Several points need your consideration:

• At 500 - 1000 nm then such systems would not be defined as 'nanoparticles' by international definition (ISO & ASTM). Sub-micron could be a better definition.
• Fe3O4 is magnetite and thus magnetic and these long range forces cause combination and settling. Steric stabilization is the best way of preserving stability - although at these larger sizes, inertial forces will be much larger than attactive van der Waals
• Fe3O4 is also dense and settles easily. Have you performed a Stokes' Law calculation? Such material of 500 - 1000 nm cannot stay in free suspension. I assume you've considered this and do not want the material to stay in free suspension? My calculations (ASTM E2490-09) indicate that the material needs to be
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You may consider coating very low SG materials with Fe3O4 instead of having the same material to the core and losing the primary advantage of nanosize.....surface area!. As correctly pointed out by Alan, if suspension is desired, the size is too big. If functionality is desired with larger particle size, porous materials such as ion exchange resins or polymeric adsorbents with large surface areas and already approved for drug delivery might be considered as base materials coated with Fe3O4.

Hope this helps.

I say the same, if the material has 500 to 1000 nm, is not a nanomaterial. Maybe, you can use iron sulphate, the method of Schwertmann, You can look for the "Iron oxides in the Laboratory. Preparation and Characterization"

Hi, you can refer this paper for preparation of Fe3O4 nanoparticles : Angew. Chem. Int. Ed. 2010, 49, 607–61. Just play with the amount of trisodium citrate.

@ Raj Kumar: from your question 'using Fecl3 and Fecl2 compounds and ammonium hydroxide as reducing agent'

Please tell us where the change in oxidation state is.....

Just improve the concentration and temperature

Less amount of ammonium hydroxide may support the enhancement in particles size.Temprature also can play a crucial role in enhancement of particles size during preparation of nanoparticles.