several methods of coating or functionalization of magnetic nanomaterials have been reported in the literature. it all depend on your intended application.

attached is a method we reported recently

Article A Sonochemical Approach to the Direct Surface Functionalizat...

Phosphonic acids should bind stronger with the NPs surface than oleic acid. You can try octadecylphosphonic acid (ODPA).

you can use different polymers, PEG, TEG, Oleic acid, PVP, PVA etc. But I suggest you to use a chemical method for coating.  Since your nanoparticles are FeNi, you should be careful about oxidation of your samples during coating process.

good luck

Thank you for all your answers. So now I know what materials I can use to coat, my final question would be how to coat. Is it as easy as pouring the surfactant and the powder together? 

Thanks for any suggestions

I think you can use hydrothermal or solvothermal method and use metal chlorides or metals acac, metal nitrates etc as starting materials. you can look in literature for synthesized methods.

If you previously prepared aggregated FeNi nanoparticles , it is not easy to coat them with polymers. I think you should prepare the nanoparticles in a hot polymer solvent.

but be careful about oxidation. Ethylene glycol can be a good candidate for start.

it depends on the facilities you have and your understanding of the techniques