Variation of the structural and the magnetic properties in Mn-doped CoFe2O4 thin films

The variations of the magnetic properties caused by Mn substitution for Fe and Co in inversespinel CoFe2O4 have been investigated by using vibrating-sample magnetometry (VSM) and conversion electron Mossbauer spectroscopy (CEMS). MnxCoFe2-xO4 and MnxCo1-xFe2O4 samples were prepared as thin films by using a sol-gel method. As the Mn composition increased, the MnxCoFe2O4 samples showed an increase of the lattice constant and underwent a cubic-tetragonal phase transition at x = 0.57. Such a phase transition is attributed to a Jahn-Teller effect on the high-spin Mn3+ (t(3)(2),e(g)(1)) ions substituting at the octahedral (Fe3+) sites. On the other hand, the MnxCo1-xFe2O4 samples were found to maintain a cubic structure up to x = 0.76, indicating that Mn2+ ions substituted at the octahedral (Co2+) sites mostly. The CEMS data indicated that a fraction of the octahedral Co2+ ions exchanged sites with tetrahedral Fe3+ ions, which was stimulated by Mn doping for both MnxCoFe2-xO4 and MnxCo1-xFe2O4. The VSM data exhibited a large increase of saturation magnetization for both MnxCoFe2-xO4 and MnxCo1-xFe2O4 compared to CoFe2O4. Such an enhancement of the magnetization can be understood in terms of a relaxation of the ferrimagnetic order caused by Co2+ migration.