Fe_4-xNi_xNb₂O₉ (x ≤ 1): Nickel impact on the magnetoelectric properties of Fe₄Nb₂O₉

We report the investigation of the Ni for Fe substitution in Fe₄Nb₂O₉ by X-ray techniques (diffraction and absorption spectroscopy), transmission electron microscopy (TEM), magnetometry and magneto (di)electric measurements. Up to x = 1 in Fe_4-xNi_xNb₂O₉, the structure remains trigonal, with a unit cell volume which decreases by −1.4% from x = 0 to 1, in accordance with the difference in the ionic radius of divalent iron and nickel whose oxidation states were obtained by XAS. Furthermore, EDX analyses and high resolution TEM confirm the homogeneous atomic distribution. The antiferromagnetic transition temperature T_N = 75 K of Fe₃NiNb₂O₉ (x = 1) is 18 K lower than that of Fe₄Nb₂O₉. The magnetic field (H) dependence of the magnetization (M) of Fe₃NiNb₂O₉ below T_N exhibits a spin-flop like at about 1T, and a more ferromagnetic-like M(H) behaviour than that of Fe₄Nb₂O₉. Similarly, below T_N, the H-dependent electric polarization (P) for the x = 0.5 and 1 samples is steeper than the P(H) curve of the x = 0 compound. This points towards the key role of such chemical substitutions to induce larger magnetoelectric coefficient.