic transition at $T_N\text{(β)\hspace{0.17em}=\hspace{0.17em}25.5°}\mathrm{K}$ . The moment alignment between $T_N\mathrm{(\alpha )}$ and $T_N\mathrm{(\beta )}$ is represented by a static linear–transverse spin wave propagating along the $a$ axis (the face‐centered description) and polarized in the direction of the $c$ axis. The wavelength of this modulation wave is 1.21 times (the $a$ spacing), being incommensurable with the chemical repetition. At $T_N\mathrm{(\beta )}$ , the modulation wavelength becomes abruptly equal to the $a$ spacing with a discontinuous increase in the rms ordered moment but without change in the moment direction. This commensurated magnetic structure persists down to 1.7°K and the saturation ordered moment is ${\text{9.2 μ}}_B$ . The DyAg₂ structure is analogous to DyAu₂ and characterized by $T_N\text{(α)\hspace{0.17em}=\hspace{0.17em}15.0°}\mathrm{K}$ ; the incommensurate moment wavelength, ${\text{1.17a; T}}_N\text{(β)\hspace{0.17em}=\hspace{0.17em}9.5°}\mathrm{K}$ ; and the saturation ordered moment, ${\text{7.4 μ}}_B$ , being much smaller than ${\text{10.0 μ}}_B$ of the free Dy³⁺ ion. The crystallographic data at 300–5°K are also presented.