Phase change caused by Jahn-Teller distortion in Ni doped MnCo2O4 spinel

Polycrystalline NixMnCo2−xO4 (x = 0, 0.25, 0.5, 0.75, and 1) powders were produced. The addition of nickel to the MnCo2O4 system modifies the magnetic ion concentration and regulates the Jahn-Teller distortion of the system. We have intended to replace octahedral Co with Ni. Beyond expectation, we found that the different types of Co were replaced by Ni with the doping concentration increasing. When the concentration of Ni doping is low, replacing the non-magnetic octahedral Co3+ (3d6, t2 g6eg0), Ni3+ (3d7, t2 g6eg1) enters octahedral sites. The bond angle between tetrahedral Co and octahedral Mn enlarges and the Neel temperature increases from 164 to 207 K. As Ni enters the octahedral site, the net magnetic moment at low temperature increases, which is believed that the sample exhibits a non-collinear magnetic configuration. As the doping Ni concentration is further increased, Ni2+ (3d8, eg4t2 g4) unexpectedly replaces the tetrahedral Co2+ (3d6, eg4t2 g4). The replacement makes the non-collinear frustrated transition temperature increase from 74 K to 87 K. The interaction between Co2+Tet-Ni3+Oct and Ni3+Oct-Mn3+Oct competes with each other to form a spin frustration at low temperature. An antiferromagnetic-dominated spin-glass phase occurs at low temperatures in the NiMnCoO4 system. The saturation magnetization in the NixMnCo2−xO4 system increases from 0.40 to 0.67 μB/formular (2 K).