Magnetic structure and magnetic phase transitions in TbPtGe2

Magnetic and neutron diffraction measurements have been performed on TbPtGe 2 at low temperatures. The compound crystallizes in the orthorhombic YIrGe2 -type structure (space group Immm); crystal structure parameters have been refined on the basis of the neutron diffraction pattern collected at T = 30 .7 K (paramagnetic region). TbPtGe2 is antiferromagnetic below T-N = 24.2 K . Below this temperature only one of the two Tb sublattices is ordered; the Tb magnetic moments localized at the 4(i) sites order with the simple prop agation vector k = 0. Below T-t1 = 11.4 K the magnetic moments at the other Tb sites, 4(h), show an ordering with the propagation vector k(1) = [0.267 7(8), 0.1312(24), 0.6989(27)]. At T-t2 = 7 K a further phase transition to a new modulated phase described by the propagation vector k(2) = [0,2584(5) , 0, 0.5895(6)] is observed.