The magnetic phase diagram of the HoCo 2X 2 (X=Ge, Si) and DyCo 2Ge 2 compounds by neutron diffraction

Abstract

The magnetic ordering of the tetragonal (I4/mmm) HoCo 2X 2 (X=Ge, Si) and DyCo 2Ge 2 compounds with the ThCr 2Si 2 type of structure has been reinvestigated by neutron powder diffraction. The HoCo 2Si 2 compound orders exclusively with the type-I antiferromagnetic structure with the wave vector q=(0, 0, 1) and the moments confined to the c-axis, while the magnetic phase diagrams of the DyCo 2Ge 2 and HoCo 2Ge 2 compounds comprise three distinct regions of magnetic ordering: a low-temperature structure also of type-I ( q 1=(0, 0, 1)), stable in the range between 2 K and T IC, a high-temperature longitudinal sine wave modulated structure with the wave vector q 2=(0 ,0, 1− q z ), stable in the range T IC– T N and a coexistence region of q 1 and q 2 around T IC. The wave vector component q z has a temperature-dependent length and locks-in by a first-order transition to the value q z =0 at T IC. In the case of the DyCo 2Ge 2 compound a spin reorientation transition was detected in the low-temperature range. The ordered moment values at 1.5 K are 7.8(1) μ B/Dy and 9.33(1) μ B/Ho for the RCo 2Ge 2 compounds and 8.8(1) μ B/Ho atom for the HoCo 2Si 2 compound at 3 K.