I have gone to MH measurement at RT of Mn-doped TiO2 samples up to 6%. It was seen that up to 2% remanence increase but after that it decreases. Why ?
It may be due to strong Mn2+-Mn2+ or Mn2+-O2--Mn2+ antiferromagnetic interaction. At lower doping concentration the magnetization comes from the isolated Mn2+ ions. However, at higher doping concentration the average distance between the Mn2+ ion decreases and the Mn2+ ions are now paired up. The pairing takes place nearest to the oxygen ligands bonded to the Mn2+ ions. A strong antiferromagnetic superexchange interaction prevents ferromagnetic alignment of the magnetic spins, resulting in the decrease in magnetization.
Please also see these papers....
http://pubs.rsc.org/en/content/articlepdf/2016/RA/C6RA06606B
http://iopscience.iop.org/article/10.1088/0022-3727/41/5/055006/pdf
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