Magnetic ordering in HoCoSi and TbCoGe

The crystal and magnetic structures of HoCoSi and TbCoGe were determined by neutron diffraction on polycrystalline samples. Both compounds exhibit the orthorhombic TiNiSi-type of crystal structure and order magnetically, at low temperatures. In HoCoSi at T = 1.7 K magnetic moments of holmium atoms form a conical spiral. It transforms with increasing temperature into a ferromagnetic structure. The Curie point of HoCoSi 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 ordering 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 moment localized on the Tb3+ ion at 1.5 K amounts to 8.9(1) mu(B) in a noncollinear ferromagnetic phase and 1.5(2) in a modulated phase.