Magnetism in and

Results of a neutron diffraction study on powder samples of and at 1.3 K and on a single crystal of flux-grown at 1.5 K are given. The magnetization and susceptibility of single crystals have been measured for both compounds from 2.0 K to room temperature. In addition a Mössbauer spectrum of was taken at 4.2 K, and the specific heat of has been measured from 0.5 K up to 25 K. and exhibit antiferromagnetic order below 4.9 K and 5.7 K, respectively, with a canted magnetic structure. The powder samples reveal two propagation vectors, , and , for as well as for . The magnetization measurements are interpreted in the mean-field approximation by taking the crystal field, spin - orbit coupling, isotropic Heisenberg exchange, magnetic dipole - dipole interaction and an external field into account. Within this model very good agreement between theory and experiment was obtained. By reproducing the antiferromagnetic - paramagnetic phase transitions with an applied external field parallel to the a- or c-direction an estimation of the effective exchange interaction between zigzag chains running along the c-direction is given. The magnetic structures, as derived from model considerations, agree with neutron diffraction results.