Magnetic ordering in the rutile molecular magnets M^II[N(CN)₂]₂ (M = Ni, Co, Fe, Mn, Ni_0.5Co_0.5, and Ni_0.5Fe_0.5)

Rietveld refinement of powder neutron diffraction data, combined with group theory considerations, is used to determine the magnetic structures of the binary metal dicyanamide, M^II[N(CN)₂]₂ where M = Ni, Co, Fe, Mn, Ni_0.5Co_0.5, and Ni_0.5Fe_0.5. Compounds with M = Mn or Fe show a canted antiferromagnetic arrangement of spin oriented in the ab crystallographic plane, with antiparallel components of the two sublattices along the a axis and parallel along the b axis. Symmetry considerations forbid an additional moment, whether compensated or not, to be present along the c axis. The compounds with fewer unpaired electrons (Co and Ni) are ferromagnets, with all moments oriented along the c axis. The mixed composition of Ni_0.5Co_0.5 displays the same collinear ferromagnetic structure as its parent compounds. However, the composition with M = Ni_0.5Fe_0.5, whose parent compounds show different magnetic behavior, does not exhibit long-range magnetic ordering down to 1.7 K. Magnetostriction was observed for the ferromagnets for which we investigated the variable temperature powder neutron diffraction. The cobalt-rich compounds show more pronounced effects, consistent with their increasing magnetocrystalline anisotropy.