XPS on Co0.95R0.05Fe2O4 Nanoparticles with R = Gd or Ho

Co0.95R0.05Fe2O4 nanoparticles were synthesized using a sol-gel approach incorporating bio-based agents and were found to be single phases adopting a cubic Fd-3m structure. XPS shows the presence of Gd3+ and Ho3+ ions. The spin–orbit splitting of about 15.4 eV observed in Co 2p core-level spectra is an indication that Co is predominantly present as Co3+ state, while the satellite structures located at about 6 eV higher energies than the main lines confirm the existence of divalent Co in Co0.95R0.05Fe2O4. The positions of the Co 3s and Fe 3s main peaks obtained by curve fitting and the exchange splitting obtained values for Co 3s and Fe 3s levels point to the high Co3+/Co2+ and Fe3+/Fe2+ ratios in both samples. The saturation magnetizations are smaller for the doped samples compared to the pristine ones. For theoretical magnetization calculation, we have considered that the heavy rare earths are in octahedral sites and their magnetic moments are aligned antiparallelly with 3d transition magnetic moments. ZFC-FC curves shows that some nanoparticles remain superparamagnetic, while the rest are ferrimagnetic, ordered at room temperature, and showing interparticle interactions. The MS/Ms ratio at room temperature is below 0.5, indicating the predominance of magnetostatic interactions.