The magnetic phase diagram of the HoCo2X2 (X = Ge, Si) and DyCo2Ge2 compounds by neutron diffraction

The magnetic ordering of the tetragonal (14/mmm) HoCo2X2 (X=Ge, Si) and DyCo2Ge2 compounds with the ThCr2Si2 type of structure has been reinvestigated by neutron powder diffraction. The HoCo2Si2 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 DyCo2Ge2 and HoCo2Ge2 compounds comprise three distinct regions of magnetic ordering: a low-temperature structure also of type-I (q, = (0, 0, 1)), stable in the range between 2 K and T-1C, 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 DyCo2Ge2 compound a spin reorientation transition was detected in the low-temperature range. The ordered moment values at 1.5 K are 7.8(1) mu(B)/Dy and 9.33(1) mu(B)/Ho for the RCo2Ge2 compounds and 8.8(1) mu(B)/Ho atom for the HoCo2Si2 compound at 3 K. (C) 2003 Elsevier Science B.V. All rights reserved.