Fe4-xNixNb2O9 (x ≤ 1): Nickel impact on the magnetoelectric properties of Fe4Nb2O9

We report the investigation of the Ni for Fe substitution in Fe 4 Nb 2 O 9 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-x Ni x Nb 2 O 9 , 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 Fe 3 NiNb 2 O 9 (x = 1) is 18 K lower than that of Fe 4 Nb 2 O 9 . The magnetic field (H) dependence of the magnetization (M) of Fe 3 NiNb 2 O 9 below T N exhibits a spin-flop like at about 1T, and a more ferromagnetic-like M(H) behaviour than that of Fe 4 Nb 2 O 9 . 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.