The Ni4-xCoxNb2O9 phase diagram: from magnetization reversal to linear magnetoelectricity

Ni4Nb2O9 and Co4Nb2O9 have two different structures derived from corundum which show different magnetic and electrical properties. The ferrimagnetism (FIM) associated with magnetization reversal (MR) of the former and the antiferromagnetism (AFM) combined with the magnetic field control of the polarization of the latter are of interest for applications in spintronics. The exploration of the Ni4-xCoxNb2O9 series was thus appealing because it presents all ingredients to generate exotic magnetic ground states as a result of competing magnetic exchanges due to the framework of the O-6 octahedra, around the magnetic transition metals, that share faces, edges and apices, leading to a structure described as the stacking of layers of honeycomb type and/or zigzag chains. This paper reports the domain of stability of each structure (Pbcn up to x = 2 and P3c1 for x >= 2.5) with the evolution of the structural and physical properties over the entire series. This overview allows a (T, x) phase diagram to be established on the base of room-temperature X-ray powder diffraction data and macroscopic magnetization characterizations in addition to dielectrical and electrical measurements. Four domains are identified increasing x, (i) Pbcn, FIM with MR, (ii) Pbcn, FIM without MR, (iii) coexistence of Pbcn and P3c1, and (iv) P3c1, AFM with linear magnetoelectric properties. Despite the change of structure, some characteristic parameters show a continuous evolution, such as the unit cell volume, which increases with x unlike the magnetic transition temperature, which decreases with x, from 77 K to 27 K.