Please see:-https://www.researchgate.net/post/Can_anyone_suggest_the_simple_method_for_the_synthesis_of_magnetite_nanoparticles

https://www.researchgate.net/post/Which_method_should_I_use_for_synthesis_Magnetite_Fe3O4_for_hyperthermia_treatment

I have not used co-precipitation method for synthesis but as far as I know that is the method for high yields, I did however synthesize magnetite nps using hydrothermal method. what is the kind of yields you are looking for?

In your case, if it is good to ignore morphological and other stability irregularities, undemanding method to synthesize Magnetite nanoparticles at reasonably good yield is Indeed Co-precipitation. You may use +2 and +3 salts of Iron as precursors. Remember maintaining pH, Inert condition and temperature are decisive. Try to maintain basic pH (Use ammonia solution 25%), do not overheat the reaction as it may result in phase change. Careful washing and separation using magnet will produce a good yield.

However, despite the difficulties in the reaction setup other methods like hydrothermal and thermal decomposition produces high-quality monodisperse shape-controlled iron oxide nanoparticles.