Magnetic ordering in the rutile molecular magnetsMII[N(CN)2]2(M=Ni,Co, Fe, Mn,Ni0.5Co0.5,andNi0.5Fe0.5)

Abstract

Rietveld refinement of powder neutron diffraction data, combined with group theory considerations, is used to determine the magnetic structures of the binary metal dicyanamide, ${M}^{\mathrm{II}}[{\mathrm{N}(\mathrm{CN})}_{2}{]}_{2}$ where $M=\mathrm{Ni},$ Co, Fe, Mn, ${\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5},$ and ${\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{0.5}.$ Compounds with $M=\mathrm{Mn}$ 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 ${\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5}$ displays the same collinear ferromagnetic structure as its parent compounds. However, the composition with $M={\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{0.5},$ 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.