Quasi-one-dimensional magnetism in [formula omitted] compounds: From Heisenberg to Ising chains.

A series of Mn x Fe 1 - x Nb 2 O 6 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 = 1.5 K , 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 , 1 2 , 0 ) . For other compositions, short-range magnetic correlations are extracted from diffuse neutron-scattering data. [ABSTRACT FROM AUTHOR]