Magnetic nanoparticles should not be put into a FEG-SEM, you may have seriously damaged the microscope. If your material can be stabilised so the NPs are captive then it may be possible, but again magnetic materials will affect the ability to control the stigmator and reduce the chance of obtaining a focussed image. Any organic/gel material will be liable to beam damage. You can either use very low probe currents to mitigate or try freezing the sample on a coldstage.