Magnetism in FeNb2O6 and NiNb2O6

Results of a neutron diffraction study on powder samples of FeNb2O6 and NiNb2O6 at 1.3 K and on a single crystal of flux-grown FeNb2O6 at 1.5 K are given. The magnetization and susceptibility of single crystals have been measured for both compounds from 2.0 K to room temperature. In addition a Mossbauer spectrum of FeNb2O6 was taken at 4.2 K, and the specific heat of NiNb2O6 has been measured from 0.5 K up to 25 K. FeNb2O6 and NiNb2O6 exhibit antiferromagnetic order below 4.9 K and 5.7 K, respectively, with a canted magnetic structure. The powder samples reveal two propagation vectors, (0 1/2 0), and (1/2 1/2 0), for FeNb2O6 as well as for NiNb2O6. The magnetization measurements are interpreted in the mean-field approximation by taking the crystal field, spin-orbit coupling, isotropic Heisenberg exchange, magnetic dipole-dipole interaction and an external field into account. Within this model very good agreement between theory and experiment was obtained. By reproducing the antiferromagnetic-paramagnetic phase transitions with an applied external field parallel to the nor c-direction an estimation of the effective exchange interaction between zigzag chains running along the c-direction is given. The magnetic structures, as derived from model considerations, agree with neutron diffraction results.