The successful methods for high-quality Iron-Oxide nanoparticles are thermal decomposition methods (leading to mono disperse NPs ) and co-precipitation methods (leading to polydispersity)

You can use various methods for the synthesis of iron nano particles e.g. co-precipitation, microemulsion, thermal decomposition, solvo thermal, sonochemical, microwave-assisted, chemical vapor deposition, combustion, carbon arc, and laser pyrolysis.

But in general the green methodology is the best. you can see the below attachments for more details. 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5245755/

Article Current methods for synthesis of magnetic nanoparticles

The most powerful technique for synthesizing the high-quality MnO2 nanoparticles is Sputtering (PVD) method which gives the more uniform particle size distribution and highly dispersed nanoparticles. Therefore, it might be used to fabricate the Contamination free and High quality Fe2O3 NPs.

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