First-principles study of exchange interactions of NiNb2O6 2 O 6 and FeNb2O6 2 O 6

Transition metal niobates MNb2O6 2 O 6 (M = Mn, Fe, Co, Ni) have attracted a lot of interest because of their low dimensional magnetic properties. We carry out a comprehensive study of the electronic and magnetic properties of NiNb2O6 2 O 6 and FeNb2O6 2 O 6 using density functional calculations. NiNb2O6 2 O 6 and FeNb2O6 2 O 6 are insulators with band gaps of 2.4 and 1.4 eV, respectively. We find that the state with the lowest total energy of FeNb2O6 2 O 6 is inconsistent with the experimental magnetic ground state. We estimate the exchange parameters for the two compounds and find that this difference is due to the presence of the relatively strong interchain exchange interactions along the a direction in FeNb2O6. 2 O 6 . We propose that the strong tilt of the magnetic structure, stemming from magnetic anisotropy, offsets the contributions of these interactions to some extent. Thus, unlike NiNb2O6, 2 O 6 , in order to describe FeNb2O6 2 O 6 correctly by Heisenberg model, the magnetic anisotropy is necessary. Our results indicate that the magnetic anisotropy plays an important role in determining the magnetic ground state for MNb2O6 2 O 6 type compounds.