Evidence for interpenetrating magnetic structures across an IC-C phase transition in Mn_0.88Fe_0.12WO₄

Neutron diffraction, specific heat and magnetization measurements were applied to study the magnetic structures in Mn_0.88Fe_0.12WO₄ using single crystal and powder samples. A commensurate antiferromagnetic structure with a propagation vector k = (1/4, 1/2, 1/2) below 13.5 K and an incommensurate structure with k = (0.23, 0.5, 0.508) between 13.5 K and 15.5 K were found to be the dominating ordered states. The temperature behaviour of the propagation vector above the IC-C phase transition shows a logarithmic behaviour. An FeWO₄-type ordering of spins with k ≈ (1/2, 0, 0) is observed below 19 K and coexists between 15.5 K and 13.5 K with the MnWO₄-type structure. Our experimental results indicate a cluster-like FeWO₄-type magnetic ordering on a diluted 3D lattice, which interpenetrates the dominating MnWO₄-type spin ordering. Below 13 K, with the majority of spins ordered in the commensurate MnWO₄-type structure, reflections which can be indexed to a unit cell doubled along the crystallographic a-axis appear again; however, these intensities cannot purely be related to Bragg diffraction by ordered local magnetic moments at the (Mn, Fe) positions.