ANTIFERROMAGNETIC ORDERING IN THE NOVEL HO3GE4 AND HOGE1.5 COMPOUNDS STUDIED BY NEUTRON-DIFFRACTION AND MAGNETIC MEASUREMENTS

The magnetic ordering of the novel binary compounds Ho3Ge4 (Er3Ge4 str ucture type, space group Cmcm) and HoCe1.5 (AlB2 structure type, space group P6/mmm) has been studied by neutron diffraction and magnetic me asurements. Antiferromagnetic ordering of Ho3Ge4 sets in below T-N = 1 2 K. The two Ho sites order independently with two distinct order para meters associated with the same wave vector q = (010). Both sublattice s have a uniaxial antiferromagnetic moment arrangement but with mutual ly perpendicular orientations. In a first step (T < T-N) the Ho-1 subl attice orders with a uniaxial moment arrangement along c. In a second step (T < T-1 = 6.6 K) the Ho-2 sublattice orders perpendicular to Ho- 1 with a uniaxial arrangement along a. Below T the resulting two-dimen sional canted antiferro-magnetic structure is described by the monocli nic magnetic space group Cp11 2(1)/m' i'(Sh(14)(82)). At 1.4 K the ord ered moment values of Ho-1 and Ho-2 are 8.9(1) mu(B)/atom and 7.2(2) m u(B)/atom, respectively. The magnetic ordering of the coexisting novel phase HoGe1.5 with the AlB2 structure type (space group P6/mmm) has b een also studied. The Ho moments order antiferromagnetically below T-N = 10 K. The collinear moment at arrangement is associated with the wa ve vector (q = 001/2) and is described by the magnetic space group Pr( 2c)11 2'/m' (Sh(11)(56)). At 1.4 K the ordered magnetic moment mu(Ho) = 6.2(6) mu(B) is confined to the (001) plane.