There are several ways like using proper surfactant but below link might give some interesting information

http://nano.iphy.ac.cn/N04/papers/NO4_papers%20all%20pdf/HJ_Gao.0826.pdf

You may be able to get it to disperse but, without agitation, the dispersed particles will quickly sediment due to their large size and density.

If your polymerization process is under stirred conditions, the particles may remain sufficient suspended.

Note that particles can be dispersed yet still sediment. My assumption is you mean dispersed such that they don't aggregate (?)

I am afraid your particles are too big to be dispersed. There should be some methods to synthesize Fe3O4 nanoparticles like hydrothermal...., if you can do that.

Use surfactants, or use ultrasound.