Quasi-one-dimensional magnetism in MnxFe1-xNb2O6 compounds: From Heisenberg to Ising chains

A series of MnxFe1-xNb2O6 compounds (0⩽x⩽1) is investigated by both X-ray and neutron powder diffraction, as well as specific-heat and magnetic measurements. The samples present orthorhombic Pbcn crystal symmetry, and exhibit weakly coupled magnetic chains. These chains are of Heisenberg type (weak anisotropy) on the Mn-rich side, and Ising-like (strong anisotropy) on the Fe-rich side. Except for 100% Fe (x=0), which has weakly-interacting ferromagnetic Ising chains, a negative Curie-Weiss temperature is obtained from the magnetic susceptibility, indicating dominant antiferromagnetic interactions. At the lowest probed temperature, T=1.5K, true long-range magnetic order is only observed for x=1, 0.8, and 0. Although the ordering is globally antiferromagnetic in all cases, the first two are characterized by a two-sublattice structure with propagation vector k=(0,0,0), while the latter presents alternatingly oriented ferromagnetic chains described by k=(0,12,0). For other compositions, short-range magnetic correlations are extracted from diffuse neutron-scattering data.