ANTIFERROMAGNETIC ORDERING IN THE NOVEL TB3GE4 (2-STEP) AND TBGE2-DELTA (ALB2-TYPE) COMPOUNDS STUDIED BY NEUTRON-DIFFRACTION AND MAGNETIC MEASUREMENTS

The magnetic ordering of the novel binary compound Tb3Ge4 with two Tb sites (Er3Ge4 structure type, space group Cmcm) has been studied by ne utron diffraction and magnetic measurements. Tb3Ge4 orders antiferroma gnetically in two steps. In a first step the Tb-1 sublattice orders be low T-N = 28 K with a collinear moment arrangement along the shortest axis a which persists down to T-1 = 16.5 K. Below T-1 a reorientation of the Tb-1 moments takes place. Simultaneously the Tb-2 moments order with a collinear arrangement along c. The ordering of the Tb-1:8(f) a nd Tb-2:4(c) sites is associated with the wavevector q = (010) and is described by the monoclinic magnetic space group Cp1121'/mi'(Sh55/11). At 1.4 K the ordered moment values of Tb-1 and Tb-2 are 8.4(2) mu(B)/ atom and 6.3(2) mu(B)/atom, respectively. The Tb-1 moment is tilted aw ay from the x-direction by an angle of 41(2)degrees and by 57(1)degree s from the y-direction within the (Oyz) plane. The resulting structure corresponds to a three-dimensional canting with six sublattices. The presence of diffuse scattering in the 16.5-53 K temperature interval i s attributed to short-range ordered magnetic domains of the mean corre lation length similar to 11.4(2) Angstrom. Fourier analysis of the dat a reveals that the first- (up to 4.5 Angstrom) and third- (6.5-7.5 Ang strom) neighbour correlations are mainly ferromagnetic; the second (4. 5-6.5 Angstrom) and fourth (7.5-8.5 Angstrom) ones are antiferromagnet ic. The magnetic ordering of the coexisting novel phase TbGe2-delta wi th the AlB2 structure type (space group P6/mrnrn) has been also studie d. The Tb moments order antiferromagnetically below T-N = 25 K. This c ollinear moment arrangement is associated with the wavevector (q = 001 /2) and is described by the magnetic space group P(2c)112'/m'(Sh56/11) . At 1.4 K mu(Tb) = 6.7(2) mu(B) and is confined to the (001) plane.