Long range magnetic forces will always overcome short range electrostatic or charge related forces. (Note: van der Waals forces are always attractive). The only route to make such a system would be a bottom-up process with steric stabilization such as a 50 kDa PEG or PEI. Then the slipping plane is located in the polymer layer.
Elida Nekovic That's a good technique, yes, especially with the high density of magnetite. It should be mandatory to carry out a Stokes' law calculation for Fe3O4 so that the size at which particles will stay in suspension can be gauged. Such a calculation (for a material of density 5.5 g/cm3) can be found in Table 1 of the ASTM standard E2490-09 (15)