You should search about the Isoelectric point (IEP). It's a pH value meaning that the spinel will be positively charged below this pH. The below link is about this subject.

Good luck

http://onlinelibrary.wiley.com/doi/10.1111/j.1551-2916.2012.05360.x/abstract;jsessionid=D0F64433EB510A3C3352770CDDDB69CF.f02t03?deniedAccessCustomisedMessage=&userIsAuthenticated=false

You may need to coat the particles with polymers of the desired charge after synthesis.

I believe the pH will be more effective in your case since you may not want a polymer cap layer.