Antiferromagnetism of RPtX (R = Ho, Er; X = Si, Ge) compounds

X-ray and neutron diffraction as well as magnetometric measurements perform ed for HoPtX and ErPtX (X = Si, Ge) compounds indicate that they are crysta lline and that they have the orthorhombic TiNiSi-type crystal structure. Th ey are antiferromagnetic at low temperatures. A collinear magnetic structur e with the propagation vector k = \1/2, 0, 1/2\, stable between 1.5 K and t he Neel point at 3.1 K is observed in HoPtSi. HoPtGe is paramagnetic at 1.8 K. A collinear magnetic order described by the wavevector k(1) = \0, 1/2, 0\ has been found in ErPtSi and ErPtGe. As the temperature rises this struc ture transforms in both compounds into a sine modulated one with the waveve ctor k(2) = \0, k(y), 0\. The Neel points of ErPtSi and ErPtGe are 4.0 and 4.1 K respectively.