MAGNETIC-ORDERING IN HOCOSI AND TBCOGE

The crystal and magnetic structures of HoCoSi and TbCoGe were determin ed by neutron diffraction on polycrystalline samples. Both compounds e xhibit the orthorhombic TiNiSi-type of crystal structure and order mag netically, at low temperatures. In HoCoSi at T = 1.7 K magnetic moment s of holmium atoms form a conical spiral. It transforms with increasin g temperature into a ferromagnetic structure. The Curie point of HoCoS i is 13 K. The magnetic moment of the Ho3+ ion is 6.5(1) mu(B) at 1.7 K. In TbCoGe, a noncollinear ferromagnetic FxCz and a sine-modulated o rdering are present at 1.5 K. The former disappears at 17.5 K and the latter at 20.5 K, which is the Neel point of TbCoGe. The magnetic mome nt localized on the Tb3+ ion at 1.5 K amounts to 8.9(1) mu(B) in a non collinear ferromagnetic phase and 1.5(2) in a modulated phase.