Magnetic homogeneity in Fe-Mn co-doped NiO nanoparticles

The effect of Fe and Mn co-doping on the magnetic properties of the antiferromagnetic (AFM)NiO nanoparticles which offer large potential for different magnetic applications have beenstudied. The Rietveld refinement fitting of powder x-ray diffractometry (XRD) patternsconfirmed the phase formation of face-centred cubic crystal structure of NiO and averagecrystallite size lies in the short range of 32–38 nm. The cavity and broadband ferromagneticresonance (FMR) measurements taken at room temperature demonstrate the smaller localmagnetic inhomogeneity for 4%Mn-4%Fe co-doped NiO nanoparticles as compared toundoped, single doped and co-doped with different concentration NiO nanoparticles. The M-Hloops revealed the room temperature ferromagnetism-like behaviour for higher Fe dopingconcentration and lower Mn doping concentration. This can be attributed to the doubleexchange interaction. The zero field cooled (ZFC) and field cooled (FC) dc magnetizationcurves showed a small surface freezing peak (atTf)at low temperatures and a blocking peak(atTB)at higher temperatures. For samples with 4%Mn-4%Fe and 2%Mn-6%Fe, the blockingpeak was found at a relatively high temperature in comparison to other samples. This can beattributed to the presence of magnetic exchange interactions which block the magnetic spinsagainst a thermal increase. The ZFC AC-susceptibility showed three peaks; a surface freezingpeak at Tf, a blocking peak at TBpeak and an anomalous peak at Txin betweenTfandTB,which was found to be most prominent for the 4%Mn-4%Fe co-doped nanoparticles. Theneutron diffraction pattern confirmed the AFM order of the core of the 4%Mn-4%Fe co-dopednanoparticles, which indicates an AFM coupling between the Fe2+and Mn2+ions and the Ni2+ions through super-exchange interaction. Therefore, the origin of TXpeak can be attributed tothe ferromagnetic coupling between the Fe2+and Mn2+ions which has a maximum strength atequal concentration. Thus, small and equal doping concentration of Fe and Mn in NiOnanoparticles increase the magnetic homogeneity which makes them attractive for magneticapplications