For the compound of wolframite structure three antiferromagnetic phases are known to exist in the absence of an external field. In this contribution we present the magnetic phase diagrams up to a field strength of 20 T. The plane spanned by the easy direction and the orientation of the twofold screw axes is exposed for the application of external magnetic fields, and the change in the topology of the phase diagrams with the variation of the specific field direction within this plane is described in detail. The application of an external magnetic field perpendicular to this plane hardly affects the stability ranges of any of the phases.

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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.

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